tag:blogger.com,1999:blog-71057286146110103252024-03-28T20:29:55.416-07:00Seismic ReflectionsA technical blog about seismic data processing (powered by GLOBE Claritas)GLOBE Claritashttp://www.blogger.com/profile/00646197047930265069noreply@blogger.comBlogger16125tag:blogger.com,1999:blog-7105728614611010325.post-35440443418497670312015-04-30T15:40:00.002-07:002015-04-30T15:40:42.656-07:00Moore's Law, Inflation, and Seismic Processing<div style="background-color: white; border: 0px; box-sizing: border-box; font-stretch: inherit; line-height: 24px; margin-bottom: 30px; padding: 0px; vertical-align: baseline;">
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In 1995 on board the <strong style="border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border: 0px; box-sizing: border-box; font-stretch: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; margin: 0px; padding: 0px; vertical-align: baseline;"><em style="border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border: 0px; box-sizing: border-box; font-stretch: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; margin: 0px; padding: 0px; vertical-align: baseline;">M/V Polar Search</em></strong> we started to do full 3D seismic processing using a single <a href="http://en.wikipedia.org/wiki/HP_9000" rel="nofollow" style="border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border: 0px; box-sizing: border-box; font-stretch: inherit; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; margin: 0px; padding: 0px; text-decoration: none; vertical-align: baseline;" target="_blank">HP Apollo 735 Workstation</a>. We had 15Gbytes of disk space, and a 66MHz clock-speed on the microprocessor. </span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">For less than $8000 I can now buy a system with 16 cores on the microprocessor, each with a clock-speed of 2GHz - in fact I can order it directly from a website.</span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Seismic processing, by its nature, is “<a href="http://en.wikipedia.org/wiki/Embarrassingly_parallel" style="border: 0px; box-sizing: border-box; font-stretch: inherit; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; margin: 0px; padding: 0px; text-decoration: none; vertical-align: baseline;" target="_blank">embarrassingly parallel</a>”. We can usually find some way of blocking a problem up trace-by-trace, shot-by-shot, offset-by-offset or in tiles so that we can have multiple computes work on it at once. These days that means multiple cores.</span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In practice, the <strong style="border: 0px; box-sizing: border-box; font-stretch: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; margin: 0px; padding: 0px; vertical-align: baseline;">$8,000 desktop workstation</strong> can crunch through seismic processing problems around <strong style="border: 0px; box-sizing: border-box; font-stretch: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; margin: 0px; padding: 0px; vertical-align: baseline;">485</strong> times faster than the HP735 could!</span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://en.wikipedia.org/wiki/Moore%27s_law" rel="nofollow" style="border: 0px; box-sizing: border-box; font-stretch: inherit; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; margin: 0px; padding: 0px; text-decoration: none; vertical-align: baseline;" target="_blank">Moore’s Law</a> – named for Gordon E Moore, co-founder of Intel – suggests that computer power should double every 2 years. This was based on the number of transistors you could get onto a chip and since 1965, when Moore suggested this was the case, computer architecture has changed a lot.</span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In the 20 years since I started on-board processing, by Moore’s law we would have hoped to see the computer speed go up by a factor of </span><span style="font-weight: bold; line-height: 18.2000007629395px; text-align: left;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">2<sup style="line-height: 0.9;">10</sup></span></span><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">, or <strong style="border: 0px; box-sizing: border-box; font-stretch: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; margin: 0px; padding: 0px; vertical-align: baseline;">1024</strong> <strong style="border: 0px; box-sizing: border-box; font-stretch: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; margin: 0px; padding: 0px; vertical-align: baseline;">times</strong> in total, which makes the factor of<strong style="border: 0px; box-sizing: border-box; font-stretch: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; margin: 0px; padding: 0px; vertical-align: baseline;"> 485</strong> seem a little low.</span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">However the HP735 would have weighed in at around US$30,000 (US$47,000 allowing for inflation).</span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">If you look at what you can get now for US$47,000, this outstrips Moore’s Law by some distance – nearly <strong style="border: 0px; box-sizing: border-box; font-stretch: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; margin: 0px; padding: 0px; vertical-align: baseline;">2850 times </strong>the processing power of that first on-board system.</span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">To put that in context, a modern system <strong style="border: 0px; box-sizing: border-box; font-stretch: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; margin: 0px; padding: 0px; vertical-align: baseline;">could process as much data in half an hour as we could process in an entire 8-week rotation</strong>, or a whole <strong style="border: 0px; box-sizing: border-box; font-stretch: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; margin: 0px; padding: 0px; vertical-align: baseline;">year’s worth of computing</strong> in just <b>3</b><strong style="border: 0px; box-sizing: border-box; font-stretch: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; margin: 0px; padding: 0px; vertical-align: baseline;"> hours!</strong></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Of course, back in 1995 we were stuck mounting 240Mbyte <a href="http://en.wikipedia.org/wiki/IBM_3480_Family" rel="nofollow" style="border: 0px; box-sizing: border-box; font-stretch: inherit; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; margin: 0px; padding: 0px; text-decoration: none; vertical-align: baseline;" target="_blank">3480 cartridges </a>to read the field data; meaning the CPU was mostly idle apart from when we were applying (then state of the art) 3D <a href="http://wiki.seg.org/wiki/Dip-moveout_correction" style="border: 0px; box-sizing: border-box; font-stretch: inherit; font-style: inherit; font-variant: inherit; font-weight: inherit; line-height: inherit; margin: 0px; padding: 0px; text-decoration: none; vertical-align: baseline;" target="_blank">Dip Move Out</a>. The surveys were smaller and we were only using 3x 1500m long cables.</span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Even so, the rate at which computing demands for seismic processing have increased is far slower than the rate at which the cost of processing has decreased. </span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">And with disk-space also increasing by a factor of 500 or more, keeping track of hundreds of tapes and standing next to the machine to mount them for each processing run is thankfully a thing of the past too.</span><br />
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<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">By: <b>Guy Maslen</b></span></span></div>
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GLOBE Claritashttp://www.blogger.com/profile/00646197047930265069noreply@blogger.com0tag:blogger.com,1999:blog-7105728614611010325.post-49038775479693795902014-05-26T19:15:00.000-07:002014-05-26T19:15:36.228-07:00Marine Processing - Part 10 | Migration (continued)<table border="1" cellpadding="0" cellspacing="0" class="MsoNormalTable" style="border-collapse: collapse; border: none; text-align: justify; width: 640px;"><tbody>
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<i><span style="color: #444444; font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This sequence of blog posts will build up into a complete description of a 2D marine processing sequence and how it is derived.</span></i></div>
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<i><span style="color: #444444; font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">No processing sequence is definitive and techniques vary with time (and software), however the idea is to provide a practical guide for applying seismic processing theory.<o:p></o:p></span></i></div>
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<i><span style="color: #444444; font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The seismic line used is TRV-434 from the Taranaki Basin offshore New Zealand. It is the same marine line used in our basic tutorial dataset.<o:p></o:p></span></i></div>
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<i><span style="color: #444444; font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The data are available from New Zealand Petroleum and Minerals, under the Ministry of Economic Development under the “Open File” System.<o:p></o:p></span></i></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The processing sequence developed so far:<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/05/marine-processing-part-1-resample.html"><b>Resample and Minimum Phase Conversion </b></a><b> </b></span></div>
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<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Reformat from field data (in this case, SEGY)</span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Apply a combined minimum-phase conversion and anti-alias filter</span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Resample to a 4ms sample interval</span></li>
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<b><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-2-initial-qc.html"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">QC and Assign Geometry</span></a></b></div>
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<b><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-3-amplitudes.html"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Amplitude Recovery</span></a></b></div>
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<b><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-4-swell-noise.html"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Swell Noise Suppression</span></a></b></div>
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<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Interpolation to 6.25m group interval, 480 channels to unwrap spatially aliased dips</span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="text-indent: -18pt;">Tau-p transform 500ms robust AGC "wrap", 960 p-values and transform range from </span><span style="line-height: 18px;">-1400ms <sup>-1</sup> to +1400 ms<sup>-1</sup></span></span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Tail mute to remove linear arrivals and linear noise</span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Predictive deconvolution: 32ms gap, 480 ms operator</span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Rho filter for low frequency compensation</span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Inverse Tau-p transfrom</span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Interpolation back to 25m group interval, 240 channels</span></li>
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<span style="color: #0000ee; font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><b><u><a href="http://seismicreflections.globeclaritas.com/2013_10_01_archive.html" target="_blank">Picki<span id="goog_939294291"></span><span id="goog_939294292"></span>ng Velocities</a></u></b></span></div>
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<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">NMO correct with 90% of primary velocity</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">Form 120-fold supergathers</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">Parabolic Radon Demultiple</span></li>
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<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">Transform range -300ms to 500ms far-offset moveout;</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;"> </span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">200 p-values</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">Multiple</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;"> </span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">removed from +24 to +500ms far-offset moveout</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">Application from 1400ms onwards</span></li>
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<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">For 60-fold CDP gathers from supergathers</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">Remove 90% NMO correction</span></li>
</ul>
<br />
<span style="line-height: 18.399999618530273px;"><b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013_10_01_archive.html">Re-pick Velocit</a></span></b></span><b style="line-height: 18.399999618530273px;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013_10_01_archive.html">ies</a></span></b><br />
<span style="line-height: 18.399999618530273px;"><b><br /></b></span></div>
</td></tr>
</tbody></table>
<br />
<div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The <a href="http://seismicreflections.globeclaritas.com/2014/04/marine-processing-part-10-migration.html">last
post</a> really covered the theoretical basics of how to apply pre-stack time
migration to the dataset, as well as the kinds of parameters you might
encounter. In practice there are typically a few more things you will need to
think about.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The first is to be aware that Kirchhoff pre-stack time
migration is kinematic; that is to say the migration operator is going to
redistribute the seismic energy across the seismic section. <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Depending on how the algorithm has been implemented you
might need to remove the <a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-3-amplitudes.html">spherical
spreading</a> part of your amplitude correction prior to the migration, and
then run a new sequence of amplitude recovery tests after the migration.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh5Wthr0Wiq-b1N2DecTlFU103EOrstZeMT7pc7_LhFj5QXPhMs6qr5gbLOc3uKreKqLPHNBaoviHxt0WxZt2_8D6upFlqfZIrJZrG3eLFwmiNqLqY2rT31XXpOQkyjUIMLhh7fnvMuwRQ/s1600/one.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh5Wthr0Wiq-b1N2DecTlFU103EOrstZeMT7pc7_LhFj5QXPhMs6qr5gbLOc3uKreKqLPHNBaoviHxt0WxZt2_8D6upFlqfZIrJZrG3eLFwmiNqLqY2rT31XXpOQkyjUIMLhh7fnvMuwRQ/s1600/one.jpg" height="632" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;">Pre-stack time migrated data from TRV-434. The panel on the left has had the spherical divergence correction removed before migration; the panel on the right has not. The cancellation of the migration operators is less effective where the amplitude correction has not been removed (red circled area) resulting in a noisier result with more artefacts</span></td></tr>
</tbody></table>
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</div>
<br />
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Another place where migration artefacts can be created is at
the start and end of the seismic line, where we have incomplete summation of
the migration operators. As we get to within one migration operator width of the
edge of the data, the quality of the migrated image starts to deteriorate until
at the very edge we only have artefacts.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This is most obvious in the “taper on” zone at the start of
the line, however you will have similar effects at the end of the line, or
where there are any gaps in the data.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh1_x6vJp5t7_XP_3ebaSLgMX0jZ-ysNK-5BvmMrWp8NWTQmHxP2vXZfeRGlQ35DmrB02BJY_rmGU5MTAsGpBUoSI7UEXHzrsJUaG13yidSEAHNFWoXcvpz6r5LGjotmFHoG_TrpjFrRLE/s1600/two.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh1_x6vJp5t7_XP_3ebaSLgMX0jZ-ysNK-5BvmMrWp8NWTQmHxP2vXZfeRGlQ35DmrB02BJY_rmGU5MTAsGpBUoSI7UEXHzrsJUaG13yidSEAHNFWoXcvpz6r5LGjotmFHoG_TrpjFrRLE/s1600/two.jpg" height="612" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;">The stacked section (left) and pre-stack time migrated section (right) from at the start of line TRV-434. Shooting direction is left-to-right. The “taper on” has been populated with migration “swings” where there is no more data to cancel out the un-needed parts of the operator (blue boxed area). The image will also be incorrectly formed where there isn’t a full migration operator to contribute to the data</span></td></tr>
</tbody></table>
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<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">These effects can be managed by scaling down the data at the
start and end of the seismic line, using a function that is approximately the
same shape as the operator. This is sometimes built into the migration routines
as an optional pre-migration scaling. The scaling can also be approximated with
a simple function that leaves the data un-scaled at full fold and ramps to zero
amplitude at zero fold.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The next thing you need to think about is the offsets to
use.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The pre-stack time migration works by forming the data into
common-offset planes; we want to make sure these will be fully populated since
gaps in the data will result in imaging artefacts.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In this case, we have data with 25m spacing between shotpoints
and 25m between receivers - which means that the full 120 channels in each shot
are divided up between two CDPs. <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">There’s a full discussion of this <a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-2-initial-qc.html">here</a>,
which shows how adjacent CDPs will have the same number of traces (60) but
different offsets.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">We have two choices. We can either: (1) migrate to 120
offset planes corresponding to each offset present in the shot record, or (2) migrate
to 60 offset planes corresponding to the fold of the CDPs.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">With 120 offset planes, we’ll have gaps that need to be
interpolated. We could do this pre-migration or allow the migration to fill in
the gaps for us, accepting that it might create some noise artefacts which we will
need to remove.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">With 60 offset planes, we’d want to choose those that minimise
the difference between the actual offset of any given trace, and the target offset
plane it will contribute to. <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The simplest way to do this is to average the offsets that
will make up each offset plane. In this case the first offset plane will be at
270.5m; half way between the offset for channel 120 (the near offset) and
channel 119 (the next offset).<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The biggest impact here is on run-time. If we take 120
offset planes and interpolate the data, the migration will take twice as long. Depending
on time pressure, sometimes migrating to less offset planes than the fold (such
as 30 planes, 100m apart) can be a good idea to get a result faster.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">If there are any strong amplitude contrasts in the data –
for example a noisy trace we missed in the edit stage – these are going to also
cause problems in the migration. The migrated energy will not sum correctly and
will leave an imprint of the migration operator in the data. <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhIAEgbzCQkqpIy4KbYxIVaA7EC58xHW8Ljqt2KtD8bv-wMkw1HIWrUI4yZq_7yWXvaOVXOEG-6dPipOj97whyX7-fuEYR5ieFOkFC4ne3onihynCKWmj7MTZ03jQ9HDCPoZXO3tvhVyJ4/s1600/three.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhIAEgbzCQkqpIy4KbYxIVaA7EC58xHW8Ljqt2KtD8bv-wMkw1HIWrUI4yZq_7yWXvaOVXOEG-6dPipOj97whyX7-fuEYR5ieFOkFC4ne3onihynCKWmj7MTZ03jQ9HDCPoZXO3tvhVyJ4/s1600/three.jpg" height="632" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;">Pre-stack time migrated data from line TRV434; the right hand panel has had a series of 7Hz Ricker wavelets added on the input data at CDP 1100, offset 1733m, which are approximately two orders of magnitude higher than the surrounding data. While these artefacts show through clearly, a single spike on a trace would be more subtle</span></td></tr>
</tbody></table>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Unlike other Pre-stack processing, you cannot avoid this
kind of amplitude-based artefact with a <a href="http://seismicreflections.globeclaritas.com/2013/04/agc-equaliser.html">removable
AGC</a> prior to the migration. This would prevent all of the amplitudes
summing correctly and would create more noise. The only solution is to <a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-3-amplitudes.html">check
the data carefully for high amplitudes</a> as we did earlier in the sequence.</span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The extent to which you can afford to test all of the
parameters and issues worked through here <a href="http://seismicreflections.globeclaritas.com/2014/04/marine-processing-part-10-migration.html">and
in the previous post</a> depends upon your time, project objectives, geological
complexity and the resources at hand.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">A good “baseline” to start off with is a 45-degree dip angle
and a 3000m aperture, and migrating to at least as many offset planes as you
have fold of coverage. With these limits the aperture will only start to
restrict the migration operator around 2-3 seconds TWT.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">It can also be worth looking at the approximate dips you
want to image. Here there are dips of about 4ms/trace on a post-stack time
migration, which (when I use the average interval velocity calculated from my
velocity functions of 2750 ms<sup>-1</sup>, and remembering that it is two way
time) corresponds to 5.5m/trace, or an angle of ~ 24<sup>o </sup>(tan<sup>-1</sup>
[5.5/12.5]).<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This 60-fold migration took 696 seconds to run on my humble
laptop (4 x 2.6Ghz cores, multithreaded to 8); this was running in parallel
using 5 processes so that I could carry on working (as <a href="http://en.wikipedia.org/wiki/Multithreading_(computer_architecture)">multi-threading</a>
is not quite the same as having real cores.)<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">While all migrations used different algorithms and scale
differently, here are some runtimes I got:<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: center;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div style="text-align: center;">
<table border="1" cellpadding="0" cellspacing="0" class="MsoTableGrid" style="border-collapse: collapse; border: none; text-align: center;">
<tbody>
<tr>
<td style="border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal" style="text-align: center;">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Fold<o:p></o:p></span></b></div>
</td>
<td style="border-left: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal" style="text-align: center;">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Angle<o:p></o:p></span></b></div>
</td>
<td style="border-left: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal" style="text-align: center;">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Aperture<o:p></o:p></span></b></div>
</td>
<td style="border-left: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal" style="text-align: center;">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Stretch<o:p></o:p></span></b></div>
</td>
<td style="border-left: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal" style="text-align: center;">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Run time<o:p></o:p></span></b></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">60<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">45 degrees<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">3000m<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">120%<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">696s<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">60<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">60 degrees<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">3000m<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">120%<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">799s<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">60<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">60 degrees <o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">6000m<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">120%<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">1183s<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">60<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">75 degrees<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">6000m<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">120%<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">1204s<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">60<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">75 degrees<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">9000m<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">120%<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt;" valign="top"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">1313s<o:p></o:p></span></div>
</td>
</tr>
</tbody></table>
</div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">As you can see, opening up both the angle and aperture
starts to make a big difference to the run-time of the migration. The diagram
in <a href="http://seismicreflections.globeclaritas.com/2014/04/marine-processing-part-10-migration.html">this
entry</a> shows how these interact and why adjusting only the angle makes
little difference. It is also worth remembering in this case that the seismic
line is less than 24km long, so wider apertures will only impact the centre of
the line.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In these examples I have not modified the stretch mute or
the degree of anti-alias filtering; these have an impact on the runtime as well
as the image quality at higher angles and bigger ranges.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In this case it makes sense to focus on the central portion
of the line – between CDPs 800 and 1500, and between 2000ms and 3500ms
two-way-time – as this was the main structural target.<o:p></o:p></span></div>
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgZYw8Z97im1-jJ3TBPNXAnEpk48eYYAp2bhAk64rSRRCl1jvzSMDHA1wr1ur7EUbWUn6YW0XYEstI01E9RPlxlwSiRf9k1fZjzziTr4oMz-ZFnmQVG2KvAg1ktEICQbqEhviXFibhzHo8/s1600/four.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgZYw8Z97im1-jJ3TBPNXAnEpk48eYYAp2bhAk64rSRRCl1jvzSMDHA1wr1ur7EUbWUn6YW0XYEstI01E9RPlxlwSiRf9k1fZjzziTr4oMz-ZFnmQVG2KvAg1ktEICQbqEhviXFibhzHo8/s1600/four.jpg" height="204" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;">A section of line TRV-434 Pre-stack time migrated, with a 15</span><sup style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;">o</sup><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;"> (left), 30</span><sup style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;">o</sup><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;"> (centre) and 75</span><sup style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;">o</sup><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;"> (right) dip limit. Aperture is constant at 6000m</span></td></tr>
</tbody></table>
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<br /></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Running a full suite of aperture and dip limit tests can be
time consuming and, as in this case, there is often a compromise on the dip
limit in terms of image quality and introduced noise. Though I’ve not shown the
full suite of tests here, the lower dip angles give a cleaner result - which
makes sense when you look at the rough dip calculation above.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">I have chosen a 30<sup>o </sup>angle and a 6000m aperture as
it seems to be the best compromise.<o:p></o:p></span></div>
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<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgN23dEzJl7ToiOBwpNe72xxPKzQ4XvSLDDkKeWPY-29eWBom7ErumURSs7f31mNnix28_UV3xkmsLsC1XaH4yI4tAapik066gi4SroiHzJjMlA0euw29NVEyihY1NunFeRhSLqx5C7kcg/s1600/five.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgN23dEzJl7ToiOBwpNe72xxPKzQ4XvSLDDkKeWPY-29eWBom7ErumURSs7f31mNnix28_UV3xkmsLsC1XaH4yI4tAapik066gi4SroiHzJjMlA0euw29NVEyihY1NunFeRhSLqx5C7kcg/s1600/five.jpg" height="500" width="640" /></a></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: left; margin-right: 1em; text-align: left;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh9r191FIFRoW5fcYsn8L_RBCp2UKoHCW1B_1C0NardGhQgT3uIGhRlRDTmPj6RZaLnaAkzo6TYXz_yWwwAvBdrtsSODgiahOSn5N8OqhO6P5XhHsoQJcwsI3HL0FE6a-t7bbS1Uur20cg/s1600/six.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh9r191FIFRoW5fcYsn8L_RBCp2UKoHCW1B_1C0NardGhQgT3uIGhRlRDTmPj6RZaLnaAkzo6TYXz_yWwwAvBdrtsSODgiahOSn5N8OqhO6P5XhHsoQJcwsI3HL0FE6a-t7bbS1Uur20cg/s1600/six.jpg" height="500" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;">Line TRV434 stacked (top), and with Pre-stack time migration applied (bottom). Both sections have a 500ms AGC applied. Migration dip limits were 35</span><sup style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;">o</sup><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;"> and the aperture used was 6000m</span></td></tr>
</tbody></table>
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<br /></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">With the migration completed - which as I mentioned in the
last entry might include re-picking velocities and re-migration of the data –
the bulk of the processing is now completed.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Post-migration
processing is sometimes termed “finalisation” and usually includes some random
noise attenuation, scaling, and filtering before creating a final SEGY output.</span></span></div>
</div>
<div>
<br /></div>
<div>
<br />
<div class="separator" style="clear: both; text-align: center;">
<span style="font-family: "Calibri","sans-serif"; font-size: 11pt; line-height: 17px;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" /></a></span></div>
<div>
<span style="font-family: "Calibri","sans-serif"; font-size: 11pt; line-height: 17px;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></span></div>
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<span style="font-family: "Calibri","sans-serif"; font-size: 11pt; line-height: 17px;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">By: <b>Guy Maslen</b></span></span></div>
</div>
GLOBE Claritashttp://www.blogger.com/profile/00646197047930265069noreply@blogger.com0tag:blogger.com,1999:blog-7105728614611010325.post-47438819300709760712014-04-28T17:39:00.001-07:002014-04-28T17:39:04.429-07:00Marine Processing - Part 10 | Migration<table border="1" cellpadding="0" cellspacing="0" class="MsoNormalTable" style="border-collapse: collapse; border: none; text-align: justify; width: 640px;"><tbody>
<tr><td style="border: 1pt solid windowtext; padding: 0cm 5.4pt; width: 182.6pt;" valign="top" width="243"><div class="MsoNormal">
<div style="text-align: left;">
<i><span style="color: #444444; font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This sequence of blog posts will build up into a complete description of a 2D marine processing sequence and how it is derived.</span></i></div>
</div>
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</div>
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<div style="text-align: left;">
<i><span style="color: #444444; font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">No processing sequence is definitive and techniques vary with time (and software), however the idea is to provide a practical guide for applying seismic processing theory.<o:p></o:p></span></i></div>
</div>
<div class="MsoNormal">
<div style="text-align: left;">
<i><span style="color: #444444; font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></i></div>
</div>
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<i><span style="color: #444444; font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The seismic line used is TRV-434 from the Taranaki Basin offshore New Zealand. It is the same marine line used in our basic tutorial dataset.<o:p></o:p></span></i></div>
</div>
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<i><span style="color: #444444; font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></i></div>
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<i><span style="color: #444444; font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The data are available from New Zealand Petroleum and Minerals, under the Ministry of Economic Development under the “Open File” System.<o:p></o:p></span></i></div>
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</td><td style="border: 1pt solid windowtext; padding: 0cm 5.4pt; width: 297.7pt;" valign="top" width="397"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The processing sequence developed so far:<o:p></o:p></span></div>
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<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/05/marine-processing-part-1-resample.html"><b>Resample and Minimum Phase Conversion </b></a><b> </b></span></div>
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<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Reformat from field data (in this case, SEGY)</span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Apply a combined minimum-phase conversion and anti-alias filter</span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Resample to a 4ms sample interval</span></li>
</ul>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<br />
<div class="MsoListParagraphCxSpLast" style="text-indent: -18pt;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><o:p></o:p></span></div>
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<b><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-2-initial-qc.html"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">QC and Assign Geometry</span></a></b></div>
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</div>
<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="text-indent: -18pt;">Assign 2D marine geometry; 12.5m CDP spacing and 60 fold </span><span style="text-indent: -18pt;">gathers</span></span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">QC shots and near trace plots</span></li>
</ul>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<br />
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<b><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-3-amplitudes.html"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Amplitude Recovery</span></a></b></div>
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<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="text-indent: -18pt;">Amplitude recovery using a T</span><sup style="text-indent: -18pt;">2</sup><sub style="text-indent: -18pt;"> </sub><span style="text-indent: -18pt;">Spherical divergence correction</span></span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Amplitude QC and Edit using peak and RMS amplitude displays</span></li>
</ul>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<br />
<div class="MsoListParagraphCxSpLast" style="text-indent: -18pt;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><o:p></o:p></span></div>
<div class="MsoNormal">
<b><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-4-swell-noise.html"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Swell Noise Suppression</span></a></b></div>
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</div>
<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Swell noise suppression using projective filtering</span></li>
</ul>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><b><br /></b><b><a href="http://seismicreflections.globeclaritas.com/2013/07/marine-processing-part-5-direct.html" target="_blank">Linear Noise Suppression</a></b></span></div>
<div class="MsoNormal">
</div>
<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Interpolation to 6.25m group interval, 480 channels to unwrap spatially aliased dips</span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="text-indent: -18pt;">Tau-p transform 500ms robust AGC "wrap", 960 p-values and transform range from </span><span style="line-height: 18px;">-1400ms <sup>-1</sup> to +1400 ms<sup>-1</sup></span></span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Tail mute to remove linear arrivals and linear noise</span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Predictive deconvolution: 32ms gap, 480 ms operator</span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Rho filter for low frequency compensation</span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Inverse Tau-p transfrom</span></li>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Interpolation back to 25m group interval, 240 channels</span></li>
</ul>
<div>
<span style="color: #0000ee; font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><b><u><a href="http://seismicreflections.globeclaritas.com/2013_10_01_archive.html" target="_blank">Picki<span id="goog_939294291"></span><span id="goog_939294292"></span>ng Velocities</a></u></b></span></div>
<div>
<span style="color: #0000ee; font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><b><br /></b></span>
<span style="color: #0000ee; font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><b><br /></b></span>
<br />
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><b><a href="http://seismicreflections.globeclaritas.com/2014/01/marine-processing-part-9-demultiples.html">Multiple Removal</a></b></span></div>
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</div>
<ul>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">NMO
correct with 90% of primary velocity</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">Form
120-fold supergathers</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">Parabolic
Radon Demultiple</span></li>
<ul>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">Transform
range -300ms to 500ms far-offset moveout;</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">
</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">200 p-values</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">Multiple</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;"> </span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">removed from +24 to +500ms far-offset moveout</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">Application
from 1400ms onwards</span></li>
</ul>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">For
60-fold CDP gathers from supergathers</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">Remove 90%
NMO correction</span></li>
</ul>
<br />
<span style="line-height: 115%;"><b><br /></b></span>
<span style="line-height: 115%;"><b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013_10_01_archive.html">Re-pick Velocit</a></span></b></span><b style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013_10_01_archive.html">ies</a></span></b><br />
<span style="line-height: 115%;"><b><br /></b></span></div>
</td></tr>
</tbody></table>
<br />
<div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In practice, we only get coherent reflections (that are in
the correct lateral position) when the rock layers are flat and
continuous.</span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">One way to think of this is to consider the “zero-offset
normal ray” section. This is where the reflected ray-path between a source and
receiver at the same location at the surface makes a right-angle with the
reflecting layer.</span></div>
</div>
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<br /></div>
<div class="separator" style="clear: both; text-align: center;">
</div>
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgKIvaGnGGPlF2Idk-s7w9lOJPFS-RbBwYiAV9DoCDa3059uEw_xCCrlHo07MuoD8Esa0JX-qo1SNO2uBRsWLwH4qR9_tL7KsM_b66kEsBGk8XBG6IL9i6KnPAPBJRuXw8wQCrFMinWaAo/s1600/one.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgKIvaGnGGPlF2Idk-s7w9lOJPFS-RbBwYiAV9DoCDa3059uEw_xCCrlHo07MuoD8Esa0JX-qo1SNO2uBRsWLwH4qR9_tL7KsM_b66kEsBGk8XBG6IL9i6KnPAPBJRuXw8wQCrFMinWaAo/s1600/one.jpg" height="336" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><i><span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">An
example of “zero offset normal rays” that illustrates why only flat events will
be correctly positioned laterally on a normal ray section. The recorder at the surface has no
directional information, so both reflections will appear on the same trace at
different times, as shown on the right</span></span></i></td></tr>
</tbody></table>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">In effect the “zero offset normal ray” section is a stack – we have corrected the source-receiver offset to zero using the </span><a href="http://en.wikipedia.org/wiki/Normal_moveout" style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">NMO correction</a><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"> on a CDP gather, and then stacked (summed) the traces together.</span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">As well as the “ray path” view we can also think of how
waves propagate based on </span><a href="http://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle" style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">Huygen’s
Principle</a><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">. Where we have discontinuities (such as faults, or intrusions)
the secondary wavelets used in Huygen’s construction will not fully
constructively or destructively interfere, giving rise to diffractions.</span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br clear="all" style="mso-special-character: line-break; page-break-before: always;" />
</span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">When looking at a stacked section, the key things to
remember are that:</span></div>
</div>
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</div>
<ul>
<li style="text-align: justify;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">dipping events will be mispositioned laterally
and appear less steep than they really are</span></li>
<li style="text-align: justify;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">the steeper the dip the greater the degree of
mispositioning</span></li>
<li style="text-align: justify;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">discontinuities (such as faults, or intrusions)
will give rise to diffractions</span></li>
<li style="text-align: justify;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">diffracted events may cross reflected events</span></li>
</ul>
<div style="text-align: justify;">
<br /></div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In order to resolve these issues we need to <a href="http://www.xsgeo.com/course/mig.htm">migrate</a> the data.<o:p></o:p></span></div>
</div>
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiR_l1Etncc_VBK1m7BdAre71J4rlQfvXqxhps2dcfxF4_3kDNdWyHifRxBHNX4eD5kEzJNMfvkSX5wOUo-JpB_wRQx-jDrif7A_qPemRh4X_ErT9bwNfPXuumQxMiGX03wuV_t2o8554o/s1600/two.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiR_l1Etncc_VBK1m7BdAre71J4rlQfvXqxhps2dcfxF4_3kDNdWyHifRxBHNX4eD5kEzJNMfvkSX5wOUo-JpB_wRQx-jDrif7A_qPemRh4X_ErT9bwNfPXuumQxMiGX03wuV_t2o8554o/s1600/two.jpg" height="270" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><i style="font-size: medium; text-align: start;"><span style="line-height: 18.399999618530273px;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Stacked (left) and migrated (right) portions of the TRV-434 seismic line. The migrated events are a little steeper and have moved laterally up-dip. Diffractions caused by faults and events that are not continuous have been collapsed. Crossing complex events have been resolved.</span></span></i></td></tr>
</tbody></table>
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">Migration uses a mathematical model to reposition data to
its correct lateral position and collapse diffractions. This usually requires some kind of velocity
model; as computation speeds have improved it has become practical to use
algorithms that can work with more complex – or realistic – velocity
models.</span></div>
</div>
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<div style="text-align: justify;">
<br /></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Post stack migrations (of the type shown above) have largely
been replaced by pre-stack migration techniques. In pre-stack migrations the data are formed
into offset planes which are migrated to their near-offset position and time,
which replaces the NMO correction and the migration stage.<o:p></o:p></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In this case the line we are working on has some fairly steep
dips and complex ray-paths; after a couple of rounds of <a href="http://seismicreflections.globeclaritas.com/2013_10_01_archive.html">velocity
picking</a> you will have come across some situations where the velocities are
hard to pick; this is because we have
diffracted events (which do not have hyperbolic moveout) as well as dipping
events.</span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Dipping events cause problems not only because the
reflection hyperbola is distorted, but because the “common mid-point” is
smeared and shifted up-dip. <o:p></o:p></span></div>
</div>
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjkIndcdjrFR1e009AiFT5PI8Olxc9YFNx_WXgiPGtvO0dGgZFocsWkJhCqgeFvNxurHTNfghCr3ljP5hK_YfEU84U9sOnhyphenhyphen8JSRdiI8Bgl42FAmopjZrlRtxxmQL5Kz7fGNfge2Zr7-yk/s1600/three.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjkIndcdjrFR1e009AiFT5PI8Olxc9YFNx_WXgiPGtvO0dGgZFocsWkJhCqgeFvNxurHTNfghCr3ljP5hK_YfEU84U9sOnhyphenhyphen8JSRdiI8Bgl42FAmopjZrlRtxxmQL5Kz7fGNfge2Zr7-yk/s1600/three.jpg" height="236" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><i><span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Illustration
of how the common midpoint (CMP, also called CDP) does not contain information
from a single sub-surface location (or bin) when the events are dipping. The
green ray paths show what we might assume is happening when collect data by its
common mid-points, however the red ray paths show the actual ray paths for the
shortest (near) and longest (far) offsets – these are both displaced up dip, by
differing amounts</span></span></i></td></tr>
</tbody></table>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">Pre-stack migration resolves this issue allowing better
velocities to be picked and improved focussing to be obtained on dipping
events. For that reason it is common to create a new velocity field after
pre-stack time migration, and, in some cases, run a second iteration of the
pre-stack time migration with that new velocity model.</span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Velocities picked on pre-stack time migrated data are a
better approximation to the actual RMS velocity of the sub-surface layers, and
so can be more useful as a starting point for depth conversion.<o:p></o:p></span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">A good way to understand the migration parameters is to look
at some impulse response functions; this will show you the shape of the
migration operator with different parameters.
You can do this by zeroing all of the traces, and placing some suitable <a href="http://subsurfwiki.org/wiki/Ricker_wavelet">ricker wavelets</a> at
different time values then running migrations.<o:p></o:p></span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br clear="all" style="mso-special-character: line-break; page-break-before: always;" />
</span></div>
</div>
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<o:p><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></o:p></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh5A_3amnfB_M7KwG9Jx-7niuokbUP0m3T6gMf-HAIYF8-ucpXDrWkBFSTMPXEV0XD2TcAwBpxLrk7cQRCClxf8OCAEYqvkCGZit5RqaNrO1DvxMSgkYganDf2s9RZsrx5AFqQyNADstvA/s1600/four.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh5A_3amnfB_M7KwG9Jx-7niuokbUP0m3T6gMf-HAIYF8-ucpXDrWkBFSTMPXEV0XD2TcAwBpxLrk7cQRCClxf8OCAEYqvkCGZit5RqaNrO1DvxMSgkYganDf2s9RZsrx5AFqQyNADstvA/s1600/four.jpg" height="640" width="578" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><i style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: medium; text-align: start;">An impulse response function showing the full pre-stack time migration operator for a trace at CDP 1100, with an offset of 1283m, and 35Hz Ricker wavelets at 1000ms, 2000ms, 3500ms, 5000ms. Since the pre-stack time migration corrects to zero offset, the operators include a correction for normal moveout. The migration was run a “open” parameters and a migration velocity of 1480 ms.<sup>-1</sup></i></td></tr>
</tbody></table>
<div class="MsoNormal" style="margin-bottom: 0.0001pt;">
<br /></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The parameters we can vary do two things. They serve to speed up the migration (by
limiting the operator in terms of its spatial extent) as well as reduce or
remove artefacts from the data.<o:p></o:p></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The main artefacts we want to eliminate are <i>wavelet</i> <i>stretch, </i>which occurs for the same reason as <a href="http://seismicreflections.globeclaritas.com/2013_10_01_archive.html">NMO
stretch which picking velocities</a>, and aliasing of the operator shape. The
operator becomes aliased because (usually) the vertical sampling of the data
(time sample interval converted to distance) is at a much higher resolution
than the horizontal sampling (the CDP spacing).<o:p></o:p></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In this case, we have a 4ms sample interval at a constant
velocity of 1480ms<sup>-1</sup>, which corresponds to a 2.96m vertical
resolution (remembering we are looking at two-way-time), and the CDP spacing is
12.5m. As a result the operator will be <a href="http://seismicreflections.globeclaritas.com/2013_06_01_archive.html">spatially
aliased</a> at high angles.<o:p></o:p></span></div>
</div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiDSetg-nygVlLgvGYxNA2AEdfcVti5c3l13yfgUskUOB0IlnqfLpk1eSDLqFFpTXNCk3dVxx3iOnGjOpu5aCT6DDxr9ym6qxjuO5GmG0Ch1qz7p18-U5R0CcQLQzmjc-e-ClMW39tTFWs/s1600/five.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiDSetg-nygVlLgvGYxNA2AEdfcVti5c3l13yfgUskUOB0IlnqfLpk1eSDLqFFpTXNCk3dVxx3iOnGjOpu5aCT6DDxr9ym6qxjuO5GmG0Ch1qz7p18-U5R0CcQLQzmjc-e-ClMW39tTFWs/s1600/five.jpg" height="288" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><i style="font-size: medium; text-align: start;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Impulse response function of a 80Hz Ricker wavelets through pre-stack time migration with a constant velocity (left) with a close up variable-area/wiggle (VAWG) display on the right showing detail of the operator at a high dip angle (right); note the wavelet of the operator caused by spatial aliasing. To migrate this without aliasing we would need to be able to represent the 80Hz wavelet horizontally at 1480 ms<sup>-1</sup>, which corresponds to a full wavelength of 18.5m. With two samples per wavelength, this would need a CDP spacing of 9.25m, as opposed to the 12.5m spacing we have</span></i></td></tr>
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">The main controls we usually have in a pre-stack time migration
are:</span></div>
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></div>
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Name<o:p></o:p></span></b></div>
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Description<o:p></o:p></span></b></div>
</td>
<td style="border-left: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 154.05pt;" valign="top" width="205"><div align="center" class="MsoNormal" style="text-align: center;">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Impact<o:p></o:p></span></b></div>
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<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 147.15pt;" valign="top" width="196"><div class="MsoNormal">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Migration
Aperture, or Range<o:p></o:p></span></span></div>
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<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 160.9pt;" valign="top" width="215"><div class="MsoNormal">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The
horizontal distance that a trace is allowed to migrate. Cuts off the migration operator at a
specific distance, usually from the CDP position of the trace
pre-migration. <o:p></o:p></span></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 154.05pt;" valign="top" width="205"><div class="MsoNormal">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Reduces how
many input traces contribute to each output trace, and so makes the migration
run faster. Reduces the ability to
image steep dip, deep events. <o:p></o:p></span></span></div>
</td>
</tr>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Migration
Angle <o:p></o:p></span></span></div>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Limits the
maximum angle of the migration, usually measured from the vertical trace at
the CDP location out to the operator, so that 90 degrees is horizontal.<o:p></o:p></span></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 154.05pt;" valign="top" width="205"><div class="MsoNormal">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Reduces how
far each trace migrates, so requires less calculation runs more quickly if
applied in the algorithm and not as a mute.
Can act as a way of reducing both stretch and aliasing, and is
approximately the maximum dip to image.<o:p></o:p></span></span></div>
</td>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Anti-Alias
Filtering<o:p></o:p></span></span></div>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Acts to
limit the operator shape where it might become aliased or distorted
spatially; usually has various degrees
(weak, medium, strong) that can be applied<o:p></o:p></span></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 154.05pt;" valign="top" width="205"><div class="MsoNormal">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Little
impact on speed of migration, as it can be computationally expensive to
compute the limits even though less data is migrated; if applied as a mute can actually slow down
the migration. Avoid artefacts where
you have rapid lateral or vertical velocity gradients, and a single dip value
is inappropriate. <o:p></o:p></span></span></div>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Stretch
Muting<o:p></o:p></span></span></div>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Acts to
limit the operator if the shape of the wavelet is too distorted in time.
Similar effect to NMO stretch.<o:p></o:p></span></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 154.05pt;" valign="top" width="205"><div class="MsoNormal">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Reduces
low-frequency artefacts in the shallow-mid portion of the data. May cause the near surface to be muted,
unless there is a “near angle protection” option.<o:p></o:p></span></span></div>
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<o:p><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></o:p></div>
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<o:p><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></o:p></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg-FGo-fjn__tWrehRX5sbjtMB1Ny5_Bkrm_WaTsfkI4eEsAetEQcOr9rT-4FdGoiTdPAQOVr9DQX1DcfF-bsLSlOKhYp2q03Fw70jE4azFZ8fn3UGfZ9VWPBn9lASjy3szzlNLl1qc5w8/s1600/six.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg-FGo-fjn__tWrehRX5sbjtMB1Ny5_Bkrm_WaTsfkI4eEsAetEQcOr9rT-4FdGoiTdPAQOVr9DQX1DcfF-bsLSlOKhYp2q03Fw70jE4azFZ8fn3UGfZ9VWPBn9lASjy3szzlNLl1qc5w8/s1600/six.jpg" height="640" width="600" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><i style="font-size: medium; text-align: start;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Pre-stack time migration operators from an impulse response function, with some limits applied. In this case the red line shows the <b>angle</b> parameter, set to 60<sup>o</sup>, and the blue vertical line shows the effect of limiting the <b>migration aperture</b> to 2500m, which in this case corresponds to 200 cdps. The input trace is at CDP 1100. With a constant 1480ms<sup>-1</sup> velocity, the <b>angle</b> parameter cuts of the migration before it is spatially aliased, although the shallow event is still stretched. No stretch mute was applied</span></i></td></tr>
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<o:p><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></o:p></div>
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<o:p><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></o:p></div>
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<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">Pre-stack time migration is a zero-offset 'image ray' migration. This means the results will
look like data that has zero offset between source and receiver. However, unlike the </span><a href="http://en.wikipedia.org/wiki/Normal_moveout" style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">NMO correction</a>,<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">
these will also be corrected for the dip of any reflectors. In t</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">he zero-offset 'image ray' migration, the rays are normal to the surface - not the rock interface.</span></div>
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgeMXrXtXPNY9thwpip3UrnxHRRvE-DFQIz_Fxdq6rWUbGZmiabe0SI0ftoaMSjgTMbHyVWrVVrkCiF4jPJOIL8b9n415fI4wxFZ6lUpd5qvTmRdN0lPZbXBjeTQb1pKQ1UmUUtzMbb5Qk/s1600/seven.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgeMXrXtXPNY9thwpip3UrnxHRRvE-DFQIz_Fxdq6rWUbGZmiabe0SI0ftoaMSjgTMbHyVWrVVrkCiF4jPJOIL8b9n415fI4wxFZ6lUpd5qvTmRdN0lPZbXBjeTQb1pKQ1UmUUtzMbb5Qk/s1600/seven.jpg" height="456" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><i style="font-size: medium; text-align: start;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Zero offset ray-paths for the normal ray (red) and images rays (orange) to a complex interface. The image ray is identical to the normal ray if the interface is horizontal. Where the interface is dipping, the image ray path starts and ends at a surface position (blue circle) that is above the reflection point on the interface. The normal ray path for the dipping interface starts and ends at a different surface location (purple circle), which is laterally miss-positioned from the reflection point</span></i></td></tr>
</tbody></table>
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<br /></div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The results of a pre-stack time migration will be zero-offset
image ray gathers; these will be “flattened gathers” if:</span></div>
</div>
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</div>
<ul>
<li style="text-align: justify;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">the input velocity model was correct</span></li>
<li style="text-align: justify;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">we don’t have to worry about </span><a href="http://en.wikipedia.org/wiki/Seismic_anisotropy" style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">anisotropy</a></li>
<li style="text-align: justify;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">we don’t have to worry about the ray paths being
refracted</span></li>
</ul>
<div style="text-align: justify;">
<br /></div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In practice, these are all likely to be approximations. The
velocities we supplied came from normal-ray gathers, and so as we identified
earlier they are contaminated by dip.</span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Beyond 45° or so, we need to start worrying about
anisotropic behaviours; this is where the simple second-order <a href="http://en.wikipedia.org/wiki/Normal_moveout">normal moveout equation</a> breaks
down and we must <a href="http://marine.veritasdgc.com/WebSite/VTechDocWeb.nsf/all/3DB4C1DC0EC3912387256CA50041FE5A/$File/0087.pdf">pick
higher order terms</a> and use anisotropic migrations. <o:p></o:p></span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Finally, if we have significant refractions as a result of
rapid lateral or vertical velocity gradients, then we need to think about using
a <a href="http://en.wikipedia.org/wiki/Seismic_migration#Depth_Migration">pre-stack
depth migration. </a> <o:p></o:p></span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">When we examine the gathers after migration we can look at
whether we need to modify or re-</span></div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">pick the velocities (sometimes called picking
residual moveout or RMO) before making a final stack. <o:p></o:p></span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">If the residual moveout is large it might be worth using the
new velocity field and running a new migration. This could be another iteration
of the same algorithm, however if there are non-hyperbolic components to the
residual moveout then you might need to consider if it is worth the time and
effort to move to a more sophisticated migration algorithm.<o:p></o:p></span></div>
</div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhRsO1NcSQNJrxGjhHO8896fx-G7BV1t-R6Pd9JvF45T8xKLQJX6PjlWc4eMkWSaX6GdTeMrIhVzEubUMFifYSgkbFmEhw7NSZygkoAUlY2XMmvXRzOromIBzOvH7hojEYxOzihuq-PakY/s1600/ten.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhRsO1NcSQNJrxGjhHO8896fx-G7BV1t-R6Pd9JvF45T8xKLQJX6PjlWc4eMkWSaX6GdTeMrIhVzEubUMFifYSgkbFmEhw7NSZygkoAUlY2XMmvXRzOromIBzOvH7hojEYxOzihuq-PakY/s1600/ten.jpg" height="396" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><i style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: medium; text-align: start;">Stacked (top), post-stack time migrated (middle) and pre-stack time migrated (bottom) sections from TRV-434, using the same velocity model and with a 500ms robust AGC applied. The key thing to observe is the improved imaging of the steeper dip, strong amplitude events and reduction in noise between the post- and pre-stack migrations (circled); this is typically where a pre-stack time migration offers advantages</i></td></tr>
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<span style="font-family: "Calibri","sans-serif"; font-size: 11pt; line-height: 17px;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">By: <b>Guy Maslen</b></span></span></div>
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GLOBE Claritashttp://www.blogger.com/profile/00646197047930265069noreply@blogger.com0tag:blogger.com,1999:blog-7105728614611010325.post-36783842244157475082014-01-23T19:38:00.002-08:002014-01-23T19:38:52.985-08:00Marine Processing - Part 9 | Demultiples<table border="1" cellpadding="0" cellspacing="0" class="MsoNormalTable" style="border-collapse: collapse; border: none; text-align: justify; width: 640px;"><tbody>
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This sequence of blog posts will build up into a complete description of a 2D marine processing sequence and how it is derived.</span></span></i></div>
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">No processing sequence is definitive and techniques vary with time (and software), however the idea is to provide a practical guide for applying seismic processing theory.<o:p></o:p></span></span></i></div>
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The seismic line used is TRV-434 from the Taranaki Basin offshore New Zealand. It is the same marine line used in our basic tutorial dataset.<o:p></o:p></span></span></i></div>
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The data are available from New Zealand Petroleum and Minerals, under the Ministry of Economic Development under the “Open File” System.<o:p></o:p></span></span></i></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The processing sequence developed so far:<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/05/marine-processing-part-1-resample.html"><b>Resample and Minimum Phase Conversion </b></a><b> </b></span></div>
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<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Reformat from field data (in this case, SEGY)</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Apply a combined minimum-phase conversion and anti-alias filter</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Resample to a 4ms sample interval</span></span></li>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><o:p></o:p></span></div>
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-2-initial-qc.html">QC and Assign Geometry</a></span></b></div>
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<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="text-indent: -18pt;">Assign 2D marine geometry; 12.5m CDP spacing and 60 fold </span><span style="text-indent: -18pt;">gathers</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">QC shots and near trace plots</span></span></li>
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-3-amplitudes.html">Amplitude Recovery</a></span></b></div>
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<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="text-indent: -18pt;">Amplitude recovery using a T</span><sup style="text-indent: -18pt;">2</sup><sub style="text-indent: -18pt;"> </sub><span style="text-indent: -18pt;">Spherical divergence correction</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Amplitude QC and Edit using peak and RMS amplitude displays</span></span></li>
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-4-swell-noise.html">Swell Noise Suppression</a></span></b></div>
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<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Swell noise suppression using projective filtering</span></span></li>
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<b><br /></b><b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/07/marine-processing-part-5-direct.html" target="_blank">Linear Noise Suppression</a></span></b></div>
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<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Interpolation to 6.25m group interval, 480 channels to unwrap spatially aliased dips</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Tau-p transform 500ms robust AGC "wrap", 960 p-values and transform range from </span></span><span style="line-height: 18px;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">-1400ms <sup>-1</sup> to +1400 ms<sup>-1</sup></span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Tail mute to remove linear arrivals and linear noise</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Predictive deconvolution: 32ms gap, 480 ms operator</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Rho filter for low frequency compensation</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Inverse Tau-p transfrom</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Interpolation back to 25m group interval, 240 channels</span></span></li>
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<span style="color: #0000ee; font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><b><u><a href="http://seismicreflections.globeclaritas.com/2013_10_01_archive.html" target="_blank">Picki<span id="goog_939294291"></span><span id="goog_939294292"></span>ng Velocities</a></u></b></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">One of the key issues in marine data is addressing the issue
of multiples. Multiples are delayed reflections that interfere with the
“primary” reflections we want to image. The delay occurs because the reflection
energy has taken a more complex and longer ray path from source to receiver,
reverberating between two layers that are highly reflective.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">While any two layers with high reflectivity can create a
multiple image, in general it is energy reverberating in the water column (i.e.
between the seafloor and the sea surface) that is the most important issue to
contend with.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">There are three main ways in which to remove multiples:</span></div>
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<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">treat the multiple as a long, ringing wavelet,
and use signal processing to simplify it</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">create a model of the multiple and try and
subtract it from the data</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">use the difference in </span><span style="color: #1f497d; font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">stacking velocity </span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">of the primary and multiple to
remove the multiple</span></li>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">We have already used the “signal processing” approach when
we applied <a href="http://seismicreflections.globeclaritas.com/2013/08/marine-processing-part-6-predictive.html">deconvolution</a>
to collapse the reverberations. So any further reduction in multiples is going
to require a different technique/s.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The modelling approaches, such as SRME (Surface Related
Multiple Elimination), usually require the primary seafloor reflection to be
the first clear signal. This limits their application on shallow water lines
such as this one, although there are some variations that can address this.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This leaves us with the velocity discrimination methods; I’m
going to use the Parabolic Radon Transform (PRT) approach. The Radon transform is a generic mathematical procedure
where input data in the frequency domain are decomposed into a series of events
in the RADON domain. The PRT has been the “workhorse” demultiple method
for many years; even when SRME is deployed it is often used to overcome
limitations in the modelling.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The Tau-P transform is a special case of the Radon transform
where Tau (intercept time) and P (slowness, slope, or parabolic curvature)
represent two axes in Tau-p space. The data are summed along a series of
parabolic tracks, each described by a curvature value (P). The user defines a
number of P values to use as well as the total P range, which can be from
positive to negative.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">One of the key elements of the PRT as a demultiple method is
that we usually apply an NMO correction of some sort to the data. This helps do
two things: (i) target the primaries and multiples – for example we can use
water velocity or a picked primary trend, and (ii) approximate the reflections
(which are originally hyperbolic) through a series of parabolic curves. <o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">There are different ways of considering the parabolic
curvature (P) however one of the easiest is to look at the relative moveout (in
ms) of the events at the far offset. This means a flat event has a P value of
zero, up-dip events (NMO corrected with too slow a velocity) have negative p
values, and down-dip events (NMO corrected with too high a velocity) have
positive P values.<o:p></o:p></span></div>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Here’s
the same synthetic as <a href="http://seismicreflections.globeclaritas.com/2013/10/marine-processing-part-7-picking.html">before</a>;
this time it has a NMO correction applied with 90% of the primary velocities so
that the primaries are up-dip and the multiples down-dip.</span></span></div>
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi-usSDtb4xt5GIdgjtsqp5DDbZr4LWF6xuTLvYYYTy6lxHKhzMJe5b9WQFLz1RTeVwZG7vuXEJSUcsyfbwtGnAg8smKHRbKalXlXJ-ERvLzRuSZXjbXg83lijTb-bVcCka-oI6808djHI/s1600/one.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi-usSDtb4xt5GIdgjtsqp5DDbZr4LWF6xuTLvYYYTy6lxHKhzMJe5b9WQFLz1RTeVwZG7vuXEJSUcsyfbwtGnAg8smKHRbKalXlXJ-ERvLzRuSZXjbXg83lijTb-bVcCka-oI6808djHI/s1600/one.jpg" height="585" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">A synthetic CDP gather corrected with 90% of the
primary NMO velocity (left) and its parabolic Radon transform (right). The
Radon transform is labelled by “far offset moveout” so that the over-corrected,
up-dip primaries correspond to negative numbers and the under-corrected,
down-dip multiples correspond to positive numbers. Where the primaries and
multiples cross over in the X-T domain, they are separated in the PRT domain
(circled)</span></td></tr>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">There are some key things to notice in this result. <o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Firstly, the PRT is only really effective when we have
enough live traces – in the shallow part of the gather (down to ~500ms) with
only 20 traces the events can still be seen, but they are smeared. Below about 500ms TWT it is hard to make the
PRT demultiple effective.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Secondly, the events form a “cross” shape in the X-T domain;
this means that we can damage the primary (or the multiple) if we don’t
transform a sufficiently large range of P-values (or we use too few values,
causing the signal to be aliased). <o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Some other general points to consider are:</span></div>
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<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">the more P-values and frequencies you have, the
larger the matrix required for the transformation. You need a different matrix
for each combination of offsets, and so regularising the data (and using a
routine that saves the matrix) can make a big difference</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">it is much more important to focus on removing
the multiples from the near offsets (as these will “stack-in”) as opposed to
the far offsets (which will tend to be supressed by stacking or “stack out”)</span></li>
</ul>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Here are some synthetic examples of the PRT demultiple in action, which show how important it is to tune the parameters defining the “multiple-zone”, and check the
results!</span></span></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjYKtDSMi4nvO_82n_v4rIOO4GpH6pMsMOVTAxuQe6PcqPUAnoWtSuU5GfFhUSJ0bmafTeWSj3XLQMZLQ3CsqgQ2012FLoMFAR4jQrCCvyQE9qwhgrnelrW-CebhaBTrWXE3bmj8pWoXVs/s1600/two.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjYKtDSMi4nvO_82n_v4rIOO4GpH6pMsMOVTAxuQe6PcqPUAnoWtSuU5GfFhUSJ0bmafTeWSj3XLQMZLQ3CsqgQ2012FLoMFAR4jQrCCvyQE9qwhgrnelrW-CebhaBTrWXE3bmj8pWoXVs/s1600/two.jpg" height="422" width="640" /></a></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgFKfxaQfLsA4k2_UlplkDOcepSqAlL9gXDm3ig6-8TrKYU8B2PKZY7jVMLWd3G5fY3Q7jkHXeNVn92ndL0IPB6AyAhAxcGrlEAmZL_AS3XYQpRDtIjr7YdZVPsU3vpZkrugZx9XgpZQfA/s1600/three.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgFKfxaQfLsA4k2_UlplkDOcepSqAlL9gXDm3ig6-8TrKYU8B2PKZY7jVMLWd3G5fY3Q7jkHXeNVn92ndL0IPB6AyAhAxcGrlEAmZL_AS3XYQpRDtIjr7YdZVPsU3vpZkrugZx9XgpZQfA/s1600/three.jpg" height="480" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">The synthetic gather (top left) with two different
PRT demultiples applied (top centre, top right). The transforms of each gather are also
displayed underneath. The demultiple
only starts at 550ms TWT to avoid damaging the shallow events as these are
poorly defined in the PRT space where the NMO stretch mute has limited the
offsets. The centre panels show the “multiple zone” as being defined between
+20 and +1600ms “far-offset moveout”, on the right this has been increased to
include dips up to -40ms moveout. This allows the deepest multiple with the
smallest moveout variation to be attacked</span></td></tr>
</tbody></table>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">From this the PRT demultiple looks promising; it can manage
even small separations between the primary and multiple as long as the
velocities have been determined accurately enough. It needs to be tested
carefully, however, if the primary and multiple are close in terms of moveout. <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Here are some selected CDP gathers from the seismic line. In
the shallow part of the section the multiples have been effectively managed by
the deconvolution, however at depth (where the primary signals are weaker)
there’s still a lot of multiple energy from more complex ray-paths.<o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The
gathers have been NMO corrected with 90% of the picked primary velocity, so we
can be sure the primaries will be up-dip, or at the very least flat.</span></span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhrIRDWOgZKzur2pYSLpUzTvppTL48eEevwDGMk3Qgxyw_Bo4pFQf9XzaCOT7AXDG03isf15HWpyvfAY7CeNxJLTHbuZkMCggVt8LTWLrRD3FhQL002xHW0dRv1rIEdmMawjOI31Vc6E60/s1600/four.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhrIRDWOgZKzur2pYSLpUzTvppTL48eEevwDGMk3Qgxyw_Bo4pFQf9XzaCOT7AXDG03isf15HWpyvfAY7CeNxJLTHbuZkMCggVt8LTWLrRD3FhQL002xHW0dRv1rIEdmMawjOI31Vc6E60/s1600/four.jpg" height="640" width="430" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">Selected CDP gathers displaying the deeper data from
about 800ms TWT onwards. The gathers are
corrected with 90% of the primary velocity. The multiples become more obvious in the deeper section from 1500ms or
so – as indicated in yellow</span></td></tr>
</tbody></table>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">To apply the PRT demultiple we need to look at the
“far offset moveout” range we need for the data overall, and to define what
part of the data we want to classify as multiples.</span></span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj0MTyv38I_g4HPHFZua0l4o0mPLgKKlnXBzkx2M7aEZrd8b0MU3R_VbDEQceAD9QxzidlvOPhGPzY8MsPI1iR7JJNFN99-mrCgvMuddexKoWOlW8h-qNcuGIfSWnSYjIaa8N5Gbonvg-w/s1600/five.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj0MTyv38I_g4HPHFZua0l4o0mPLgKKlnXBzkx2M7aEZrd8b0MU3R_VbDEQceAD9QxzidlvOPhGPzY8MsPI1iR7JJNFN99-mrCgvMuddexKoWOlW8h-qNcuGIfSWnSYjIaa8N5Gbonvg-w/s1600/five.jpg" height="640" width="462" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">CDP gathers NMO corrected with 90% of the stacking
velocity with some example measurements of positive and negative far-offset
moveout</span></td></tr>
</tbody></table>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In this case I’m going to use a range of -300ms to +500ms,
and as the primary is over-corrected (through applying a Normal Moveout
correction with 90% of the primary velocity) I’m going to classify the multiple
as having a far-offset moveout between +24 and +500ms.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">I’ll also need to specify how many P-values to use; one way
to approach this is to think about the total moveout range we are transforming,
and what each P-value will represent. <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">With a total range of 800ms (from -300ms to +500ms) I’m
going to use 200 P-values; this means that each P-value corresponds to 4ms of
moveout at the far offset (which is the sample interval.)<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Finally I’m going to form the data into supergathers. As I
described in the post on <a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-2-initial-qc.html">geometry
</a>we have different offsets in the “odd” and “even” numbered CDPs, each of
which has 60 traces. If we combine these
into a single supergather with 120 traces then each gather will have the same
offset distribution, and we’ll have better resolution in the PRT domain as
well.<o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Here
are the results – remembering that the demultiple is only applied
from 1400ms onwards:</span></span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgF4q6PSxdQZEPY_Gj9MGh9r70Jss0MA6Ayvqw6djn0ssRgtjVlSJuXR0NcxzDmSWIT6TEmTx7r8a5r5BCf7ICHDjSItlHVrYZVDTVrBJCvWpLH5WE57f3txLRG1Vpl_31oUturOnfbgZ8/s1600/six.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgF4q6PSxdQZEPY_Gj9MGh9r70Jss0MA6Ayvqw6djn0ssRgtjVlSJuXR0NcxzDmSWIT6TEmTx7r8a5r5BCf7ICHDjSItlHVrYZVDTVrBJCvWpLH5WE57f3txLRG1Vpl_31oUturOnfbgZ8/s1600/six.jpg" height="604" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">Test of PRT demultiple shown on two CDP gathers (500
and 501) which were formed into a single 120-trace supergather for the
application. The gathers on the right
have had the mutliples with a far-offset moveout between +24 and +500ms
supressed; the data had a NMO correction
with 90% of the picked primary velocity applied prior to the PRT demultiple</span></td></tr>
</tbody></table>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This seems to have done a pretty efficient job at cleaning
up the gathers; it will be a lot easier to pick the deeper velocities now,
which should help with the imaging.<o:p></o:p></span></div>
<div style="text-align: justify;">
</div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">While it is important to check the results on the stacked
data as well (with this dataset these are actually relatively small), the
majority of the multiple ‘stacks out’ and our main goal is to make it easier to
get an improved velocity analysis.</span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiPmEkFzyjDnYdqbuOrwSFPX9q-oEzUnS0-EoT0mOyAO4dytKZPJQLY6TLQzXjUUjQ294L49ZsKG8kXR-DBRXSoeqDWKi9okxoF6Nl0K1TGfFUGdXnM_FIGH8Pss_j0cqMb_-yZQWrumaM/s1600/seven.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiPmEkFzyjDnYdqbuOrwSFPX9q-oEzUnS0-EoT0mOyAO4dytKZPJQLY6TLQzXjUUjQ294L49ZsKG8kXR-DBRXSoeqDWKi9okxoF6Nl0K1TGfFUGdXnM_FIGH8Pss_j0cqMb_-yZQWrumaM/s1600/seven.jpg" height="382" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">Velocity semblance spectra for CDP 500 before (left)
and after (right) the PRT demultiple has been applied; removal of the “ringing”
multiple chain (circled in red) has helped in differentiating primaries from
multiples</span></td></tr>
</tbody></table>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Having used our first-pass velocities to help remove the
multiples we can now go back and check or re-pick the deeper velocities to
further improve the image.</span><o:p></o:p><br />
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span>
<div class="separator" style="clear: both; text-align: center;">
<span style="font-family: "Calibri","sans-serif"; font-size: 11pt; line-height: 17px;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" /></a></span></div>
<div style="text-align: start;">
<span style="font-family: "Calibri","sans-serif"; font-size: 11pt; line-height: 17px;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div style="text-align: start;">
<span style="font-style: italic;"></span><br /></div>
<span style="line-height: 115%;"></span><br />
<div style="text-align: center;">
<span style="font-family: "Calibri","sans-serif"; font-size: 11pt; line-height: 17px;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">By: <b>Guy Maslen</b></span></span></div>
</div>
GLOBE Claritashttp://www.blogger.com/profile/00646197047930265069noreply@blogger.com0tag:blogger.com,1999:blog-7105728614611010325.post-41166568235757756782013-12-19T13:08:00.002-08:002013-12-19T13:08:54.135-08:00Marine Processing - Part 8 | Picking Velocities II<table border="1" cellpadding="0" cellspacing="0" class="MsoNormalTable" style="border-collapse: collapse; border: none; text-align: justify; width: 640px;"><tbody>
<tr><td style="border: 1pt solid windowtext; padding: 0cm 5.4pt; width: 182.6pt;" valign="top" width="243"><div class="MsoNormal">
<div style="text-align: left;">
<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This sequence of blog posts will build up into a complete description of a 2D marine processing sequence and how it is derived.</span></span></i></div>
</div>
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</div>
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<div style="text-align: left;">
<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">No processing sequence is definitive and techniques vary with time (and software), however the idea is to provide a practical guide for applying seismic processing theory.<o:p></o:p></span></span></i></div>
</div>
<div class="MsoNormal">
<div style="text-align: left;">
<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></i></div>
</div>
<div class="MsoNormal">
<div style="text-align: left;">
<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The seismic line used is TRV-434 from the Taranaki Basin offshore New Zealand. It is the same marine line used in our basic tutorial dataset.<o:p></o:p></span></span></i></div>
</div>
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<div style="text-align: left;">
<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></i></div>
</div>
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<div style="text-align: left;">
<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The data are available from New Zealand Petroleum and Minerals, under the Ministry of Economic Development under the “Open File” System.<o:p></o:p></span></span></i></div>
</div>
</td><td style="border: 1pt solid windowtext; padding: 0cm 5.4pt; width: 297.7pt;" valign="top" width="397"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The processing sequence developed so far:<o:p></o:p></span></div>
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/05/marine-processing-part-1-resample.html"><b>Resample and Minimum Phase Conversion </b></a><b> </b></span></div>
<div class="MsoNormal">
</div>
<ul>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Reformat from field data (in this case, SEGY)</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Apply a combined minimum-phase conversion and anti-alias filter</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Resample to a 4ms sample interval</span></span></li>
</ul>
<br />
<div class="MsoListParagraphCxSpLast" style="text-indent: -18pt;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><o:p></o:p></span></div>
<div class="MsoNormal">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-2-initial-qc.html">QC and Assign Geometry</a></span></b></div>
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</div>
<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="text-indent: -18pt;">Assign 2D marine geometry; 12.5m CDP spacing and 60 fold </span><span style="text-indent: -18pt;">gathers</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">QC shots and near trace plots</span></span></li>
</ul>
<br />
<div class="MsoNormal">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-3-amplitudes.html">Amplitude Recovery</a></span></b></div>
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</div>
<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="text-indent: -18pt;">Amplitude recovery using a T</span><sup style="text-indent: -18pt;">2</sup><sub style="text-indent: -18pt;"> </sub><span style="text-indent: -18pt;">Spherical divergence correction</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Amplitude QC and Edit using peak and RMS amplitude displays</span></span></li>
</ul>
<br />
<div class="MsoListParagraphCxSpLast" style="text-indent: -18pt;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><o:p></o:p></span></div>
<div class="MsoNormal">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-4-swell-noise.html">Swell Noise Suppression</a></span></b></div>
<div class="MsoNormal">
</div>
<ul>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Swell noise suppression using projective filtering</span></span></li>
</ul>
<div class="MsoNormal">
<b><br /></b><b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/07/marine-processing-part-5-direct.html" target="_blank">Linear Noise Suppression</a></span></b></div>
<div class="MsoNormal">
</div>
<ul>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Interpolation to 6.25m group interval, 480 channels to unwrap spatially aliased dips</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Tau-p transform 500ms robust AGC "wrap", 960 p-values and transform range from </span></span><span style="line-height: 18px;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">-1400ms <sup>-1</sup> to +1400 ms<sup>-1</sup></span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Tail mute to remove linear arrivals and linear noise</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Predictive deconvolution: 32ms gap, 480 ms operator</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Rho filter for low frequency compensation</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Inverse Tau-p transfrom</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Interpolation back to 25m group interval, 240 channels</span></span></li>
</ul>
<div>
<span style="color: #0000ee; font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><b><u><a href="http://seismicreflections.globeclaritas.com/2013_10_01_archive.html" target="_blank">Picki<span id="goog_939294291"></span><span id="goog_939294292"></span>ng Velocities</a></u></b></span></div>
<div>
<span style="color: #0000ee; font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><b><br /></b></span></div>
</td></tr>
</tbody></table>
<br />
<div>
<br /></div>
<div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In the <a href="http://seismicreflections.globeclaritas.com/2013_10_01_archive.htm">last
post</a> we looked at some of the things that you need to bear in mind when you
are performing velocity analysis. <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This post continues with that theme and looks at how to
identify some key issues as part of our quality control checks.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The most important thing to remember is that whilst we are
trying to pick the velocities of seismic reflections, there are other signals
that we can pick by mistake.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The main problems we’ll see are:</span></div>
<div class="MsoNormal" style="text-align: justify;">
</div>
<ul>
<li><b style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">Multiples</b><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">
- low velocity signals that are a delayed version of the primary. The delay is
caused by the reflection energy reverberating between two strong reflectors,
typically the sea surface and seafloor. There’s a nice diagram of this </span><a href="http://en.wikipedia.org/wiki/File:Multiple_Reflection.jpg" style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">here</a>.</li>
<li><b style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify; text-indent: -18pt;">Diffractions
- </b><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify; text-indent: -18pt;">where the energy is scattered (off an object or discontinuity); the tails
of the diffraction have anomalously high velocities</span></li>
</ul>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The velocity we are picking is the “<b>stacking velocity</b>” – that is to say, it is the velocity which in
conjunction with the <a href="http://en.wikipedia.org/wiki/Normal_moveout">hyperbolic
NMO equation</a> is going to flatten the CDP gathers and allow us to stack the
data.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The “<b>stacking
velocity</b>” has no other meaning apart from this, however in the situation
where the source-receiver offset is small (which is generally less than 45
degrees) and the rock layers are horizontal and isotropic, the “<b>stacking velocity</b>” approximates the “<b>root mean square velocity</b>” (<b>RMS velocity</b>). </span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This in turn is the RMS
of the (P-wave)<b> “interval velocity”</b>
within each of the isotropic layers. There’s
a useful page on these relationships <a href="http://www.xsgeo.com/course/velan.htm">here.</a><o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Most velocity analysis software will display the interval
velocities while you work, which can act as a guide if you know the interval
velocities you might expect in some common rock types.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEilFk_1WL7OtyKP4444EDuQKOxhG9qm7t9ZmCjxM-h_Ot_Vvfa5-gvm_FHZmFjWsJL1CXby8ixGSBbsvYyGKtQYOxguHo6kV1mgeRYvAlXf9Kb2QfNdbvxWu9qxXRj_L1WKjO7JnQBxPZk/s1600/one.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEilFk_1WL7OtyKP4444EDuQKOxhG9qm7t9ZmCjxM-h_Ot_Vvfa5-gvm_FHZmFjWsJL1CXby8ixGSBbsvYyGKtQYOxguHo6kV1mgeRYvAlXf9Kb2QfNdbvxWu9qxXRj_L1WKjO7JnQBxPZk/s1600/one.jpg" height="640" width="528" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;">Interval velocities (blue) plotted alongside the stacking velocities (black) on a semblance display</span></td></tr>
</tbody></table>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">While the P-wave velocities for different lithologies vary
significantly, the majority of multiples are caused by the water column. This
leads to a corresponding interval velocity which matches that of seawater. Some
typical ranges and values you might find are given below:</span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<table border="1" cellpadding="0" cellspacing="0" class="MsoTableLightGridAccent2" style="border-collapse: collapse; border: none; text-align: justify; width: 616px;">
<tbody>
<tr>
<td style="border-bottom: solid #C0504D 2.25pt; border: solid #C0504D 1.0pt; mso-border-bottom-themecolor: accent2; mso-border-themecolor: accent2; padding: 0cm 5.4pt 0cm 5.4pt; width: 125.9pt;" valign="top" width="168"><div class="MsoNormal">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Layer<o:p></o:p></span></b></div>
</td>
<td style="border-bottom: solid #C0504D 2.25pt; border-left: none; border-right: solid #C0504D 1.0pt; border-top: solid #C0504D 1.0pt; mso-border-bottom-themecolor: accent2; mso-border-left-alt: solid #C0504D 1.0pt; mso-border-left-themecolor: accent2; mso-border-right-themecolor: accent2; mso-border-top-themecolor: accent2; padding: 0cm 5.4pt 0cm 5.4pt; width: 336.2pt;" valign="top" width="448"><div class="MsoNormal">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Expected
Interval Velocities in ms<sup>-1</sup> <o:p></o:p></span></b></div>
</td>
</tr>
<tr>
<td style="background: #EFD3D2; border-top: none; border: solid #C0504D 1.0pt; mso-background-themecolor: accent2; mso-background-themetint: 63; mso-border-themecolor: accent2; mso-border-top-alt: solid #C0504D 1.0pt; mso-border-top-themecolor: accent2; padding: 0cm 5.4pt 0cm 5.4pt; width: 125.9pt;" valign="top" width="168"><div class="MsoNormal">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Seawater<o:p></o:p></span></b></div>
</td>
<td style="background: #EFD3D2; border-bottom: solid #C0504D 1.0pt; border-left: none; border-right: solid #C0504D 1.0pt; border-top: none; mso-background-themecolor: accent2; mso-background-themetint: 63; mso-border-bottom-themecolor: accent2; mso-border-left-alt: solid #C0504D 1.0pt; mso-border-left-themecolor: accent2; mso-border-right-themecolor: accent2; mso-border-top-alt: solid #C0504D 1.0pt; mso-border-top-themecolor: accent2; padding: 0cm 5.4pt 0cm 5.4pt; width: 336.2pt;" valign="top" width="448"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Around 1480ms<sup>-1</sup> in
general, although there is some variation due to salinity and temperature. I’ve
seen velocities as high as 1540ms<sup>-1</sup> on data from Antarctica, for
example<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid #C0504D 1.0pt; mso-border-themecolor: accent2; mso-border-top-alt: solid #C0504D 1.0pt; mso-border-top-themecolor: accent2; padding: 0cm 5.4pt 0cm 5.4pt; width: 125.9pt;" valign="top" width="168"><div class="MsoNormal">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Seafloor
sediment<o:p></o:p></span></b></div>
</td>
<td style="border-bottom: solid #C0504D 1.0pt; border-left: none; border-right: solid #C0504D 1.0pt; border-top: none; mso-border-bottom-themecolor: accent2; mso-border-left-alt: solid #C0504D 1.0pt; mso-border-left-themecolor: accent2; mso-border-right-themecolor: accent2; mso-border-top-alt: solid #C0504D 1.0pt; mso-border-top-themecolor: accent2; padding: 0cm 5.4pt 0cm 5.4pt; width: 336.2pt;" valign="top" width="448"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The soft, seafloor sediments
typically ramp up with compaction; from seawater velocity (1480ms<sup>-1</sup>)
very gradually up to 1750-2000ms<sup>-1</sup> - depending on thickness and age <o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="background: #EFD3D2; border-top: none; border: solid #C0504D 1.0pt; mso-background-themecolor: accent2; mso-background-themetint: 63; mso-border-themecolor: accent2; mso-border-top-alt: solid #C0504D 1.0pt; mso-border-top-themecolor: accent2; padding: 0cm 5.4pt 0cm 5.4pt; width: 125.9pt;" valign="top" width="168"><div class="MsoNormal" style="text-align: left;">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Partially
compacted Sandstone and Shale<o:p></o:p></span></b></div>
</td>
<td style="background: #EFD3D2; border-bottom: solid #C0504D 1.0pt; border-left: none; border-right: solid #C0504D 1.0pt; border-top: none; mso-background-themecolor: accent2; mso-background-themetint: 63; mso-border-bottom-themecolor: accent2; mso-border-left-alt: solid #C0504D 1.0pt; mso-border-left-themecolor: accent2; mso-border-right-themecolor: accent2; mso-border-top-alt: solid #C0504D 1.0pt; mso-border-top-themecolor: accent2; padding: 0cm 5.4pt 0cm 5.4pt; width: 336.2pt;" valign="top" width="448"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Partially compacted sediments still
have fluids trapped within the pores and so the velocities can have a large
range. 2500ms<sup>-1</sup> is a typical value but it can push up to 5000ms<sup>-1</sup>
in older rocks such as the “Old Red Sandstone” from the Devonian Period, where
the pores have been closed by pressure and diagenesis <o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid #C0504D 1.0pt; mso-border-themecolor: accent2; mso-border-top-alt: solid #C0504D 1.0pt; mso-border-top-themecolor: accent2; padding: 0cm 5.4pt 0cm 5.4pt; width: 125.9pt;" valign="top" width="168"><div class="MsoNormal">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Limestone<o:p></o:p></span></b></div>
</td>
<td style="border-bottom: solid #C0504D 1.0pt; border-left: none; border-right: solid #C0504D 1.0pt; border-top: none; mso-border-bottom-themecolor: accent2; mso-border-left-alt: solid #C0504D 1.0pt; mso-border-left-themecolor: accent2; mso-border-right-themecolor: accent2; mso-border-top-alt: solid #C0504D 1.0pt; mso-border-top-themecolor: accent2; padding: 0cm 5.4pt 0cm 5.4pt; width: 336.2pt;" valign="top" width="448"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Thin, reefy, limestones tend to
have velocities close to 3700ms<sup>-1</sup>, while bulk limestones (or
chalks) can be higher<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="background: #EFD3D2; border-top: none; border: solid #C0504D 1.0pt; mso-background-themecolor: accent2; mso-background-themetint: 63; mso-border-themecolor: accent2; mso-border-top-alt: solid #C0504D 1.0pt; mso-border-top-themecolor: accent2; padding: 0cm 5.4pt 0cm 5.4pt; width: 125.9pt;" valign="top" width="168"><div class="MsoNormal">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Salt
(Halite)<o:p></o:p></span></b></div>
</td>
<td style="background: #EFD3D2; border-bottom: solid #C0504D 1.0pt; border-left: none; border-right: solid #C0504D 1.0pt; border-top: none; mso-background-themecolor: accent2; mso-background-themetint: 63; mso-border-bottom-themecolor: accent2; mso-border-left-alt: solid #C0504D 1.0pt; mso-border-left-themecolor: accent2; mso-border-right-themecolor: accent2; mso-border-top-alt: solid #C0504D 1.0pt; mso-border-top-themecolor: accent2; padding: 0cm 5.4pt 0cm 5.4pt; width: 336.2pt;" valign="top" width="448"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Usually a constant 4500ms<sup>-1</sup>,
although you have to be aware of things like anhydrite that can “raft” in the
salt if it becomes ductile<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid #C0504D 1.0pt; mso-border-themecolor: accent2; mso-border-top-alt: solid #C0504D 1.0pt; mso-border-top-themecolor: accent2; padding: 0cm 5.4pt 0cm 5.4pt; width: 125.9pt;" valign="top" width="168"><div class="MsoNormal">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Basalt
and Granite<o:p></o:p></span></b></div>
</td>
<td style="border-bottom: solid #C0504D 1.0pt; border-left: none; border-right: solid #C0504D 1.0pt; border-top: none; mso-border-bottom-themecolor: accent2; mso-border-left-alt: solid #C0504D 1.0pt; mso-border-left-themecolor: accent2; mso-border-right-themecolor: accent2; mso-border-top-alt: solid #C0504D 1.0pt; mso-border-top-themecolor: accent2; padding: 0cm 5.4pt 0cm 5.4pt; width: 336.2pt;" valign="top" width="448"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">“Bulk” igneous rocks tend to
have high interval velocities of around 5500-6500ms<sup>-1</sup> <o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="background: #EFD3D2; border-top: none; border: solid #C0504D 1.0pt; mso-background-themecolor: accent2; mso-background-themetint: 63; mso-border-themecolor: accent2; mso-border-top-alt: solid #C0504D 1.0pt; mso-border-top-themecolor: accent2; padding: 0cm 5.4pt 0cm 5.4pt; width: 125.9pt;" valign="top" width="168"><div class="MsoNormal">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Anhydrite<o:p></o:p></span></b></div>
</td>
<td style="background: #EFD3D2; border-bottom: solid #C0504D 1.0pt; border-left: none; border-right: solid #C0504D 1.0pt; border-top: none; mso-background-themecolor: accent2; mso-background-themetint: 63; mso-border-bottom-themecolor: accent2; mso-border-left-alt: solid #C0504D 1.0pt; mso-border-left-themecolor: accent2; mso-border-right-themecolor: accent2; mso-border-top-alt: solid #C0504D 1.0pt; mso-border-top-themecolor: accent2; padding: 0cm 5.4pt 0cm 5.4pt; width: 336.2pt;" valign="top" width="448"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Anhydrite layers typically have
very high velocities, around 7000ms<sup>-1</sup></span> <span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><o:p></o:p></span></div>
</td>
</tr>
</tbody></table>
<div class="MsoNormal" style="text-align: justify;">
<div style="text-align: justify;">
<br /></div>
</div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">As well as incorrectly identified signals, we also have to
watch out for simple mistakes and errors.
We can do this by looking both at the velocities (usually termed an <b>isovel plot</b>) and the stacked section.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Anomalously high or low values, relative to the overall
trend, indicate a miss-pick or problem with the velocity field and show up as “bulls eyes”.
These must be edited or removed. <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjIuINp6qB3ca0-K_EQf6c1GQzteurh137_JLcJ8vfnz-t9rbrhikZAUQkutLLNW7vRSatzGAePe8xD7vPyG3RdSVYfH0eQScoQkQkEzUwHdABP5CUN333alUw-NEaQSNXSoHc0a4Y5sFA/s1600/two.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjIuINp6qB3ca0-K_EQf6c1GQzteurh137_JLcJ8vfnz-t9rbrhikZAUQkutLLNW7vRSatzGAePe8xD7vPyG3RdSVYfH0eQScoQkQkEzUwHdABP5CUN333alUw-NEaQSNXSoHc0a4Y5sFA/s1600/two.jpg" height="323" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;">A single velocity miss-pick (2500ms</span><sup style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;">-1</sup><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;"> rather than ~1700ms</span><sup style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;">-1</sup><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;">) on the isovels plot (left), and the corresponding impact on the stack (right, circled). The result is dimmed/faded reflectors in that region</span></td></tr>
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">While it is easy to see a single miss-picked velocity on
this occasion, when dealing with a missed trend it can be much more challenging
to spot. It is important to carefully compare the current stack to the previous
one, looking for areas where there are improvements and places where the stack
quality has degraded.</span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">It’s also important to look at the stack while you are
picking; a sudden lateral change in velocities can dramatically change the
overall primary trend, but the multiples may be close to the primary trend you
have been following.<o:p></o:p></span></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhqAdhqfvMefcPlxncRblMa-j00dv53q9FJRsl6gUfNoADba502IePorsv-yJepEVfSZ8n7mv-Mu9ZMHQerFc23B8xU3sVrW_ZSjJj9l8B-0fcNmbBH04hjnivoef2VaDtxwcDBK-rJUeU/s1600/three.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhqAdhqfvMefcPlxncRblMa-j00dv53q9FJRsl6gUfNoADba502IePorsv-yJepEVfSZ8n7mv-Mu9ZMHQerFc23B8xU3sVrW_ZSjJj9l8B-0fcNmbBH04hjnivoef2VaDtxwcDBK-rJUeU/s1600/three.jpg" height="640" width="588" /></a></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhwaEsSidog88U8kCm08dX_oFqB21JDCZjYZ5cLajcE-lqNFLavNF8n3UivKkk085NW0i5OypcDxZj05C-Z76GydMHRTi3D54sKIFdIEI74mS8jkMIqdEv7JLlVraGFQSCONZN8fk3JZLA/s1600/four.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhwaEsSidog88U8kCm08dX_oFqB21JDCZjYZ5cLajcE-lqNFLavNF8n3UivKkk085NW0i5OypcDxZj05C-Z76GydMHRTi3D54sKIFdIEI74mS8jkMIqdEv7JLlVraGFQSCONZN8fk3JZLA/s1600/four.jpg" height="640" width="588" /></a></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhDE3SsNOz2d2lFGS1HSrXeBiAAsU6lBuE1z6vJqgjh86seZsF2cj9Smh0EBv4X0SzY8MQFLFp_u3qNKl3WVXBP-PVSmbcKarCjZ0saIkP6aFu3_DMkwsIChsxpFeWJaZN_O35ccE7ZL20/s1600/five.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhDE3SsNOz2d2lFGS1HSrXeBiAAsU6lBuE1z6vJqgjh86seZsF2cj9Smh0EBv4X0SzY8MQFLFp_u3qNKl3WVXBP-PVSmbcKarCjZ0saIkP6aFu3_DMkwsIChsxpFeWJaZN_O35ccE7ZL20/s1600/five.jpg" height="640" width="588" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;">The brute section (top), an incorrect velocity trend (middle) and improved picks (bottom); centred on the “tricky” area of the line i.e. the main thrust fault. The incorrect trend has followed slow multiples and fast diffractions, and so misses the high-velocity overthrust (around CDP 1320).</span></td></tr>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Of course, the stack is only one guide. We also need to look
at NMO corrected gathers to make sure that the primaries are flat, the
diffractions are up-dip, and the multiples are down-dip. It can be hard to
judge “flatness” with a single velocity so comparing different velocity fields,
or looking at 95%, 100%, and 105% of the stacking velocities, side by side is worthwhile.<o:p></o:p></span></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEitxBVUnktmIrFaWhUe3ugiwxdJvVBuSYHoifwh4W6Jhm4Wj-iMJM3f3QhO1dc8RNIXyf2UcBKNqHox9o4zcQ0MaAbsstPrG5twulp8oGWQS8dPHLe1f6YixaN7z__8cusYR_F2FJzwuVQ/s1600/six.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEitxBVUnktmIrFaWhUe3ugiwxdJvVBuSYHoifwh4W6Jhm4Wj-iMJM3f3QhO1dc8RNIXyf2UcBKNqHox9o4zcQ0MaAbsstPrG5twulp8oGWQS8dPHLe1f6YixaN7z__8cusYR_F2FJzwuVQ/s1600/six.jpg" height="324" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;">Two CDP gathers (900 and 1800): NMO corrected with 95% (left), 100% (centre), and 105% (right) of the stacking velocities. CDP 900 looks flattened with 100%, but above 1000ms TWT, CDP 1800 looks to have been picked too slowly; some upward dip remains. CDP 1800 looks to be best flattened with 105% of the stacking velocity, above 1000ms TWT.</span></td></tr>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">At this stage in the processing the limitations we have on
picking are the presence of multiples, diffractions, and dip. The velocities in
and around the over-thrust at the high CDP end of the line vary rapidly, and it
is difficult to identify the correct trend.<o:p></o:p></span></div>
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;">To
improve further, we need to use the existing velocity field to remove the
multiples, and then move on to pre-stack imaging of the data.</span></div>
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<span style="font-family: "Calibri","sans-serif"; font-size: 11pt; line-height: 17px;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" /></a></span></div>
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<span style="font-family: "Calibri","sans-serif"; font-size: 11pt; line-height: 17px;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">By: <b>Guy Maslen</b></span></span></div>
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GLOBE Claritashttp://www.blogger.com/profile/00646197047930265069noreply@blogger.com0tag:blogger.com,1999:blog-7105728614611010325.post-83303312691199228472013-10-17T15:31:00.002-07:002013-10-17T15:31:19.273-07:00Marine Processing - Part 7 | Picking Velocities<table border="1" cellpadding="0" cellspacing="0" class="MsoNormalTable" style="border-collapse: collapse; border: none; text-align: justify; width: 640px;"><tbody>
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This sequence of blog posts will build up into a complete description of a 2D marine processing sequence and how it is derived.</span></span></i></div>
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">No processing sequence is definitive and techniques vary with time (and software), however the idea is to provide a practical guide for applying seismic processing theory.<o:p></o:p></span></span></i></div>
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The seismic line used is TRV-434 from the Taranaki Basin offshore New Zealand. It is the same marine line used in our basic tutorial dataset.<o:p></o:p></span></span></i></div>
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The data are available from New Zealand Petroleum and Minerals, under the Ministry of Economic Development under the “Open File” System.<o:p></o:p></span></span></i></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The processing sequence developed so far:<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/05/marine-processing-part-1-resample.html"><b>Resample and Minimum Phase Conversion </b></a><b> </b></span></div>
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<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Reformat from field data (in this case, SEGY)</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Apply a combined minimum-phase conversion and anti-alias filter</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Resample to a 4ms sample interval</span></span></li>
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-2-initial-qc.html">QC and Assign Geometry</a></span></b></div>
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<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="text-indent: -18pt;">Assign 2D marine geometry; 12.5m CDP spacing and 60 fold </span><span style="text-indent: -18pt;">gathers</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">QC shots and near trace plots</span></span></li>
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-3-amplitudes.html">Amplitude Recovery</a></span></b></div>
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<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="text-indent: -18pt;">Amplitude recovery using a T</span><sup style="text-indent: -18pt;">2</sup><sub style="text-indent: -18pt;"> </sub><span style="text-indent: -18pt;">Spherical divergence correction</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Amplitude QC and Edit using peak and RMS amplitude displays</span></span></li>
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-4-swell-noise.html">Swell Noise Suppression</a></span></b></div>
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<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Swell noise suppression using projective filtering</span></span></li>
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<b><br /></b><b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/07/marine-processing-part-5-direct.html" target="_blank">Linear Noise Suppression</a></span></b></div>
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<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Interpolation to 6.25m group interval, 480 channels to unwrap spatially aliased dips</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Tau-p transform 500ms robust AGC "wrap", 960 p-values and transform range from </span></span><span style="line-height: 18px;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">-1400ms <sup>-1</sup> to +1400 ms<sup>-1</sup></span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Tail mute to remove linear arrivals and linear noise</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Predictive deconvolution: 32ms gap, 480 ms operator</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Rho filter for low frequency compensation</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Inverse Tau-p transfrom</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Interpolation back to 25m group interval, 240 channels</span></span></li>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">We have now reached a stage where the “pre-processing” of
the data is complete, and we have “cleaned” the shot gathers. The next stage is
to create a model of the seismic velocities in the sub-surface. <o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This velocity field will be used in three main ways:</span></div>
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<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">to apply the </span><a href="http://en.wikipedia.org/wiki/Normal_moveout" style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">Normal Moveout Correction</a><span class="MsoHyperlink" style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">,</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;"> as part for the </span><a href="http://www.xsgeo.com/course/acq.htm" style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">Common MidPoint</a><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;"> (CMP, also
called Common Depth Point, CDP) processing approach, and thus improve the
signal-to-noise ratio</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">to help remove the multiples (delayed copies of
the primary reflection signals caused by reverberation in the sub-surface)</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">to focus the seismic energy and form a crisp
image - at the moment some of the signal is scattered off the sub-surface
geology</span></li>
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;">For the majority of processing sequences, the velocity field
is created iteratively. This allows us to improve velocities for the pre-stack
time migration; if there are steep dips we may have to pick velocities again.</span><br />
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">I’ll cover velocity analysis in a few different posts – this
first part will make use of synthetics and look at some of the issues we are
going to face.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The basic mechanics of velocity analysis are broadly similar
in all packages. At a given CDP location we test-apply different NMO velocities
and review the results in multiple analysis windows. We can then select the
appropriate velocity, at a given two-way-time value, that best flattens the
hyperbolic reflection in the CDP gather.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The main analysis tools that are used are:<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><b>Constant Velocity
Gathers (CVG) – </b>the CDP is displayed in a series of panels, each is NMO
corrected using a different constant velocity. The user picks the velocity
which best flattens the gather at a given two-way-time value.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><b>Constant Velocity
Stacks (CVS) </b>– a range of CDPs centred on the analysis location (typically
11 or 19 CDPs) are displayed as panels, each is NMO corrected using a different
constant velocity. The user picks the “best” stacked image at each two-way-time
value.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><b><span style="line-height: 115%;">Velocity Spectra</span></b><span style="line-height: 115%;"> –
a graphical display that shows a measure of trace-to-trace coherence once the
NMO at a given velocity correction is applied to the data, creating a contour.
This is usually semblance, but can also be stacked amplitude or other coherence
measures. The user can then pick a velocity function by focusing on the “bright
spots” caused by primary reflections.</span></span></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiT_xdWBYn8v5-Wi82LlM2-iOqpyBQ1JD4oG1evaX6KVdWJiTh1hDY5Zg3AFaNXZTEgb0uTmkP1_rCDtAlf3D1cGlqPTO0cKqS2SucZM6TyVAqeomEM-0iuJ5w9ojSvfp0Qnekf55cMI0A/s1600/one.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="540" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiT_xdWBYn8v5-Wi82LlM2-iOqpyBQ1JD4oG1evaX6KVdWJiTh1hDY5Zg3AFaNXZTEgb0uTmkP1_rCDtAlf3D1cGlqPTO0cKqS2SucZM6TyVAqeomEM-0iuJ5w9ojSvfp0Qnekf55cMI0A/s640/one.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Calibri, sans-serif; line-height: 115%;"><span style="font-size: small;">A synthetic CDP gather (left) and its semblance
spectrum (right). The velocities are displayed at the top of the semblance plot.
The first event is at 400ms (TWT) with a velocity of 1480ms<sup>-1</sup> (a
typical seawater velocity). Stacking velocities generally, although not always,
increase with depth</span></span></td></tr>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Other techniques for picking velocities include using a
predetermined velocity function as a “guide”. Modifying this function, by
applying a fixed percentage scaling for example, can be used to create a series
of “variable velocity” panels (either stacks or NMO corrected gathers). These functions
generally form a “fan” with a narrow range of velocities in the near-surface,
and a wider range at depth. <o:p></o:p></span></div>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The
“variable velocity stacks” (VVS) and “variable velocity gathers” (VVG) are
probably the most widely used tools for velocity analysis. There is a small
risk that the velocity “fan” can be too narrow (missing the primary velocity
trend), however modern tools allow you to recalculate this as a check. Most
tools also allow the user to “mix and match” these displays, as well as
test-apply velocity functions.</span></span></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhzQjDWTRclwM01TMdGaV-XmgHdknu-HMvqgDOr9kyTLEfMV6zXKaB37Mu_I3kAJBK3tA4EI5QnrVXKvnJd5pyKd3_O_wb_NpaTOZsjByOqU5IasQ1CS1mkOezT0Qw320x4Nn-UoepF3DE/s1600/two.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="378" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhzQjDWTRclwM01TMdGaV-XmgHdknu-HMvqgDOr9kyTLEfMV6zXKaB37Mu_I3kAJBK3tA4EI5QnrVXKvnJd5pyKd3_O_wb_NpaTOZsjByOqU5IasQ1CS1mkOezT0Qw320x4Nn-UoepF3DE/s640/two.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">A typical set of velocity analysis displays windows,
with semblance based on a “fan” of velocities around a central function (left),
variable velocity gathers that match this fan (centre) and an NMO corrected
gather with velocities displayed (right). Note how the gathers are over-corrected
(up dip) at low velocities, flat on the central function, and under-corrected
(down dip) at high velocities</span></td></tr>
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Velocities and Depth<o:p></o:p></span></b></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In the near surface, there is a long ray-path difference
between the near-offset reflection and the far offset – this means there is a
big “moveout difference” between the near and far offsets. At depth, this
variation is reduced. As a result, not only is there less moveout difference at
depth, the accuracy to which we can determine the velocity also reduces.</span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In
practice this means that while we can get a good stacking velocity at depth
quite easily, the velocity we pick for this from location to location could
vary a lot in the deep part of the section. This can produce a “saw tooth”
appearance for the deeper velocities that won’t impact on the stack, but could
cause issues with other processes that use the velocity field.</span></span></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjjIxgcPRHbUAk6T-UREc4dEUbzNiG9Ti94tYQO4v8snBb3Ru0pmBwpmi802oiIiZP9wpU_TX2dRziHCZS2-lrm2yR78EZIhhAatTZgh2Sk0DQYdepSIwu8i5d3_Z7tYc_4OEYEOuN-0uc/s1600/three.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="294" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjjIxgcPRHbUAk6T-UREc4dEUbzNiG9Ti94tYQO4v8snBb3Ru0pmBwpmi802oiIiZP9wpU_TX2dRziHCZS2-lrm2yR78EZIhhAatTZgh2Sk0DQYdepSIwu8i5d3_Z7tYc_4OEYEOuN-0uc/s640/three.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">Impact on velocity resolution and accuracy with
two-way-time. The green ray-paths correspond to the far offset, and the pink
ray-paths to the near offset (left). For a shallow event these ray-paths are
very different, but at depth they converge. As a result, on the gather (centre)
we see less moveout-difference with depth, making it harder to accurately
determine the velocity and giving larger “bright spots” on the semblance
spectra (right)</span></td></tr>
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">NMO Stretch<o:p></o:p></span></b></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In the first 1500ms of the data and where the velocities are
low, you will have very large NMO corrections – so large that the correction at
the top and base of an event can be very different. This “stretches” seismic
events and in doing so creates artificially low frequencies that need to be addressed;
otherwise it will reduce the frequency of the stacked image.<o:p></o:p></span></div>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">We
can address this in two ways: (i) using a “stretch mute percentage” which
removes the stretched part of the wavelet once it has been distorted too much,
or (ii) by manually picking a mute. You need to be aware of how the data is being
muted while you are picking velocities.</span></span></div>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgNZYVmrZ-U3YrsRmyEMnhBcwJqPjgODl7mcQz1vxlkZG4rECeutXwL0Qhw-_D56jk_7_RLepdcPTAYSlA09eZFmYmXHoUYZIrqLwzWMHblHDNb3sedudfoEskFsn42uJVtwz6xwYTu4_A/s1600/four.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="580" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgNZYVmrZ-U3YrsRmyEMnhBcwJqPjgODl7mcQz1vxlkZG4rECeutXwL0Qhw-_D56jk_7_RLepdcPTAYSlA09eZFmYmXHoUYZIrqLwzWMHblHDNb3sedudfoEskFsn42uJVtwz6xwYTu4_A/s640/four.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">A synthetic CDP gather (left) with NMO applied (right).
Note now the reflection signal “smears” at far offsets, especially in the
shallow part of the section where the NMO correction is the largest – this is
“NMO stretch”. It arises when the NMO correction at the top of the reflection
signal is significantly larger than that at the bottom of the reflection</span></td></tr>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Accuracy with Offset<o:p></o:p></span></b></div>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The
NMO equation is an approximation. As a result its accuracy decreases with
increasing offset; meaning it’s harder to flatten gathers at far offsets. By
adding a fourth order term to the equation (in addition to the second-order
velocity term) the accuracy of the reflection moveout can be improved, allowing
you to pick velocities at those far offsets.</span></span></div>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiS00O37zOYXUKU0smnohugESHav_Ev8o16M9ss_WJmJGXkIna9oBTV1wA5Vn-YHVLI06zK8oaP-2Y1fRUJtqR5dB58EHP7zy8ibCYb8AOLAommL2U4iTkfUe-7cTu_kDZukdjEECGPbRs/s1600/five.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="510" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiS00O37zOYXUKU0smnohugESHav_Ev8o16M9ss_WJmJGXkIna9oBTV1wA5Vn-YHVLI06zK8oaP-2Y1fRUJtqR5dB58EHP7zy8ibCYb8AOLAommL2U4iTkfUe-7cTu_kDZukdjEECGPbRs/s640/five.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">A gather with offsets from 100m to 3075m (left). A gather
with doubled offsets, from 200m to 6150m (right). The doubled spectra on the
right show improved sensitivity at depth, as a result of the larger offset. However
the large NMO correction and stretch factors distort the semblance in the
shallow</span></td></tr>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Other Signals<o:p></o:p></span></b></div>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The
synthetic we are using is of course very “clean” – there are only reflections
(and white noise) present, with no other signals to complicate velocity
picking. In practice, even just adding in sea-floor multiples can make the
primary velocities much more difficult to pick. </span></span></div>
<div style="-webkit-text-stroke-width: 0px; color: black; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: justify; text-transform: none; white-space: normal; word-spacing: 0px;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="-webkit-text-stroke-width: 0px; color: black; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; margin-left: auto; margin-right: auto; text-align: center; text-transform: none; white-space: normal; word-spacing: 0px;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhJ902S6pj9-NqrQTdkOBKtoBUiZQh-NAjAQPoVonvnLTlR3fBTtm2uW19DhknJK-lGxkAu_-WniRCHhRlqWzcEafOpad_35H7gjjtcYXhlYaoSl7X5T13mzsUxc8OWAohDWPVYRkjDhS4/s1600/six.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="390" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhJ902S6pj9-NqrQTdkOBKtoBUiZQh-NAjAQPoVonvnLTlR3fBTtm2uW19DhknJK-lGxkAu_-WniRCHhRlqWzcEafOpad_35H7gjjtcYXhlYaoSl7X5T13mzsUxc8OWAohDWPVYRkjDhS4/s640/six.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">A more complex synthetic including seafloor multiples,
with no NMO correction applied (left), the correct primary NMO correction
applied (centre), and the semblance spectrum (right). Note how the multiples,
with a lower velocity, remain under-corrected on the centre panel </span></td></tr>
</tbody></table>
<div style="-webkit-text-stroke-width: 0px; color: black; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: justify; text-transform: none; white-space: normal; word-spacing: 0px;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div class="MsoNormal" style="-webkit-text-stroke-width: 0px; color: black; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: justify; text-transform: none; white-space: normal; word-spacing: 0px;">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Picking Velocities on
Line TRV-434<o:p></o:p></span></b></div>
<div class="MsoNormal" style="-webkit-text-stroke-width: 0px; color: black; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: justify; text-transform: none; white-space: normal; word-spacing: 0px;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The line we are working with has a water depth that
corresponds to about 80ms TWT; this means short-period multiples as well as
inter-bed multiples. The data is also not flat, so the shapes of the reflection
hyperbolae will be distorted. Finally, we can also expect some distorted
velocities from the diffractions, which will give high velocity anomalies.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">A few tips when picking:</span></div>
<div class="MsoNormal" style="text-align: justify;">
</div>
<ul style="-webkit-text-stroke-width: 0px; color: black; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-transform: none; white-space: normal; word-spacing: 0px;">
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">pick velocity control points no closer than
100ms TWT apart; closer than this it can be hard to ensure that you are not
“cycle” skipping, and the interval velocities calculated between points as a QC
check start to become unstable</span></li>
</ul>
<ul style="-webkit-text-stroke-width: 0px; color: black; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-transform: none; white-space: normal; word-spacing: 0px;">
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">seismic velocities almost always increase with
depth. You can get small inversions, and with a near-constant velocity (such as
in the near-surface sediments in deeper water) these are not worth worrying
about. Large inversions usually require a big step down in interval velocity,
and you need to consider what geology could produce that change</span></li>
</ul>
<ul style="-webkit-text-stroke-width: 0px; color: black; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-transform: none; white-space: normal; word-spacing: 0px;">
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">very large velocity “kicks” up or down probably
indicate some kind of non-reflection signal.</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">
</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">Multiples give a low velocity trend, and diffractions will give an
abnormally high velocity</span></li>
</ul>
<ul style="-webkit-text-stroke-width: 0px; color: black; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-transform: none; white-space: normal; word-spacing: 0px;">
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">look at the stacks carefully when picking to
make sure you are looking at a primary reflection</span></li>
</ul>
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;">We typically pick velocities at a fixed increment – say 2km
or 4km – and interpolate. This works reasonably well, but where the geology
varies rapidly (e.g. a submarine canyon), a tighter increment over that area
may be more suitable.</span><br />
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="-webkit-text-stroke-width: 0px; color: black; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: justify; text-transform: none; white-space: normal; word-spacing: 0px;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">On this line, there is a sedimentary sequence on the low CDP
end of the line down to around 3000ms TWT, where we start to lose velocity
sensitivity. Under this the velocities are faster. On the high CDP end the
structure is more complex, with a metamorphic basement thrust fault block that
has been pushed over the sedimentary packages.</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">The complexity of the line makes it challenging, especially
when there is still multiple contamination.</span></div>
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<br /></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">When picking velocities you may also be able to generate an
“NMO Stretch Mute” to remove the worst of the NMO stretch distortions; ideally
you should do this in conjunction with your velocities, although you can pick
it separately on NMO corrected CDP gathers.<o:p></o:p></span></div>
<div style="-webkit-text-stroke-width: 0px; color: black; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: justify; text-transform: none; white-space: normal; word-spacing: 0px;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">I’ll
discuss how to check and edit your velocities in another post, but for now,
here’s the kind of stacked section you should be aiming at with these data:</span></span></div>
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<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiB1BjRHGFMjQ2yotuPbF1KlBhFzzAPKqRmd89uj3_LyOGoLPLjyIAGg2nGQHCPRzd6OVw9E8U-cqkxxZDkFY5azxTmlU3TeqIu8oTBudl9U6V75iuibNJJh4iQnDpz64LLJrz50jPtuP4/s1600/seven.jpg" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" height="550" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiB1BjRHGFMjQ2yotuPbF1KlBhFzzAPKqRmd89uj3_LyOGoLPLjyIAGg2nGQHCPRzd6OVw9E8U-cqkxxZDkFY5azxTmlU3TeqIu8oTBudl9U6V75iuibNJJh4iQnDpz64LLJrz50jPtuP4/s640/seven.jpg" width="640" /></a></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgA3uLgXkkqF4TPDitzntxT-nXGVk6IfO1fUbzJKymWTZTuLxXRiJaUE1rGsmxIPmeqAzeHhwigtdVwJVs0RQtkaGuSRDRFKROSuPwpKBU5EV_OPdqFayX8A3a-J3UWxfgr9vyNMCNj9tU/s1600/eight.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="550" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgA3uLgXkkqF4TPDitzntxT-nXGVk6IfO1fUbzJKymWTZTuLxXRiJaUE1rGsmxIPmeqAzeHhwigtdVwJVs0RQtkaGuSRDRFKROSuPwpKBU5EV_OPdqFayX8A3a-J3UWxfgr9vyNMCNj9tU/s640/eight.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">Two stacks of TRV434, using a single velocity function
(above) and picked velocities (below). Both have a 120% stretch mute applied; note
the improved resolution and reduction in low-frequent, reverberant multiples
using the picked velocities </span></td></tr>
</tbody></table>
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<div class="separator" style="clear: both; text-align: center;">
<span style="font-family: "Calibri","sans-serif"; font-size: 11pt; line-height: 17px;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" /></a></span></div>
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<span style="font-style: italic;"></span><br />
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<span style="font-family: "Calibri","sans-serif"; font-size: 11pt; line-height: 17px;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">By: <b>Guy Maslen</b></span></span></div>
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</div>
GLOBE Claritashttp://www.blogger.com/profile/00646197047930265069noreply@blogger.com2tag:blogger.com,1999:blog-7105728614611010325.post-7758398927024186282013-08-13T20:19:00.001-07:002013-08-13T20:19:44.467-07:00Marine Processing - Part 6 | Predictive Deconvolution<table border="1" cellpadding="0" cellspacing="0" class="MsoNormalTable" style="border-collapse: collapse; border: none; text-align: justify; width: 640px;"><tbody>
<tr><td style="border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 182.6pt;" valign="top" width="243"><div class="MsoNormal">
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This sequence of blog posts will build up into a complete description of a 2D marine processing sequence and how it is derived.</span></span></i></div>
</div>
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</div>
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<div style="text-align: left;">
<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">No processing sequence is definitive and techniques vary with time (and software), however the idea is to provide a practical guide for applying seismic processing theory.<o:p></o:p></span></span></i></div>
</div>
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></i></div>
</div>
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<div style="text-align: left;">
<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The seismic line used is TRV-434 from the Taranaki Basin offshore New Zealand. It is the same marine line used in our basic tutorial dataset.<o:p></o:p></span></span></i></div>
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></i></div>
</div>
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<div style="text-align: left;">
<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The data are available from New Zealand Petroleum and Minerals, under the Ministry of Economic Development under the “Open File” System.<o:p></o:p></span></span></i></div>
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</td><td style="border-left: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 297.7pt;" valign="top" width="397"><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The processing sequence developed so far:<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/05/marine-processing-part-1-resample.html"><b>Resample and Minimum Phase Conversion </b></a><b> </b></span></div>
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<ul>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Reformat from field data (in this case, SEGY)</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Apply a combined minimum-phase conversion and anti-alias filter</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Resample to a 4ms sample interval</span></span></li>
</ul>
<br />
<div class="MsoListParagraphCxSpLast" style="mso-list: l0 level1 lfo1; text-indent: -18.0pt;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><o:p></o:p></span></div>
<div class="MsoNormal">
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-2-initial-qc.html">QC and Assign Geometry</a></span></b></div>
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</div>
<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="text-indent: -18pt;">Assign 2D marine geometry; 12.5m CDP spacing and 60 fold </span><span style="text-indent: -18pt;">gathers</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">QC shots and near trace plots</span></span></li>
</ul>
<br />
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-3-amplitudes.html">Amplitude Recovery</a></span></b></div>
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<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="text-indent: -18pt;">Amplitude recovery using a T</span><sup style="text-indent: -18pt;">2</sup><sub style="text-indent: -18pt;"> </sub><span style="text-indent: -18pt;">Spherical divergence correction</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Amplitude QC and Edit using peak and RMS amplitude displays</span></span></li>
</ul>
<br />
<div class="MsoListParagraphCxSpLast" style="mso-list: l0 level1 lfo1; text-indent: -18.0pt;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><o:p></o:p></span></div>
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-4-swell-noise.html">Swell Noise Suppression</a></span></b></div>
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<ul>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Swell noise suppression using projective filtering</span></span></li>
</ul>
<div class="MsoNormal">
<b><br /></b>
<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/07/marine-processing-part-5-direct.html" target="_blank">Linear Noise Suppression</a></span></b></div>
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</div>
<ul>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Interpolation to 6.25m group interval, 480 channels to unwrap spatially aliased dips</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Linear noise removal via Tau-P domain muting, with removable 500ms robust AGC "wrap", 960 p-values and transform range from </span></span><span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">-1400ms <sup>-1</sup>
to +1400 ms<sup>-1</sup></span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Interpolation back to 25m group interval, 240 channels</span></span></li>
</ul>
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</tbody></table>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The processing sequence we have developed so far gives us
the ideal input for predictive (or gap) <a href="http://www.xsgeo.com/course/decon.htm">deconvolution</a>; it is minimum phase,
has the swell noise and strong amplitude linear noise removed, and much of the
spatially aliased high frequency dipping events have been eliminated as well. <o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">On marine datasets, the main goal of predictive
deconvolution is in part to collapse any residual effects caused by the
limitations of the tuned airgun array, and to help suppress short-period reverberations
in the wavelet. These reverberations occur mainly from energy that has
“multiple” reflections from the sea-surface and seafloor, but they can also be
from inter-bed multiples if there are strong reflectors that are close
together.<o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In this dataset I suspect there are also some mode converted
energy – specifically P-S-P mode conversions – where, especially in the high
shot point end of the line, the basement overthrust creates the right
conditions for this to happen.</span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
</div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">The
basic tool we have for looking at the reverberations in a dataset is the
autocorrelation function. It uses a sliding window of fixed length window to mathematically
compare the trace with itself, often over a specific data range. The
autocorrelation function is always symmetrical about time zero where there is a
strong peak. Subsequent peaks indicate where a time-shifted version of the
trace is similar to the original.</span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">
</span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="line-height: 17px;"><br /></span></span></div>
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhPmL0x0ybw_Rokjn5C0nbxfKxWkhxGhIkuW0X6_kiZfHXWZH2O4plGh1T2u2Pby7aPunMG7Zn9Xfhr3DxN7ExxnFblOavfwAsmh6Fi51ommvo5JFuMtrFHCnL0RRsB9OPPzdpsCAWTW8A/s1600/one.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhPmL0x0ybw_Rokjn5C0nbxfKxWkhxGhIkuW0X6_kiZfHXWZH2O4plGh1T2u2Pby7aPunMG7Zn9Xfhr3DxN7ExxnFblOavfwAsmh6Fi51ommvo5JFuMtrFHCnL0RRsB9OPPzdpsCAWTW8A/s640/one.jpg" width="566" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">Shots from the start and end of the line with an
auto-correlation appended to the bottom. The design window for the
autocorrelation function is indicated between the blue and yellow lines</span></td></tr>
</tbody></table>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="line-height: 17px;"><br /></span>
</span></div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">When working with deconvolution, this kind of display should
be your standard approach. I’ve used a bit of ‘trickery’ here in that I have reduced
the record length to 5500ms (for display purposes) and then extended it by
100ms to create a gap between the shots and their autocorrelations. <o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">For the design window, I have defined the start gate using a
calculation based on offset (using the speed of sound in water as 1500ms<sup>-1</sup>
and shifting this down by 200ms), and then made the gate length 2500ms.<o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">You can define the gates manually, but on marine data I
prefer to create a gate that is tied to offset and, if needed, shifted by the
water bottom. In doing so, if you see an anomalous result, it is easier to
back-track and adjust – and of course on large multi-line projects it’s less
work.<o:p></o:p></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The design gate needs to:<o:p></o:p></span></div>
<div style="text-align: justify;">
</div>
<ul>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">be at least 5-7 times the length of the
auto-correlation</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">avoid any very strong reflections – usually just
the seafloor, but there can be others</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">contain reflections – if you can’t see
reflections in the gate window, you will get a bad result</span></li>
</ul>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In this case I’ve got an autocorrelation length of 300ms
which should be enough to show the reverberations caused by the water bottom
(at about 80ms); note how reverberant the data is on SP900.<o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The reason to focus on the autocorrelation is that it is not
just a quality control parameter – it is also used to design the deconvolution
operator we will apply. <o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">You can use more complex designs – such as having multiple
design windows one above and one below a strong unconformity – but the problem
can become this limits the design window and hence the autocorrelation length
that is viable. A long autocorrelation
gives a more stable result!<o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The other key parameter, as well as the length of the
operator (defined in turn by the autocorrelation), is the predictive gap. In
this case, we are not aiming to do much in the way of wavelet shaping or
whitening, so a longer multi-sample gap is preferable to a short one.<o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This is where things become very subjective. Some people
have strong views on the gap being tied to particular values, or to the first
or second zero crossing of the auto-correlation function and so on – however
all deconvolution code is different, and my advice is to *always* test the gap.<o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">There are three basic approaches to deconvolution we need to
test:<o:p></o:p></span></div>
</div>
<div class="MsoListParagraphCxSpFirst" style="margin-left: 54.0pt; mso-add-space: auto; mso-list: l0 level1 lfo1; text-indent: -18.0pt;">
<div style="text-align: justify;">
</div>
<ul>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">we can work one trace at a time, in the X-T
domain</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">we can average autocorrelation functions over
multiple traces, or even a shot</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">we can apply deconvolution in the Tau-P domain</span></li>
</ul>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The first of these is the usual marine “work horse”, but in
situations where the data is noisy the trace-averaging approach can be
effective. Tau-P domain deconvolution is
a special case, as we’ll discuss later.<o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">For the XT domain approaches, I generally start with operator
tests using a 24ms gap; I run these from about 1.5x the first peak on the
autocorrelation function up to the largest value that makes sense given the
design criteria. In this case I might look at 150ms, 250ms and 300ms.<o:p></o:p></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Once I have an operator, I then test gaps – usually 8ms,
16ms, 24ms 32ms and 48ms, perhaps with a spiking (one sample) gap as well.</span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The results tend to be pretty subjective, and depend on the
interpreter’s needs, but 24ms is a fairly standard choice.<o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div style="text-align: justify;">
<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">I’m
not going to fill this post with images of different deconvolution test panels
on shots and stacks – you can see those in</span> <a href="http://books.google.co.nz/books/about/Seismic_Data_Analysis.html?id=ceu1x3JqYGUC" style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">Yilmaz</a><span style="font-family: Calibri, sans-serif;"><span style="font-size: 15px; line-height: 17px;"> (</span></span></span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">you should probably have access to a copy, I’ve never worked
anywhere that didn’t have one available).</span></div>
<div>
<div style="text-align: justify;">
<br /></div>
<div style="text-align: justify;">
<br /></div>
<div id="ftn1">
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhEeCPRFsu1QHxV_zp3xRPHCBkD4NoRdUsy_nwDM_hkodalVg1CPDKAWNwmczTBne6VKt05b2lDFIG7Ur50jf6rANJboybx2jF7vrPhxLHiGE3MdZcwZr2fUr950OwtMQmOpU4xD4FxW_c/s1600/two.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhEeCPRFsu1QHxV_zp3xRPHCBkD4NoRdUsy_nwDM_hkodalVg1CPDKAWNwmczTBne6VKt05b2lDFIG7Ur50jf6rANJboybx2jF7vrPhxLHiGE3MdZcwZr2fUr950OwtMQmOpU4xD4FxW_c/s640/two.jpg" width="536" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">Shots from the start and end of the line; a 24ms gap,
300ms operator XT domain deconvolution </span><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">applied. Start/end design gates displayed (blue,
yellow lines)</span><span style="line-height: 115%;"> </span></span></td></tr>
</tbody></table>
<div class="MsoFootnoteText">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Tau-P domain deconvolution is a little different. It is based on the idea that the multiples
are more periodic in the Tau-P domain than in X-T, but has the additional advantage
that you don’t have the same restriction on design gate lengths at far offsets
– and hence can have a longer, more stable operator.<o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The design process is the same as with the X-T domain, but
in general a longer gap (32ms or 48ms) works better. In general, Tau-P domain
deconvolution is a lot more effective that X-T domain.<o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In this case I’ve tested operators from 400ms to 500ms, and
gaps of 24ms, 32ms and 48ms. These tests
are a lot slower to run, of course.<o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoFootnoteText">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">In
practice the 500ms operator and 32ms gap gave the best result.</span></div>
</div>
<div class="MsoFootnoteText">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><br /></span></div>
</div>
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhv02Ba1w_jdI49nGQiTtcTDNCGVD1X4vwTIxt0-AHICJLBCQR0PW3cq_0Z5AAAQLL7y5a2HNl7gJCazRez7xmJsR1e19zqXIOtKBLY7FIfCdd7hRqpxxmdRqBwiqap3cTDXA-ODLp2sKY/s1600/three.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhv02Ba1w_jdI49nGQiTtcTDNCGVD1X4vwTIxt0-AHICJLBCQR0PW3cq_0Z5AAAQLL7y5a2HNl7gJCazRez7xmJsR1e19zqXIOtKBLY7FIfCdd7hRqpxxmdRqBwiqap3cTDXA-ODLp2sKY/s640/three.jpg" width="508" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Shot record from start and end of the line with no deconvolution</span></td></tr>
</tbody></table>
<div class="MsoFootnoteText">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><br /></span></div>
</div>
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjTjPGYz52dOZSAizjnKFndqEXJq9c4h8AMi3f8fztnTrJavdaLnnTuxaco69g1h1ZP2aT8jrLyhw8qU0aVmfCdW3LWKtP5j2ibZObqFCi6dPOk42rjhXFv0Oi-WmQVB3_hs50YtJ5h07M/s1600/four.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjTjPGYz52dOZSAizjnKFndqEXJq9c4h8AMi3f8fztnTrJavdaLnnTuxaco69g1h1ZP2aT8jrLyhw8qU0aVmfCdW3LWKtP5j2ibZObqFCi6dPOk42rjhXFv0Oi-WmQVB3_hs50YtJ5h07M/s640/four.jpg" width="508" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Shot record from start and end of the line with 24ms gap, 300ms operator XT deconvolution</span></td></tr>
</tbody></table>
<div class="MsoFootnoteText">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><br /></span></div>
</div>
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj4eMTnbumy8KxIe9XYspwsXyLOgSOdfjzbIP_jqaYkRPssAYpGOa0ylnNRWIxCP0cFyvMUq_GWm7cU6Ln3c70DQssOPuRks5BqauNmEtMLw-Bc_XB3XH-xx1eQFNfEczGeaym0HmuSnLg/s1600/five.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj4eMTnbumy8KxIe9XYspwsXyLOgSOdfjzbIP_jqaYkRPssAYpGOa0ylnNRWIxCP0cFyvMUq_GWm7cU6Ln3c70DQssOPuRks5BqauNmEtMLw-Bc_XB3XH-xx1eQFNfEczGeaym0HmuSnLg/s640/five.jpg" width="508" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Shot record from start and end of the line with 32ms gap, 500ms operator Tau-P deconvolution</span></td></tr>
</tbody></table>
<div class="MsoFootnoteText">
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">In practice, the differences are relatively minor between
the X-T and Tau-P domain deconvolution results. This is partly because we have
already applied Tau-P domain linear noise suppression, which can have a big
impact on how effective the deconvolution is.</span></div>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Ultimately the choice of what to use depends on the time and
resources you have available – the Tau-P domain deconvolution is
computationally expensive, but if you are using Tau-P domain linear noise
suppression, these methods can be combined at that stage.</span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Running a second deconvolution on common receiver gathers
can also help improve the effectiveness of the result; if you have used
shot-ensemble or Tau-P domain deconvolution in the first pass.</span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
</div>
<div class="MsoFootnoteText">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">It’s
also important to review stacked sections – either the entire line or just key
areas – with these tests, to ensure that the results on the stacks match what
you require.</span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><br /></span></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiL8YmNankftXfEW9q0cliAYgDl0-ICuUbye4NbwRLxXAl985SEA1tTKgD6F9qFjUbkKYZi3n7C-Gca7osm8RrViCUVEd7zzXkKkR5Oi7mIJgXwQ3hFkBRRKusS_wCRuA7wtmyIvETWo5U/s1600/six.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" height="552" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiL8YmNankftXfEW9q0cliAYgDl0-ICuUbye4NbwRLxXAl985SEA1tTKgD6F9qFjUbkKYZi3n7C-Gca7osm8RrViCUVEd7zzXkKkR5Oi7mIJgXwQ3hFkBRRKusS_wCRuA7wtmyIvETWo5U/s640/six.jpg" width="640" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Stacked section with: constant velocity, stretch mute, amplitude recovery and swell noise recovery (no deconvolution)</span></td></tr>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi44XLTqnnbAJF2ZB_6psjdb9j4YLQB8vD-nFHFIx0THTV2hSGZbFW3nFMrDq6eD8o_UmZBSLs9NXdvvgP8BG8pANolkaqZTadWDiiRFOzVjQG4_hk0zmETOfoJ4knR1Ba9UCUm3xhX85Y/s1600/seven.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" height="552" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi44XLTqnnbAJF2ZB_6psjdb9j4YLQB8vD-nFHFIx0THTV2hSGZbFW3nFMrDq6eD8o_UmZBSLs9NXdvvgP8BG8pANolkaqZTadWDiiRFOzVjQG4_hk0zmETOfoJ4knR1Ba9UCUm3xhX85Y/s640/seven.jpg" width="640" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Stacked section from above with Tau-P domain muting and deconvolution applied</span></td></tr>
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<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" /></a></span></div>
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GLOBE Claritashttp://www.blogger.com/profile/00646197047930265069noreply@blogger.com3tag:blogger.com,1999:blog-7105728614611010325.post-3785879051836588982013-07-16T20:09:00.001-07:002013-07-16T20:09:53.976-07:00Marine Processing - Part 5 | Direct Arrivals, Refractions, and Linear Noise<table border="1" cellpadding="0" cellspacing="0" class="MsoNormalTable" style="border-collapse: collapse; border: none; text-align: justify; width: 640px;">
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This sequence of blog posts will build up into a complete
description of a 2D marine processing sequence and how it is derived.</span></span></i></div>
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></span></i></div>
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">No processing sequence is definitive and techniques vary with
time (and software), however the idea is to provide a practical guide for
applying seismic processing theory.<o:p></o:p></span></span></i></div>
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></i></div>
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The seismic line used is TRV-434 from the Taranaki Basin
offshore New Zealand. It is the same marine line used in our basic tutorial
dataset.<o:p></o:p></span></span></i></div>
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></i></div>
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<i><span style="color: #444444;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The data are available from New Zealand Petroleum and
Minerals, under the Ministry of Economic Development under the “Open File”
System.<o:p></o:p></span></span></i></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The processing sequence developed so far:<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/05/marine-processing-part-1-resample.html"><b>Resample and Minimum Phase
Conversion </b></a><b> </b></span></div>
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</div>
<ul>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Reformat from field data (in this case, SEGY)</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Apply a combined minimum-phase conversion and
anti-alias filter</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Resample to a 4ms sample interval</span></span></li>
</ul>
<br />
<div class="MsoListParagraphCxSpLast" style="mso-list: l0 level1 lfo1; text-indent: -18.0pt;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><o:p></o:p></span></div>
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-2-initial-qc.html">QC and Assign Geometry</a></span></b></div>
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<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="text-indent: -18pt;">Assign 2D marine geometry; 12.5m CDP spacing
and 60 fold </span><span style="text-indent: -18pt;">gathers</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">QC shots and near trace plots</span></span></li>
</ul>
<br />
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-3-amplitudes.html">Amplitude Recovery</a></span></b></div>
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</div>
<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="text-indent: -18pt;">Amplitude recovery using a T</span><sup style="text-indent: -18pt;">2</sup><sub style="text-indent: -18pt;"> </sub><span style="text-indent: -18pt;">Spherical
divergence correction</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Amplitude QC and Edit using peak and RMS
amplitude displays</span></span></li>
</ul>
<br />
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><o:p></o:p></span></div>
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-4-swell-noise.html">Swell Noise Suppression</a></span></b></div>
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<ul>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Swell noise suppression using projective
filtering</span></span></li>
</ul>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">We need to address and remove linear noise from the shot
record; specifically the direct and refracted arrivals. If these are strong
events, there can be reverberated “copies” (i.e. multiples) that can also cause
problems and need to be removed.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">There are three basic approaches we can take for this:</span></div>
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</div>
<ul>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">we can remove the direct and refracted arrivals
by applying a front mute to the data; zeroing all of the samples above an event
that we pick</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">we can try to isolate the linear events in the
FK domain and then remove them</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">we can try to isolate the linear events in the
Tau-P domain and then remove them</span></li>
</ul>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Using a mute to remove the direct and refracted arrivals is by
far the easiest method.<o:p></o:p></span></div>
<span style="line-height: 115%;"><div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Here’s
an example of how you might pick the front mute on a shot at the start of the
line:</span></span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiZ_GtENjKP5CfqpWDnXkRUqu5K815tDizU0vERXgkpIetBOrkjACzfwvU8CAKaTERbwXVpCHDBQTR13YjvTQpJKrSP9fzcG5CBYhYTqVRnr1wF4tE_26XxxY611Pb_zVi_DVkSbLs8sYs/s1600/1.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiZ_GtENjKP5CfqpWDnXkRUqu5K815tDizU0vERXgkpIetBOrkjACzfwvU8CAKaTERbwXVpCHDBQTR13YjvTQpJKrSP9fzcG5CBYhYTqVRnr1wF4tE_26XxxY611Pb_zVi_DVkSbLs8sYs/s640/1.jpg" width="424" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">A typical "front mute" for removal of the direct and refracted arrival. Data above the blue line will be zeroed.</span></td></tr>
</tbody></table>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div style="text-align: justify;">
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">There are a few tricks to picking the mute, especially in
shallow water. <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The first thing to remember is that the mute will need to
have a taper applied; depending on your
software this may start at the time you have picked, end at that time, or be
centred on that time.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The taper is needed to “soften” the boundary from the “hard”
zeros to the actual data. Without the taper we could “clip” waveforms
introducing artificially high frequencies to the dataset leading to artefacts
when we apply further processing.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">I’ve also picked the mute to avoid the few inner traces.
This is important in order to preserve the sea-floor event, but it can be
difficult to identify on shallow water datasets. Even if you can spot the
event, the taper can suppress the amplitudes of this event depending on how it
is applied. <o:p></o:p></span></div>
<span style="line-height: 115%;"><div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;">We
need to keep at least two traces on our shot gathers, because when we sort from
120-fold shots to 60-fold CDPs, the near channel (120) is only going to be the
near trace in half of the CDPs – in the other half it will be the
second-nearest channel (119).</span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div style="text-align: justify;">
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">At this stage it’s important not to be too harsh or
aggressive with the mute – we can always tidy things up later but once you have
removed data, you can’t get it back without re-running.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">You need to step through the data checking the refraction
mute you have picked is valid, adding in more control points where needed to
spatially vary the mute with the geology. This is especially important if the
water depth varies.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The problem with a mute is that it limits the information
from the far offsets. Reflection energy is present in the far offsets and we
can use it for picking velocities, imaging and AVO work, but we can’t see it
under all that noise! <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The next two approaches are based on linear dips either in
the <a href="http://www.xsgeo.com/course/basic.htm#fourier">2D fourier
transform</a> (FK) domain or the <a href="http://www.xsgeo.com/course/basic.htm#tp">linear Tau-P</a> domain. <o:p></o:p></span></div>
<span style="line-height: 115%;"><div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;">Here
are two shot records, from the start and end of the line, plotted out with
their </span><a href="http://www.xsgeo.com/course/basic.htm#fourier" style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;">FK response</a><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;">
underneath:</span></div>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;"><br /></span></div>
<div class="separator" style="clear: both; text-align: center;">
</div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjN3R1B3mEYqu9ew3Rqp5HmcBpq11EsZbW4AYq1igoMzKgpgpBcnYd93coqV901OABsfI3F8A1P6Y3L_J62SeIXhOwO58mHWebm673ws5c34Je2fu__y3NsuqKSflMehWGThMVeqjffZ2E/s1600/2.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjN3R1B3mEYqu9ew3Rqp5HmcBpq11EsZbW4AYq1igoMzKgpgpBcnYd93coqV901OABsfI3F8A1P6Y3L_J62SeIXhOwO58mHWebm673ws5c34Je2fu__y3NsuqKSflMehWGThMVeqjffZ2E/s640/2.jpg" width="387" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">The FK response of two shot records
(which have had gain recovery and swell-noise supression applied). The linear
direct and refracted arrivals we want to remove map into the marked out areas. Above about 31 Hz the signal becomes spatially
aliased. Note the additional linear noise on SP 800 throughout the record,
caused by multiples of the refracted arrivals</span></span></td></tr>
</tbody></table>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;"><br /></span></div>
<div style="text-align: justify;">
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The biggest issue in removing this dipping energy is spatial
aliasing. Spatial aliasing takes place when the apparent, linear dip of an
event is so steep that it is impossible to tell from frequency and wavenumber
alone, which way the event is dipping.</span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">For example, consider a 31 Hz sine wave signal; this has a time
period of (1/31) = 32.25 ms.<o:p></o:p></span></div>
<span style="line-height: 115%;"><div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;">Below
are three examples of a 31 Hz sign wave. In the first panel they are unshifted, the
second panel has a 10 ms/trace shift applied, and the third panel has a
17 ms/trace shift applied.</span></div>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;"><br /></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjaGMOJzpX-ILFHuw_UV9x5BjX60KcSZKNw6D-AEvhViIaISFPPqaZ7bRIBqfBvFwz7sy1TF1nXuo5o0CAdpU8732c1XINSGrMxsS63iFxLaJQkcSUvtes5Y8vhVFyV3vyk99NHPz9uLQY/s1600/5.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjaGMOJzpX-ILFHuw_UV9x5BjX60KcSZKNw6D-AEvhViIaISFPPqaZ7bRIBqfBvFwz7sy1TF1nXuo5o0CAdpU8732c1XINSGrMxsS63iFxLaJQkcSUvtes5Y8vhVFyV3vyk99NHPz9uLQY/s640/5.jpg" width="430" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Three panels showing 20 traces with a 31 Hz sine wave. The
first panel has no shifts applied, the second has a 10ms/trace shifts and the
third has 17ms/trace shifts. The dip direction is obvious on the second panel,
but ambiguous on the third panel – the data are spatially aliased</span></span></td></tr>
</tbody></table>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;"><br /></span></div>
<div style="text-align: justify;">
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">On the second panel the dip direction can be clearly seen as
upper-left to lower-right, but on the third panel (where we have shifted by
approximately half the time period) it is hard to discern the dip direction - it
could also be lower left-to-upper right.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This is an example of spatial aliasing, and shows its
relationship with frequency. With 25m between traces, 17ms/trace corresponds to
25/17=1.470m/ms, or 1470ms<sup>-1</sup>, which is approximately the speed of
sound in water.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In other words, with a 25m spacing between channels, the
linear direct arrival with an apparent velocity of around 1500ms<sup>-1</sup>
will be spatially aliased over 31Hz or so, which is exactly what we see on the
shot records.</span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Spatially aliased linear noise – especially of things like
the tail end of diffractions – is one of the significant causes of apparently
“random” noise – if we can address this we will end up with a much cleaner
section with improved signal strength.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<span style="line-height: 115%;"><div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">If
we only address the non-aliased part of the direct arrival with an FK domain mute,
we get something like this on SP 900.</span></span></div>
<div style="font-size: 11pt; text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: 11pt; line-height: 115%;"><br /></span></div>
<div style="font-size: 11pt; text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: 11pt; line-height: 115%;"><br /></span></div>
</span></div>
</span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgNrJej5ziLhvEStfs_tqSOtLEeWZQ_pHgmhbKnpYzRVa-1lDRxy_B2lp15SZU2-2dF4H7OvttK3LTmrspPj-p12poUfTBAOZLk_r-UEIeIrZwPtPlzzXkjAIRQYBD3Qar0QfmigjJdC20/s1600/6.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgNrJej5ziLhvEStfs_tqSOtLEeWZQ_pHgmhbKnpYzRVa-1lDRxy_B2lp15SZU2-2dF4H7OvttK3LTmrspPj-p12poUfTBAOZLk_r-UEIeIrZwPtPlzzXkjAIRQYBD3Qar0QfmigjJdC20/s640/6.jpg" width="602" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Impact of using an FK domain mute on
SP 900 to remove the non-aliased part of the direct and refracted arrivals
only. Above is the FK domain display, and below the shot record before (left)
and after (right) the FK domain mute. The spatially aliased energy is
characterised by a “reverse dipping chevron” effect, and still contaminates the
data</span></span></td></tr>
</tbody></table>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;"><br /></span></div>
</span><div>
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">While the spatially aliased component is an issue, this
approach has cleaned up a lot of the reverberating linear noise trains below
the first set of refracted arrivals which couldn’t be touched by the front
mute.</span></div>
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">To get around the aliasing issue we can design a more
complex mute shape in the FK domain, we still have some reflection energy around
the K=0 axis at 50 Hz or so, which is going to be contaminated by the spatially
aliased direct arrival.<o:p></o:p></span></div>
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Of course, one answer is to have a smaller trace spacing; if
we had a spacing of 6.25 m, then (approximately) 1500 m/s water bottom arrival
would have dip of 6.25/1500 = 4.16 ms/trace.
This in turn means that only data with a time period less than 8.32 ms
would be aliased, or about 120 Hz.<o:p></o:p></span></div>
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Some modern data is acquired with this trace spacing, which
is a great help in addressing this issue.<o:p></o:p></span></div>
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">We
can try and simulate this by interpolating the data from a 25 m channel interval
up to a 6.25 m interval, and then using an appropriate FK domain mute, ideally
with an “<a href="http://seismicreflections.globeclaritas.com/2013/04/agc-equaliser.html">AGC
wrap</a>” to avoid amplitude issues.</span></span></div>
</div>
</span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhYnyjRCwgL7XBgR6OjjYZJzEOD-l3wLUemXa4gnjH5_CyyB9KiaiqddIcEZhPdkj46Kltohyphenhyphen9STyo6-Zm2HK7PqNYSm8ZoskTyRgl4946VYVN6U2TwBezXufmTTEXzE9xa9_QM1Qe0aKQ/s1600/10.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="428" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhYnyjRCwgL7XBgR6OjjYZJzEOD-l3wLUemXa4gnjH5_CyyB9KiaiqddIcEZhPdkj46Kltohyphenhyphen9STyo6-Zm2HK7PqNYSm8ZoskTyRgl4946VYVN6U2TwBezXufmTTEXzE9xa9_QM1Qe0aKQ/s640/10.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">The FK transform of SP 900 after the data has been
interpolated from 25 m group interval (receiver spacing) to 6.25 m to unwrap the
spatial aliasing of the direct and refracted arrivals</span></span></td></tr>
</tbody></table>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div style="text-align: justify;">
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The interpolation scheme here is relatively fast to run – it
is a simple polynomial fit to the amplitudes at each time value, with an NMO
applied to remove aliasing - and so
leaves a residual aliased arrival, but critically this crosses the K=0 line at
a much higher frequency (80 Hz+) at the top of the range we are expecting. We
can design an FK domain mute to preserve the reflections and reject the linear
noise much more effectively. <o:p></o:p></span></div>
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
Comparing
the FK results we get this:</span></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEisweullf9_-i8YQTlc37CDZkU_WhGlU6r1ogq_znuxeVjJvzk4oBDVy1678gTM7wzx2Le6HmZ8g4XVNBo51_gQKTTx-oxLM390CPSRz9l0KHeNxvPAPZsEt4DbWbny2jCAOv8gxiY6RNc/s1600/11.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="228" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEisweullf9_-i8YQTlc37CDZkU_WhGlU6r1ogq_znuxeVjJvzk4oBDVy1678gTM7wzx2Le6HmZ8g4XVNBo51_gQKTTx-oxLM390CPSRz9l0KHeNxvPAPZsEt4DbWbny2jCAOv8gxiY6RNc/s640/11.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">SP 900, with no FK muting (left), FK muting with 25 m
trace spacing (centre) and FK muting applied with interpolation to 6.25 m trace
spacing (right); the dipping noise can
be removed from the shot record more effectively with interpolation</span></span></td></tr>
</tbody></table>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div style="text-align: justify;">
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The interpolated FK domain route is a lot more effective
than simply applying an FK mute on the interpolated channels, and offers more
advantages over the basic X-T domain front mute as linear noise throughout the
shot record is also addressed. <o:p></o:p></span></div>
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The final approach to compare is the Tau-P domain muting
(also termed “linear RADON”); we face the same spatial aliasing issues in the
Tau-P domain as we did with the FK, so again interpolating the data to remove
these will make a big difference. <o:p></o:p></span></div>
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
In
the Tau-P domain we can vary what linear dips we want to remove with the time
(Tau) value, giving a lot more control than the FK domain.</span></span><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<div>
<br /></div>
<div>
<br /></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEil_YpmNSnL_MICDBp3YBx52WKrqOk9WrOdqi1KIbTvFR8ucP7xARdNpPf6ShyGAcIdf32lha5Syr31pgCAMrnf-8_WVozCus4L2I5ni3yDhGD79UvNd0gbvOujak8WzvqFswEdkTXwKFE/s1600/14.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEil_YpmNSnL_MICDBp3YBx52WKrqOk9WrOdqi1KIbTvFR8ucP7xARdNpPf6ShyGAcIdf32lha5Syr31pgCAMrnf-8_WVozCus4L2I5ni3yDhGD79UvNd0gbvOujak8WzvqFswEdkTXwKFE/s640/14.jpg" width="492" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Tau-P transform of SP 900, with an interpolation to
6.25 m trace spacing applied first. 960 P values have been used, with the traces
labelled by their linear velocity. The transform includes both negative and positive
dips (remembering channel 1 is the far offset) ; a “tail mute” has been picked
to isolate the reflection data (higher apparent velocities) from the linear
noise (lower values) </span></span></td></tr>
</tbody></table>
<div>
<br /></div>
<div>
<!--[if !supportFootnotes]--><div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Using high P-value counts than input data can help to remove
aliasing issues, and so in this case I have used 960 P-values from a
480-channel shot. I’ve also displayed the linear velocities over the transform,
in this case I used -1400ms<sup>-1</sup> to 1400ms<sup>-1 </sup>to cover both
positive and negative dips. The refractions and direct arrivals have negative
dips because channel 1 is the far offset (this may be software dependent). The
channel number is now the “P trace number” and the times have been padded as
part of the Tau-P transform.</span></div>
<div class="MsoNormal">
<br /></div>
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">To
attack the linear noise, I have designed a “tail mute” (green line) below which
the data will be zeroed. As with other mutes (FK or XT), there will be a taper
applied to the data which needs to be taken into account in the mute design.</span></span></div>
<div>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjY8QA7vtEeLJuRuZtNJ0HpwDO0sb-0AFJcgO2fl0XrLSGhjCzH-9jl4JI8ECm_tYdsfk7jOwGPvngqtZVMveEPGmOTg4epH4qua0sfzQExOlW7DVE1OjSdIhYcKA48Q-qq8OUfqz0JNYk/s1600/15.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="276" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjY8QA7vtEeLJuRuZtNJ0HpwDO0sb-0AFJcgO2fl0XrLSGhjCzH-9jl4JI8ECm_tYdsfk7jOwGPvngqtZVMveEPGmOTg4epH4qua0sfzQExOlW7DVE1OjSdIhYcKA48Q-qq8OUfqz0JNYk/s640/15.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">SP 900 with no muting applied (left), FK domain muting
(centre) and Tau-P domain muting (right) to address linear noise such as direct
and refracted arrivals. In both cases the data has first been interpolated to a
6.25 m interval, and had an “<a href="http://seismicreflections.globeclaritas.com/2013/04/agc-equaliser.html">AGC
wrap</a>” applied</span></span></td></tr>
</tbody></table>
<div>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div>
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">When we apply the FK or Tau-P approaches we may also remove
the seafloor event as the water depth is so shallow; it’s usually possible to
avoid this by having a “start time” for the muting process.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In practice the choice of which approach to apply depends on
the a number of factors, including the time you have available to test and
apply different approaches, as the computation time of different transforms can
be significant. The approach I have suggested is quite CPU heavy – to the
extent you’d need to run it in parallel – and so key tests can be whether a
smaller level of interpolation or fewer P-values can get the same results on
your data.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">You should check the results from each approach on shot
records from the whole line, as well as stacks.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Other processes are also relevant – if you took the <a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-4-swell-noise.html">FK
approach to removing swell noise</a> you could combine this with an
interpolation and FK-domain mute to address the direct/refracted arrivals and
save computation time. Similarly we may need to use the Tau-P domain for
deconvolution and can apply that transform only once, which saves processing
time.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In this case, I’ve selected the Tau-P domain result to move
forward with because of the improvement on the low frequencies (and the chance
we’ll use it later on), even though it is expensive computationally.<o:p></o:p></span></div>
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To
see the impact of the processing so far, it is also important to compare the
stacked </span></span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;">sections.</span></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjkc8k7qqxx71hkJ0Get_kzUICIsZ6HUBFTOZzfq557yZz2r0jKshzIYTFNjxzD56KDsndWd-cDvUpIOpEttUTPghnWi5a5_fFiasd0ouks3SiamD-fji1GXsM9ci95_DJ4buBzIYzcc_Y/s1600/18.jpg" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" height="550" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjkc8k7qqxx71hkJ0Get_kzUICIsZ6HUBFTOZzfq557yZz2r0jKshzIYTFNjxzD56KDsndWd-cDvUpIOpEttUTPghnWi5a5_fFiasd0ouks3SiamD-fji1GXsM9ci95_DJ4buBzIYzcc_Y/s640/18.jpg" width="640" /></a></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhz2gQSF4hbzLG8e0MYzS86KUBn37g3pl_IdV8_KSulMjzhthD_pGPk2YkDDb4Hhj880s7-IE_Zp2ht_JNOtNfxUNlnydvR3mYDcFgVowsSl-kBZ_hNvxbsk5xD3TOFon4n-ymbUZMS7TI/s1600/19.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="550" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhz2gQSF4hbzLG8e0MYzS86KUBn37g3pl_IdV8_KSulMjzhthD_pGPk2YkDDb4Hhj880s7-IE_Zp2ht_JNOtNfxUNlnydvR3mYDcFgVowsSl-kBZ_hNvxbsk5xD3TOFon4n-ymbUZMS7TI/s640/19.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Stacked sections: Brute Stack (Spherical Divergence
only) and the current stack, with additional swell noise removal and Tau-P
domain linear noise removal. The stacks have a constant velocity and fixed-percentage
stretch mute applied. Note the impact of removing the steep-dip noise on the
deeper section<br /><br /><br /></span></span></td></tr>
</tbody></table>
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<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" /></a></span></div>
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<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></span></div>
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<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">By: <b>Guy Maslen</b></span></span></div>
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GLOBE Claritashttp://www.blogger.com/profile/00646197047930265069noreply@blogger.com6tag:blogger.com,1999:blog-7105728614611010325.post-24367549215856627802013-06-27T16:36:00.002-07:002013-06-27T16:36:57.012-07:00Marine Processing - Part 4 | Swell Noise Removal<br />
<span style="font-style: italic;">These blog posts will build up into a complete description of a 2D marine processing sequence. They are based on our tutorial datasets, which in turn came from the New Zealand Government’s Ministry of Economic Development under the “Open File” System.</span><br />
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">The processing sequence we have developed so far is:</span></div>
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<b><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><a href="http://seismicreflections.globeclaritas.com/2013/05/marine-processing-part-1-resample.html" target="_blank">Resample & Minimum Phase Conversion</a></span></b></div>
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<!--[if !supportLists]--><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">-<span style="font-size: 7pt;">
</span><!--[endif]-->apply a combined minimum phase conversion and
anti-alias filter<o:p></o:p></span></div>
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<!--[if !supportLists]--><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">-<span style="font-size: 7pt;">
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interval<o:p></o:p></span></div>
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<a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-2-initial-qc.html" target="_blank"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><b>Initial QC & Geometry</b></span></a></div>
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<!--[if !supportLists]--><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">-<span style="font-size: 7pt;">
</span><!--[endif]-->assign a simple straight line 2D marine geometry<o:p></o:p></span></div>
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<!--[if !supportLists]--><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">-<span style="font-size: 7pt;">
</span><!--[endif]-->check the geometry by plotting offsets over the
shots and near traces<o:p></o:p></span></div>
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<a href="http://seismicreflections.globeclaritas.com/2013/06/marine-processing-part-3-amplitudes.html" target="_blank"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><b>Amplitudes & Spikes</b></span></a></div>
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<!--[if !supportLists]--><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">-<span style="font-size: 7pt;">
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spherical divergence correction<o:p></o:p></span></div>
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<!--[if !supportLists]--><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">-<span style="font-size: 7pt;">
</span><!--[endif]-->trace edits using peak-amplitude analysis to
identify noisy shots and channels<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In this post I’m going to continue cleaning up the shot records,
to get them ready for more signal processing, by addressing some of the noise
issues we identified earlier.
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Here’s a shot record with a swell noise burst, plotted
alongside its <a href="http://wiki.seg.org/index.php/Dictionary:F-k_domain">FK
response</a>.<o:p></o:p></span></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgXbmuokSX-xT_dNKX4yI-Y3BOxtbYhZEqEmBmSRzhixziNPx5pgL3ipXcuQbxycX0q_F1_1oylz8B_B7VjXxJmIgVN6uXWcsoj4LNqRVG8pTB9h2gLYWF4oNL3P9N0CPVUubgfGck80LQ/s1155/one.jpg" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" height="572" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgXbmuokSX-xT_dNKX4yI-Y3BOxtbYhZEqEmBmSRzhixziNPx5pgL3ipXcuQbxycX0q_F1_1oylz8B_B7VjXxJmIgVN6uXWcsoj4LNqRVG8pTB9h2gLYWF4oNL3P9N0CPVUubgfGck80LQ/s640/one.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">A shot record (left) and its FK spectrum (right); the shot record shows various types of noise,
including swell noise (red), which has a low frequency and broad spatial
frequency; tail-buoy jerk (white) which
has a low spatial frequency and negative (tail-to-head) dip, and the
direct/refracted arrivals (purple) which are spatially aliased</span></td></tr>
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">The swell noise shows up as low frequency (in this case less
than 5 Hz) energy with a broad spatial frequency band (indicated in red). This
overlaps with the tail-buoy jerk (white) which is also low frequency, but dips
from the tail of the cable to the head. These often occur together, with liquid-filled
seismic streamers; modern streamers achieve their neutral buoyancy through foam
not oil. The motion of the tail-buoy over the sea swell sets up waves that
propagate through the cable, creating the noise.</span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The direct and refracted arrivals are highly <a href="http://www.xsgeo.com/course/basic.htm#fk">spatially aliased</a> (purple)
and wrap back over the reflected signals we want to preserve; the aliased data
crosses the K=0 axis at about 62.5 Hz, which is at the high end of our frequency
band. We can also see some periodicity in the FK plot indicating short period
multiples, as well as the back dipping (tail-to-head) reflections caused by
structure we extend below the K=0 line.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The combination of the XT and FK displays suggest approaches
we can use to tackle these noise issues, the main one of which is swell noise.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">We can:<o:p></o:p></span></div>
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<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">throw data away by editing the traces that have
swell noise present</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">address the swell noise and tail-buoy jerk by
removing only those frequencies</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">address the swell noise and tail-buoy jerk by
muting them in the FK-domain; retaining the low frequency signal (along with
some noise) around the K=0 axis</span></li>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">There is also another technique we can use called “projective
filtering”; this takes a moving time- and space- window and looks at the
frequency content, aiming to locate and scale back anomalous low frequency
noise within a trace automatically.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Firstly I’ll define a window below the direct and refracted
arrivals for the technique to be applied in. This will avoid introducing
artefacts into the data, which can happen at sharp amplitude boundaries. Most
software allows you to design a spatial application gate in this way, above (or
below) which the process isn’t applied.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Secondly, when I go into the FK domain I’ll use a <a href="http://seismicreflections.globeclaritas.com/2013/04/agc-equaliser.html">removable
AGC</a> which will also help to avoid any amplitude complications.<o:p></o:p></span></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjaDS7VxZ8QPqLoYNYZqqiCJsIGjwT02rcexD0i9O6G8Il4xBkexZl4eNKW10X35h9nyR-NSTb3K5SytZp3QpvGTSQqZdl6gNp2JRE01duTiZ4YPcciFN_Yftyh5Uq-Hp3ohFksnPPd9NQ/s979/two.jpg" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" height="382" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjaDS7VxZ8QPqLoYNYZqqiCJsIGjwT02rcexD0i9O6G8Il4xBkexZl4eNKW10X35h9nyR-NSTb3K5SytZp3QpvGTSQqZdl6gNp2JRE01duTiZ4YPcciFN_Yftyh5Uq-Hp3ohFksnPPd9NQ/s640/two.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">FK Domain mute designed to remove swell noise and
tail-buoy jerk, while preserving low frequency reflections. Data above the red line will be muted in the
FK domain</span></td></tr>
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">Both of these approaches will help to remove artefacts.</span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">For the FK approach, the mute I have picked is designed to
be above the direct and refracted arrivals, including the aliased components;
we’ll worry about those later.</span></div>
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: right; margin-left: 1em; text-align: right;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiVfx5WebzAvCYLMrsq3X0hRBFlK3ANnEFStQcQmtu2bNqyd6yRgRbc6NdgBEQ9QICevX8gDhlN95mezLHahxWBqcJ2Zdnh5MeEddDcQbEhgBehyphenhyphenRoEJhRBTh27vAy1gG_mBVzigUqFPec/s1370/three.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="474" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiVfx5WebzAvCYLMrsq3X0hRBFlK3ANnEFStQcQmtu2bNqyd6yRgRbc6NdgBEQ9QICevX8gDhlN95mezLHahxWBqcJ2Zdnh5MeEddDcQbEhgBehyphenhyphenRoEJhRBTh27vAy1gG_mBVzigUqFPec/s640/three.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">A single shot with (from left to right) no swell noise
suppression, a 5 Hz-10 Hz low cut filter, an FK Mute (with AGC “wrap”) and
projective filtering applied to combat swell noise. The blue line is the peak amplitude within a
“deep” window, showing how the swell noise has been removed</span></td></tr>
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">These are the four basic processes we can test; we could of
course vary the low-cut filters, or modify how the FK mute was applied –
however the projective filtering is very effective in this case.</span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">It is also important to look at what is being removed from
the data by calculating “difference” plots to make sure that we are not
removing signal.<o:p></o:p></span></div>
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhz0CdX2k_LAV2ZbbAWXir0l-E7d6zCR4wRGpBxsVt4yWZC5J7Pjwy4HgzuN4d0zfUgdCAykXfPeLb_tPmWdUrG1mRPfJxAaDg1LQK4wEMRq2UMK5Vd2ZKCOenghmbn45lon76BJnhrWEo/s1018/four.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhz0CdX2k_LAV2ZbbAWXir0l-E7d6zCR4wRGpBxsVt4yWZC5J7Pjwy4HgzuN4d0zfUgdCAykXfPeLb_tPmWdUrG1mRPfJxAaDg1LQK4wEMRq2UMK5Vd2ZKCOenghmbn45lon76BJnhrWEo/s640/four.jpg" width="572" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">A single shot record with (from left to right) no
swell noise applied, a low cut 5 Hz-10 Hz band pass filter, and the difference
plot showing what has been removed. The
swell noise is being removed, but so is some reflected energy. The blue line shows the peak amplitude on the
trace</span></td></tr>
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">If we compare the band-pass filter approach to the
projective filtering, we can see that this removes less signal:</span></div>
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<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: right; margin-left: 1em; text-align: right;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj5WMaGkfwlaEKmg4uJWx9LefO39_R04uZDayoKcmmm7grkLzMC_-bizgNX51dnltxuti-pD4AHW7OEQIxfJINBt_OHwTFQCBHgp-bPhcJRsHB7AY4-sB1t-OAQZJtsFgGWYtk2mbVlb5A/s1060/five.jpg" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" height="614" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj5WMaGkfwlaEKmg4uJWx9LefO39_R04uZDayoKcmmm7grkLzMC_-bizgNX51dnltxuti-pD4AHW7OEQIxfJINBt_OHwTFQCBHgp-bPhcJRsHB7AY4-sB1t-OAQZJtsFgGWYtk2mbVlb5A/s640/five.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">A single shot record with (from left to right) no
swell noise applied, projective filtering, and the difference plot showing what
has been removed. The swell noise is
being attenuated, but without removing any of the reflection energy</span></td></tr>
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">Finally, here are the FK plots from the input data and the
same shot with projective filtering applied (as an additional check):</span></div>
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<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: right; margin-left: 1em; text-align: right;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhq4h36rD90GxUYaGeFto4RiIv-mchdcM2hilcs69gowLElCDl8h_Zl6-pTTa9rt-e1ruA1sC7qnAb6Mu2kIG__G9f_fqFITUz0JXdp8tWHonV3PQGCBFKGB7I_GuB37t5XO7KE-_5KLxw/s1200/six.jpg" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" height="480" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhq4h36rD90GxUYaGeFto4RiIv-mchdcM2hilcs69gowLElCDl8h_Zl6-pTTa9rt-e1ruA1sC7qnAb6Mu2kIG__G9f_fqFITUz0JXdp8tWHonV3PQGCBFKGB7I_GuB37t5XO7KE-_5KLxw/s640/six.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">The FK spectra of the input shot record (left) and
after projective filtering (right); the signature of the swell noise and
tail-buoy jerk (red) have been attenuated</span></td></tr>
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">If you were testing this, in practice you would use more
than one shot; a selection of shots from along the line, ideally ones that you
have identified as having swell noise issues as part of the noise QC tests you
have run so far.</span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In this case I’ll continue processing using the projective
filtering. If you didn’t have this
available then varying the filter (or the frequency “cut” for the FK mute) can
increase how harshly the swell noise is attenuated, but of course can cost you
signal.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">One of the techniques you can employ is to select just the
traces with significant swell noise for the harshest filtering. In our software you can store the peak
amplitude in a trace header and then select traces where this exceeds a given
value.</span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">Now
we have de-spiked and “clean” shot records. The next thing we need to address
is the strong linear noise from the direct and refracted arrivals.</span></div>
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<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" /></a></span></div>
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<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">By: <b>Guy Maslen</b></span></span></div>
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GLOBE Claritashttp://www.blogger.com/profile/00646197047930265069noreply@blogger.com0tag:blogger.com,1999:blog-7105728614611010325.post-61193307673494923972013-06-10T22:09:00.001-07:002013-06-10T22:09:59.603-07:00Marine Processing - Part 3 | Amplitudes & Spikes<br />
<span style="font-style: italic;">These blog posts will build up into a complete description of a 2D marine processing sequence. They are based on our tutorial datasets, which in turn came from the New Zealand Government’s Ministry of Economic Development under the “Open File” System.</span><br />
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">As a recap, so far I have discussed:<o:p></o:p></span></div>
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<!--[if !supportLists]--><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">-<span style="font-size: 7pt;">
</span><!--[endif]-->marine source signatures, minimum phase
conversion and resampling<o:p></o:p></span></div>
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<!--[if !supportLists]--><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">-<span style="font-size: 7pt;">
</span><!--[endif]-->assigning the 2D geometry, and some initial QC
checks of the data<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The next thing we need to look at is the seismic trace
amplitudes. </span></div>
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">The strength of the seismic reflections we see decreases
with the depth of the reflecting surface.</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"> </span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">There are several reasons for this but the main ones are
that:</span></div>
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<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">as the wavefield spreads it loses energy; this
is known as spherical divergence. The same amount of energy is spread out over
a larger surface - like blowing up a balloon</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">
</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;"> </span></li>
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<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">as the energy travels, some of it is reflected
at each interface and only a proportion continues; depending on the angle of
incidence, some of this energy is <a href="http://en.wikipedia.org/wiki/Zoeppritz_equations" target="_blank">converted to S-waves, refractions, and so on</a></span></li>
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<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">as the energy travels it is scattered; the Earth
is not homogenous and when seismic waves encounter variations, wave fronts are
distorted and energy is deflected in all possible directions – seismic
scattering</span></li>
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">Typically there are two different corrections needed to
account for this; a spherical divergence correction and a gain function. </span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">These vary with geology, and so for any given
dataset they are something we need to test and confirm before applying. </span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">If you
are new to processing it is worth a quick <a href="http://seismicreflections.globeclaritas.com/2013/02/the-art-of-testing.html" target="_blank">recap of testing approaches</a> before you carry on.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">There are usually many options for the spherical divergence
correction; terms related to powers of the seismic velocity (V),
offset (X) and time (T). This is a large parameter space to search from the
outset.</span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Linear gain testing is usually based on a series of <a href="http://en.wikipedia.org/wiki/Decibel" target="_blank">decibel</a> per second functions (as
opposed to amplitude scalars) which offers a simple way to express an
exponential gain function.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">A typical test “suite” would be to test a few of different
spherical divergence corrections such as T<sup>2</sup>, T<sup>2</sup>V and
perhaps T<sup>2</sup>V<sup>2</sup>, as well as linear gains of 1dB/second,
1.5dB/second and 2dB/second, and then review the results and adjust them if
needed.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Here’s the output of a test sequence for different Spherical
Divergence corrections. Corresponding amplitude decay curves are also shown. I’ve
put a full trace balance onto each display so we can clearly see the relative
amplitudes panel-to-panel.<o:p></o:p></span></div>
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<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="font-size: small;">A single shot record with: no spherical divergence correction (top); a <span style="line-height: 115%;">T<sup>2</sup> function spherical divergence (middle)
and a T<sup>2</sup>V spherical divergence correction (bottom). The amplitude decay functions are plotted alongside as power spectra in dB. The T<sup>2</sup>V
panel (bottom) appears a little over-gained in the last 1000 ms.</span></span><span style="font-size: x-small; line-height: 115%;"> </span><span style="font-size: x-small;"> </span></span></td></tr>
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;">In this case, the middle panel with the T</span><sup style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;">2 </sup><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;">function
looks to be better scaled than the T</span><sup style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;">2</sup><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;">V function; this is
more obvious on the amplitude decay power spectra and curves. </span><br />
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;">In practice you
should apply these tests to a selection of shots from the whole line, ideally
looking at different geological settings.</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;"> </span><br />
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">It’s a good idea to generate a stack with your chosen amplitude
recovery applied as well. <o:p></o:p></span></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgffiEb0gwtsaeSjlKJ2hAovo6BIp-KZH-6j0QRMVSMSyH35J6xwX-pn_SQXkHjy4Z3W8btVouNEbK5gMKdy00ZZQOH1-WV2f5rqR91woxiE2C6rFC2wYnKFuqM24phOfSox29eAfX1hc4/s1600/five.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="550" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgffiEb0gwtsaeSjlKJ2hAovo6BIp-KZH-6j0QRMVSMSyH35J6xwX-pn_SQXkHjy4Z3W8btVouNEbK5gMKdy00ZZQOH1-WV2f5rqR91woxiE2C6rFC2wYnKFuqM24phOfSox29eAfX1hc4/s640/five.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small;">Brute stack with no true amplitude recovery (top); </span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;">T</span><sup style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;">2</sup><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;"> spherical divergence correction applied (bottom). Both sections have </span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;">only</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;"> had a balance applied, to enable comparison.</span></td></tr>
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">The stack sho</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">ws that perhaps we could apply an additional
linear gain, but as the shots are reasonably well balanced we can make that
choice later if needed.</span></div>
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The other aspect of amplitude to consider is the presence of
unusually high amplitude noise bursts.
There are a variety of possible causes, from sharks biting the cable
though to digital spikes caused by loose connections in the cable (on older
surveys). If we leave these in place they can cause significant problems, as
can be seen in the example in the blog entry on <a href="http://seismicreflections.globeclaritas.com/2013/04/agc-equaliser.html" target="_blank">Automatic Gain Control</a> <o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">One way to check for these “spike” problems is to look at
the peak amplitude on every trace. This
analysis needs to be in a window that is under the direct and refracted
arrivals, as these are high amplitude. Some
software allows you to plot this as a graph over the data, but the best
approach is to display it as a graphical colour coded display, with channel on
one axis and the shotpoint number on the other.<o:p></o:p></span></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgp9gLeeeFGtb0MphMzbrnIqkutrcydHEDDt9xvp4zBR6_q8QsHK2UEH3vaSoT_ERehPSEhlEC3x-pFdzgX3qqbcd0zBYiTVapkABhQinzRXPgoddWmuftdABdoW2IrcWl9Q9MCbZPpjYw/s1600/six.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="470" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgp9gLeeeFGtb0MphMzbrnIqkutrcydHEDDt9xvp4zBR6_q8QsHK2UEH3vaSoT_ERehPSEhlEC3x-pFdzgX3qqbcd0zBYiTVapkABhQinzRXPgoddWmuftdABdoW2IrcWl9Q9MCbZPpjYw/s640/six.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;">The peak amplitude on each trace, displayed as a colour coded grid (right); the X-axis is the channel number, and the Y-axis is the shot point number. </span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;">The single red square is at shot point 114, channel 116.</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;"> </span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;">This shot record is displayed (left); y</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; text-align: justify;">ou can see the swell noise burst on the inner channels that corresponds to these higher amplitudes.</span></td></tr>
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<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">The amplitude display plots are a useful way to look at the
data, especially if there are a lot of spiking traces. Swell-noise bursts will
tend to dip from upper-left to lower-right, as the cable is pulled through the
area where the wave is breaking. Vertical lines are bad channels – you can see
some unusually faint channels on this display.</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"> </span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Bad shots show up as horizontal bars – these can be caused
by corrupted files or “misfires” where one or more airguns fire out of
sequence. On older systems electrical
interference or short-circuits can corrupt the whole record.<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Bad channels and bad shots are the key things to edit out;
things like swell noise bursts can be addressed in other ways.<o:p></o:p></span></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiI6ehkV6-OHYEHIeMqqTJcOhcOz__dqfc82qDqDnpdoPPYLF4f8a2CGZPu6gicG38xHdv33Agl0iJNimxvy7XLxjmdLTVnEicovzjmbvi7sh8OzFBJEAvVZ_9S-Scnq7sluOZnS_LiyRk/s1600/seven.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="548" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiI6ehkV6-OHYEHIeMqqTJcOhcOz__dqfc82qDqDnpdoPPYLF4f8a2CGZPu6gicG38xHdv33Agl0iJNimxvy7XLxjmdLTVnEicovzjmbvi7sh8OzFBJEAvVZ_9S-Scnq7sluOZnS_LiyRk/s640/seven.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;"><span style="font-size: small;">A shot record with a swell noise burst on the near channels [102-114] (left); the shot record has a spherical divergence correction applied. </span></span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;"><span style="font-size: small;">The swell noise (which is at the surface) is much higher in amplitude than the reflections, especially in the last 1000 ms of data. In the FK plot (right), the swell noise can be seen as a broad stripe at the top of the plot – it is low frequency (mainly under 5 Hz) and has a broad spatial frequency between +/- 1.</span></span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-align: justify;"><span style="font-size: x-small;"><div style="text-align: justify;">
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<span style="font-size: 11pt; line-height: 115%;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-size: 11pt; line-height: 115%;">In
the next blog, we’ll look at removing noise from the shot records, including
swell noise and the direct/refracted arrivals.</span></div>
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<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" /></a></span></div>
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<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></span></div>
<span style="font-style: italic;"></span><br />
<div style="text-align: center;">
<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">By: <b>Guy Maslen</b></span></span></div>
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GLOBE Claritashttp://www.blogger.com/profile/00646197047930265069noreply@blogger.com0tag:blogger.com,1999:blog-7105728614611010325.post-22460554705235875672013-06-05T20:00:00.002-07:002016-09-13T21:06:01.503-07:00Marine Processing - Part 2 | Initial QC & Geometry<br />
<span style="font-style: italic;">These blog posts will build up into a complete description of a 2D marine processing sequence. They are based on our tutorial datasets, which in turn came from the New Zealand Government’s Ministry of Economic Development under the “Open File” System.</span><br />
<span style="font-style: italic;"><br /></span>
<span style="font-style: italic;"><br /></span>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Following on from the <a href="http://seismicreflections.globeclaritas.com/2013/05/marine-processing-part-1-resample.html" target="_blank">previous post</a> in this series - now that we have minimum phase shot records at a 4 ms
sample interval, we need to review the observer’s logs and do some quality
control checks on the data.</span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"> <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">We’ve already covered a lot of the <a href="http://seismicreflections.globeclaritas.com/2013/02/getting-started-with-marine-data.html" target="_blank">basics for a 2D marine line</a> so we can focus now on the specifics of the seismic
line I’m working with:<o:p></o:p></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
<ul style="margin-top: 0cm;" type="disc">
<li class="MsoNormal" style="text-align: justify;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">FFIDs are the same as
shotpoints in this case; from 100 to 975 with none missing<o:p></o:p></span></li>
<li class="MsoNormal" style="text-align: justify;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">There are 120 channels,
with channel 1 furthest from the boat. The group interval is 25 m, and the
near offset is 258 m. This makes the
far offset (119 x 25) + 258 = <b>3233 m</b>.<o:p></o:p></span></li>
</ul>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">When we sort the data to Common Depth Points (CDP), also
known as Common Mid Points (CMP), the “natural” CDP spacing is half the
receiver spacing; so in this case 12.5 m.<o:p></o:p></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">The number of traces in a CDP gather is referred to as the <i>fold</i>. This is sometimes expressed as a
coverage percentage: single-fold = 100% coverage, sixty-fold = 6000% coverage,
and so on. <o:p></o:p></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Fold = (Receiver
Spacing x Number of Receivers) / (2 x shot spacing)<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: center;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Fold = (25 x 120) /
(2 x 25) = <b>60</b> <o:p></o:p></span></div>
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<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">For more on seismic acquisition in practice, including fold
and CDP, visit <a href="http://www.xsgeo.com/course/acq.htm" target="_blank">this web page</a>. <o:p></o:p></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">The first things to do are to look at the data – a few shots
along the line and the near-trace-plot is a good start. Here’s what our shots look like:<o:p></o:p></span></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhFQDin8ioAIzdQcOJuNWgY887ynvX_QH0WbQUdTsletBg8u-qegxq7vrg-vwHZVu6bYdg_v-McRoHqkpY8IJ4IQImlTVICaZqmHFMZAt8O6EMgMMRvcyXTl0J1wktZ0a3WR4UwrWhUeeM/s1600/one.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><img border="0" height="371" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhFQDin8ioAIzdQcOJuNWgY887ynvX_QH0WbQUdTsletBg8u-qegxq7vrg-vwHZVu6bYdg_v-McRoHqkpY8IJ4IQImlTVICaZqmHFMZAt8O6EMgMMRvcyXTl0J1wktZ0a3WR4UwrWhUeeM/s640/one.jpg" width="640" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Every 100th raw shot from the line with shotpoint number and channel labelled at the top </span></td></tr>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">There are only 120 traces per shot record, and no sign of
any traces that have a trace type not equal to one (trace type 1 = live), so we
don’t have to worry about any additional non-seismic auxiliary traces.</span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">It is also a good idea to check the number of traces; in
this case I have 120 traces per shot, and (975-100) + 1 = 976 shots – so there
should be 117120 traces in total. <o:p></o:p></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Looking at the data, there are actually quite a lot of
issues on these shots we are going to need to think about as we process the
line, partially because the geology is quite complex.<o:p></o:p></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">First of all the water depth is quite shallow, so much so that
you can’t easily tell apart the seafloor reflection, direct arrival and
refracted seafloor. <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Looking at the direct arrivals and refractions, these get
quite a lot worse as we go from low shotpoint numbers to high ones; we also
seem to have more reverberations and lower frequencies on the high shotpoint
numbers. All of this suggests hard rock
with a high seismic velocity coming up towards the seafloor.<o:p></o:p></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">At time 4000 ms – 5000 ms, you can see some low frequency
rumbles; on shotpoint 200 there’s some energy that dips from the tail of the
cable down towards the head. This is referred to as “tailbuoy jerk” as it is
caused by the tailbuoy riding over waves and jerking the cable. There’s also
the “low frequency rumble” of swell noise.<o:p></o:p></span></div>
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<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Detail of SP 400: the low frequency, high energy "stripes" are swell noise</span></td></tr>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjRzT1VO7q1cR5bohA5e-IPTNcWV8QmSCkv84_E7zM6iPMbBZCrcouhCCgnNnHffn99aAZeqeq7i5rJTykklaShY77yaS4zb_zUn8Ys0tyAEfalWuCVM3i_ePK-_VYASPa6noiIAiIdXbo/s1600/three.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjRzT1VO7q1cR5bohA5e-IPTNcWV8QmSCkv84_E7zM6iPMbBZCrcouhCCgnNnHffn99aAZeqeq7i5rJTykklaShY77yaS4zb_zUn8Ys0tyAEfalWuCVM3i_ePK-_VYASPa6noiIAiIdXbo/s640/three.jpg" width="545" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Detail of SP 200: minor swell noise along with tailbuoy jerk, a low frequency dipping event (upper left to middle of the plot)</span></td></tr>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhMaTxMoqm6lXlH8XUvt-OFypti4Gu0LP_8CKnGLYZRkZw4Ry4LABq_csbN_a3VIfmEsAZJNk9-c5IU6zEWnHgtuLu-LVuqAbmE7uv6bQbJHavWZ6vvDvWf_E4_yfvDHxWxMvHYZSHP_Js/s1600/four.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhMaTxMoqm6lXlH8XUvt-OFypti4Gu0LP_8CKnGLYZRkZw4Ry4LABq_csbN_a3VIfmEsAZJNk9-c5IU6zEWnHgtuLu-LVuqAbmE7uv6bQbJHavWZ6vvDvWf_E4_yfvDHxWxMvHYZSHP_Js/s640/four.jpg" width="544" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Detail of SP 400: linear direct and refracted arrivals, as well as (hyperbolic) reflections</span></td></tr>
</tbody></table>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhkpyyP0frs73vhMiH2JDMzWTPjrhGcvB9Z8BgGZE8OMmBGBokf0kpci_U21sTYcHSp3tai55JRYYXiVQhRyaS_smNBY0QrNRkmoL8PcqM3-HT8FaClxHlgiWFZ6R7aVxp56efB43MZ-tI/s1600/five.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhkpyyP0frs73vhMiH2JDMzWTPjrhGcvB9Z8BgGZE8OMmBGBokf0kpci_U21sTYcHSp3tai55JRYYXiVQhRyaS_smNBY0QrNRkmoL8PcqM3-HT8FaClxHlgiWFZ6R7aVxp56efB43MZ-tI/s640/five.jpg" width="544" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Detail of SP 900: the faster (less steeply dipping) refracted arrivals, and loss of signal showing how the geology changes </span></td></tr>
</tbody></table>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">As part of the processing sequence we will have to manage
all of these – everything that is not a seismic reflection is effectively
noise.<o:p></o:p></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">The next thing we need to do is to sort out the geometry, so
that we can compare what we think the geometry should be to the actual data. In
most software the setting up of a 2D marine geometry is pretty simple – you
assume the streamer is being towed directly behind the boat (and so the offset
is a simple function of the channel number), and the CDP number is a function
of the shot number and the offset.<o:p></o:p></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">In our case the OFFSET = 258 + ((CHANNEL - 120) x 25), as
there are 120 channels that are 25 m apart and channel 120 is the near offset,
258 m from the source.<o:p></o:p></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">The midpoint between this near channel and the first shot is
129 m, or a little over 10 CDP spacings.<o:p></o:p></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">The midpoint between the far channel (at 3233 m offset) is
1661.5 m, or a little under 133 CDP spacings.<o:p></o:p></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">If we set the first CDP to be 100 (corresponding to the far
offset on the first shot), then the trace generated by the near channel will be
at CDP number 233. For the second shotpoint, we will have moved along by 25 m,
(or 2 CDPs) and so the far channel will correspond to CDP 102, and the near
channel will correspond to CDP 235, and so on.<o:p></o:p></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">There are a couple of things to note about this. If we take
two CDPs from the middle of the line (say CDP 400 and 401), then they look like
this:<o:p></o:p></span><br />
<br /></div>
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<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiP7GHGCUXCxAtx4DpPvqYyCxE_2UpvMkMAOSNXOfhs-P3BbK3cS5d9Z2AHqO9riX41-5dfTjlSV6v5bX3K3YPn4ihcZXdbLh90uPtwJ_cIhF2lgbmf06Td8C8Lkbo2SGSnF49JoAQ_dM4/s1600/table1.jpg" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiP7GHGCUXCxAtx4DpPvqYyCxE_2UpvMkMAOSNXOfhs-P3BbK3cS5d9Z2AHqO9riX41-5dfTjlSV6v5bX3K3YPn4ihcZXdbLh90uPtwJ_cIhF2lgbmf06Td8C8Lkbo2SGSnF49JoAQ_dM4/s640/table1.jpg" width="640" /></a></div>
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">The key things to note are:</span></div>
<div class="MsoNormal">
</div>
<ul>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Odd and even CDP numbers have different offset
values, but the same number of traces</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">The spacing between traces in the CDP gather is
50m, i.e. every second receiver</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">The even CDPs don’t have the “near offset” trace
in them</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">The odd CDPs don’t have the “far offset” trace
in them</span></span></li>
</ul>
<br />
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">This pattern becomes important later on in the processing
sequence, especially for processes that work on common offset planes.<o:p></o:p></span></div>
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<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Having assigned the offsets, there are three things we can
look at as a check:<o:p></o:p></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
<ul style="margin-top: 0cm;" type="disc">
<li class="MsoNormal" style="text-align: justify;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">shots<o:p></o:p></span></li>
<li class="MsoNormal" style="text-align: justify;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">the near trace plot<o:p></o:p></span></li>
<li class="MsoNormal" style="text-align: justify;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">a “brute” stack<o:p></o:p></span></li>
</ul>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjMZpCk9A2heFwvxGHggfUfnQSGwmLK9op5Rtsi9rN3hdfIZAw4BrISbyopHFF5Y4lqJF3OgRsB1EHONHPqWd6ro5I3i_KkVtnl37QWFB3tJIlqC7KW3nPh1nSyT8Mhm08oKucBMQufl30/s1600/six.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><img border="0" height="386" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjMZpCk9A2heFwvxGHggfUfnQSGwmLK9op5Rtsi9rN3hdfIZAw4BrISbyopHFF5Y4lqJF3OgRsB1EHONHPqWd6ro5I3i_KkVtnl37QWFB3tJIlqC7KW3nPh1nSyT8Mhm08oKucBMQufl30/s640/six.jpg" width="640" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Shot records displayed with the direct arrival time calculated from the offset (displayed as a red line). In this case the speed of sound in seawater (1500 m/s) was used to convert the offset in metres to a two-way-time value</span></td></tr>
</tbody></table>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhmZf6Jm7f6PmMeIHAPjFoxnlCLZnINCzuLJPgcE24Kj77BY9vpvYr9sJQF1HWMI4mFEQE2UgSWVI5RrtxckRJZEISMPz6mdvSNsKmc_sV3SedwurJ5kcpW76CEDcnsh-5RR1EbsOYd5pQ/s1600/seven.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhmZf6Jm7f6PmMeIHAPjFoxnlCLZnINCzuLJPgcE24Kj77BY9vpvYr9sJQF1HWMI4mFEQE2UgSWVI5RrtxckRJZEISMPz6mdvSNsKmc_sV3SedwurJ5kcpW76CEDcnsh-5RR1EbsOYd5pQ/s640/seven.jpg" width="458" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Offset (red line) overlaid on shot record; it matches the first break, meaning it is correct </span></td></tr>
</tbody></table>
<div style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">On both the shot record and near trace plot displays, we can
calculate the time of the theoretical direct arrival based on the offset and
the speed of sound in seawater, which is usually in the range
1480-1500 m/s. By plotting this as a line
on top of the seismic, we can verify the offsets and the timing of the shots.</span></div>
<div style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEipMW5Ef5AF6trhF60PadEjO_RszNDw_jqMDDy5OdIE8hbBbW2VbRyFwz7GaMsxqoL9TaQFQRpycct3PqRZz9Q2bPtOzhFRlclZ2IbAQutWID1Tvt27vdn2jqVElW6uuRTgr2dEeAxB6kA/s1600/eight.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><img border="0" height="358" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEipMW5Ef5AF6trhF60PadEjO_RszNDw_jqMDDy5OdIE8hbBbW2VbRyFwz7GaMsxqoL9TaQFQRpycct3PqRZz9Q2bPtOzhFRlclZ2IbAQutWID1Tvt27vdn2jqVElW6uuRTgr2dEeAxB6kA/s640/eight.jpg" width="640" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Part of the near-trace (channel 120) plot with the predicted time of the direct arrival (as calculated from the offset - displayed in red); this aligns well with the first arrival of the data, confirming the near offset and the absence of any gun delay </span></td></tr>
</tbody></table>
<div style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">To create a “brute” stack you will need a basic velocity
model; this should be a single simple function. The velocity model is often drawn
from some regional geological information – you may also have the capability to
make some rough velocity measurements with a “hyperbolic” ruler on the shot
records, for example. You could also output semblance spectra from the shots
and pick a velocity model from that, to be used to create a brute stack. </span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">The basic approach is to gather the data by CDP number,
flatten the hyperbolic reflection events using the <a href="http://en.wikipedia.org/wiki/Normal_moveout" target="_blank">normal moveout (NMO)</a> correction, and then stack (sum) the data to improve the signal to noise ratio.<o:p></o:p></span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">The sequence is:</span></div>
<div class="MsoNormal">
</div>
<ul>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Read in the minimum phase data at 4ms sample
interval</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Apply the
geometry we have defined</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Sort the data to CDP</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">NMO correct with a simple function</span></span></li>
<li style="text-align: justify;"><span style="text-indent: -18pt;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Stack the data</span></span></li>
</ul>
<br />
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">When you apply the NMO, you can use an “automatic” stretch
mute at this stage. These are used to avoid low frequency artefacts on the data
when the NMO correction is large in the shallow portion of the section. As a side effect, they will also tend to
remove refracted data.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">In this case I’ve used a velocity
function that has only a few points:</span><br />
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span>
<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg9RhXrkl_Ab9UedddXnyEYrTV0M528K5Q6Bf-vEiKaYjJk9qVxKQAuff3vz3fUH-og8x0hyQY5eoM2EISZk7oYk5SfudF0CoiLzwXAQH8ihROLapKRzIDJEDi_W-EmS5IgZufb5evK14I/s1600/table2.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg9RhXrkl_Ab9UedddXnyEYrTV0M528K5Q6Bf-vEiKaYjJk9qVxKQAuff3vz3fUH-og8x0hyQY5eoM2EISZk7oYk5SfudF0CoiLzwXAQH8ihROLapKRzIDJEDi_W-EmS5IgZufb5evK14I/s640/table2.jpg" width="490" /></a></div>
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"> </span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">The “brute stack” gives an initial look at the structure of
the line; it is a good idea to plot the “fold” of coverage (the number of
traces in each CDP) on top of the stack as a check.</span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"> <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">Now we have the geometry resolved and checked, we can start
to look at the data in more detail, and start to test some different processing
sequences to make improvements.</span></div>
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<span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><br /></span></div>
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhHwpJRrMTHF0LW46oiPuJOIK6foOLS8nVYkWqQ5C8GH7akyy41hTEyGAxhc_PcQWbVdnNqLMEsFEF4yWgntcL_b9aoPUPuo1wZwlqHjlY91TKan0W35UKPtAokVbGtmjGxRi3drdYHHCI/s1600/nine.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;"><img border="0" height="452" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhHwpJRrMTHF0LW46oiPuJOIK6foOLS8nVYkWqQ5C8GH7akyy41hTEyGAxhc_PcQWbVdnNqLMEsFEF4yWgntcL_b9aoPUPuo1wZwlqHjlY91TKan0W35UKPtAokVbGtmjGxRi3drdYHHCI/s640/nine.jpg" width="640" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">The brute stack of the line, with the fold pattern overlaid (multiplied by 10); the data is un-scaled but you can still see the basic geological structure. The CDP numbers and shotpoint positions are both labelled.</span></td></tr>
</tbody></table>
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<div style="text-align: justify;">
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<span style="font-family: "calibri" , "sans-serif"; font-size: 11.0pt; line-height: 115%;"><span style="font-family: "helvetica neue" , "arial" , "helvetica" , sans-serif;">By: <b>Guy Maslen</b></span></span></div>
GLOBE Claritashttp://www.blogger.com/profile/00646197047930265069noreply@blogger.com11tag:blogger.com,1999:blog-7105728614611010325.post-49689982878832326802013-05-19T20:51:00.001-07:002014-04-28T17:27:03.862-07:00Marine Processing - Part 1 | Resample & Minimum Phase Conversion<br />
<div style="border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-element: para-border-div; padding: 1.0pt 4.0pt 1.0pt 4.0pt;">
<div class="MsoNormal" style="border: none; mso-border-alt: solid windowtext .5pt; mso-padding-alt: 1.0pt 4.0pt 1.0pt 4.0pt; padding: 0cm;">
<i>These blog posts will build up into a complete description of a 2D
marine processing sequence. They are
based on our tutorial datasets, which in turn came from the New Zealand
Government’s Ministry of Economic Development under the “Open File” System.<o:p></o:p></i></div>
</div>
<div class="MsoNormal">
<br />
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Marine seismic data is typically recorded at a 2 ms (0.002 s)
sample interval, giving a <a href="http://wiki.seg.org/index.php/Dictionary:Alias">theoretical maximum
frequency of 250 Hz</a> that can be accurately recorded;<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<div class="MsoNormal" style="text-align: center;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Sampling frequency
(F) = 1/Period (T) = 1/0.002 = 500<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: center;">
<a href="http://www.blogger.com/blogger.g?blogID=7105728614611010325" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"></a><a href="http://www.blogger.com/blogger.g?blogID=7105728614611010325" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"></a><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Nyquist Frequency (f<sub>N</sub>)
= Sampling frequency (F)/2 = 500/2 = <b>250 Hz</b><o:p></o:p></span></div>
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<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">We don’t do this because we actually have those frequencies
– in practice anything over 90-100 Hz is pretty optimistic – but instead to
avoid issues with aliased noise.</span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">If we record data such that the maximum frequency is 250 Hz,
but then resample to a 4 ms sample interval and filter out energy above around 120 Hz,
we can be confident that there are no aliased frequencies in our raw data.</span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">As a result, this is typically the very first processing
step; in fact we may do this before we really take much of a look at the data.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Processing software tends to have a resample routine
with a built in <a href="http://wiki.seg.org/index.php/Dictionary:Alias_filter">anti-alias filter</a>.
The main concern with using this is that you have no idea how good (or bad) it
is. Some “built in” filters are very old in software terms i.e. still using as
little as 51 input points. In general, fewer input points leaves more noise (ringing)
in the data after applying the filter.</span><br />
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh8NegVxMEo2ecDr0eKm-e3VEhTt50pa72gtxiF9AgCcvZvWiCeznmQAm7GVUtprkHAdMXQR4C1rr4G4ld1ylv54alICYK-0t6ZvBpOPNOhdFpEj4ch260FLlZBuzVCxdp5EzJ3Fwbwb3k/s1600/51point.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh8NegVxMEo2ecDr0eKm-e3VEhTt50pa72gtxiF9AgCcvZvWiCeznmQAm7GVUtprkHAdMXQR4C1rr4G4ld1ylv54alICYK-0t6ZvBpOPNOhdFpEj4ch260FLlZBuzVCxdp5EzJ3Fwbwb3k/s640/51point.jpg" height="640" width="527" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Variation in the number of input points and resulting filtered output (Image from Gaussian Waves - <a href="http://www.gaussianwaves.com/2010/11/moving-average-filter-ma-filter-2/">http://www.gaussianwaves.com/2010/11/moving-average-filter-ma-filter-2/</a>)</td></tr>
</tbody></table>
<br />
<a href="http://www.blogger.com/blogger.g?blogID=7105728614611010325" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"></a><a href="http://www.blogger.com/blogger.g?blogID=7105728614611010325" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"></a><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">You also may not know what phase the filter has, and even if you do, your raw
data is likely to be mixed phase. <o:p></o:p></span><br />
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">So I prefer not to use the built in filters. I tend to
create my own resample filter each time, and save it (along with a screen grab of the
displays) for my report so that I know what has been applied. <o:p></o:p></span><br />
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">I also combine this stage with the minimum-phase
conversion. You are likely to have been
supplied with a source signature at the original sample interval, so we can
combine the high-cut filter we want with the minimum-phase conversion filter
derived from the source signature.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Typically, the starting point for this is a far-field
source-signature from the acquisition company, which will probably look
something like this:<o:p></o:p></span></div>
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<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgNhSFk0-zcU0wh4ZrLCX1xSZTEMTEc8sbooeD7fkDnKj_OOkFt5P_gGqdfjHDQUlF9NFF3iWOSAnQTi8Ew-AADxrShzprPN-U_ZL2gptZhJB9EAufQzz6Se7tOgJPmKM7WePPAFNcnsXk/s1600/one.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgNhSFk0-zcU0wh4ZrLCX1xSZTEMTEc8sbooeD7fkDnKj_OOkFt5P_gGqdfjHDQUlF9NFF3iWOSAnQTi8Ew-AADxrShzprPN-U_ZL2gptZhJB9EAufQzz6Se7tOgJPmKM7WePPAFNcnsXk/s640/one.jpg" height="342" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">A typical marine far-field source signature in a
“wavelet calculator”; the time samples are on the left, and the frequency
content on the right. Note the frequency
“notch”, identified by the cursor at 125 Hz. The signature looks pretty close to a minimum
phase wavelet, as you might expect.</span></td></tr>
</tbody></table>
<br />
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">When loading up the source signature it is important to look
out for any “time shift” applied; in this case the time of the first sample was
given as -64 ms in the header of the source signature file.</span><br />
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The key thing to notice with this signature is the “notch”
in the frequency spectrum. This arises
because of the “<a href="http://wiki.seg.org/index.php/Dictionary:Ghost">ghost</a>”
effect (and is thus referred to as the “ghost notch”). Energy travels up from
the source and is reflected off the sea surface. When this energy combines with the original
down-traveling energy, the waveshape is changed and the “ghost” is created.<o:p></o:p></span><br />
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">You can calculate the dominant frequency of the “ghost notch”
easily enough, by considering the travel time from the source to the sea
surface and back again.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In this case, the source is being towed at 6 m, so the
distance traveled is 2 x 6 m = 12 m.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The speed of sound in sea water is about 1500 m/s, which
gives a ‘delay time’ of 0.008 seconds (8 ms).<o:p></o:p></span></div>
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<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This time period (T) corresponds to a frequency of 1/0.008
or 125 Hz.<o:p></o:p></span><br />
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<br />
<div class="separator" style="clear: both; text-align: center;">
</div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEix27KhuGsbngnmWMm6bmzev9kbiDiP5yyqaOLDq6mSWn1gycuFm78sO1cAWvZR_-NMFcFj86eDzoH-rniO-MueVYXRA6Kol0ZSSBs-5OjcCU2cjUguE-qse8HO4axCYJqd0DpysUJRmnc/s1600/notch.JPG" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEix27KhuGsbngnmWMm6bmzev9kbiDiP5yyqaOLDq6mSWn1gycuFm78sO1cAWvZR_-NMFcFj86eDzoH-rniO-MueVYXRA6Kol0ZSSBs-5OjcCU2cjUguE-qse8HO4axCYJqd0DpysUJRmnc/s640/notch.JPG" height="370" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Effect of towing at 6 m depth i.e. 8 ms delay (Image from Mohamed in 'Ghost Reflections' forum discussion - <a href="http://forum.detectation.com/viewtopic.php?f=28&t=1144">http://forum.detectation.com/viewtopic.php?f=28&t=1144</a>) </td></tr>
</tbody></table>
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This of course is one reason why 6 m is a common tow depth –
the ghost effect is used as a natural anti-alias filter to suppress energy around
the Nyquist frequency (f<sub>N</sub>) and make aliasing less likely.<o:p></o:p></span><br />
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Over time, however, it has become common to tow the
receivers slightly deeper than the source; the main reason being that the
bubbles created by an airgun array can interfere with the sonic ranging systems
used to position the cables.</span><br />
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjZmrck9TOHk_QLmOabHP0cv3I9GRs1rOvcmTujXvzFyFo-7E64TAGeTtkJXprbQw-fBHjMG-3xCkQlfrcpuNhtSmEGNjS-t5BQG-NUoYT0gYRhlW-2B4eoa4Wso0tg-A26VI39bNP7BUE/s1600/aerial_shot.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjZmrck9TOHk_QLmOabHP0cv3I9GRs1rOvcmTujXvzFyFo-7E64TAGeTtkJXprbQw-fBHjMG-3xCkQlfrcpuNhtSmEGNjS-t5BQG-NUoYT0gYRhlW-2B4eoa4Wso0tg-A26VI39bNP7BUE/s400/aerial_shot.jpg" height="320" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">A 3D marine vessel from the air during a crew change
(photo courtesy of Tim Mason); the two source
arrays can be seen as they have three
closely spaced “wakes”; the starboard
source has just fired (black arrow), but the bubbles from the previous three
shots (orange arrows) can still be seen on the sea surface. This indicates that
the source is towed shallower than the streamers.</span></td></tr>
</tbody></table>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">The signature I have shown includes the source ghost, but not
the receiver ghost, where a delayed signal that has reflected off the
sea-surface will also be recorded.</span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjNitI5YANL2we1A6QU-3-lHgCnarU8UJCUIXpLBFb31GrEHvTBsvQzo_COf6hpShNVjctn7w0KL5FJrGV_kISSyqE-4LpQusrp0TLAB-B8iU6eXDG31YIqeDLL2yPBJBGbnFRI9wwGzxE/s1600/ghost_reflections.gif" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjNitI5YANL2we1A6QU-3-lHgCnarU8UJCUIXpLBFb31GrEHvTBsvQzo_COf6hpShNVjctn7w0KL5FJrGV_kISSyqE-4LpQusrp0TLAB-B8iU6eXDG31YIqeDLL2yPBJBGbnFRI9wwGzxE/s400/ghost_reflections.gif" height="253" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Ray paths showing the possible routes for ghost reflectors (Image from Mohamed in 'Ghost Reflections' forum discussion - <a href="http://forum.detectation.com/viewtopic.php?f=28&t=1144">http://forum.detectation.com/viewtopic.php?f=28&t=1144</a>)</td></tr>
</tbody></table>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">This is pretty common, and you need to check
carefully. If this is the case, you’ll need to model the receiver ghost as
well.</span><br />
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">To do this, simply create a spike (of amplitude +1) at time
zero and a second spike (of amplitude -1) at the appropriate time for the
receiver ghost to arrive (twice the receiver depth). <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">For example, with a 7 m tow depth and 1500 m/s seawater
velocity this would be 14/1500 = 0.00933 s. The result will look something like this:<o:p></o:p></span><br />
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhjzxj8vndg7_vPDQIeTgcOi_WlXjaFRtiwULJInb6y9FPDJitwCHuIByKnMTRM4JW835mT4v_pnqjl3zadkkaD_jehglr20f-ZXWL2pJhkA2TxKbk8XBV4Mp_w8mNEspk8dFDfaSHJXBw/s1600/two.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhjzxj8vndg7_vPDQIeTgcOi_WlXjaFRtiwULJInb6y9FPDJitwCHuIByKnMTRM4JW835mT4v_pnqjl3zadkkaD_jehglr20f-ZXWL2pJhkA2TxKbk8XBV4Mp_w8mNEspk8dFDfaSHJXBw/s400/two.jpg" height="351" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">The original signature plotted (above) with a modeled
receiver ghost for a streamer depth of 7 m, created by making a spike at
time zero and a negative spike at the arrival time of the ghost. Note the ghost notch is now at 112.5 Hz.</span></td></tr>
</tbody></table>
</div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This illustrates another key issue with the ghost – it has a
low frequency effect as well. <o:p></o:p></span><br />
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">To create the combined signature, you need to convolve these
two together, but we also need to apply a time shift to the modeled
signature. In the supplied information
it, it tells me that the time of the first sample is -64 ms, so I have to apply
this “delay” to correct the signature.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj5Cip-qJk753K-X4w3Bd4FzNfna38I7GUWbsKb_nz1M5OSIZHp7BtORh71eLF0zrMaiANSQi9p1ahJvezusisjOAxmk6a_FWcFYpcLRuPP75VB-OePM17_IgM21-AmqBx-i8grCiDz3rI/s1600/three.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj5Cip-qJk753K-X4w3Bd4FzNfna38I7GUWbsKb_nz1M5OSIZHp7BtORh71eLF0zrMaiANSQi9p1ahJvezusisjOAxmk6a_FWcFYpcLRuPP75VB-OePM17_IgM21-AmqBx-i8grCiDz3rI/s640/three.jpg" height="339" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">The modeled signature including the receiver ghost
effect. The notch has broadened, and the
wavelet shape is now far from the “minimum phase” ideal. The start delay of 64 ms has also been removed
from the data.</span></td></tr>
</tbody></table>
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">As I mentioned before, this additional modelling of the
receiver ghost is a critical step that is often left out; the results as you
can see can look a little like a zero phase wavelet. I have seen situations where no phase
correction was applied, but in looking at the seabed reflection the
interpreters have assumed the data were zero phase corrected.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">We can calculate the minimum phase equivalent of this, as a
starting point for our idealised signature.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhV1MGMB3G8I-SBGT9HVh4o8MASwrvY26Nn5mkedez16_6VgV4khUqQaWH39EMr85V3jcctY0FvEQZ7Gffrdq6yKryMkX1SQthDnMAsd9O1t-kL62qAf0l_sGb280F71Vl5ns-Bt_cjoME/s1600/four.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhV1MGMB3G8I-SBGT9HVh4o8MASwrvY26Nn5mkedez16_6VgV4khUqQaWH39EMr85V3jcctY0FvEQZ7Gffrdq6yKryMkX1SQthDnMAsd9O1t-kL62qAf0l_sGb280F71Vl5ns-Bt_cjoME/s640/four.jpg" height="340" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">The minimum phase equivalent of the model source
signature, including the ghost and start time shift.</span></td></tr>
</tbody></table>
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">While this represents our idealised source wavelet, we have
one more thing to do before creating a conversion filter. If we are going to include the anti-alias
high-cut filter in here as well, then we need to create the minimum phase
equivalent of the signature with the appropriate minimum phase filter high-cut
filter. I typically use an 80 Hz-100 Hz
high cut.<o:p></o:p></span><br />
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">A couple of points on the design of this filter; I tend to
use the minimum phase equivalent of a butterworth filter, with at least 200
points. While butterworth filters are
pretty stable, the minimum phase formulations are not terribly minimum phase at
higher orders – hence the additional step of making a minimum phase
equivalent.</span><br />
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">As always, you need to check the filter you have created
carefully – look for “ringing” in the frequency domain, which appear as
“ripples” on top of the pass-band of the filter; if you see these you may need
to adjust the slope of the high-cut or lengthen the filter.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhgFTl_UhdnWRixUzRiBqCad1OCW8o0IJIT71rsbqy6VIB40LUx1KOsSXdS-N_TdoDnovPd7RVPBo4QhJtshiDOulgvB85pBu2PdXODGl2q_9z53WrAiyfOUeG_csCm0luj2GhkIURZxWY/s1600/five.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhgFTl_UhdnWRixUzRiBqCad1OCW8o0IJIT71rsbqy6VIB40LUx1KOsSXdS-N_TdoDnovPd7RVPBo4QhJtshiDOulgvB85pBu2PdXODGl2q_9z53WrAiyfOUeG_csCm0luj2GhkIURZxWY/s640/five.jpg" height="340" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">The minimum phase equivalent of an 80 Hz-90 Hz high cut
filter, which will be used to create the anti-alias effect.</span></td></tr>
</tbody></table>
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Finally, we can convolve this filter with our minimum-phase
equivalent of the time-shifted signature, that now includes the receiver ghost;
this is our idealised output signature:<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgu2NklyrfErlJMfyMXqk2ZaRAijoy30OL28fD3CCG0cVQW9dW1I7zEfr5jPOvhqSITgbVL_OODcj0c0etvZTxp-6NOzrnVts9J3uEjPkCFpF6LKIdTtRnw58Z2kTjIzSHDqDfXVkthadI/s1600/six.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgu2NklyrfErlJMfyMXqk2ZaRAijoy30OL28fD3CCG0cVQW9dW1I7zEfr5jPOvhqSITgbVL_OODcj0c0etvZTxp-6NOzrnVts9J3uEjPkCFpF6LKIdTtRnw58Z2kTjIzSHDqDfXVkthadI/s640/six.jpg" height="339" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">The idealised signature, created by including the
receiver ghost and time shift, computing the minimum phase equivalent, and
convolving this with a minimum phase high cut anti-alias filter.</span></td></tr>
</tbody></table>
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The final stage is to design a matching filter to go from
the time-shifted signature with the receiver ghost, to our new idealised
version – and to apply and test that this works! There is usually a function in
a wavelet calculator that allows you to make a “matching” or “conversion”
filter.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEioEcYnjm0rOk8UxmhGYgB0UI-_FHBbLTSzQ-cgpDM6lv4c_kNqC9jVrS4mIwr4raWxLX0yAeH95hlKyAbxeHGjMltVGqejShuOhTisg8Aujdu8osVPDlxz6avuK9ChR2aW7Xb3yAHBF8k/s1600/seven.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEioEcYnjm0rOk8UxmhGYgB0UI-_FHBbLTSzQ-cgpDM6lv4c_kNqC9jVrS4mIwr4raWxLX0yAeH95hlKyAbxeHGjMltVGqejShuOhTisg8Aujdu8osVPDlxz6avuK9ChR2aW7Xb3yAHBF8k/s640/seven.jpg" height="560" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">The final combined minimum-phase conversion and
anti-alias filter (top) and the test application of this to the modeled source
signature, including time shift and modeled receiver ghost.</span></td></tr>
</tbody></table>
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The last step is to save this filter so that you can apply
it (convolve it) with the seismic traces.
One final trap for the unwary here is to save the wrong thing – one of
the saved results you have been working though, or the test result instead of
the filter. If you do this and it’s not
spotted, you can have to re-run the whole project.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">If you wanted to go to zero-phase the process would be much
the same – instead of a minimum phase equivalent of the source signature, you
would calculate the zero-phase version and work with that – the anti-alias
filter would also need to be zero phase, of course.</span><br />
<br />
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 18px;">Now, you can successfully resample the data from 2 ms to 4 ms sample interval, and pass it on in a controlled phase way to the rest of the processing sequence.</span><br />
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 18px;"><br /></span>
<br />
<div>
<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: 12pt;"><br /></span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" /></a></span></div>
<div>
<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div style="text-align: center;">
<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">By: <b>Guy Maslen</b></span></span></div>
<o:p></o:p></div>
GLOBE Claritashttp://www.blogger.com/profile/00646197047930265069noreply@blogger.com10tag:blogger.com,1999:blog-7105728614611010325.post-35616564329898004022013-04-29T17:15:00.003-07:002014-02-26T13:28:54.956-08:00AGC: The Equaliser<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">For a while, if you were in trouble and no-one else could
help, you could turn to <a href="http://www.imdb.com/title/tt0088513/">The
Equaliser</a>, who would turn up and resolve your problems.<o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In seismic processing, our problems are often amplitude
related. <o:p></o:p></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">High amplitudes on a screen display or plot can make it
difficult to see fine detail elsewhere, even after we have applied amplitude
recovery methods such as linear gain and spherical divergence corrections.<o:p></o:p></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">At this point, we can call on our very own Equaliser: the
Automatic Gain Control or AGC.<o:p></o:p></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">An <a href="http://wiki.seg.org/index.php/Dictionary:Fig_A-25">AGC balances out
amplitudes across a whole trace</a>, using a sliding window. An average value for the amplitude is
calculated inside the window, and then a scale factor calculated to normalise
this to a fixed value, usually 1.0. The
window then slides down by a sample and the process is repeated.<o:p></o:p></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The window (or gate) is typically 250-500ms in length; short gates have a more pronounced effect. </span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">This
is an exceptionally “brute force” approach but it is very effective – we used it a lot on hard-copy wiggle-trace plots, which were particularly
difficult to scale properly.</span></div>
</div>
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<!--StartFragment-->
<br />
<div class="MsoNormal">
</div>
</div>
<span lang="EN-NZ" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;">
</span><br />
<div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><br /></span></div>
</div>
<div>
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiO6rOac409xoaZoOWtr7bxSfz8SMQ3S9-ROMFqoJjPQXBhPSgl6zdDOapBrQZ6-MwGXOm-02g6MIj20Xo6S2i5N7Cm-h0arI12rglxL_CjIBZlE4qduykvxjRo9_ydRZ3Idd-Mu-jKWDg/s1600/agc_1.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiO6rOac409xoaZoOWtr7bxSfz8SMQ3S9-ROMFqoJjPQXBhPSgl6zdDOapBrQZ6-MwGXOm-02g6MIj20Xo6S2i5N7Cm-h0arI12rglxL_CjIBZlE4qduykvxjRo9_ydRZ3Idd-Mu-jKWDg/s640/agc_1.jpg" height="528" width="640" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">A marine shot record, with no AGC (left) and
AGC applied (right).</span></span></td></tr>
</tbody></table>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">AGC is great when you haven’t got the time to scale a plot
properly – just drop it on the data and you can see everything! </span></div>
<div class="MsoNormal">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">AGC will “blow up” the noise in the water column (or above
the direct/refracted arrivals on land data) which can surprise the
inexperienced. It can be a good idea to
have muted this off before showing the displays to clients.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">That said, a simple AGC has some downsides. Yes it is fast to calculate, but uses the mean
value as the basis for the average amplitude. As a result you get a “scaling shadow” just above and just below a
strong event. </span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;">You
can get around this by using a “robust” (or median) AGC – it takes a bit longer
to run but by using the median value instead of the mean, it has fewer issues.</span></div>
</div>
<div style="text-align: justify;">
<span style="line-height: 17px;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh5FRuxcLo5nKXh1IJjmAV1lBBsmv7HgJ6hY2-d-U3mA1WeEH_JUeEoBRReo0aSIkUmLHtiDqufgrutzhXwykv0BtYE4h5XBA_1q5an-m-HgR5Z4psND1gX1Rcev88xOfsTc3PsiSKFtgQ/s1600/agc_2.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh5FRuxcLo5nKXh1IJjmAV1lBBsmv7HgJ6hY2-d-U3mA1WeEH_JUeEoBRReo0aSIkUmLHtiDqufgrutzhXwykv0BtYE4h5XBA_1q5an-m-HgR5Z4psND1gX1Rcev88xOfsTc3PsiSKFtgQ/s640/agc_2.jpg" height="529" width="640" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Normal AGC (left) compared to a robust AGC (right) –
the robust AGC doesn’t have the scaling shadows around the strong amplitude
refracted and direct arrival events.</span></span></td></tr>
</tbody></table>
<div style="text-align: justify;">
<span style="line-height: 17px;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Of course sometimes the AGC scaling shadow can actually be
useful – it makes it easier to pick out refractions on noisy land data for
example. </span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">In general, AGC is cheap and
nasty as a gain recovery method.</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">
</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">While it lets us see structure clearly, the subtlety of amplitude
variation is lost. </span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">With interpretation
workstations allowing you to toggle an AGC on or off, and the rise of
quantitative interpretation methods like </span><a href="http://wiki.seg.org/index.php/Dictionary:Amplitude_variation_with_angle/offset_(AVA/AVO)" style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">AVO</a><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">
and </span><a href="http://wiki.seg.org/index.php/Dictionary:Acoustic_impedance_section" style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">inversion</a><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">, AGC has fallen out of favour.</span></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">It is more common now to use a combination of
spherical divergence correction and linear gains to recover amplitudes more correctly. The former corrects for the spreading of the
source wavefield with time, and the latter for losses arising from things like
scattering, inelasticity, as well as mode conversion of P-waves into S-waves or
refractions.</span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">These “true amplitude recovery” processes are usually tested early
on in a processing sequence (often in combination) and retain much of the “amplitude
character” of a trace that AGC can swamp.<o:p></o:p></span></div>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"></span><br />
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="line-height: 115%;">In
addition we can also select parameters for these gain recovery techniques based
on quantitative analysis – such as amplitude decay curves or even in a
statistically robust way using surface-consistent scaling techniques, which
helps with their accuracy.</span></span></div>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">
</span>
<br />
<div class="separator" style="clear: both; text-align: center;">
</div>
<div style="text-align: justify;">
<br /></div>
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgHzmhVaZrA_X2yR-BNcXWRyZIXoY6C0P6GpqD_OodoxIRYyk21sE7A9KDgpNfxPim-LXStTQI3XPBXqU8g0wV1SXuTraXHPQZ9suo_I14UKQGmxImpusgGR0MxlGqs053aXMNqy8q6YHI/s1600/agc_3.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgHzmhVaZrA_X2yR-BNcXWRyZIXoY6C0P6GpqD_OodoxIRYyk21sE7A9KDgpNfxPim-LXStTQI3XPBXqU8g0wV1SXuTraXHPQZ9suo_I14UKQGmxImpusgGR0MxlGqs053aXMNqy8q6YHI/s640/agc_3.jpg" height="529" width="640" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Robust AGC (left) compared to amplitude
recovery (right); in this case a T<sup>2
</sup>spherical divergence correction coupled with a 1 dB/second linear gain
function.</span></span></td></tr>
</tbody></table>
<br />
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">However, AGC should not be neglected, as it remains a useful
trick in the processors toolbox for managing difficult datasets with big
amplitude variations.<o:p></o:p></span></div>
<span style="line-height: 115%;"></span><br />
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;">Consider
these shots – the RMS amplitude of each trace is plotted over the top as a
graph:</span></span></div>
<span style="line-height: 115%;">
</span>
<br />
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjeOwYQZxFLfKvO0ZfldbfQScEXJ7Vu3AQpaxRLh-R-TWSnoBz6eKK076EGjp82UppwZYSjJc0X9QN2OXwrIazujy5CQ33YDLs4uN3CWSYL4So71rg3z13A8Vs9WyD3AXK_FTVk9K0TyPQ/s1600/FOUR.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjeOwYQZxFLfKvO0ZfldbfQScEXJ7Vu3AQpaxRLh-R-TWSnoBz6eKK076EGjp82UppwZYSjJc0X9QN2OXwrIazujy5CQ33YDLs4uN3CWSYL4So71rg3z13A8Vs9WyD3AXK_FTVk9K0TyPQ/s1600/FOUR.jpg" height="632" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; line-height: 14.720000267028809px;">A few shot records, one of which has a nasty surprise in the middle!</span></td></tr>
</tbody></table>
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;">As you can see from the RMS amplitude plots, one of
the traces has extreme values; this doesn’t show all that well on the seismic
display, and you might miss it. Apply an
FK filter, however, and you get a nasty result!</span></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div style="text-align: justify;">
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi1SmO1-TymHShlCdAAPENh7X6Cp593lp6S6o1Z8X9a7oxZzdVawX39ws5t4Fy_kBqj03dLxMQOD1DGslCI9dJjj8F-_p28ZfRiQ7hhVHqbVHm8zXCWT9WrScdV-lAdCHT0QbVINk-x-oE/s1600/FIVE.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi1SmO1-TymHShlCdAAPENh7X6Cp593lp6S6o1Z8X9a7oxZzdVawX39ws5t4Fy_kBqj03dLxMQOD1DGslCI9dJjj8F-_p28ZfRiQ7hhVHqbVHm8zXCWT9WrScdV-lAdCHT0QbVINk-x-oE/s1600/FIVE.jpg" height="632" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; line-height: 14.720000267028809px;">The same shots with an FK filter applied; the extreme amplitudes have given us an impulse response function of the FK filter, spreading the high amplitude noise across the whole shot.</span></td></tr>
</tbody></table>
</div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The dipping noise has gone from the dataset, but the high
amplitudes of a single trace have produced an impulse response function for the
FK filter that has contaminated the whole shot.
It is an extreme example, but highlights the issue. <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;">We
can combat this using a removable AGC. </span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;">This is like a regular AGC except the scalar applied to each window is
stored; we have one scalar value for each sample so this can be time
consuming. </span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;">We can then “back off” the
AGC, recovering the original amplitudes by removing the scalar values one gate
window at a time.</span></span></div>
<span style="line-height: 115%;">
</span>
<br />
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div style="text-align: justify;">
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhLcnaSu-vkODLx5KMjY1c4P9evw9VwOl3gomUZFnvxeOe-hE9iuKWSYDx-bWdv9M9F0LM8jAXSVHevTASJcJ28j_oCCOTy_rVUoWeODZu1u2Du8kvah2i4kziLbB92_KsJHsWxGwp8LVY/s1600/SIX.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhLcnaSu-vkODLx5KMjY1c4P9evw9VwOl3gomUZFnvxeOe-hE9iuKWSYDx-bWdv9M9F0LM8jAXSVHevTASJcJ28j_oCCOTy_rVUoWeODZu1u2Du8kvah2i4kziLbB92_KsJHsWxGwp8LVY/s1600/SIX.jpg" height="632" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: small; line-height: 14.720000267028809px;">The same shots with an FK filter applied with a ”removable AGC wrap” – that is an AGC before the FK filter which stores the gate scalars, and an “un-AGC” afterwards to recover the original amplitudes.</span></td></tr>
</tbody></table>
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This “AGC” wrap approach can save a lot of time when
managing a large number of spikes, or when data has big variations in amplitude
that cannot be compensated for easily in other ways. It is also reasonably “AVO friendly” in that
the amplitudes are well preserved. <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">While this example is extreme, the AGC wrap “equaliser” can
be a good way to deal with noisy land datasets when you need to apply FK or
Tau-P domain processes, and is well worth remembering as an alternative to
time-consuming despikes or geologically driven scaling.<o:p></o:p></span></div>
<br />
<span style="line-height: 115%;"></span>
<span style="line-height: 115%;">
</span>
<br />
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;">It
doesn’t have as good </span><a href="http://www.youtube.com/watch?v=xzjJ4FccWAc" style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;">theme
music</a><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%;"> as the 80’s original though.</span></span></div>
<span style="line-height: 115%;">
</span>
<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"></span><br />
<div style="text-align: justify;">
<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: 12pt;"><br /></span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" /></a></span></div>
<div style="text-align: justify;">
<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></span></div>
<div style="text-align: center;">
<span style="font-family: "Calibri","sans-serif"; font-size: 11.0pt; line-height: 115%; mso-ansi-language: EN-NZ; mso-bidi-font-family: "Times New Roman"; mso-bidi-language: AR-SA; mso-fareast-font-family: Calibri; mso-fareast-language: EN-US;"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">By: <b>Guy Maslen</b></span></span></div>
</div>
GLOBE Claritashttp://www.blogger.com/profile/00646197047930265069noreply@blogger.com4tag:blogger.com,1999:blog-7105728614611010325.post-71066588925092781702013-02-27T15:05:00.001-08:002013-02-27T15:05:08.286-08:00Getting Started with Marine Data<span style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span>
<span style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Two of the
“Seven Habits of Highly Successful People” are:</span></span><br />
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
</div>
<ul>
<li><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Start with the end in mind</span></li>
<li><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-indent: -18pt;">Put first things first</span></li>
</ul>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">These are pivotal
in seismic data processing; spotting bad assumptions mid-way through processing
is much harder than trapping them upfront.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">I’m going to
describe some of the things to look out for at the start of a marine seismic processing
project, with a particular focus on reprocessing of older data.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The first thing
you need to know about is the sequence of events that takes place when the data
is recorded in the field. A seismic vessel needs to move through the water at a
speed of 5 knots. This means that the ship covers the typical shot-point
interval of 25 metres in about 10 seconds. This speed also ensures that the
streamer can be held at the right depth by the depth controllers (i.e. the
paravanes or ‘birds’).<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The locations of
every shot to be fired are pre-loaded into the navigation system; when the
source is in the right place (on older data this might be when the ship’s
antenna is in the right place) the navigation system triggers the gun
controller to fire the shot. Worth a small mention that prior to the main
‘burst’ there are some small variations in the airgun; this is referred to as
the ‘precursor’ and should be removed in the processing when creating a minimum
phase equivalent (filter) of the original source wavelet. <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">At the same time
that the shot is fired, some of the key navigation information needs to be
“captured” and go into the header of the digital seismic record. This is trivial on modern systems, but back
in the 1980's (where 48K of <st1:stockticker w:st="on">RAM</st1:stockticker> was
pretty racy) this was much harder as the data was written directly to tape. At
this point the data is recorded (usually with a back-up copy) and the system is
reset for the next shot.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The Observers on
the boat (often one junior and one senior per shift) keep track of what is happening by
making notes of anything and everything that might impact the data. They also
check that the data is “within specification” - the contract will outline key
parameters like tow depths, sea state, minimum source configuration and so
on. This information is recorded into
the Observer’s Log aka: 'Obs. Log'.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The exact
position of the cable (or cables) is unknown at the time of shooting. The
locations are calculated "offline" using a combination of sensors such as:
compasses along the cable length, acoustic pods which send range-finding
‘pings’, and laser range finders that measure angle and distance to ‘targets’. These
are attached to floats at the end of the cable (tail-buoys) or on the airgun
string.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This information
is used to create an ASCII file of the positional data, usually in P1/90 or
P1/84 formats. I’ll cover this at a
later stage in another post.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span lang="EN-AU"> </span> </span></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The Observer’s
Log is a key piece of “metadata” that can be extremely useful to the
processor.</span><br />
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"> <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiPu8J_kYKmmSLvkogKQMQ09CA-4gMP1ilytbey8bm-DdOdEf6Obt0byHKrE9cx7WHdFDDLtM6s8EyysKmOXZ3JPvI87khYYb3aj9d7poFrUsVykqDFAG7Vp_ahWaKCw9OEXCMS2UeN-fo/s1600/obs_log.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiPu8J_kYKmmSLvkogKQMQ09CA-4gMP1ilytbey8bm-DdOdEf6Obt0byHKrE9cx7WHdFDDLtM6s8EyysKmOXZ3JPvI87khYYb3aj9d7poFrUsVykqDFAG7Vp_ahWaKCw9OEXCMS2UeN-fo/s640/obs_log.jpg" width="448" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Example of an Observer's Log entry</span></td></tr>
</tbody></table>
</div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Modern Observer's Logs
are digital, but older ones are hand written on paper forms. That can mean three things – obscure jargon, the
odd mistake, and illegible handwriting! <o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<table border="1" cellpadding="0" cellspacing="0" class="MsoTableGrid" style="border-collapse: collapse; border: none; text-align: justify; width: 100%px;">
<tbody>
<tr>
<td style="border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Spec.<o:p></o:p></span></div>
</td>
<td style="border-left: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Specifications
- usually “out of spec.” meaning the data is not meeting requirements. This
might be tow depths, sea state, noise levels, wind, feather angle, airgun
pressure or airgun array volume. The
client doesn’t pay for “out of spec.” data, and gaps need to be re-shot<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">DNP<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Do Not Process
- bad or noisy files, ‘warm up” shots, or other data that isn’t being paid
for and should be excluded<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">NR<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Noise Record -
a recorded file where the source didn’t fire to look at noise levels<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">BOT<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Beginning Of
Tape - usually with tape, shotpoint and file numbers <o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Warm-up<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Shots fired to
warm up the airguns, to avoid misfires<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><st1:stockticker w:st="on"><span lang="EN-AU">SOL</span></st1:stockticker><span lang="EN-AU"><o:p></o:p></span></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Start Of Line<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">FSP<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">First Shot
Point<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">FCSP<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">First
Chargeable ShotPoint - on a reshoot, the first new shotpoint (after the
required overlap) the client can be charged for<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">M/F<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Misfire - the
airgun array fired incorrectly in time or volume<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">A/F<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Autofire - the
airgun is discharging incorrectly, automatically<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">SI<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Seismic
Interference – caused by the signals from another seismic vessel some
distance off.<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Ship Noise<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Hydrophones
were developed to detect ships and submarines by their propeller noise; they
still do this very well!<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">FA<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Feather Angle –
the angle at which the cable deviates (feathers) from directly behind the
vessel; often caused by currents, tides and wind. This is one of the
“specifications” <o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">LGSP<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Last Good
ShotPoint - before a breakdown or failure<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">LCSP<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Last
Chargeable ShotPoint<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">FGSP<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">First Good
ShotPoint - after a breakdown or failure
<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><st1:stockticker w:st="on"><span lang="EN-AU">EOT</span></st1:stockticker><span lang="EN-AU"><o:p></o:p></span></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">End Of Tape -
usually with tape, shotpoint and file numbers <o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">EOL<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">End Of Line<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">LSP<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Last ShotPoint<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">FFID<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Field File
Identification Number – the File Number on Tape<o:p></o:p></span></div>
</td>
</tr>
<tr>
<td style="border-top: none; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 21.5%;" valign="top" width="21%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">SP<o:p></o:p></span></div>
</td>
<td style="border-bottom: solid windowtext 1.0pt; border-left: none; border-right: solid windowtext 1.0pt; border-top: none; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; padding: 0cm 5.4pt 0cm 5.4pt; width: 78.5%;" valign="top" width="78%"><div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">ShotPoint - the
unique number for each shotpoint<o:p></o:p></span></div>
</td>
</tr>
</tbody></table>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<br />
<div style="text-align: right;">
</div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjH9x68dqMhKi6Ji3zVOhiOSkiJGzQdpyvmQmXiEWt3HCpV7j5yLGJewWGVGYexXGgOryZJ4FhhmHw7Q8wDpCplhVc9weXJg5eBiwsoV84n7Btb2xOmsf5OFVeHjnc9t6rhcigfDiqkFXI/s1600/feather_angle.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjH9x68dqMhKi6Ji3zVOhiOSkiJGzQdpyvmQmXiEWt3HCpV7j5yLGJewWGVGYexXGgOryZJ4FhhmHw7Q8wDpCplhVc9weXJg5eBiwsoV84n7Btb2xOmsf5OFVeHjnc9t6rhcigfDiqkFXI/s640/feather_angle.jpg" width="376" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Diagram illustrating Feather Angle (taken from "<b>Applied Seismology: A Comprehensive Guide To Seismic Theory And Application</b>" by M. Gadallah & R. Fisher)</span></td></tr>
</tbody></table>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Things to check
or look out for on the Observer’s Log include:<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="margin-left: 36.0pt; mso-list: l0 level1 lfo1; tab-stops: list 36.0pt; text-align: justify; text-indent: -18.0pt;">
<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">shooting geometry – number of
receivers, the distance from the centre of the source array to the centre of
the first receiver group, the receiver group separation, maximum offset, and
nominal shot point interval</span></li>
</ul>
<ul>
<li><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="text-align: justify;">field file identification
numbers (FFIDs) – are they the same as the shotpoint numbers for the whole line? It’s usually true for modern systems but for
older ones you may need to renumber the shotpoints. Shots can be missed for a
variety of reasons, but the line is usually “in spec.” until multiple shots are “dropped”</span><span style="text-align: justify;"> </span></span></li>
</ul>
<ul>
<li><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-align: justify;">gun delay - some (older)
systems start to record data at a fixed time before the airgun fires. This introduces a fixed “delay” that you will
need to remove before processing</span></li>
</ul>
<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">recording delay - this is when the
airgun is fired and there is a set time delay before the start of recording (to
save space on the tape)</span></li>
</ul>
<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">bad traces, noise issues or bad
files</span></li>
</ul>
<ul>
<li><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-align: justify;">seismic recording format,
sample rate and record length</span></li>
</ul>
<ul>
<li><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-align: justify;">near- and far-channel - is
channel 1 the near of far channel from the vessel?</span></li>
</ul>
<ul>
<li><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-align: justify;">shotpoint numbers -
incrementing or decrementing?</span></li>
</ul>
<ul>
<li><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; text-align: justify;">filters - were any applied in
the field?</span></li>
</ul>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">When you start
to read the seismic data, you can then match it back against the Observer’s Log
to make sure that everything is as it should be at this first critical stage.<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The basic checks
you need to do are:</span><br />
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="margin-left: 36.0pt; mso-list: l0 level1 lfo1; tab-stops: list 36.0pt; text-align: justify; text-indent: -18.0pt;">
<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">trace count - does the number
of traces make sense? Calculate this first of all from [LSP-FSP+1] x [number of
channels], and then confirm that any missing shots are fully accounted for by
the Observer’s Log. Too many traces
might mean you have read some DNP, warm-up or noise records.</span></li>
</ul>
<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">near-trace plot (NTP) – create a plot that shows just the first channel. Is the time of the direct arrival correct? A quick “measure and calculate” can highlight a potential issue such as a gun-delay you have missed, or an incorrect near-offset definition</span></li>
</ul>
<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">shots - look at ideally at least one shot per cable length; check that the direct arrival has the right dip based on the receiver spacing and the speed of sound in water). You can also look for noise problems and/or display shots that are recorded as having issues in the Observer’s Logs.</span></li>
</ul>
</div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU" style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">A few traps for
the unwary:<o:p></o:p></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="margin-left: 36.0pt; mso-list: l0 level1 lfo1; tab-stops: list 36.0pt; text-align: justify; text-indent: -18.0pt;">
<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span lang="EN-AU">some data has no low-cut field
filter; all you will see is 1-2Hz or
lower “swell noise” and very little else</span><span lang="EN-AU">; drop a quick filter over
your selected data to QC; 5Hz low cut
should be fine</span></span></li>
</ul>
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi4Ha-YVJHmGqA15lR-KzF8nrvqgqOOwwTkeo8nOg8ZcqFzVgVjNhx8j7w2NG9DSawj5EYwLdgKwWnNbrLhym6w8YDkzgeJq_QMRRCdS4-SGoUXubAorNSXbXGc1PxDqCO9lyAQck79EHE/s1600/no_infield_filter.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi4Ha-YVJHmGqA15lR-KzF8nrvqgqOOwwTkeo8nOg8ZcqFzVgVjNhx8j7w2NG9DSawj5EYwLdgKwWnNbrLhym6w8YDkzgeJq_QMRRCdS4-SGoUXubAorNSXbXGc1PxDqCO9lyAQck79EHE/s640/no_infield_filter.jpg" width="478" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Shot record with no low-cut field filter</span></td></tr>
</tbody></table>
<div class="MsoNormal" style="margin-left: 36.0pt; mso-list: l0 level1 lfo1; tab-stops: list 36.0pt; text-align: justify; text-indent: -18.0pt;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="margin-left: 36.0pt; mso-list: l0 level1 lfo1; tab-stops: list 36.0pt; text-align: justify; text-indent: -18.0pt;">
<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">older data might have odd file
numbering; I’ve encountered Octal file
numbers (0,1,2,3,4,5,6,7,10,11 etc) as well as numbers that roll from 000-999
and then loop around to 000. You’ll need
to renumber these carefully based on the Observer’s Logs.</span></li>
</ul>
</div>
<div class="MsoNormal" style="margin-left: 36.0pt; mso-list: l0 level1 lfo1; tab-stops: list 36.0pt; text-align: justify; text-indent: -18.0pt;">
<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">auxiliary (extra) channels
contain things like near hydrophone measures and “time breaks”. They are
usually flagged as not being seismic data and ignored, but sometimes they are
included, so your channels numbers in a shot might go 1-7 and start again with
1-240. More editing and renumbering.</span></li>
</ul>
</div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="MsoNormal" style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">When you start off with a new seismic line and a
new vintage, taking the time to carefully check the items mentioned above can prevent
a great deal of grief later on!</span><br />
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: 12pt;"><br /></span>
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; font-size: 12pt;"><br /></span><br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" /></a></div>
<div>
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></div>
<div style="text-align: center;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">By: <b>Guy Maslen</b></span></div>
</div>
<span lang="EN-AU" style="font-size: 12pt;">
</span>GLOBE Claritashttp://www.blogger.com/profile/00646197047930265069noreply@blogger.com1tag:blogger.com,1999:blog-7105728614611010325.post-73655067048579423932013-02-13T18:44:00.001-08:002013-02-13T19:04:49.506-08:00The Art of Testing<br />
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Seismic
processing is often described as more of an art than a science. <o:p></o:p></span></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">I suspect this
is because, in practice, the Earth is a complex place. Fundamentally, seismic
processing assumes a flat earth made up of homogeneous layers – sometimes a
single layer – that have constant properties.
If it were truly that simple, the entire endeavour would be completely
automated.<o:p></o:p></span></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">It is the
variability of seismic data that makes processing a challenge and at the heart
of this is testing and applying a processing sequence.<o:p></o:p></span></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">For the new
processor, testing is probably the biggest challenge. It’s not just what to test and how to test
it, but also keeping track of what you have tested, what worked well, and then
applying it to all of the data. <o:p></o:p></span></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Experts make
this look easy but that’s largely because they have developed habits over a
number of years that help them to avoid making mistakes. Rest assured that most
of these habits only came about <i>because
they made the mistakes</i> <i>in the first
place!<o:p></o:p></i></span></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">The aim of this
blog post is to help new processors develop good habits when it comes to
testing, so as to avoid making the mistakes in the first place.<o:p></o:p></span></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">What kind of
mistakes are we talking about? Here are
some of the classics:<o:p></o:p></span></span></div>
<div class="MsoNormal" style="text-align: justify;">
</div>
<ul>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">creating a spectacular test
section, then overwriting the processing flow so you have no idea how you did
it</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">using the wrong (or different)
input files for the tests, with different pre-processing applied so that it is
not a fair comparison</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">not being able to explain at a
later stage why you chose a particular approach</span></li>
<li><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; text-indent: -18pt;">putting the test sequence into
“production” processing, and getting a different result</span></li>
</ul>
<br />
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">In my experience,
testing is seldom well-documented in a final processing report. Many reports
don’t discuss the things that were tried <i>which
didn’t work;</i> this can be very important information at a later date. They
can also describe what tests were applied, but not what issue was trying to be addressed
and why a given result was chosen.<o:p></o:p></span></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<div class="MsoNormal" style="text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Here are my top
ten testing suggestions, to help you become a better processor:<o:p></o:p></span></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<ol start="1" style="margin-top: 0cm;" type="1">
<li class="MsoNormal" style="mso-list: l1 level1 lfo2; tab-stops: list 36.0pt; text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><b><i><span lang="EN-AU">Pick your test
line/area carefully</span></i></b><span lang="EN-AU"><o:p></o:p></span></span></li>
</ol>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">There is always a push to use a test-line that is in the main area
of interest, e.g. the well location or the target area. From a processing flow point of view you
ideally want a line that has the greatest geological variation, or the worst
noise contamination. Try and balance
these needs if you can; it’s better to test on more lines than be unpleasantly surprised
by the results!<o:p></o:p></span></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<ol start="2" style="margin-top: 0cm;" type="1">
<li class="MsoNormal" style="mso-list: l1 level1 lfo2; tab-stops: list 36.0pt; text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><b><i><span lang="EN-AU">Keep a Testing Log</span></i></b><span lang="EN-AU"><o:p></o:p></span></span></li>
</ol>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Create a log of the tests you have run; this can be a test file, a
Word Document, even a hand written journal, but it needs to be done. Treat each test as an experiment, and log the
objective, method, results and conclusion.
Put the science back in to the art of testing.<o:p></o:p></span></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgufqh0VaeJQ1t4VhPtFxa-4ClKOulbaRcVvZTpDuLU_dQ41oz6cg9SXS3BQmLTUs-rIG-c5WnjhAnTn0302mZXpzxn2ghUXN72KksGNMidemb8hf2C_Tlqnr7b3flN_p2Q9j-5ZAEeKIk/s1600/testlog.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="398" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgufqh0VaeJQ1t4VhPtFxa-4ClKOulbaRcVvZTpDuLU_dQ41oz6cg9SXS3BQmLTUs-rIG-c5WnjhAnTn0302mZXpzxn2ghUXN72KksGNMidemb8hf2C_Tlqnr7b3flN_p2Q9j-5ZAEeKIk/s640/testlog.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Example of a basic Testing Log</td></tr>
</tbody></table>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<br /></div>
<ol start="3" style="margin-top: 0cm;" type="1">
<li class="MsoNormal" style="mso-list: l1 level1 lfo2; tab-stops: list 36.0pt; text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><b><i><span lang="EN-AU">Do not re-use processing
flows</span></i></b><span lang="EN-AU"><o:p></o:p></span></span></li>
</ol>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Create a new processing flow for each test, and preserve the
original. The flow – and any log file –
is a vital part of the “audit trail” that you will need if you have to
back-track. Recording the name of each
testing flow in your testing log will save a huge amount of time and effort if
things go wrong.<o:p></o:p></span></span></div>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<br /></div>
<ol start="4" style="margin-top: 0cm;" type="1">
<li class="MsoNormal" style="mso-list: l1 level1 lfo2; tab-stops: list 36.0pt; text-align: justify;"><b><i><span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Number your tests<o:p></o:p></span></span></i></b></li>
</ol>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Give each test a unique number.
I tend to use double digits 00, 01, 02 – I’ve never been on a project
yet where I needed 99 different tests!
Record this number in the Testing Log.
Tests are not just processing flows, but can include things like a mute
you have designed (and tested the application of) or a velocity function you
picked.<o:p></o:p></span></span></div>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<br /></div>
<ol start="5" style="margin-top: 0cm;" type="1">
<li class="MsoNormal" style="mso-list: l1 level1 lfo2; tab-stops: list 36.0pt; text-align: justify;"><b><i><span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Number each test panel<o:p></o:p></span></span></i></b></li>
</ol>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">I extend the numbering system so that each sub-test has a unique number
as well. The “00” sub-test is always
“nothing applied”, and then all the other parameter variations have a separate
number. So for example, if my test 04
was “deconvolution before stack” my test numbers might be:<o:p></o:p></span></span></div>
<div class="MsoNormal" style="margin-left: 72.0pt; text-align: justify;">
<br /></div>
<div class="MsoNormal" style="margin-left: 72.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">04_00: nothing applied<o:p></o:p></span></span></div>
<div class="MsoNormal" style="margin-left: 72.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">04_01: 160ms operator, 24ms gap<o:p></o:p></span></span></div>
<div class="MsoNormal" style="margin-left: 72.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">04_02: 200ms operator, 24ms gap<o:p></o:p></span></span></div>
<div class="MsoNormal" style="margin-left: 72.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">04_03: 240ms operator, 24ms gap<o:p></o:p></span></span></div>
<div class="MsoNormal" style="margin-left: 72.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">etc.<o:p></o:p></span></span></div>
<div class="MsoNormal" style="margin-left: 72.0pt; text-align: justify;">
<br /></div>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This allows you to extend the testing sequence as much or as little
as you need.<o:p></o:p></span></span></div>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<br /></div>
<ol start="6" style="margin-top: 0cm;" type="1">
<li class="MsoNormal" style="mso-list: l1 level1 lfo2; tab-stops: list 36.0pt; text-align: justify;"><b><i><span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Use these numbers in your
processing flows<o:p></o:p></span></span></i></b></li>
</ol>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Make the start of each testing flow the unique test number; the use
of “double digits” will help to keep these in order when you list them. Some systems let you run “panel tests” with
repeat copies of data, but that’s not always applicable for every test. Some of
your tests might end up with a test and sub-test number like this: <b>08_02_radon200.job<o:p></o:p></b></span></span></div>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<br /></div>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Because you have a Testing Log, you won’t need to try and think up
(or remember) long and convoluted processing flow names that describe the test
or the parameters. <o:p></o:p></span></span></div>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<br /></div>
<ol start="7" style="margin-top: 0cm;" type="1">
<li class="MsoNormal" style="mso-list: l1 level1 lfo2; tab-stops: list 36.0pt; text-align: justify;"><b><i><span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Use the same numbering
for input and output datasets <o:p></o:p></span></span></i></b></li>
</ol>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">If you extend this numbering technique to the output datasets from
each test, then you can easily trace an output file back to where it was
created. The datasets have a link to
both the test number, and the processing flow, which is simple and easy to
remember. What was the output from <b>08_02_radon200.job</b>? Why it was <b>08_02_radon200.cdps</b>!<o:p></o:p></span></span></div>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">This greatly reduces the chance of a labelling error or mistake when
reviewing results and making decisions. <o:p></o:p></span></span></div>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<br /></div>
<ol start="8" style="margin-top: 0cm;" type="1">
<li class="MsoNormal" style="mso-list: l1 level1 lfo2; tab-stops: list 36.0pt; text-align: justify;"><b><i><span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Number your metadata
files <o:p></o:p></span></span></i></b></li>
</ol>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Metadata is “data about data”. In the context of testing, it’s all
the “other” files and information you create, like mutes, velocities, horizons,
filters, design gates, and picks. Number
these too! If <b>08_02_radon200.job</b>
uses the mute you picked as test number 05, then the mute file should start
with 05 in the name. Applying the wrong “metadata” is one of the main causes of
incorrect processing. <o:p></o:p></span></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<ol start="9" style="margin-top: 0cm;" type="1">
<li class="MsoNormal" style="mso-list: l1 level1 lfo2; tab-stops: list 36.0pt; text-align: justify;"><b><i><span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Create a flow diagram
showing how tests are related<o:p></o:p></span></span></i></b></li>
</ol>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Simple test numbering really makes this easy; but it is useful as a
check to make sure that parameters can actually be compared, especially when
looking from one technique to another.<o:p></o:p></span></span></div>
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi6VEPQJTjKfFajvE7vOTQuvvExexwz3PCkDo7vbmlyjpI9YsrSc7XKlQ0814Oz830ZoQzkqhP15bEcGHSUJN4EB29S9fe5GWjQBm0EVO9M9k3uBx5Gd64gV5htbWuBMXiCVfsuP-zLbkU/s1600/flowchart.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="457" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi6VEPQJTjKfFajvE7vOTQuvvExexwz3PCkDo7vbmlyjpI9YsrSc7XKlQ0814Oz830ZoQzkqhP15bEcGHSUJN4EB29S9fe5GWjQBm0EVO9M9k3uBx5Gd64gV5htbWuBMXiCVfsuP-zLbkU/s640/flowchart.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Example flow diagram for demultiple processing routes</td></tr>
</tbody></table>
<br />
<div class="MsoNormal" style="margin-left: 36.0pt; text-align: justify;">
<span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">A flow diagram is useful for spotting omissions or errors, as well
as explaining what you did, and why.<o:p></o:p></span></span></div>
<div class="MsoNormal" style="text-align: justify;">
<br /></div>
<ol start="10" style="margin-top: 0cm;" type="1">
<li class="MsoNormal" style="mso-list: l1 level1 lfo2; tab-stops: list 36.0pt; text-align: justify;"><b><i><span lang="EN-AU"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Extend tests until you
find the ‘end members’<o:p></o:p></span></span></i></b></li>
</ol>
<span lang="EN-AU"></span><br />
<div style="text-align: justify;">
<span lang="EN-AU"><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">To be sure that you are not looking at a “local
minima” when testing – especially with things like demultiple, filters, gain
control and so on – it is a good idea to push the testing of the parameters so
that the results look terrible.</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"> </span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">These
“end member” tests are quickly discarded, but show you have covered the full
spectrum of possibilities.</span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"> </span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">Test panels
that show artefacts, or over processed data help to assure that the overall
sequence will be effective.</span></span></div>
<span lang="EN-AU">
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0Z1zm_UpcLxDy6E-yqDoN57zbx16RxlEaE4JO717XIs8Bec3scaOQDtKLI8t89HIBR79lVkvKEhTQDNLI1IanOviQVR4B1bfvHbVgygWORxeUSbeYkLDGKotqSfNjV-kwBb424mSHtyw/s1600/guy_maslen.jpg" /></a></div>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></div>
<div style="text-align: center;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;">By: <b>Guy Maslen</b></span></div>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></div>
<div style="text-align: justify;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif;"><br /></span></div>
</span>GLOBE Claritashttp://www.blogger.com/profile/00646197047930265069noreply@blogger.com3tag:blogger.com,1999:blog-7105728614611010325.post-20085208823543083012013-02-06T18:18:00.001-08:002013-02-13T16:30:17.302-08:003D Land Geometry<div class="separator" style="clear: both; text-align: center;">
</div>
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%; text-align: justify;">Mention the
word “geometry” and people think about shapes and angles, and maybe the time
they outsmarted an opponent on the pool table. Like so many other terms in
seismic processing, “geometry” carries a slightly different meaning in that it
describes the relationship between geophysical data (e.g. shot records) and
coordinate information (the specific position on the Earth where that shot took
place).</span><br />
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; line-height: 115%;">Both the
shooting and where the shooting occurs are handled by the acquisition crew,
while relating the two is often left to someone in the processing centre as not
all field crews have the capability to check the navigation/seismic merge in
the field. It’s not that common for the person processing the data to also be
in the field acquiring it, so it can be easy to fall into a ‘field vs. office’
mentality when errors are found. “They can fix that in the processing” is
uttered from time to time by field crews so that they don’t have to fix a
problem encountered during acquisition. Alternatively you might hear “if the
acquisition crew had just done a better job this wouldn’t be an issue” from a
processor, complaining about the extra work. The truth lies somewhere in the
middle. Despite every effort being taken in the field during acquisition, from
time to time issues do arise that lead to the data being recorded with errors
and this is something that you, the processor, have to deal with.<o:p></o:p></span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><br /></span></div>
</div>
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<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; line-height: 115%;">The field
can sometimes be a harsh environment. When things go wrong it can range from being
soaked in stormy weather, to guerrilla encounters and a coup d’état! Not to
mention the long days, temperamental equipment and pesky local wildlife; one
observer log entry read “two exploded camels” as reason for a misfire. Nevertheless
a lot of time can be lost in the processing centre having to solve for
navigation errors (e.g. two stations with the same location) that really
shouldn’t exist. Field or office, everyone makes mistakes and you need to have
steps in place to lessen the effects. <o:p></o:p></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; line-height: 115%;"> <o:p></o:p></span></div>
</div>
<div class="MsoNormal">
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; line-height: 115%;">“Garbage in,
garbage out” is a famous computer axiom which basically states that if invalid
data is entered into a system, the resulting output will also be invalid. Pretty
straightforward really. It should then come as no surprise that if you
incorrectly set up your geometry (one of the first steps in processing) your
results will be incorrect. Don’t underestimate the importance of quality
control (QC) at this stage; the price of rewinding an entire project because
the geometry was wrong is not one you want to pay. <o:p></o:p></span><br />
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; line-height: 115%;"><br /></span></div>
</div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="line-height: 115%;">When
merging seismic data with coordinate data you are actually setting up a
database (of sorts) which contains all of the survey information; such as </span><span style="line-height: 115%;">shot
point definitions, source and receiver coordinates, and elevations. This ‘database’
is accessed for a variety of uses including refraction statics calculations,
CDP binning, creating CDP gathers, and populating SEG-Y trace headers.</span></span></div>
<div class="separator" style="clear: both; text-align: center;">
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<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: justify;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhFJag5_CR_1Aw1ixugPob5UbmTpR529jqC7pe7AToPfn4lazb7FZLb4YKc6tC4odCHuUezk0kydmKg5MpuvzMSkuaFKe9U56p885-OYzvg_SKpAhA6Pl1-wyE2BbUYHlhBYbVvT2NXlZ4/s1600/geom_query.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhFJag5_CR_1Aw1ixugPob5UbmTpR529jqC7pe7AToPfn4lazb7FZLb4YKc6tC4odCHuUezk0kydmKg5MpuvzMSkuaFKe9U56p885-OYzvg_SKpAhA6Pl1-wyE2BbUYHlhBYbVvT2NXlZ4/s640/geom_query.png" width="484" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Geometry database summary</span></td></tr>
</tbody></table>
<div style="text-align: left;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%; text-align: justify;">With seismic
processing in general, it’s good practice to follow robust QC procedures and there’s
no exception when it comes to setting up and proofing geometry. Here are three useful methods:</span></div>
<div style="text-align: left;">
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%; text-align: justify;"><br /></span></div>
<div style="text-align: justify;">
<ul><span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">
<li style="text-align: justify;"><span style="line-height: 115%;">Review
the diagnostic text and check that values are consistent. Typical errors are
likely to come from: seismic numbering, navigation numbering, shot location,
and receiver spread. If an error message pops up, read it! It may be telling
you something important about your data. An example of this was where a client
was ignoring warning messages and was unable to build the geometry. The message
was trying to inform them of a 6 m elevation difference from one station to the
next. As it turns out the survey was shot in two phases and in between, a flood
had washed away tons of sediment in the area, leaving a small chasm. Moral of
the story, read the observer logs and pay attention to pop ups. Good software
will be checking data consistency for you and alert you when it isn’t.</span></li>
</span></span></ul>
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">
</span></span></div>
<span style="line-height: 115%;">
</span>
<br />
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: justify;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEirKr6ZO1UDIpBQ18Pu0SLmenqwfw3pUtzJiduqDlTzCfbMmR5EHvmyUzjhVhdAQZmgMsjVThy9JCcS5GKe0_8-QsI8tIxVe_ZdgR44rUr2z8zEVb61MoYjN42zRJiIkExs_08m6qieXOw/s1600/diagnostic.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" height="430" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEirKr6ZO1UDIpBQ18Pu0SLmenqwfw3pUtzJiduqDlTzCfbMmR5EHvmyUzjhVhdAQZmgMsjVThy9JCcS5GKe0_8-QsI8tIxVe_ZdgR44rUr2z8zEVb61MoYjN42zRJiIkExs_08m6qieXOw/s640/diagnostic.png" width="640" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Shot position on map with live update diagnostic information</span><br />
<br /></td></tr>
</tbody></table>
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%; text-align: justify;"></span><br />
<ul><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%; text-align: justify;">
<li><span style="line-height: 115%;">You’ve heard it again and again – “A picture is worth
a thousand words” – the adage that it is easier to absorb large amounts of data
quickly through visualization. If you graphically represent the information by
producing a plot/map it will help you highlight any obvious inconsistencies. You
can display source/receiver positions and quickly identify erroneous positions;
e.g. a station that is shifted 30 metres away from the spread. It would pay to check
the observer logs as there may have been a valid reason for relocating the
position, such as a creek in the way. Other useful information to display
graphically include: source/receiver elevations, first break picks, CDP fold, and
bin locations.</span></li>
</span></ul>
<span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 115%; text-align: justify;">
</span>
<br />
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: justify;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhsA9fuQ_sx6ytYki3tLH-Gz3lFuob3PDi84T9BS9FcPySLHEg17I2ynw_qQCEQ2NxmGIBSup3Wxr-rKxPa7sBEax8i9xynb_jPBp25Z8Tm_1q43tjfoUXRTDzfP77FWdANoQ_SvcPFiE0/s1600/src_rec_pos.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" height="439" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhsA9fuQ_sx6ytYki3tLH-Gz3lFuob3PDi84T9BS9FcPySLHEg17I2ynw_qQCEQ2NxmGIBSup3Wxr-rKxPa7sBEax8i9xynb_jPBp25Z8Tm_1q43tjfoUXRTDzfP77FWdANoQ_SvcPFiE0/s640/src_rec_pos.png" width="640" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Map showing source (red) and receiver positions</span></td></tr>
</tbody></table>
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: justify;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjbeTjYiF4D3J0rUAHB0GBYHKdjHJnPjG6fnVaPqo5wdgRtt_mBvmbndWJMUupquwBe-NQLcf_mjD46gtAXotrX9-McgBskyCWFVgOq6IhZXuDqpx4ArW3hu0CE5XY2ASiJ65Gm0GqezjM/s1600/cmp_pos.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" height="438" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjbeTjYiF4D3J0rUAHB0GBYHKdjHJnPjG6fnVaPqo5wdgRtt_mBvmbndWJMUupquwBe-NQLcf_mjD46gtAXotrX9-McgBskyCWFVgOq6IhZXuDqpx4ArW3hu0CE5XY2ASiJ65Gm0GqezjM/s640/cmp_pos.png" width="640" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Map showing Common Mid Point positions</span><br />
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;"><br /></span>
</td></tr>
</tbody></table>
<div style="text-align: justify;">
</div>
<ul>
<li style="text-align: justify;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="line-height: 115%;">You’ll need to check if the geometry, once applied to
the data, is correct. Start by overlaying the offset from the trace headers onto
the seismic traces. This can be done by applying an estimated linear velocity
to convert offset to time. Check that the apex of the offset values, match the
apex of the seismic record. You can also apply a linear moveout (LMO) making
sure that your velocity estimate gives you roughly horizontal first-breaks when
the LMO is applied. Now you can cycle through the shots and if the header
offsets are incorrect, the </span><span style="line-height: 115%;">LMO
will be out of sync and there will be a clear 'step' at zero-offset.</span></span></li>
</ul>
<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: justify;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg8Y9E9tfQSHd-fnsI7D_ihiyszT2s-AArRR-9iOScx70M8xctl_sJ1iR4bUIpieg6YH_K22DZPSehC7QdL4-wau-zTE7V6J10GDPSIs7aqS9A244bgNB6i06291EuHRwusj64Edx0B8jM/s1600/apex.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><img border="0" height="523" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg8Y9E9tfQSHd-fnsI7D_ihiyszT2s-AArRR-9iOScx70M8xctl_sJ1iR4bUIpieg6YH_K22DZPSehC7QdL4-wau-zTE7V6J10GDPSIs7aqS9A244bgNB6i06291EuHRwusj64Edx0B8jM/s640/apex.png" width="640" /></span></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;">Offset from trace headers (red) does not match shot record; in this case channel numbers are not matched correctly against stations</span><br />
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; font-size: small;"><br /></span></td></tr>
</tbody></table>
<div style="text-align: left;">
<div style="text-align: left;">
<div style="text-align: justify;">
<span style="line-height: 115%;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">Ideally you want the geometry to be loaded into the
trace headers before it reaches you, and for it to be correct. Unfortunately
it’s best to assume that it is incorrect and get into the habit of checking! If
it hasn’t already been applied in the field you should receive the coordinate
information in the form of Shell Processing Support (SPS) files. The SPS format
was developed by Shell Internationale Petroleum Maatschappij as a common standard
for positioning-data, between their 3D land field crews and the processing
centres. The format proved durable enough that it was adopted by the Society of
Exploration Geophysicists (SEG) Technical Standards Committee in 1993 </span></span><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 18px;">(</span><a href="http://www.seg.org/documents/10161/77915/seg_sps_rev0.doc" style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 18px;" target="_blank">SEG SPS Revision</a><span style="font-family: 'Helvetica Neue', Arial, Helvetica, sans-serif; line-height: 18px;">).</span></div>
<div style="text-align: left;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="line-height: 115%; text-align: justify;"><br /></span></span></div>
<div style="text-align: justify;">
<span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif; line-height: 115%; text-align: justify;"><span style="line-height: 115%; text-align: left;">Most SPS files today are generated automatically by the seismic recording
instrumentation</span><span style="border: 1pt none windowtext; line-height: 115%; padding: 0cm; text-align: left;"> and </span><span style="line-height: 115%; text-align: left;">consist of a set of three
files; a Source file [s], a Receiver file [r], and a Cross Reference file [x].
There is also an optional fourth file for comments. The Receiver file contains
information about the geophones such as their ID, type, and position (Easting,
Northing, and Elevation). The Source file contains information about the
seismic source, namely its position and ID. The Cross Reference file contains
information about the Shot ID, and the source and receivers associated with
that Shot ID.</span></span></div>
<div style="text-align: left;">
<br /></div>
</div>
</div>
<span style="border: 1pt none windowtext; line-height: 115%; padding: 0cm;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;">
</span></span>
<br />
<div style="text-align: justify;">
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<span style="border: 1pt none windowtext; line-height: 115%; padding: 0cm;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="line-height: 115%;">At the end of the day,
QC’ing your geometry is just one of those things that has to be done; kind of
like flossing your teeth. Yes, you could not do it and just hope for the best,
but I’m willing to bet you’d end up in a situation where pulling teeth (or what
feels a lot like it) is involved.</span></span></span><br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj2z-hffSqmFuL5iyFMfUlIrljf3KdcpiegZUAOTcAGlZr53Rzz76QxKGhXKKgYaOI9KJVBGIyXDN8TJQ7d_BfXDz7tCUZF71bp-79jzSAKSMnqQ4NAnHFvs3jfQ1nNK2rIJ6oA8Zc70LY/s1600/299_45255561272_8865_n.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj2z-hffSqmFuL5iyFMfUlIrljf3KdcpiegZUAOTcAGlZr53Rzz76QxKGhXKKgYaOI9KJVBGIyXDN8TJQ7d_BfXDz7tCUZF71bp-79jzSAKSMnqQ4NAnHFvs3jfQ1nNK2rIJ6oA8Zc70LY/s1600/299_45255561272_8865_n.jpg" /></a></div>
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<span style="border: 1pt none windowtext; line-height: 115%; padding: 0cm;"><span style="font-family: Helvetica Neue, Arial, Helvetica, sans-serif;"><span style="font-size: large; line-height: 115%;"><span style="line-height: 115%;">By: Dhiresh Hansaraj</span></span></span></span></div>
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</span></span>GLOBE Claritashttp://www.blogger.com/profile/00646197047930265069noreply@blogger.com3