## Wednesday, 5 June 2013

### Marine Processing - Part 2 | Initial QC & Geometry

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.

Following on from the previous post 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.

We’ve already covered a lot of the basics for a 2D marine line so we can focus now on the specifics of the seismic line I’m working with:

• FFIDs are the same as shotpoints in this case; from 100 to 975 with none missing
• 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 = 3233 m.
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.

The number of traces in a CDP gather is referred to as the fold. This is sometimes expressed as a coverage percentage: single-fold = 100% coverage, sixty-fold = 6000% coverage, and so on.

Fold = (Receiver Spacing x Number of Receivers) / (2 x shot spacing)
Fold = (25 x 120) / (2 x 25) = 60

For more on seismic acquisition in practice, including fold and CDP, visit this web page

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:

 Every 100th raw shot from the line with shotpoint number and channel labelled at the top

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.

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.

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.

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.

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.

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.

 Detail of SP 400: the low frequency, high energy "stripes" are swell noise

 Detail of SP 200: minor swell noise along with tailbuoy jerk, a low frequency dipping event (upper left to middle of the plot)

 Detail of SP 400: linear direct and refracted arrivals, as well as (hyperbolic) reflections

 Detail of SP 900: the faster (less steeply dipping) refracted arrivals, and loss of signal showing how the geology changes

As part of the processing sequence we will have to manage all of these – everything that is not a seismic reflection is effectively noise.

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.

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.

The midpoint between this near channel and the first shot is 129 m, or a little over 10 CDP spacings.

The midpoint between the far channel (at 3233 m offset) is 1661.5 m, or a little under 133 CDP spacings.

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.

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:

The key things to note are:
• Odd and even CDP numbers have different offset values, but the same number of traces
• The spacing between traces in the CDP gather is 50m, i.e. every second receiver
• The even CDPs don’t have the “near offset” trace in them
• The odd CDPs don’t have the “far offset” trace in them

This pattern becomes important later on in the processing sequence, especially for processes that work on common offset planes.

Having assigned the offsets, there are three things we can look at as a check:

• shots
• the near trace plot
• a “brute” stack

 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

 Offset (red line) overlaid on shot record; it matches the first break, meaning it is correct

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.

 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

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.

The basic approach is to gather the data by CDP number, flatten the hyperbolic reflection events using the normal moveout (NMO) correction, and then stack (sum) the data to improve the signal to noise ratio.

The sequence is:
• Read in the minimum phase data at 4ms sample interval
• Apply the geometry we have defined
• Sort the data to CDP
• NMO correct with a simple function
• Stack the data

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.

In this case I’ve used a velocity function that has only a few points:

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.

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.

 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.

By: Guy Maslen

1. If I have different group interval along the streamer e.g 1 m between channel 1 snd 16 then 2m between channel 17 to 24, how can I work the offset and CDP point? Near offset is only 1m and shot interval 4m.
Thank youin advance for the suggestion.

Rgds,
Mike

2. That can be tricker - depending on what software you are using.
One approach is to do the following:

- assign the offset header based on the channel number; hopefully you can interpolate this from a table in some way in your software

- calculate the distance along the line for the shot point; so once you have numbered for any missing shots you can just multiply the shotpoint number by 4.0 metres and store this as a header shuch as SHOT_X

- calculate the distance along the line for each receiver, which is the shot location minus the offset (cable behind the boat..) and store this as the REC_X header

- now you can use the SHOT-X and REC-X values to calculate the midpoint location (SHOT-X + REC-X)/2

- and you can then calculate a CDP number from this be dividing by your chosen CDP spacing

That said, you will have to be very careful in your processing. A lot of routines like FK might assume a constant trace spacing in a shot or CDP and come unstuck with your variable offset distribution.

A better aporoach would probably be to regularise the offsets by either a k-filter and trace drop on the first half of the cable (to 16 offsets, 2m spacing) or using an interpoation routine on the second part (to give 32 channels at 1m spacing); if you have a good pre-stack regularisation routine this is probably the best way to go.

Guy

3. This comment has been removed by the author.

4. Hi Guy,

I have one question for you. How to calculate near offset for 2D marine project? I know, we can get near offset value from observer log, but another option is by calculation to get the right near offset value. Please advise.

Hidayah

5. Hello Guy,

Thanks for these very helpful blogs, a welcome addition to the 2D Marine Processing tutorial!

I'm currently working predominantly on shallow ultra high resolution seismic data (<0.25 s). The effect of swell in the sense of changing source and receiver elevation resulting from wave movement, is heavily present in this data. This is normally attacked by very precise first breaks picking, determining the dominant swell wavelength and applying a spatial bandpass filter and the differences being applied as static corrections.

Does GLOBE Claritas have good functionality to fight these swell effects and regain coherence in my data?

Many thanks!

Cheers,
Roeland

1. Hi Roeland - glad you are finding them useful.

We've had similar issues in the past with high resolution data, and addressed them broadly as you describe.

You could pick first breaks (or digitise the direct arrival on common channel gathers), then put these into trace headers (ADDPICK/ADDDIG); smoothing functions are via hdrmath (again on common channels or shots) which uses NUMPY functions, and then apply shifts as you indicate.

An alternative approach is to think of this as a residual statics issue. For example, LMO correct with water velocity, and then apply trim statics (RESIDS) to flatten/align the direct arrivals. CDP domain trim statics would achieve the same result.

Essentially you are accounting for a (non-surface consistent) variation in the water column, which is a much easier problem than land statics as the velocity is known. The advantage of using trim statics is you don't need to pick first breaks or events.

6. Hi,
The far offset is actually 3233m, not 3323m.

-Thanks for the article, though.
Nathan B.

1. Good catch - absolutely right and I've update the post! Thanks, Guy

7. hi,
how we can calculate the optimum near offset?

thanks
andrew

8. Hi Andrew - sorry about the delayed response!

Interesting question though - I suspect that for the most part for deep-tow marine data it has tended to be defined as "as close to the boat as we can without risking tangling the equipment or having too much noise from the engines and residual airgun bubbles"

Near offsets are important for modern multitude techniques like SRME, which require a projection of the shot and receiver data back to zero offset. The more interpolation you have to do, the harder this is.