Wednesday 27 February 2013

Getting Started with Marine Data

Two of the “Seven Habits of Highly Successful People” are:

  • Start with the end in mind
  • Put first things first

These are pivotal in seismic data processing; spotting bad assumptions mid-way through processing is much harder than trapping them upfront.

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.

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’).

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.

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 RAM 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.

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'.

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.

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.
The Observer’s Log is a key piece of “metadata” that can be extremely useful to the processor.
Example of an Observer's Log entry

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! 

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
Do Not Process - bad or noisy files, ‘warm up” shots, or other data that isn’t being paid for and should be excluded
Noise Record - a recorded file where the source didn’t fire to look at noise levels
Beginning Of Tape - usually with tape, shotpoint and file numbers
Shots fired to warm up the airguns, to avoid misfires
Start Of Line
First Shot Point
First Chargeable ShotPoint - on a reshoot, the first new shotpoint (after the required overlap) the client can be charged for
Misfire - the airgun array fired incorrectly in time or volume
Autofire - the airgun is discharging incorrectly, automatically
Seismic Interference – caused by the signals from another seismic vessel some distance off.
Ship Noise
Hydrophones were developed to detect ships and submarines by their propeller noise; they still do this very well!
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”  
Last Good ShotPoint - before a breakdown or failure
Last Chargeable ShotPoint
First Good ShotPoint - after a breakdown or failure 
End Of Tape - usually with tape, shotpoint and file numbers
End Of Line
Last ShotPoint
Field File Identification Number – the File Number on Tape
ShotPoint - the unique number for each shotpoint

Diagram illustrating Feather Angle (taken from "Applied Seismology: A Comprehensive Guide To Seismic Theory And Application" by M. Gadallah & R. Fisher)

Things to check or look out for on the Observer’s Log include:

  • 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
  • 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”   
  • 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
  • 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)
  • bad traces, noise issues or bad files
  • seismic recording format, sample rate and record length
  • near- and far-channel - is channel 1 the near of far channel from the vessel?
  • shotpoint numbers - incrementing or decrementing?
  • filters - were any applied in the field?

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.

The basic checks you need to do are:

  • 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.
  • 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
  • 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.

A few traps for the unwary:

  • some data has no low-cut field filter;  all you will see is 1-2Hz or lower “swell noise” and very little else;  drop a quick filter over your selected data to QC;  5Hz low cut should be fine

Shot record with no low-cut field filter

  • 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.
  • 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.

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!

By: Guy Maslen

1 comment:

  1. Greetings! This is my 1st comment here so I just wanted to give a quick shout out and tell you I truly enjoy reading your posts. Can you suggest any other blogs/websites/forums that deal with the same subjects? Thanks!
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