As we've finally got around to talking about processing, we'll start with a typical processing flow, showing all of the processes applied to one "line" of seismic data. This flow is based on a typical marine 2D line, but could apply almost equally to any type of processing.
The processes necessary for the geometrical correction of the data are shown in red, whilst the others (in yellow) are really just cosmetic processes designed to improve the final result. Many of the processes are optional, and other processes may be used within the sequence. We will discuss each process in detail in the following pages and Chapters.
Field Data
1 Transcription Conversion of the field data to an appropriate internal format.
2 S.O.D Correction A static correction so that time zero is the time that the shot was fired.
3 Signature Deconvolution
Replacement of the source signature with a more desirable wavelet.
4 Initial Gain Recovery
An initial correction for the spherical spreading of the signal with time.
5 Resample Anti-aliased and resampled to the highest sample period commensurate with the desired frequency range.
6 Edit Removal of bad traces / shots and correction of any polarity
180 reversals.
7 Multichannel Filtering
Attenuation of shot-generated noise, improvement in spatial coherency.
8 CMP Gather Input of geometry, and calculation of traces from every common mid-point.
9 De-Multiple Removal of long period multiple reflections.
10 Dip Moveout Correction of the spread of data within one CMP for dipping events.
11 Deconvolution Removal of short period multiple and frequency balancing.
12 NMO Correction Correction of curvature on events due to differing trace offsets.
13 Mute Removal of first-break noise.
14 Equalisation Amplitude normalisation.
15 CMP Stack Summation of all data from one common mid-point.
16 Datum Correction Final static correction to the final datum.
17 Final Gain Recovery
Adjustment of the initial gain recovery to compensate for velocity changes.
18 Multichannel
Filtering Additional coherency enhancement (spatially).
19 Deconvolution Final frequency balancing.
20 Migration Re-positioning of dipping events to their correct spatial position.
21 Spectral Shaping Adjustments to the final amplitude / phase spectrum.
22 Bandpass filter Limiting of frequencies to the useful signal range.
23 Equalisation Amplitude normalisation.
Final Display
Although many of these processes are optional the CMP (or incorrectly named CDP) stack (item 15) is essential when we are processing multi-fold data. The processes prior to this stage are called (logically) pre-stack processes, and those after post-stack processes. It's useful to distinguish between these two parts of the processing sequence, as there may be a 60:1 or more reduction of data at the stack stage.
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Transcription
The first stage of any processing sequence requires that the data from the field be loaded and checked.
Remember that the field data has been acquired at enormous cost, and that the field tapes and associated data (if they are the original copies) must be treated with some respect!
On more than one occasion the field data from marine surveys has been dropped overboard during data transfer, and, again more than once, field tapes have been over-written in the processing centre!
All processing systems have their own internal format for the storage of data. Even with the current trend towards cheap storage devices, it may still be impossible to store all of the data on disk, and tape-to-tape systems are still quite common. Let's repeat an earlier exercise and calculate the storage requirements for some typical field data.
If we assume 6 seconds of recording, at a 2 ms sample period, then we have 3000 samples
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for each trace. If these are stored in 32-bit longwords then the total storage for one trace (excluding any header information) will be about 12,000 bytes.
Again, we'll assume a 240 channel recording system, which gives us 240 x 12,000 bytes, or about 3 megabytes (with headers) for each field record. We've already got enough to fill two 3½" floppy disks with data.
Now we'll assume a 25 m shotpoint interval which, for a 30 km seismic line gives us 1200 shots. Multiplying up again, we now have a total of 3,600 megabytes (3.6 Gb) for just the field data for this line. If we want to keep other processed versions of this line then we'll need as much again for each stage before stack (unless we're resampling the data).
When one considers that a complete marine survey may consist of 1,000 or more kilometres of data (120 Gb), and we will normally be processing many lines simultaneously, you begin to see the storage problem. With multiple streamer 3D surveys the problem gets totally out of hand!
So, our initial processing stage must read all of the field data and convert it into our internal format.
If the data has been recorded in a
multiplexed format (ordered by channel, not trace) then the computer has to have enough temporary storage (ideally memory) to read the entire record, and then re-order it.
We need to check this initial stage to ensure that every trace of every shot has been read correctly. Many of the problems at this stage (hopefully getting fewer) are related to the physical condition of the recording media. The reprocessing of old data that has not been stored correctly may require the resources of one of the specialised companies that can, hopefully, ensure that as much data as possible can be read from the field tapes.
We also need to compare the list of shots read from the tape against those listed in the various field reports (normally the Observer's Logs). Although each shot is identified by a shotpoint number, these may not be recorded on the tape. Instead a Field Record Number may be used which might be designed to match the shotpoint number but which might, occasionally, get out of step.
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At the end of this stage we must have a complete list of the shotpoints and traces now in our internal format - errors at this stage will affect everything that follows!
What kind of problems can we encounter even if we read the data (apparently) correctly?
Most of these are fairly obvious since they usually involve some kind of mis-interpretation of the data on the field tapes. If, for example, we are expecting (and the field tape headers agree) data recorded at 2 ms sample period, and in fact it has been recorded at 4 ms, then we'll only get half the number of samples we expected.
If the computer gets confused about the actual format of the numbers on the tape, then the output will normally be very "un-seismic" looking. A scan error (where one multiplex scan of the channels gets out of "sync"), will generally mean that we'll have to omit that shot from any processing, but, we should look at the data as soon as possible to confirm this.
The following display shows four examples of loaded data, using SEG-Y as an example.
Note the (sometimes fairly subtle) errors when we read data in the wrong format. The abbreviation "FP" indicates floating-point data.
There are one or two other considerations that may be required by data from Vibroseis or other (more unusual) sources - we'll look at these briefly before moving on through the processing sequence.
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