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Loading SEG-Y files is something of an art. The more different files you load, the more of an expert you become. There are so many issues and there's so little time to explain. The problem with SEG-Y is that the format is: * rather difficult to understand * highly incomplete (although Rev. 1 cures the worst shortcomings) * very common In 2002, a SEG committee has defined a revised standard which is aimed to be completely backward compatible - this is referred to as 'SEG-Y Revision 1'. You can download it from www.seg.org. Original SEG-Y standard didn't even handle files, it was about tapes only. And the conceptual model was 2D seismics. Of course 3D became very common so everybody tried to accomodate the extra and different data you need from 3D in the existing SEG-Y format. The idea is to put some info in the textual tape header, and put things like inline and crossline numbers in the trace headers - somewhere. Last but not least, a lot of SEG-Y files that we get are seriously wrong, with bad sample intervals, start times, data formats etc. etc.
This means that you need all kinds of tools to:
* inspect and scan the file
* tell OpendTect where's what (and overrule some things)
* store that setup to be able to quickly read similar files SEG-Y files explained First start with some concepts. The basic SEG-Y file consists of:
* A textual 'tape header' of 3200 bytes
* A binary 'tape header' of 400 bytes
* Then any number of traces, each trace having:
- A (binary) 'trace header' of 240 bytes
- Trace data (size depends on #samples and sample format) Text Header The "Text Header" has those 3200 bytes used as 40 lines each of them with 80 columns (characters). You can read a lot of useful info there, but don't trust it, it's commonly typed by hand, and can contain mistakes. If not, it's created using a script done by the operator (at processing) and can also be wrong. This can be encoded using EBCDIC (non-ASCII), hence the term " HEDCDIC Header" is often used. You can read that using "Examine" in OD. Binary Header The Binary Header is a bit more trustworthy, but it doesn't contain too much info. You can read the "Number of Samples" and "Sample Rate" there. Also how the "Amplitudes" are stored (32-bit IBM Floating Point, 16-bit, 8-bit, etc). Trace Header In the Trace Header (one per trace) the structure goes: Trace Header 1 ------ Amplitude Samples for Trace 1-------
Trace Header 2 ------ Amplitude Samples for Trace 2 ------
........................
Trace Header N ----- Amplitude Samples for Trace N ------ END OF FILE
Byte locations Before loading you have to identify in the Trace Header the following data (for common SEG-Y files):
For 2D: - SP Number: (usually repeats twice for on-shore seismic because of the Trace/SP relationship) so in Trace Header 1 = 10
Trace Header 2 = 10
Trace Header 3 = 11
Trace Header 4 = 11
Trace Header 5 = 12 and so on.
For off-shore seismic Tr/SP relationship is usually 1.
- CDP Number is usually = 2*SP or 10*SP
- The Number of Samples is usually located at position 115 (16-bit).
- The Sample Rate at 117 (16-bit)
- Co-ordinates or positioned at byte location 73 for the X co-ordinate value, at 77 the Y, often both repeated at 81 and 85 respectively.
For 3D: - Inline and crossline number:
The inlne number usually increases each N traces (where N= number of CrossLines)
Thr CrossLine number increases constantly (usually by 1), until the end of the Inline, and then returns to the first value, and starts increasing again. inline-crossline
100-2001
100-2002
100-2003
......
100-2200
101-2001
101-2002
......
- SP/CDP/IL/XL are usually stored between the first position of the start of Trace Header and position 25
- The Number Of Samples usually located at position 115 (16-bit) - The Sample Rate at 117 (16-bit)
- Co-ordinates or positioned at byte location 73 for the X co-ordinate value, at 77 the Y, often both repeated at 81 and 85 respectively. In OpendTect In OpendTect click 'import-seismics-SEGY-3D' (or 2D), and click the "define.." button. Then a rather large window pops up which allows you to: * Select the SEG-Y file
* Click on 'Examine'
* Define where to find the correct byte locations in the file (see above or press the help button to find out what's what)
* Overrule certain things that may be incorrect
* Scan the file and check the result (and redo this workflow if necessary by changing the byte locations) Note 1: examine the file first, then set what you think are the correct settings, only then you can do a meaningful scan.
Note 2: The help button on the window will lead you to the help for this window (yes, there is documentation).
When you think you have everything right, you press OK. Then you can subselect part of the data, specify an output format, scale the data etc. Now enter a name for the output cube (or line set) and then pressing OK should start the loading process.
Before you can load anything, you have to have a good survey setup. If all you have is one or a few similar SEG-Y files, you often want to setup the survey exactly according to the boundaries, coordinates and steps present in the SEG-Y file(s). For that, there is an option to set up the survey from a scan of a SEG-Y file.In the survey selection window, press 'New', then in that dialog, select 'Scan SEG-Y file' and press 'Go'. You'll have to define the bits and pieces like described above. Make sure inline, crossline aswell as coordinates are correct! That's about it. There are many details and things that can go wrong, like extra headers (Rev.1 can have those which are handled nicely but there are also just bad SEG-Y files) or Rev.1 tape labels ending up in your file (your SEG-Y file then starts with 128 garbage bytes which you need to get rid of). Bad programs write coordinates as floating point numbers (very much against the SEG-Y standard), ignore the start time, trace scaling, Rev.1 flag, etc. etc.
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OpendTect v3.2