GEPH316 Field Data: Survey description

GEPH316 Seismic Data Processing

Field Data Workshop 2000

4 Survey description


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Survey data format

Most seismographs will record seismic data files in one of the SEG data formats such as SEG-Y. The binary data files from the Geometrics ES1225 Seismograph data format are converted into the international SEG-Y format for data processing. It is important to know the number format of the data file, whether it is

integer,
floating point or
compound format.

Other important survey parameters are

trace length
sample interval
number of channels.

If these are not known and used correctly, the data files will give wrong velocity information and may not be usable at all.

Survey data description

The seismic reflection survey on March 3rd 2000 consisted of 25 shot points, running at 1m intervals from 0m to 24m.
For each shot, the 12 geophones were placed at 2m intervals, the first at an offset distance of 14m from the shot. For example, for the shot at 0m, the geophones ran from 14m to 36m.
At most of the geophone points, 4 Hz geophones were used. For trace 6, a 100 Hz geophone was used (only one 100 Hz geophone was available).
At most of the shot points, 2 sledgehammer blows were used. At the shot points 06m and 07m, 1 Buffalo gun shot was used.
The seismic arrivals were digitised at a sampling interval dt=250us to give seismic traces 1000 points long with a total time of 250ms.
The data was digitised with 8 bit accuracy, giving values from -127 to + 127.
The total number of data traces in the line is 25 shots * 12 geophones = 300 traces.
In addition to the main seismic line, there were two reversed shots at 14m and 36m into geophones from 14m to 36m, stored in separate data files. The reversed shots were for use as a reversed refraction line with the plus-minus method to give some velocity and depth information.
This gave a total of 27 shot files.

Building the data file

The individual seismic data files, each containing 1 shot and 12 geophone traces, were stored on PC floppy disks using a portable PC.
The binary files were transferred from the PC floppy disks to the Unix system using an FTP file transfer facility.
The binary data files, in the Geometrics ES1225 Seismograph data format, were converted into the international SEG-Y seismic data file format, using the seismic11.for Fortran program written by Brian Robinson.
The SEG-Y seismic data file format is very easy to manipulate, as each geophone trace has its own individual header containing information on the shot and geophone positions etc.
The 25 SEG-Y reflection files were packed together into one big seismic line file using the Unix cat command e.g.

cat s00.seg >> line
cat s01.seg >> line
...


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