SuHeaderModify

This page describes the usage of Seismic Unix for modifying header values in a seismic section (eg: adding offset information to a section that has none). For a detailed discussion of Seismic Unix, see the SU documentation and archives at »http://www.cwp.mines.edu/cwpcodes/

The following procedure adds offset information to a segy file that doesn't contain any. Its best intended for a seismic section that does not have a regular shot spacing.

• The first step is to read in the SEGY data, which should be in standard IBM format (the "endian=0" option is used for little endian machines like an Intel or AMD processor. The default is "endian=1" which is for big endian machines such as a Sun Sparc).

segyread tape=segyfile.sgy endian=0

• next get the header values of interest, and write to the 'traceInfo.txt' file. This will be an ASCII file that can be modified by hand.

sugethw key=tracl,offset output=geom > traceInfo.txt

• now merge the modified 'traceInfo.txt' information into the SEGY file. First convert it to a binary file.

a2b < su_trace_offsets.txt > su_trace_offsets.bin

• then merge with the original segy data, and write out a new file.

segyread tape=segyfile.sgy endian=0 > sufile.su
sushw < sufile.su infile=su_trace_offsets.bin key=tracl,offset |
segyhdrs |
segywrite tape=segyoutput.sgy endian=0

• these steps (and some others) are included in the following script, and other python scripts.
• »upload/su_add_offset.sh Unix style Bash script to run all SU and python programs.
• »upload/columnMerge.py python script to merge two text files, the GNU program 'join' does the same operation, and is probably far superior.
• »upload/horizonOffset.py calculates the offsets for each trace in a "geoquest card image 7" horizon, relative to a given trace.

If the segy file has a regular shot spacing, it is much easier to add offsets to the file using sushw. This example sets the offset of the first trace to 20, and the next to 70 (20+50), then 130, and so on. The two numbers set in the "b" array correspond to the keywords set in "key". Therefore it is processing every trace. The "endian=0" flag is necessary on a small endian machine such as linux on an intel / amd computer.

#!/bin/bash
segyread tape=segyfile.sgy endian=0 > sufile.su
sushw < sufile.su key=offset,tracl a=20, b=50,1 |
segyhdrs |
segywrite tape=segyoutput.sgy endian=0