GEPH316 Field Data: Trace headers

GEPH316 Seismic Data Processing

Field Data Workshop 2000

7 Trace headers


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Defining geometry and trace headers

It is important that all traces in the seismic data file are correctly labelled in terms of shot positions, geophone positions, offset distances and cdp points in order that all subsequent processing steps are performed correctly.

Viewing a trace header

It is useful to be able to view the header parameter information associated with a trace using the interactive suedit header editing command. Each trace has its own header. Inspect the trace headers for shot 05m using the following lines, entered one at a time, and try to understand what some of the parameters mean.

suedit <s05
?
                  ( just a carriage return)
p
q

The header for the last trace in the group is shown first, in this case trace 12.
The question mark ? gives some help on the suedit command.
When carriage return is pressed, the header for the previous trace is displayed.
p gives a crude tabulated plot of the start of the trace.
Write down the main parameters for shot 05, such as sx, gx, offset, dt, ns, etc.
Do the parameters appear to have the correct values? Note that the offset distance is measured as (gx-sx) from the shot sx forwards to the geophone gx.
The letter q quits the suedit command as for the Unix more command.
If you are not sure what the header words mean, use the sukeyword command to get more information, as in the following example.

sukeyword cdp

Changing trace headers

The sushw header command can be used to set new header words, using the stepping trace number in the calculations. This is very useful if a mistake was made when creating the headers in the field.
The suchw header command can be used to change or create new header words, based on existing header work values. Type the suchw command to investigate the calculation formula.

suchw

In the case of our data, the words sx, gx and offset have their correct values. However, the cdp header word needs to be calculated to specify the cdp position for each trace. It can be calculated as the average of the shot and geophone positions, sx and gx. Check that you understand the a, b, c and d coefficients which give the expression of the formula as cdp=(0+1*sx+1*gx)/2.
Use the following command to create the cdp header word values.

suchw <s05 >s05.cdp key1=cdp key2=sx key3=gx a=0 b=1 c=1 d=2

Viewing trace header words

It is useful to be able to view certain header parameters associated with all the traces in a section, using the sugethw get header words command. It will display the values of the selected keywords.
Using the following command to display the shot and geophone positions for a set of traces.

sugethw <s05.cdp key=sx,gx,cdp,offset

Labelling the whole section

Convert the whole of line to contain correct cdp points, replace the old line with the new cdp labelled line, then remove the unwanted files.

suchw <line.resamp >line.cdp key1=cdp key2=sx key3=gx a=0 b=1 c=1 d=2
ls
rm line.resamp
rm s05.cdp
ls


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