GEPH316 Field Data: NMO Correction

GEPH316 Seismic Data Processing

Field Data Workshop 2000

17 NMO Correction


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NMO correction of one gather

Investigate the seismic unix NMO normal moveout correction command sunmo to see how it operates.

sunmo

It is easiest to see the effects of NMO correction by working on a single gathered section for one CDP point.
Get a clean copy of the gather for the CDP point cdp=20m, sorted into offset order.
Display the gather for the CDP point cdp=20m, for comparison later with the NMO corrected version.

suwind < line.sorted > cdp20  key=cdp min=20 max=20
suxwigb < cdp20 title='CDP gather at 20m' windowtitle='CDP gather at 12m' label1='Time (s)' label2='Trace = (Offset-12)/2 m' &

Now try NMO correction on a single gather cdp20, using the velocity picks in the file stkvel.nmo and display the corrected section.

sunmo < cdp20 > cdp20.nmo par=stkvel.nmo
suxwigb < cdp20.nmo  title='NMO corrected gather at cdp=20m' windowtitle='NMO corrected gather at cdp=20m' label1='Time (s)' label2='Trace = (Offset-12)/2 m' &

The NMO corrected section should have flattened the events which were dipping in the original section.
It will only do this correctly for reflection hyperbola which have the correct curvature for the particular stacking velocities and times in the parameter file.
The NMO corrected section will have blanked out areas in the top right hand corner. These areas have been muted because they would contain false events due to the processing involved in the NMO correction. Consult the book by Yilmaz for a fuller explanation of the reasons for this.
You can produce printed copies of these gathers using:

supswigp < cdp20 title='CDP gather at 20m' label1='Time (s)' label2='Trace = (Offset-12)/2 m' | lp -d mono
supswigp < cdp20.nmo  title='NMO corrected gather at cdp=20m'  label1='Time (s)' label2='Trace = (Offset-12)/2 m' |  lp -d mono

NMO correction of the whole line

Apply NMO correction to the whole seismic line to produce line.nmo.
Use the same velocity picks in the file stkvel.nmo.
Then inspect the corrected line using suxmovie.

sunmo < line.sorted > line.nmo par=stkvel.nmo
suxmovie < line.nmo title='NMO model corrected line' windowtitle='NMO model corrected line' label1='Time (ms)' label2='Trace numbers' &

Inspect the line, using the mouse to expand particular parts of the line.
Each individual gather should look much more horizontal.
The upper right hand ends of each gather may be muted.
This prevents false events being created by the NMO correction.
You can produce a printed copy of the NMO corrected line using:

supsmovie < line.nmo title='NMO model corrected line'  label1='Time (ms)' label2='Trace numbers' | lp -d mono

Remove the unwanted graphical windows and files.

zap xmovie
zap xwigb
rm cdp*
ls


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