GEPH316 Field Data: Summary

GEPH316 Seismic Data Processing

Field Data Workshop 2000

21 Summary


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Summary: the complete processing sequence

This page is provided to enable the complete processing sequence, applied to the seismic line, to be repeated quickly and easily.
The steps may be applied in their totality or individually, depending on the stage reached in the processing.

Setting the environment

You can use the following 4 lines to set up quickly the environment your X terminal session for this practical.

setenv CWPROOT /home/ufs1/51/robinsob/cwp
set path=($path /home/ufs1/51/robinsob/cwp/bin)
cd $scratch
cp /home/ufs1/51/robinsob/geophysics/seismic/geometrics/refl000303/line .

Viewing the data

Use the following suximage command to get a labelled plot of the whole line of data.
Use the following supsmovie command sequence to print the whole line.

suximage  < line title='Line in raw form' label1='Time (s)' label2='Trace number' &
supsimage < line title='Line in raw form' label1='Time (s)' label2='Trace number' | lp -d mono

Use the following suwind commands to make working files s05 containing just the traces for the hammer shot at 05m and s06 containing just the traces for the Buffalo gun shot at 06m.
Use the following suxwigb commands to plot the shots at 05m and 06m.
Use the following supswigp command sequences to print the shots.

suwind <line >s05 key=sx min=05 max=05
suwind <line >s06 key=sx min=06 max=06

suxwigb <s05 title='Normal hammer shot at 05m' label1='Time (s)' label2='Trace number' &
suxwigb <s06 title='Buffalo gun shot at 06m' label1='Time (s)' label2='Trace number' &

supswigp <s05 title='Normal hammer shot at 05m' label1='Time (s)' label2='Trace number' > s05.ps
supswigp <s06 title='Buffalo gun shot at 06m' label1='Time (s)' label2='Trace number' > s06.ps
lp -d mono s05.ps
lp -d mono s06.ps

Resampling the traces

Resample the whole line and keep the old file.

suresamp <line >line.resamp dt=0.0005 nt=400

CDP header labels

Use the following suchw command to create the correct cdp point header values.

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

Killing noisy traces

It appears that trace 90 is particularly noisy and should be removed. Use the sukill command to remove it. Then plot the new version of the line to check that the correct trace was removed.
When all the correct traces have been killed, replace the original version of the line with the new version.

sukill <line.cdp >line.kill min=90 nkill=1
suximage < line.kill title='Line without noisy trace' label1='Time (s)' label2='Trace number' &
suxwigb < line.kill title='Line without noisy trace' label1='Time (s)' label2='Trace number' &
mv line.kill line.cdp

Frequency spectra

One of the first considerations with seismic data is to inspect the frequency content of the traces. This can be calculated using the suspecfx frequency spectrum command, which computes the f frequency amplitude spectrum for each trace x position.
The easiest form of resampling is to specify the new sampling interval dt in seconds and number of time points nt that are required. Use the following command to resample shot 05, so that a better spectrum may be produced directly. The trace is being 'decimated' from dt=0.00025s to dt=0.0025s, so the number of points required in the trace will drop from nt=1000 to nt=100.
Apply a ramp to the trailing 10% end of the traces, from 225ms to 250ms, and plot the tapered traces. As the leading ends of the traces are flat and zero, they do not need any tapering. Then recompute and display the smoother spectra.

suresamp < s05 > s05.resamp dt=0.0025 nt=100
suramp   < s05.resamp > s05.taper tmax=0.225 
suxwigb  < s05.taper title='s05 shot' &
suspecfx < s05.taper | suxwigb title='s05 spectrum' &

Produce a similar spectrum for shot 06. Compare this spectrum with the shot 05 spectrum and explain the reasons for the differences.

suresamp < s06 > s06.resamp dt=0.0025 nt=100
suramp   < s06.resamp > s06.taper tmax=0.225 
suxwigb  < s06.taper title='s06 shot' &
suspecfx < s06.taper | suxwigb title='s06 spectrum' &

Frequency filtering

Apply the following band-pass filter, with cut-offs below 60Hz and above 300Hz half-way down the taper, to the complete seismic line to reduce unwanted high frequency noise.

subfilt < line.cdp > line.filter  fstoplo=50 fpasslo=70 fpasshi=240 fstophi=260 verbose=1
suxwigb < line.filter  title="Line band-pass filtered betwen 60 and 250 Hz"  label1="Time (s)" label2="Trace numbers" &

Static corrections

Separate out the data for shots 06 and 07.
Resample all these traces so that the starting time is 2.45ms earlier at -2.45ms, but keep the number of points fixed at 400 and the sampling interval fixed at 0.5ms.
Then display the data panels before and after the resampling.

suwind < line.filter > s06 key=sx min=06 max=06
suwind < line.filter > s07 key=sx min=07 max=07
suxwigb < s06 title="s06 original" label1="Time (s)" label2="Trace numbers" &
suxwigb < s07 title="s07 original" label1="Time (s)" label2="Trace numbers" &

suresamp < s06 > s06.static nt=400 dt=0.0005 tmin=-0.0035
suresamp < s07 > s07.static nt=400 dt=0.0005 tmin=-0.002
suxwigb < s06.static title="s06 static shifted" label1="Time (s)" label2="Trace numbers" &
suxwigb < s07.static title="s07 static shifted" label1="Time (s)" label2="Trace numbers"

Move the hammer shots s00 - s05 at the left end of the line into the new static corrected line called line.static, using the suwind window command.
Move the static corrected buffalo gun shots s06 - s07 onto the back of the new static corrected line, using the normal Unix cat command.
Move the remaining hammer shots s08 - s24 at the right end of the line onto the back of the new static corrected line, using the suwind window command.
View the new static corrected line to see whether the shots s06 - s07 match the rest of the line better than previously.

suwind < line.filter > line.static key=sx min=00 max=05
cat < s06.static >> line.static
cat < s07.static >> line.static
suwind < line.filter >> line.static key=sx min=08 max=24

suxwigb < line.filter title="Line before statics correction" label1="Time (s)" label2="Trace numbers" &
suxwigb < line.static title="Line after statics correction" label1="Time (s)" label2="Trace numbers" &

F-K filtering

No dip filter so pass through the data to the next stage.

cp line.static line.velfilter

Common-offset gather

Use the following commands to select and plot the common-offset gathers with offsets of 14m, 20m, 34m and 36m:

suwind < line.velfilter > offset14 key=offset min=14 max=14
suwind < line.velfilter > offset20 key=offset min=20 max=20
suwind < line.velfilter > offset34 key=offset min=34 max=34
suwind < line.velfilter > offset36 key=offset min=36 max=36

Use the following lines to try the suxwigb presentation of the data.

suxwigb < offset14 title='Offset gather at 14m' windowtitle='Offset gather at 14m' label1='Time (s)' label2='Trace numbers' &
suxwigb < offset20 title='Offset gather at 20m' windowtitle='Offset gather at 20m' label1='Time (s)' label2='Trace numbers' &
suxwigb < offset34 title='Offset gather at 34m' windowtitle='Offset gather at 34m' label1='Time (s)' label2='Trace numbers' &
suxwigb < offset36 title='Offset gather at 36m' windowtitle='Offset gather at 36m' label1='Time (s)' label2='Trace numbers' &

Use the following lines to try the suxmovie presentation of the data.

suxmovie < offset36 title='Offset gather at 36m' windowtitle='Offset gather at 36m' label1='Time (s)' label2='Trace numbers' &

Use the following lines if you want print-outs of the offset gathered sections.

supswigp < offset36 title='Offset gather at 36m'  label1='Time (s)' label2='Trace numbers' | lp -d mono
supsmovie < offset36 title='Offset gather at 36m' label1='Time (s)' label2='Trace numbers' | lp -d mono

Common-midpoint gather

The whole line can be sorted so that it is in order of increasing CDP points and also so that the traces are in the order of increasing offset for each CDP point.

susort < line.velfilter cdp offset > line.sorted
suxmovie < line.sorted title='Line sorted into CDP gathers'  windowtitle='Line sorted into CDP gathers' label1='Time (s)' label2='Trace numbers' &

Use the following line if you want a print-out of the CDP gathered line.

supswigp < line.sorted title='Line sorted into CDP gathers' label1='Time (s)' label2='Trace numbers' | lp -d mono

Use the following to pick out the typical CDP gather at cdp=20m, plot it and inspect it.

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

Use the following line if you want a print-out of the CDP gathered section.

supswigp < cdp20 title='CDP gather at 20m' label1='Time (s)' label2='Trace numbers' | lp -d mono

Velocity analysis

Get a copy of the velocity analysis shell script called velan.

cp /home/ufs1/51/robinsob/geophysics/seismic/geometrics/refl000303/velan .

In the following command, the parameters for the velocity analysis are:
- use the cdp gathered data in file line.sorted
- start at cdp 20
- finish at cdp 30
- step every 5 cdp points
- each gather is 12 fold

velan line.sorted 20 30 5 12

The results of the velocity analysis are stored in the file stkvel.nmo.
Use the ue Microemacs text editor to clean up the file, so all the cdp points are on one line.

ue stkvel.nmo

When you have finished, the file should look similar to the following.
Use Escape + z to save the file and exit from the editor.

cdp=20,25,30
tnmo=0.001467,0.01907,0.05035,0.1031
vnmo=408.1,931.4,847.7,1287
tnmo=0.0009777,0.03324,0.04009,0.07137,0.1501
vnmo=418.6,1005,983.7,1224,1538
tnmo=0.0009777,0.03275,0.06893,0.1383
vnmo=408.1,962.8,1015,1528

NMO correction

Apply NMO correction to the whole seismic line to produce line.nmo.
Use the velocity picks in the file stkvel.nmo.

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

You can produce a printed copy of the NMO corrected line using:

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

Residual Statics

Stacking the line

Stack the appropriate data traces in the line and do a wiggle plot of the line.

sustack < line.nmo > line.stack key=cdp normpow=1.0
suxwigb < line.stack title='Final stacked line' windowtitle='Final stacked line' label1='Time (s)' label2="CDP position (m)" &

The subtleties of the reflecting interfaces are not emphasised by the wiggle plot. More features can be seen on the xmovie plot.

suxmovie < line.stack title="Final stacked line" label1="Time (s)" label2="CDP position (m)" &

The two versions of the line may be printed using the following commands.

supswigp  < line.stack title="Final stacked line" label1="Time (s)" label2="CDP position (m)" | lp -d mono
supsmovie < line.stack title="Final stacked line" label1="Time (s)" label2="CDP position (m)" | lp -d mono

AGC correction of final section

The following AGC may be applied. It uses an automatic gain amplification with a time window of 0.05s, which makes each 0.05s section of each trace have the same maximum amplitude.

sugain < line.stack > line.gain agc=1 wagc=0.05
suxmovie < line.gain title='AGC line with agc=1 wagc=0.05' windowtitle='AGC line with agc=1 wagc=0.05' label1='Time (s)' label2="CDP position (m)" &

You can plot the data in wiggle trace format using the following command.

suxwigb < line.gain title='AGC line with agc=1 wagc=0.05' windowtitle='AGC line with agc=1 wagc=0.05' label1='Time (s)' label2="CDP position (m)" &

You can produce printed copies of this line in wiggle and movie format using:

supswigp < line.gain title='AGC line with agc=1 wagc=0.05' windowtitle='AGC line with agc=1 wagc=0.05' label1='Time (s)' label2="CDP position (m)" | lp -d mono
supsmovie < line.gain title='AGC line with agc=1 wagc=0.05' windowtitle='AGC line with agc=1 wagc=0.05' label1='Time (s)' label2="CDP position (m)" | lp -d mono

Cleaning up

Remove the unwanted graphical windows and files.

zap xmovie
zap xwigb
ls


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