GEPH316 Field Data: Statics

GEPH316 Seismic Data Processing

Field Data Workshop 2000

12 Statics


Previous: Frequency filtering data	Up: Contents	Next: F-K spectra

Preliminary statics

It is important to apply static time corrections to the seismic section to correct for
- the varying height/depth of the geophones and shotpoints.
- the varying thickness of the weathered zone.
For shallow reflection surveys, it is unusual to correct for the varying thickness of the weathered zone because discovering this thickness is usually part of the target for the seismic survey.
For this survey, the ground surface was a gently sloping plane, which does not require any correction, provided that all times and depths are relative to the dipping ground surface.
However, shots 06 and 07 used the buffalo gun source with the shot at about 1m below the ground surface, so the total travel time for all the traces is reduced for these shots.

Datum statics

To correct for the vertical positions of the shots and geophones, the SU package uses the sustatics command.
This uses the vertical z position in the trace headers to correct for the vertical positions of the shots and geophones.
Use the sustatics command to understand the preliminary static correction details.

sustatics

If we knew the depths of the buffalo gun shots, their depth parameter in the trace headers for shots s06 and s07 could be included and used for the static corrections, using the surface layer velocity.
We did not record the exact depth of the shots, so a different method is required.

Inspecting the data

Extract a file containing the shots s05 - s08, where s05 and s08 are normal hammer shots and s06 and s07 are the buffalo gun shots.
Plot the shots s05 - s08 as a wiggle section.

suwind < line.filter > s0508 key=sx min=05 max=08
suxwigb < s0508 title="s05 to s08" label1="Time (s)" label2="Trace numbers" &

Inspect the 4 shots and notice the earlier arrivals of the traces for shots s06 and s07.
Use the mouse to select the top 0.050s of the traces and try to measure the average time shift between the hammer arrivals and the shotgun arrivals.
In particular, measure the start time of the first troughs for geophone 1 (at offset 14m) for the 4 shots.
Average the time shift differences between the hammer shots and shotgun shots.
Because the depth of layers changes across this area, the time differences will vary from left to right and an average difference must be taken.
The following work assumes that the average time shift is about 2.45ms = 0.00245s.

Correlating the data

One method to determine the average time shift is to use cross-correlation of traces.
Use the suxcor command to understand the cross-correlation details.

suxcor

Extract a file containing the first trace for normal hammer shot 05, which will provide a reference trace.

suwind < s05 > s05.01 key=offset min=14 max=14

Extract a file containing the first geophone traces for the four shots 05 - 08, which will provide the test traces.

suwind < s0508 > s0508.01 key=offset min=14 max=14

Now perform the cross-correlation of the reference trace with the first geophone traces for the four shots and plot the correlations.

suxcor < s0508.01 > s0508.corr sufile=s05.01
suxwigb < s0508.corr title="S05 - S08 correlations" label1="Time (s)" label2="Trace numbers" &

The correlations will have a total length of 0.4s due to the length of 0.2s for both the reference and the test traces.
The zero lag correlation occurs around 0.2s.
The correlation wavelet is longer than the original wavelet, but its highest peak now occurs in the centre of the correlation.
Measure the time shifts between these largest peaks to determine the static shift required. Note that there appears to be a linear trend across the four shots.
It appears that the static shift required for shot S06 is about 3.5ms and for shot S07 is about 2.0ms.

Shifting the shots

To make all the shots compatible, it is necessary to move the buried shotgun shots up to the ground surface to be comparable to the hammer shots.
This requires the addition of extra time/data points to the front of the shots 06 and 07, so that all the arrivals are shifted later in time.
Separate out and display the data for shots 06 and 07.

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" &

Resample all these traces so that the starting time for shot S06 is 3.5ms earlier at -3.5ms, but keep the number of points fixed at 400 and the sampling interval fixed at 0.5ms. Similarly, the starting time for shot S06 is 2.0ms earlier at -2.0ms.
Then display the data panels before and after the resampling.

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" &

Arrange the two data panels side by side, occupying half of the screen each.
Notice that the resampled data has extra data points added at the front. It is not clear why they are noisy.
The time scale reflects the original arrival times, but this effect will be lost when all the data is recombined.

Building the static corrected line

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.

suwind < line.filter > line.static key=sx min=00 max=05

Move the static corrected buffalo gun shots s06 - s07 onto the back of the new static corrected line, using the normal Unix cat command.

cat < s06.static >> line.static
cat < s07.static >> line.static

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.

suwind < line.filter >> line.static key=sx min=08 max=24

View the new static corrected line to see whether the shots s06 - s07 match the rest of the line better than previously.

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" &

Remove the old windows and unwanted files using:

zap xwigb
rm s0508
rm s06*
rm s07*
ls


Previous: Frequency filtering data	Up: Contents	Next: F-K spectra

Go to <--->