Sunday, August 4, 2013

Old Main seismic

Thursday 1 August 2013

Caleb, Richard and Chris made the first fledgling seismic field efforts with new Geometrics Geode equipment.  It was just a test, but a symbolic one on the front lawn of Old Main, the University of Arkansas signature building completed in 1875.

Data
Old Main shot record composed from 3 shots into a 24-phone spread.  Increasing offset direction is NE. Reference velocities are indicated by yellow line (335 m/s, air wave speed) and red line (1500 m/s, water wave speed). 

Old Main shot record scanned for dispersive surface wave phase velocities.  The characteristic curves can be used to analyze for subsurface layering.  
Site

The test was laid out as a walk-away noise test with 24 single component geophones spaced 1 m apart, an off-end shot 1m from the last phone, then 2 further shots at 25 m and 40 m from the first shot location.  Shot walk-away direction is NE. As a source, we used a sledge hammer striking a 1" thick steel plate. One thing we determined for sure, the source plate needs a tough rubber coating to hold down air waves: It was very hard on the ears to be the source hammer man!


Google Earth view of seismic test location.  The Geoscience department is currently housed in Stone House South, but moving to renovated Ozark Hall in August 2013.
Aquisition

This was the first time for Caleb and Richard to lay out a seismic profile, plant geophones, wrestle the cable into place, and all the rest.  They will be the core of my seismic team and quickly be able to train others.  Here are a few photos of them in action.

All of the gear fits nicely in the back of my old 1989 Dodge Dakota. But we may need a modern geophysical truck for more remote locations.

Richard (L) with source hammer and Caleb wearing a steel plate neclace.

Caleb laying out receiver flags.

A rookie mistake unrolling the geophone cable.  It won't happen twice.
Packing up and Richard horse-collaring the cable, which Chris showed him how to do from experience in 1978 on a Western Geophysical crew.  Back then, horse-collaring was industrial practice.  With modern commercial systems having many thousands of channels, cables are too big to collar or the crew may be wireless and thus have no cables at all. But collaring is still a useful skill for near-surface seismic work.

Seismic Unix processing makefile


go:
segyread tape=./49477.sgy \
| segyclean \
| suflip flip=2 \
| sushw key=gx a=1 c=1 j=1 \
| sushw key=offset a=1 c=1 j=1 \
> 1.su
surange < 1.su
segyread tape=./49479.sgy \
| segyclean \
| suflip flip=2 \
| sushw key=gx a=26 c=1 j=1 \
| sushw key=offset a=25 c=1 j=1 \
> 2.su
surange < 2.su
segyread tape=./49482b.sgy \
| segyclean \
| suflip flip=2 \
| sushw key=gx a=51 c=1 j=1 \
| sushw key=offset a=49 c=1 j=1 \
> 3.su
surange < 3.su
fcat 1.su 2.su 3.su > shot.su
surange < shot.su
sugain < shot.su tpow=1 epow=1 \
| suximage perc=99 \
 label1="Time (s)" label2="Offset (m)" \
 windowtitle="Shot oldmain1" &
sugain < shot.su pbal=1  epow=2 \
| supsimage perc=99 \
 label1="Time (s)" label2="Offset (m)" \
 title="Shot oldmain1" \
 d1s=.2 d2s=.2 \
 grid1=dot grid2=dot \
 d1num=.02 d2num=5 \
 curve=c1.txt,c2.txt npair=2,2 curvecolor=yellow,red \
 x1end=.3 \
> shot.eps
ps2pdf shot.eps 
sugain < shot.su pbal=1 \
| suphasevel fv=100 nv=200 dv=5 fmax=250 \
| suamp mode=real \
| supsimage style=normal \
 label1="Frequency (Hz)" label2="Phase Velocity (m/s)" \
 title="Shot oldmain1: Dispersion" \
 d1s=.2 d2s=.2 \
 grid1=dot grid2=dot \
 height=4 \
 d1num=20 d2num=100 \
> disper.eps
ps2pdf disper.eps

2 comments:

geophix said...

Great blog!

Anonymous said...

once you get the dispersion curve, what are you using to pick phase velocities and to convert it to S wave velocity profile?