ReflectionGPH492

By: Jonathan Payne

Peter Bernhard

Eve Marie Hirt

California Wash Fault•Length: 32 km

•Average Strike: N15 deg E

•Sense of Movement: Normal

•Dip Direction: W

•Scarp is Discontinuous West facing with a height ranging from 2.1 to 9.8 m

•Slip Rate: Between 0.2 and 1.0 mm/yr

Methods at California Wash• Survey line using seismic cable with

48 channels spaced 2 m apart

• Each channel contains 6 geophones running parallel to the seismic cable

• Waves produced by hitting a steel plate with a 7 kg sledge hammer 10 times at each source point (gives a 10 stack record)

• Data collected on a Bison unit when digital trigger on sledge hammer is activated by impact on steel plate

• Seismic line consists of 14 source points taken every 4 before first geophone, then at every channel on the line, and 12 source points every 4 m after the last geophone

California Wash ProcessingBpfilter – 80-400 Hz Bpfilter – 80-400 Hz + dipfilter

CA Wash Line 1 CMPstack

NMO velocitiesDix Interval Velocities

~1100

~1300 ~1300

~1600~1500

~1100

~1700

~2200

NMO velocities = 80 m basin depth @ 0.10 s and 1580 m/s

?

?

EW

CMPstack using refraction velocity results(900 m/s to .005 s; 1300 m/s to .15 s)

Original CMPstack: is more clear than the artificial result

California Wash Conclusions

• We were able to image a discontinuity in the shallow reflector

• We identified two deeper reflectors at approximately 80 m depth

• If true, these outline a fault scarp, and a rough graben

Astor Pass

• Geothermal Project- Focus on the tufa tower located northwest of Needle Rocks.

• Tufas appear to be fault controlled and are expressed as a linear feature on strike with mapped faults adjacent the pass.

•Goal to image the orientation and geometry of the tufa in the subsurface.

Methods at Astor Pass

• Same as California Wash with the following exceptions:– Geophones were

placed 3 m apart– Two continuous lines,

288 m in length– Sources points taken

at every channel starting at west end of line and every 6 m after the last geophone extending 75 m

Plane 25, w/ dip filterPlane 20 w/o dip filter

Astor Pass Single Plane Examples

W E

cvStack of APRL w/ dip filter: stacking velocity 1,200 m/s

• 1,200 m/s is a generally useful stacking velocity

• ~0.2 s * 1,200 m/s = 120 m

• Other than these two areas, is no coherency in our cvStack…

CMPstack based on seisopt velocity model (visual picking)

W E

Optim seismic line 11

Conclusions

• Our reflection processing did not reflect the presence of tufa

• Cvstacking method works well for flat or dipping reflectors, but relies on the continuity of the reflector

• At Astor Pass the reflectors do not continue from plane to plane, making it nearly unrealistic to ‘flatten’ them using a common velocity

• This could be due to widely varying Vp at the surface generating a separate velocity profile at each shot point, or lateral variability in the subsurface (ie disturbance by tufa development)

• The alluvial, pluvial sediments at the surface are highly attenuating and do not transmit much energy for reflections

• Error Sources: Gain would need to be high to detect signals, this also amplifies noise (ie bad coupling, movement on the line, wind)