Reflection GPH492 By: Jonathan Payne Peter Bernhard Eve Marie Hirt.
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Reflection GPH492 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
CA Wash Line 1 CMPstack NMO velocitiesDix Interval Velocities ~1100 ~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
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
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 V p 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)