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Reflection GPH492

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

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Reflection GPH492

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  1. ReflectionGPH492 By: Jonathan Payne Peter Bernhard Eve Marie Hirt

  2. 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

  3. 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

  4. California Wash Processing Bpfilter – 80-400 Hz Bpfilter – 80-400 Hz + dipfilter

  5. CA Wash Line 1 CMPstack W E ? ? NMO velocities Dix Interval Velocities ~1100 ~1100 ~1300 ~1300 ~1700 ~1600 ~1500 ~2200 NMO velocities = 80 m basin depth @ 0.10 s and 1580 m/s

  6. 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

  7. 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

  8. 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.

  9. 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

  10. Astor Pass Single Plane Examples Plane 20 w/o dip filter Plane 25, w/ dip filter

  11. E W 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…

  12. CMPstack based on seisopt velocity model (visual picking) W E

  13. Optim seismic line 11

  14. 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)

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