SEISMIC IMAGING OF ALLUVIAL FANS

Alluvial fans contain a wealth of information about the tectonic history of western North America. The University of Kansas has acquired both shallow seismic reflection and GPR data across the active Emigrant Peak fault on the east side of Fish Lake Valley, Nevada, and shallow seismic data has been acquired in a similar, but more challenging environment in Queen Valley in California The faults that bound these basins are part of a system of faults that control 25% of the strain between the North American plate and the Pacific Plate. The active fault in Fish Lake Valley that controls a great deal of this movement is assumed to be a normal fault that aids in the transfer of regional right-lateral deformation associated with the Death Valley/Fish Lake Valley fault zone.. Two 2-D seismic lines in Fish Lake Valley were acquired with a depth penetration of approximately 200m using a 30.06 caliber rifle source. The main line in Fish Lake Valley was over 400m in length and the cross line over 150m in length. CMP bins were 0.25m in size. Similar parameters were utilized in Queen Valley, but data acquisition was limited, due to field conditions. Data were processed to migrated images and imported into an industry-standard reflection interpretation package. The seismic images contain numerous reflections, grouped in packages of short reflectors of different amplitudes and dip orientations.. The preliminary analyses indicated that the seismic data interpretations simply support the surface mapping data, however this may now be in question. It is now probably appropriate to look at a theory that both compression as well as extension events have been recorded by the seismic data that occurred during the deposition of the upper 200m of sediment in these areas...More work needs to be done to resolve whether this is a real geological issue or a seismic processing issue. Also, field work on large alluvial fans is rather inefficient when using a down-hole gun. The presence of large animal burrows and boulders are problematic. So, use of a land streamer, and weight drop or vibrator on roads would probably be more appropriate, if resolution can be maintained.