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ABSTRACT: Coincident P and Sh Reflections from Basement Rocks at Coso Geothermal Field

Peter E. Malin, M. C. Erskine

In mid-1989 we designed and collected four seismic reflection/refraction profiles that addressed the crustal structure of the Coso geothermal field. The two main east-west and north-south profiles crossed at the southeasternmost base of Sugar Loaf Mountain. Both in-line and cross-line Vibroseis and explosion data were recorded on each of these approximately 12-mi lines. This was accomplished with the simultaneous operation of two 1024-channel sign bit recording systems while four medium-weight P-wave vibrators traversed each line and 16 drilled charges were set off. A single recording system was used on each of the other profiles, one of which was a 4-mi-long, 700-channel P-wave profile connecting the Sierran front to the center of Rose Valley, and the other of which was 2-mi-long, 240-channel SH-wave line running slightly southeastward from the junction of the main lines. Rough CMP stacks of the data that included depth varying velocities at 2-mi lateral intervals were completed in the field. Reprocessing is being done to improve the statics, velocities, signal character, and migrated signal positions.

The main targets of the profiles were (1) exploring the reflection character of the crack/fluid system governing the steam field, (2) examining the Tertiary sedimentary basins in Rose Valley and Coso Wash, (3) testing the structural relationship of the Mesozoic Sierran basement to the Paleozoic sediments of the miogeosycline east of Coso, and (4) probing for significant middle and lower crustal reflector. In the shallow section, reflections that appear related to the doming of the Coso Range can be seen, along with horizons that must be associated with fracture zones and dykes in the basement intrusive rocks. The expected thickness of approximately 1 mi of Neogene sediments in Rose Valley was confirmed, with a possible underlying layer of deformed Paleogene

strata. Coso Wash turns out to contain no less than 3000 ft of (Coso formation?) strata, dipping slightly more than 10° southwestward. At greater depths (20000-30000 ft), several laterally coherent reflections extend across large fractions of the main P-wave profiles. Moreover, assuming a Poissonian P/S-wave velocity ratio, the S-wave profile appears to have at least one major reflector coincident to the P-wave profiles. Based on surface outcrop and dip, this reflector may relate to the westward extension of the sediments to the east. Some reflectors are seen below this depth, but resolution of their details awaits final processing and modeling of the entire data set.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990