NEW SEISMIC IMAGING OF THE COSO GEOTHERMAL FIELD, EASTERN CALIFORNIA

Abstract

New multifold seismic reflection data from the central Coso Range, eastern California, image brittle faults and other structures in Mesozoic crystalline rocks that host a producing geothermal field. The reflection data were processed in two steps that incorporate new seismic imaging methods: (1) Pwave first arrivals in the seismic data were inverted for subsurface acoustic velocities using a non-linear simulated annealing approach; and (2) 2-D Velocity tomograms obtained from the inversions were employed in pre-stack Kirchhoff migration to produce accurate, depth-migrated images of subsurface structure. Three-dimensional visualizations of the velocity structure show that discrete low velocity zones are associated with the two major producing areas of the Coso field, and that the producing zones are separated by a ìridgeî of relatively higher velocity rock. The low velocity zones are interpreted to be zones of pervasive fracturing and/or hydrothermal alteration associated with localized hydrothermal circulation. The presence of the intervening ìridgeî of high velocity rock, which presumably is relatively intact and unaltered bedrock, may indicate long-term compartmentalization of flow in the field. Kirchhoff depth-migrated seismic images reveal substantial coherent reflectivity in the upper 6 to 8 km of the crust. The Coso Wash fault, a Quaternary-active fault that is a locus of surface geothermal activity, is well-imaged as a moderately dipping reflector that terminates against a reflector or reflective zone at approximately 4 km depth. The 4 km reflective zone can be traced laterally beneath the geothermal field, and it may represent the brittle-ductile transition. The Kirchhoff images also reveal a prominent highamplitude reflector at 6 km depth directly beneath the geothermal field. The reflector exhibits slight antiformal geometry and appears to be restricted to the northern part of the producing zone. The 6 km reflector tentatively is interpreted to be a pocket of magmatic brine or partial melt.