ANALYSIS OF GRAVITY AND MAGNETOTELLURIC DATA FROM THE COSO GEOTHERMAL FIELD, SOUTHEAST CALIFORNIA

The Coso geothermal field is one of the major geothermal energy producing fields in the United States and is located within Coso Range area in the southeast-central California between the Sierra Nevada and the Basin and Range province. The Coso geothermal field is found in a region that with recent volcanism that began about 4 Ma and entered a bimodal period about 1Ma. The young volcanic rocks and hot springs found in this region indicate the presence of a shallow heat source that has been proven by drill holes that tap the shallow high temperature fluids used for electricity production. Although many geophysical and geochemical studies have helped to define the geothermal reservoir, and led to successful drilling, the location and size of the source of heat is controversial. We reevaluate and integrate the existing geophysical data to characterize the reservoir and the heat source using different approaches than the previous studies. In this study, we derive and interpret 3-D density models from gravity data and 2-D resistivity models from MT data previously collected in the Coso geothermal field area. Our data show zones of both low resistivity and low density at about 6 km depth in the Devils Kitchen and the Coso Hot Springs areas. We interpret these zones to indicate the presence of cooling magmatic material that provides the heat for the geothermal system for the Coso geothermal plant. A zone marked by high resistivity and low density is implied to lie directly above the interpreted magma body extending to within 1 km depth below the surface in the reservoir region where it is capped by a low resistivity clay zone. In addition, density models show the high density bodies are correlated with volcanic peaks or mountains which may imply a region of dense mafic intrusions or dikes.