Electromagnetic Soundings for Geothermal Resources in Dixie Valley, Nevada

Abstract

An electromagnetic (EM) sounding survey was performed over a region encompassing the Dixie Valley geothermal field to map the subsurface resistivity in the geothermal field and surroundings area. The EM survey, consisting of 19 frequency domain depth soundings made with the LBL EM-60 system, was undertaken to explore a narrow region adjacent to the Stillwater Range to a depth of 2 to 3 km. Lithologic and well log resistivity information from well 66-21 show that EM interpretation the section can be reduced to a three layer model consisting of moderately resistive alluvial sediments, low resistivity lacrustrine sediments, and high resistivity Tertiary volcanics and older rocks. This three layer model was used as a starting point in interpreting EM sounding data. Variations in resistivity and thickness provided structural information and clues to the accumulation of geothermal fluids. The interpreted soundings reveal a 1 to 1.5 km deep low resistivity zone spatially associated with the geothermal field. The shallow depth suggests that the zone detected is either fluid leakage or hydrothermal alteration, rather than high temperature reservoir fluids. The position of the low resistivity zone also conforms to changes in depth to the high resistivity basal layer, suggesting that faulting is a control on the location of productive intervals.