3-D Inversion of MT Data from the Raft River Geothermal Field--Preliminary Results

Abstract

Geothermal systems are, in many cases, characterized by geoelectric structures with discernible anomalous zones, related to the presence of fluids and conductive alteration minerals. The MT method has been successfully used since the 1970’s to image subsurface electrical resistivity, from which in turn can be delineated structural controls and system characteristics such as the clay cap and fluid distributions. Toward this purpose, a 3-D magnetotelluric (MT) survey of 105 stations was carried out at the Raft River Geothermal area in southern Idaho, to further characterize the field generally, fault distributions, and paths of or barriers to fluid flow. This work was done in support of developing the EGS potential of the field, as part of a Department of Energy-funded project. An electrical resistivity model of the area was constructed from the MT data using a 3-D inversion program on a multicore workstation. The model is interpreted using available lithological logs and geochemical data. Discernible in the resistivity structure are separate low-resistivity zones, attributed to increased fluids and clay alteration, within the basement from which the geothermal field produces, and within the overlying sediments, proximal to the wells. The inferred compartmentalization of the field is similar in nature to that inferred from fluid geochemistry analysis. A much deeper conductor with possible lower crustal sources is seen also which appears to correlate with a similar feature in the regional Earthscope MT project.