Toward Constraining Major Fault Zone and Heat Source Structures Beneath the Mineral Mountains and Roosevelt Hot Springs Area, Utah

The Utah Frontier Observatory for Research in Geothermal Energy (FORGE) was selected in 2018 by the U.S. Department of Energy to explore the technological feasibility of creating and maintaining an Enhanced Geothermal System (EGS) reservoir in nominally hot, dry rock. Phase 2b of FORGE involved the acquisition of a number of geophysical datasets directed at the intended drilling target and for more regional-scale heat sources in the extensional Great Basin province. These data include 122 wideband magnetotelluric (MT) stations deployed in areal coverage merged with existing sites over the adjacent Mineral Mountains (MMs) to the east. Preliminary inversion of these data using the HexMT 3D finite element algorithm revealed a number of conductive lineaments and bodies, dominantly trending slightly west of north in the upper several kilometers of the MMs, corresponding to steep mapped fracture sets in the area. A deeper anomaly under the MMs appears to project toward and reach the Roosevelt Hot Springs hydrothermal reservoir powering the PacifiCorp Blundell geothermal power plant. Though subtle and undergoing further testing, the body may be associated with the recent (0.5-0.8 Ma) eruptive centers of the MMs. Constraints on its properties would assist in resolving heat sources for the Roosevelt hydrothermal system and the FORGE site. Using HexMT, we test two categories of permissible geometries of this body through a set of synthetic studies: 1., horizontal and planar, consistent with laccolith and sill-type geometries; and 2., nearly equant ellipsoidal, consistent with small plutonic geometries. For a specified 10% error floor on the impedance elements, the ability to distinguish anomalous geometries within a surrounding halfspace is being explored along with anomaly volumes, aspects, and magnitude. For each geometry, forward data misfit with existing data will be reported. This will meet an additional goal to bound the range of geometries consistent with the MT data.