Using 3D Gravity Inversion Modeling to Iteratively Refine 3D Geologic Maps: The Nevada Play-Fairway Project, Southern Gabbs Valley, Nevada

Southern Gabbs Valley in western Nevada emerged as a particularly promising geothermal prospect from Phase 1 research for the Nevada Play Fairway Project, a project focused on regional-scale geothermal exploration in the Great Basin, Nevada, USA. Subsequently in Phase 2, detailed geologic mapping was conducted and high-resolution gravity, magnetic, and magnetotelluric data were collected in order to identify the specific areas that were most likely to host geothermal activity. Six temperature gradient wells were ultimately sited based on these analyses and were drilled in May 2018. Hot (114.4 °C and 124.9 °C) equilibrated temperatures at ~150 m depth and convective gradients were measured in two of these wells, confirming the presence of a blind hydrothermal system in southern Gabbs Valley. Continued exploration of this promising prospect necessitates a detailed understand of the complex 3D architecture of the local fault system and geologic section in order to understand the deeper hydrothermal system. Building upon detailed 2D geologic mapping and geologic cross sections, interpretation of gravity and magnetic anomalies, gravity and magnetic maximum horizontal gradient maps, and 2D profile modeling of gravity and magnetic data, we construct a 3D geologic map of southern Gabbs Valley. 3D gravity inversion modeling is employed to refine the 3D geologic map and to evaluate the 3D map and constrain the ‘uncertainty space’ i.e., the range of 3D geologic interpretations that are permissible given the existing data. As is the case in early exploration in many ‘greenfield’ geothermal prospects, deep subsurface data are absent at southern Gabbs Valley, so this study represents a 3D, data- and hypothesis-driven method for geological characterization and geophysical interpretation during early exploration that can be applied to other blind geothermal prospects. The resulting subsurface map can be used to guide, and be refined by, addition exploration efforts.