Mapping Geothermal Permeability Using Passive Seismic Emission Tomography Constrained by Cooperative Inversion of Active Seismic and Electromagnetic Data

Abstract

Preliminary results from a DOE-funded Subsurface Technology and Engineering R&D (SubTER) project to advance imaging and characterization of geothermal permeability are encouraging. Newly acquired electromagnetic data were used to develop 3-D resistivity volumes and dense, passive seismic arrays were used to create energy volumes with passive seismic emission tomography (PSET). These new datasets are being combined with historic drilling, active seismic, and potential fields data to generate 3-D permeability maps. Additionally, cooperative inversion of active seismic data and electromagnetic data is aimed at optimizing their utility for generating robust velocity models and potentially enhancing imaging of geothermal pathways and fluids. Final passive seismic and electromagnetic data collections were completed at two U.S. Geothermal Inc. projects in October 2017. At San Emidio, NV the study area partly overlaps a geothermal resource that currently produces 9 net megawatts per hour. Drilling to the south of the currently exploited geothermal resource and within the SubTER study area has discovered a hotter resource than previously identified anywhere within the San Emidio Valley. Permeability associated with the new resource coincides with energy anomalies defined by PSET and with low resistivity anomalies defined by electromagnetic data while the structural setting of these anomalies is constrained by drilling, gravity, and geophysics. At Crescent Valley a mineralized range-front fault system hosts hot springs that discharge near-boiling water with chalcedony geothermometer temperature of 314F. Energy anomalies defined by PSET and low resistivity anomalies defined by electromagnetic data occur down-dip from and basinward of the hot springs. These anomalies are located where range-front-parallel faulting potentially intersects faulting associated with a NNW-trending horst that is defined by drilling and geophysics. In addition to the range-front-fault-hosted hot springs, geothermal fluid also is associated with the horst underlying the valley floor based on blow outs of historic mineral exploration core holes. Preliminary assessment of integrated datasets suggests that a robust 3-D permeability map can be created using electromagnetic and dense passive seismic datasets when they are combined with drilling and potential fields datasets that help constrain the structural and geological controls on geothermal permeability. Cooperative inversion of seismic and electromagnetic datasets can refine generation of velocity models to optimize processing and interpretation of passive seismic data. The project aims to refine a methodology that will provide a general tool set applicable to geothermal exploration and exploitation.