Passive Seismic Emission Tomography Applied to Mapping of Geothermal Permeability

摘要

The utility of passive seismic emission tomography for mapping geothermal permeability has been tested at two locations in northern Nevada, San Emidio and Crescent Valley. The San Emidio study area overlaps a geothermal field in production since 1987 and also a new, hotter resource recently drilled to the south of the production field. In contrast, the subsurface at the Crescent Valley study area is poorly constrained by drilling with geothermal potential mainly indicated by geofhermometry of hot spring discharges, shallow, high heat flow, and reports of blown out mineral exploration wells. Newly acquired magnetotelluric data and passive seismic data collected with tightly spaced geophone arrays are combined with historic drilling, active seismic, and potential fields data to generate 3-D permeability maps. A cooperative inversion methodology has been developed using active seismic, magnetotelluric, and gravity data in order to produce more robust velocity models for passive seismic data processing without requiring expensive 3-D active seismic surveys. The cooperative inversion estimates velocities from other geophysical data where no prior seismic velocity information is available. A robust 3-D permeability map can be created using magnetotelluric 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 magnetotelluric datasets provides more robust and location-specific velocity models that optimize processing and interpretation of passive seismic data while also increasing the utility of magnetotelluric data for identifying structures that control geothermal permeability. Ongoing work will compare and contrast the passive seismic emission tomography results from simple versus refined velocity models.