APPLICATION OF POTENTIAL FIELD GEOPHYSICAL DATA TO STUDY THE GEOTHERMAL RESOURCES IN THE WIND RIVER VALLEY, WA

Geophysical investigations were conducted in the Wind River Valley (WRV) in south-central Washington as part of Phase II activities for the Department of Energy’s Geothermal Technologies Office geothermal play fairway analysis project in the Washington Cascades. For this effort, a suite of 604 new high precision gravity stations and four new ground-magnetic transects were collected within and adjacent to the WRV. These data were combined with sparse pre-existing gravity data and older ground- and aero-magnetic data to further enhance the geophysical dataset in preparation for 2¾-D forward modeling. Our modeling employed the use of the gravity and magnetic data, that highlight contrasts in density and magnetic properties (susceptibility and magnetic remanence), respectively, in conjunction with detailed geologic mapping and rock property data to map and model structural and lithologic discontinuities that may be important to geothermal fluid flow.

2¾-D modeled cross sections and detailed analysis of gridded gravity and magnetic data, employing the identification of maximum horizontal gradients and matched filtering, reveal structural features that were previously unknown and help resolve other features that were only obscurely identified in existing mapping due to little or no surface expressions. The intersection of the NW-striking Wind River fault and NE-striking Shipherd fault zone is well defined in the potential field data, as characterized by linear trends of maximum horizontal gradients, and coincides with the location of St. Martin Hot Spring (and source for Carson Hot Spring Resort). The data also reveals that WRV is bound to the NE and SW by subparallel structures. These structures and the trend of WRV mark the boundary of a major crustal discontinuity that separates low density/high magnetization rocks to the SW from high density/low magnetization rocks to the NE. Furthermore, the gravity data also reveals that the Shipherd fault zone marks the SE boundary (NE-striking) of this major crustal discontinuity, dividing low density rock to the SE from higher density rocks to the NW. The newly acquired potential field data in the WRV has aided in better constraining the subsurface geology and location of major structures, which are key in identifying potential locations of enhanced geothermal fluid flow.