Geodetic Measurements and Numerical Modeling of Deformation at Raft River Geothermal Field, Idaho, U.S.A.

Abstract:

To measure time-dependent deformation at the Raft River geothermal field in Cassia County in Southwestern Idaho, we analyze interferometric synthetic aperture radar (InSAR) data acquired between 2006 and 2015 by several satellite missions, including: Envisat, ALOS, TerraSAR-X, and TanDEM-X. The resulting time-series analysis indicates that the deformation began in late 2007, shortly after a 13-megawatt geothermal power plant began commercial production. The rate of deformation appears to be decreasing over time since 2008. The resulting maps of deformation show primarily uplift with some subsidence. The uplift signal is located in an ~8-km-by-5-km area centered near three injection wells that recycle produced brine into the Salt Lake formation, which consists of Miocene-Pliocene lacustrine deposits, volcanic tuffs, and lava flows. Subsidence occurs in an adjacent ~4-km-by-4-km area to the northwest. These two signatures remain in the same location in all of the well-correlated interferometric pairs since 2008. Although all production wells are also located inside the area experiencing uplift, most of them are close to the boundary that separates the two areas, and likely associated with the steeply dipping Bridge Fault zone. We explore the relative roles of thermal (T), and hydrological (H) processes on mechanical deformation (M). To do so, we use finite element based numerical models to calculate the time-dependent deformation field due to thermal contraction/expansion of rock (T-M coupling), and changes in pore pressure (H-M coupling).