Deformation at Brady Hot Springs (Nevada) geothermal field measured by time series analysis of InSAR data

Abstract:

We analyze interferometric synthetic aperture radar (InSAR) data acquired between 1997 and 2014 (by the ERS, Envisat, ALOS and TerraSAR-X/TanDEM-X satellite missions) to measure and characterize time-dependent deformation at the Brady Hot Springs geothermal field in Western Nevada due to net extraction of fluids. The long axis of the ~4 km by ~1.5 km oval shaped subsiding region coincides with the strike of the predominant normal fault system at Brady. Within this bowl of subsidence, the interference pattern shows several smaller features with length scales of the order of ~1 km. These smaller features are spatially associated with the intersections and overlaps of some of the mapped fault segments. This type of signature occurs consistently in all of the well-correlated interferometric pairs spanning several months. To model the deformation, we explore several different observable quantities, including the spatial derivative of the range change (dimensionless), and the (unwrapped) range change (in mm). The results from inverse modeling suggest that the deformation is a result of compaction associated with a decline in pore-fluid pressure. This phenomenon occurs in shallow lithologic units and/or highly damaged regions where fault segments mechanically interact. Such damaged zones are expected to be vertically extensive along the faults, providing a high permeability conduit to the deep reservoir tapped by production wells. Using time series analysis, we test the hypothesis that changes in the net rate of geothermal production drive the observed deformation.