Geodetic Measurements and Numerical Models of Deformation at Coso Geothermal Field, California, USA

We measure transient deformation at the Coso geothermal field in south-central California using interferometric synthetic aperture data acquired between 2004 and 2016 by the Envisat and Sentinel-1A satellite missions. All well-correlated interferometric pairs show subsidence, with rates as high as 30 mm/year, over a large 75 km2 circular area surrounding the field below which most of the seismicity associated with geothermal production is located. The deformation signature remains in the same location throughout the 12 year interval. Time-series analysis of multiple interferometric pairs reveals continuous subsidence. A decrease in the subsidence rate after 2010 corresponds to a decrease in the net production rate. Using three-dimensional, fully numerical, multiphysics models, we explore the coupling between deformation and geothermal production. We seek to distinguish between two possible mechanisms: (i) decreasing pore-pressure following net extraction of fluids, or (ii) decrease in temperature of presumably fractured reservoir rock. Irrespective of the mechanism, a contracting ellipsoidal reservoir located at a depth of 2 km, with a volume of 80 km3 or less is required to explain the geodetic observations. Almost 90% of the seismicity beneath the field occurs within this 80 km3 ellipsoid.