Temporal variations of Vp/Vs-ratio at The Geysers geothermal field, USA

Abstract

A comprehensive database of earthquakes and associated phase arrivals was generated from data acquired by a 34-station seismic geophone network at The Geysers geothermal field, USA, from 2004 to 2011. This database is comprised of several 100,000s events and 1,000,000s of P- and S-wave travel time picks. A high-precision sub-set of earthquake data was selected to analyze temporal changes in Vp/Vs-ratio and seismic P- and S-wave velocities (Vp and Vs, respectively) throughout the entire reservoir. Relatively low Vp/Vs values, found for 2004 and 2005, were followed by an increase of approximately 6% after which the Vp/Vs-ratio remained at the elevated level through 2011. The inception of a pipeline project to resupply the reservoir with water coincided with the first Vp/Vs-ratio measurements in 2004. It was found that the temporal variations in Vp/Vs-ratio reveal a high correlation to the total volume of injected water throughout the entire reservoir. Between 2004 and 2007, the observed correlation exhibits a one-year lag of the Vp/Vs-ratio relative to the injected water volume, while the correlation is near perfect between 2007 and 2011. The end of the observed lag in 2007 coincides with the reported coalescence of injected water plumes. Comparing the observed increase in Vp/Vs-ratio to an earlier study that reported a 9% decrease in Vp/Vs-ratio between 1991 and 1994, it can be concluded that three years after the inception of the pipeline project the fluid saturation appears to have been successfully increased throughout the reservoir.