Three Dimensional Seismic Tomography at The Geysers Geothermal Field, CA, USA

Abstract

Three-dimensional Vp/Vs-ratio structure is presented for The Geysers geothermal field using seismic travel-time data. The data were recorded by the Lawrence Berkeley National Laboratory (LBNL) using a 34-station seismic network. Results for the year 2011 indicate low Vp-Vs-ratios in the central section of The Geysers within and below the current reservoir. The extent of the Vp/Vs anomaly deceases with increasing depth. Spatial correlation with micro-seismicity, used as a proxy for subsurface water flow, indicates the following. Swarms of seismicity correlate well with areas of higher Vp/Vs estimates, while fewer events are located in region of low Vp/Vs estimates. This result supports past observations that high and low Vp/Vs-ratios are related to water and gas saturated zones, respectively. Furthermore, it is observed that Vp/Vs is inversely correlated to Vs but uncorrelated to Vp, leading support to laboratory measurements on rock samples from The Geysers that observe an increase in shear modulus while the core samples are dried out.