Spatiotemporal variations of in situ V/V ratio within the Salton Sea Geothermal Field, southern California

The Salton Sea Geothermal Field is located on the southeastern shore of Salton Sea and has been facilitated with many geothermal units in the past few decades. In this study, I investigate possible effects of the long-term production process on seismic parameters by using a high-resolution approach to estimate the near-source compressional- to shear-wave velocity ratios (Vp/Vs). I apply this method to similar earthquake clusters based on waveform cross-correlation data and obtain robust results for six event clusters in the study area. The resulting in situ Vp/Vs ratios vary from 1.510 to 1.811 with the calculated uncertainties below 0.02. I examine possible relations between these Vp/Vs ratios and other types of data. The observed spatial correlation with the heat flow and temporal correlation with well production data indicate that the near-source Vp/Vs ratios strongly depend on the subsurface natural properties, but are influenced by the amount of extraction and injection volume throughout the operational period. This study suggests that the in situ estimation method can be used to detect changes in near-source Vp/Vs ratios caused by exploitation of geothermal areas and is a potentially valuable tool for geothermal resource monitoring.