Updated Shallow Temperature Survey and Resource Evolution for the Coso Geothermal Field

Abstract

The Navy Geothermal Program Office completed a 2 meter temperature probe survey at and in proximity to the Coso geothermal field in 2019. This survey augmented our understanding of the larger shallow temperature anomaly associated with Coso and noted variations over time in the shallow ground temperature within the Coso field. These data were compared to other recently acquired data at Coso including a LiDAR lineation analysis and surface deformation measured with InSAR. A working hypothesis is that shallow temperatures, both directly above and proximal to the Coso geothermal field, have changed since the 1980s as the field has been produced and fluid levels have been drawn down. Specifically, within the Main Flank where the shallowest, longest running production has occurred within the field, the shallow temperature has a decrease in shallow temperature at present, compared to shallow high temperatures prior to production. Furthermore, the results of the survey demonstrate that the subsidence within the Main Flank detected with InSAR using satellite data, seem to coincide with a drop in shallow ground temperature. There may also be an increase in shallow temperature in the East Flank of the field that does not coincide with subsidence or inflation. This type of survey, paired with data sets that the Geothermal Program Office regularly analyzes, including microseismicity, will be collected in multiple phases to fill in data gaps and to achieve the most thorough representation of shallow temperatures within the Coso volcanic and geothermal field. Additionally, two of the largest earthquakes in the U.S. in the last 20 years occurred in early July of 2019 within the valley directly south of Coso, followed by numerous aftershocks. We analyzed the earthquake data recorded by the Coso seismic network for trends in the geothermal field and its surroundings, leading up to the M6.4 and M7.1 events on July 4 and 5, 2019, respectively. An overall deepening of regional seismicity was detected, as well as events at the northwestern end of the M7.1 fault rupture prior to either event. Interestingly, the Coso volcanic field experienced a non-detectable change in microseismicity around these historic events.