| Keywords |
InSAR, subsidence, surface deformation, leveling, surveys, geothermal fields, Coso, Salton Sea, Heber, North Brawley, Imperial Valley |
| Abstract |
Surface vertical deformation rates obtained from InSAR (Interferometric Synthetic Aperture Radar) and ground-based surveys are compared for four geothermal fields in California: Coso, Salton Sea, Heber, and North Brawley. They are also compared with the time series of monthly production and injection provided by the California Geologic Energy Management Division (CalGEM)/California Department of Conservation. The InSAR estimates are obtained by applying the SqueeSAR technique developed by TRE Altamira Inc. to scenes from the Envisat, Sentinel, and TerraSAR-X satellites, covering various non-overlapping time periods between 2003 and 2019. The subsidence data for the Coso geothermal field, provided by the U.S. Navy Geothermal Program Office (GPO), are available from 13 surveys since 1988, four of which have been carried out since 2006 when the satellite coverage began for this area. More than 70 GPS stations have been surveyed over this period. Both the InSAR and the subsidence surveys indicate decreasing subsidence rates with time, along with decreasing production and injection rates. Only two leveling surveys, in 2006 and 2009, were carried out during the Envisat time period for Coso (2006-2010), and only one survey, in 2017, was done during the Sentinel time period (2015-2019). For this reason, comparisons between subsidence surveys and InSAR rates strictly within the periods covered by the satellite data are characterized by significant uncertainty. For Envisat, the average difference between the InSAR and survey rates within 200 m of the GPS stations is 0.7 ± 0.3 mm/year. For Sentinel, the difference is larger, 3.1 ± 0.3 mm/year, but this is due to the decreasing subsidence with time, while the satellite period occupies the second half of the survey period used for comparison. The leveling data for the three other geothermal fields – Salton Sea, North Brawley, and Heber in the Imperial Valley of southern California – are available annually since 1998, 2009, and 1994, respectively. They are provided by the Imperial County Department of Public Works (ICDPW). In addition to Envisat and Sentinel data for all three fields, we also investigated data from the TerraSAR-X satellite (September 2012 – September 2013) over two of the geothermal fields, Salton Sea and North Brawley. When the periods with satellite data and survey measurements closely match, average differences between the two types of measurements are mostly within ±3.5 mm/year, with standard deviations of up to 1.1 mm/year. To our knowledge, this study is the first systematic comparison between vertical deformation rates from InSAR and ground-based surveys in geothermal fields of California. Such comparisons make it possible to evaluate to what extent ground-based and space-based geodetic measurements in geothermal fields agree with each other. This is important because at present, InSAR measurements are available tens of times within a year, while the leveling surveys are conducted at most once per year (Imperial Valley) and once every three or more years (Coso). |