APPLICATIONS OF RADAR INTERFEROMETRY TO DETECT SURFACE DEFORMATION IN GEOTHERMAL AREAS OF IMPERIAL VALLEY IN SOUTHERN CALIFORNIA

摘要

InSAR (interferometric synthetic aperture radar) is applied in Imperial Valley of southern California to detect and characterize surface deformation in existing geothermal fields, possible future geothermal developments, and around faults. The data used are from the Envisat satellite, collected over the period 2003-2010. The specific InSAR technique applied, SqueeSAR, identifies permanent and distributed scatterers (PS and DS), which play the role of numerous benchmarks throughout the study area. Deformation time series are obtained at thousands of individual scatterer locations. Their slopes represent the annual deformation rates. The technique is particularly suitable for vegetated and rural areas, thus providing results from the agricultural lands of Imperial Valley, where conventional InSAR methods have not worked before. The SqueeSAR results are first obtained in the line-of-sight (LOS) to the satellite. Using measurements from two geometries, ascending and descending, makes it possible to decompose the two LOS movements into vertical and horizontal displacements. Significant subsidence is observed at all geothermal fields operating during the period of satellite data (2003-2010). These include the CalEnergy units of the Salton Sea geothermal field, and Ormat's Heber and East Mesa geothermal fields. At Heber, uplift is also detected in an area adjacent to the subsidence. The results from SqueeSAR are in agreement with the ground-based measurements from the annual leveling surveys carried out at both Salton Sea and Heber. Regional GPS data and relocated earthquakes are also used to inform our analysis results. Furthermore, baseline surface deformation is detected at sites, where production either started after, or at the end of the period covered by satellite data. These are Hudson Ranch - I at the Salton Sea geothermal field (operated by Energy Source since early 2012) and North Brawley (operated by Ormat since 2010). The baseline at sites of geothermal potential is also determined, such as areas of interest to the U.S. Navy Geothermal Program Office and Orita (formerly East Brawley). In addition, the SqueeSAR measurements reveal differential movements on both sides of faults, clearly marking the Superstition Hills and Imperial faults, as well as parts of the San Andreas fault. The surface displacements are attributed to the ongoing regional extension due to the relative movements of the North American and Pacific plates, as well as to localized tectonic deformation associated with fault networks, pull-apart basins, and rotational blocks. Finally, we observe effects caused by an October 2006 aseismic event on the Superstition Hills fault, and by a M7.2 earthquake that occurred south of the U.S. - Mexico border in April 2010. We conclude that InSAR provides unprecedented information on surface deformation in Imperial Valley, as long as suitable techniques, such as SqueeSAR, are used to tease out signals amidst the extensive agriculture. Such observations can be effectively used for pre-production reservoir assessment, feedback to mitigate any environmental impact that might occur at operating fields, and exploration efforts to identify suitable drilling targets.