| Abstract: |
As part of a DOE project, we have applied satellite radar interferometry (InSAR) to detect surface deformation in the San Emidio geothermal field, Nevada. The specific method used, SqueeSARTM, is the latest innovation in the field of InSAR, which allows obtaining deformation time series at locations of permanent and distributed scatterers (PS and DS, respectively). The PS points are relatively small objects such as buildings, wellheads, boulders, etc., which remain coherent from one satellite scene to another. The DS are coherent areas covering several pixels emitting weaker signals than the PS, but still have acceptable signal-to-noise ratios. The PS and DS play the role of numerous benchmarks, at which surface deformation rates can be determined. Three sets of radar data were used covering an area of 60 km2. The datasets included imagery from the European ERS-1, ERS-2, and Envisat satellites. They consisted of 38 ERS-1/2 scenes acquired from descending orbits (satellite moving north to south) during the period May 1992 – January 2001, 53 descending Envisat images collected between June 2004 and April 2010, and 45 ascending (satellite moving south to north) Envisat scenes from the period October 2003 – June 2010. Due to the desert environment of the study area, numerous PS and DS were identified - more than 180,000 from the ERS descending, 212,000 from the Envisat descending, and 166,000 from the Envisat ascending scenes. Surface deformation at the individual PS and DS locations is first determined in the line-of-sight (LOS) direction to the satellite; i.e., in terms of movement towards or away from it. The availability of Envisat data from two orbit geometries (descending and ascending) makes it possible to decompose the LOS deformation into vertical displacements and movements in the west-east horizontal direction. Due to the steep look angle of the satellites (~21o to 22o), the vertical and the LOS movements are very similar. However, the west-east horizontal component of deformation is only revealed by the decomposition procedure. The LOS time series were used to derive surface deformation rates at all individual PS and DS locations, while vertical and west-east horizontal rates were extracted from the combinations of descending and ascending LOS rates. Distinct areas of subsidence and uplift are outlined, with changing spatial patterns in time, especially from the older ERS data set to the newer Envisat data. We observe surface deformation in the area of the production wells, and profiles transecting faults indicate distinct signals, likely resulting from hydrological control. Many of the deformation time series exhibit obvious seasonal patterns, with varying amplitudes throughout the study area. We continue to examine these uniquely rich results for clues to describe the geothermal resource at San Emidio. |