The Santa Clara Valley is a sedimentary basin whose stratigraphy forms a series of interbedded aquifers and aquitards. The primary confined aquifer is located at a depth interval of 60-300 m and consists of unconsolidated alluvium over the semi-consolidated alluvium of the plio-pleistocene Santa Clara formation. Consolidated bedrock of the Franciscan complex is found at greater depths.
Land uplift and subsidence in the valley due to the recharge and withdrawal of fluids is well documented by several public agencies such as the Santa Clara Valley Water District (SCVWD) and the USGS. An increase in the withdrawal of water from the aquifer and a decrease in rainfall for the first half of the twentieth century resulted in a substantial drop in well levels and a corresponding land subsidence of ~4 m. Recovery efforts over the past quarter century, such as the import of water from outside sources and the construction of percolation ponds, have allowed water levels to partially recover. Preservation of the aquifer requires the continual monitoring through extensometers, well water levels, and level line observations. Most of the initial subsidence was due to inelastic, unrecoverable compaction of fine silts and clay layers. Some elastic rebound can be expected with the recovery of water levels.
InSAR is an attractive method for monitoring land subsidence because of its spatial coverage and precision. However, individual interferograms are often difficult to interpret because the amount of deformation that is observed is highly dependent on the time of the season that the first and second SAR scenes are acquired. We have process over 150 interferograms for this region spanning the period from 1992 to 2000. To help synthesize the information contained in the data set of interferograms, we have developed a time-series methodology. A linear inversion solves for the incremental deformation between synthetic aperture radar (SAR) scene acquisitions. The resulting time series has a temporal resolution on the order of months. A nonlinear range-change signal is extracted from the ERS InSAR data without imposing a model of the expected deformation. The time-series methodology facilitates the spatial and temporal interpretation of deformation in regions where the deformation is complex, such as the superposition of tectonic and land subsidence signals. Over an eight-year period, ~4 cm of land uplift is observed due to recharge of the confined aquifer.
|Tools||InSAR, leveling, well water depths, extensometers|
|Geographic Location||Santa Clara Valley, California|
|Group Members Involved||David Schmidt, Roland Bürgmann|