InSAR is a powerful technique that uses radar data acquired at different times to measure land-surface deformation, or displacement, over large areas at a high level of spatial detail and a high degree of measurement resolution. InSAR displacement maps (interferograms), in conjunction with other hydrogeologic data, have been used to determine aquifer-system characteristics for areas where surface deformation is the result of stress induced changes in the granular skeleton of the aquifer system. Interferograms and measurements of aquifer-system compaction from borehole extensometers, and ground-water levels in wells in Santa Clara Valley, California, have shown that land-surface changes caused by aquifer-system deformation for September 23, 1992-August 2, 1997, are elastic (reversible): During the summer when water levels are declining, the land surface subsides, and during the winter when water levels are recovering, the land surface uplifts, resulting in no net surface deformation. Interferograms used with fault maps of Santa Clara Valley and of Las Vegas Valley, Nevada, have shown that the extent of regional land-surface changes caused by aquifer-system deformation may be partially controlled by faults. Interferograms of Yucca Flat, Nevada, show subsidence associated with the recovery of elevated hydraulic heads caused by underground weapons testing at depths of more than 600 meters.
For these selected case studies, continuing or renewed deformation of the aquifer system is coupled with pore-fluid-pressure changes. When applied stresses (water-level changes) can be measured accurately for periods that the interferograms show displacement, stress-strain relations, and thus bulk storage properties, can be evaluated. For areas where additional ground-water-level, land-surface-elevation, aquifer-system-compaction, or other environmental data are needed, the interferograms can be used as a guide for designing appropriate monitoring networks. Aquifer-system properties derived from stress-strain relations and identification of hidden faults, other structural or stratigraphic controls on deformation and ground-water flow, and other hydrogeologic boundaries in the flow system can be used to constrain numerical ground-water flow and subsidence simulations. Managing aquifer systems within optimal limits may be possible if regions susceptible to ground-water depletion and the accompanying land subsidence can be identified and characterized.