IMPROVED UNDERSTANDING OF DEFORMING AQUIFER SYSTEMS AND GROUND-WATER FLOW USING INSAR AND GPS

GPS provides accurate and updated geodetic control for hydraulic heads measured in wells, and for land surface measured at geodetic monuments over aquifer systems. Repeat GPS surveys and interferometric synthetic aperture radar (InSAR), especially when used with other hydrogeologic information, improve mapping, monitoring and modeling of crustal motion and ground-water flow accompanying the deformation of aquifer systems. This deformation can be described and interpreted using poroelastic theory relating the coupled hydraulic and mechanical responses of aquifer systems to anthropogenic (e.g. pumping) and natural (e.g. climatic) stresses. Spatial and temporal measurements of ground displacements over deforming aquifer systems are used to 1) monitor recoverable land subsidence and uplift and permanent subsidence; 2) define structural and stratigraphic controls on deformation and (or) flow; 3) map the distribution of compressible sediments; 4) estimate aquifer-system hydraulic properties (e.g. S_s and K_v); and 5) constrain models of ground-water flow and aquifer-system deformation.

The principal advantages of InSAR include high spatial detail (30-90 m pixels) and extent (100 km swaths), and high measurement resolution (5-10 mm in the satellite line-of-sight). These advantages facilitate InSAR to improve upon most of the uses listed above using GPS alone. The chief disadvantages with InSAR currently are 1) the limited temporal coverage (1992 to present with a repeat orbit cycle of 35 days for the European Space Agency satellites though not every orbit results in SAR images suitable for interferometry); 2) temporal decorrelation of the signal from changing atmospheric and ground-cover conditions; and 3) the inability to resolve component horizontal displacements. The principal advantage of GPS is the capability to measure real-time 3-D component displacements, though it is limited to a single site for continuous GPS or a small network of sites for individual campaigns.