GPS AND INSAR REVEAL HETEROGENEOUS AND ANISOTROPIC AQUIFER CONDITIONS DURING AN AQUIFER TEST

A 62-day cyclical aquifer test conducted in Mesquite, Nevada, in thick highly compressible alluvial deposits was monitored for surface deformations using high-precision GPS receivers at various radii from the pumping well. Of particular interest were the obvious radial and tangential horizontal components of strain and deformation. An eigenvalue analysis of these deformation signals implied an anisotropic aquifer system with a 3:1 strain ratio and the major axis of the hydraulic conductivity ellipsoid in an east northeast direction. Numerical modeling of flow and deformation appeared to confirm the anisotropic behavior of the aquifer system as both simulated deformation and drawdown closely matched the observed water levels at the pumped well and the temporal deformation responses at each GPS receiver station. However, one anomaly that could not be adequately explained was the observed motion at the pumped well. The physics of aquifer response due to pumping inherently implies that the pumping well should remain stationary in an infinite aquifer, even with an anisotropic aquifer. Recently, InSAR data for the site over the period spanning the aquifer test became available. The interferogram indicates that vertical deformation is not centered at the pumped well but is skewed toward the south, in the same direction as the tangential motion observed with the GPS data. Furthermore, the vertical deformation appears to be truncated just north of the pumped well and may indicate the presence of a buried fault that terminates mechanical deformation. To better understand the hydromechanical processes influencing the observed deformations, ABAQUS software is being used to simulate the potentially complex mechanical boundaries at the site that include faults, fractures and other discontinuities. Clearly, InSAR has provided a clearer understanding of potential geological controls that were not evident from GPS alone.