Inverse modeling of interbed storage parameters using land subsidence observations, Antelope Valley, California

Kurzfassung

Land subsidence above subsurface reservoirs has been observed in many locations all over the world. The observed surface displacements accompany reservoir pressure changes induced by the resource exploitation or management. In recent years satellite radar interferometry (InSAR) has increasingly complemented or replaced traditional methods for detecting and characterizing the surface displacement field above reservoirs. We have used land-subsidence observations from repeatedly surveyed benchmarks and InSAR in Antelope Valley, Mojave Desert, California to estimate interbed storage parameters controlling the subsidence in a previously calibrated regional ground-water flow and subsidence model (MODFLOW). Using the non-linear parameter estimation program UCODE, we have estimated compaction time constants varying spatially from $3.8$ to $285$ years. A linear estimation of the inelastic skeletal storage coefficients yielded values between $0$ and $0.09$. We found that subsidence observations over long time periods were necessary to constrain the large compaction time constants in Antelope Valley. Because historical subsidence data cannot yet be provided by space-geodetic techniques such as InSAR the time-constant estimates in our model were constrained primarily by observations from benchmarks. Incorporating the resulting parameter estimates into the MODFLOW ground-water flow and subsidence model significantly improved the agreement between simulated and observed subsidence, whereas the model-simulated aquifer heads proved to be very insensitive to our modifications of the interbed storage parameters. Both the accuracy of the estimated parameters and the resulting models' ability to predict subsidence over short time periods is hampered by the fact that the simulated hydraulic heads are often not representative of the actual aquifer hydraulic heads. These errors constitute the primary limitation of the approach presented here.