Incorporation of unsaturated zone effects in coupled hydrogeophysical modelling of gravity anomalies caused by pumping tests

Gravity is a promising tool for the monitoring of water storage changes on the near surface. A direct application is the use of gravity data for monitoring of pumping tests in unconfined aquifers. Previous authors have proposed solutions for the gravity anomaly caused by water table drawdown. Many of them are based on analytical solutions that would find limitations in real conditions. In preceding works, we presented two numerical solutions using a finite element model (FEM) for the simultaneous and coupled solving of gravity changes caused by water pumping in unconfined aquifers. Both models were developed under the assumptions of instantaneous and complete drainage when water table drops. However, recent works have shown the importance of accounting for the unsaturated zone in the interpretation of pumping tests from unconfined aquifers. As a result, we have extended our previous models to account for variable saturation in the coupled hydrogravimetrical model. We present an application to a real test well known in the literature for which we simulate the expected gravity anomaly when the unsaturated zone is accounted for. We observe that, even for this example with very low pumping rates, the measurable anomaly would be above detectability limits of current instrumentation.