Simulated gravitational response to hydraulic testing of unconfined aquifers

The gravitational response to hydraulic testing of shallow unconfined aquifers (<50 m) has been simulated to assess whether gravity surveying can be used as a practical tool to detect groundwater extraction. A general expression is derived to calculate the response based on the gravitational attraction of the drawdown cone. The expression can accommodate any radially symmetric drawdown solution that assumes instantaneous release of groundwater above the moving water table as exemplified by Neumanʹs solution. Based on simulations, hydraulic testing of shallow unconfined aquifers with typical properties produces gravitational responses on the order of several tens of microGals (1 μGal=1×10−8 m/s2) at the pumping well. The response at the 1 μGal level can extend hundreds of meters away from the pumping well. At a distance of 1 m, survey resolutions between 0.002 to 0.15 and 0.03 to 2 μGal are required to delineate incremental drawdown at the 0.02- and 0.3-m levels, respectively, below the point of measurement.