Simulating the gravity change caused by groundwater-storage change

Gravity changes, measurable with great precision, are readily simulated from spatially-discretized hydrological models through cell-by-cell calculation of the gravitational effect of the mass change in each model cell. This makes such measurements attractive for model calibration. New Fortran software, Heavy, models the forward-model response for a user-supplied MODFLOW simulation of transient groundwater flow. Thus, Heavy is a tool both for real-world model calibration with field data, and for testing the value and sensitivity of repeat microgravity measurements for model calibration. Heavy post-processes the head output file written by MODFLOW using the input/output routines from MODFLOW. It is orders of magnitude faster than previous efforts using interpreted languages such as Python or MATLAB. This presentation provides an overview of the software, demonstrates simulated gravity from real-world MODFLOW examples, and tests the accuracy of the widely-used linear Bouguer-slab approximation (i.e., 1 m of water = 42 µGal of gravity change) relative to calculating gravity change from the 3-dimensional spatially-distributed storage changes simulated by MODFLOW.