Direct measurement of subsurface mass change using the variable baseline gravity gradient method

Abstract Time‐lapse gravity data provide a direct, nondestructive method to monitor mass changes at scales from centimeter to kilometer. But, the effectively infinite spatial sensitivity of gravity measurements can make it difficult to isolate the signal of interest. The variable baseline gravity gradient method, based on the difference of measurements between two gravimeters, is an alternative to the conventional approach of individually modeling all sources of mass and elevation changes. This approach can improve the signal‐to‐noise ratio for many applications by removing the contributions of Earth tides, loading, and other signals that have the same effect on both gravimeters. At the same time, this approach can focus the support volume within a relatively small user‐defined region of the subsurface. The method is demonstrated using paired superconducting gravimeters to make for the first time a large‐scale, noninvasive measurement of infiltration wetting front velocity and change in water content above the wetting front.