Using ground-based gravity measurements to monitor changes in terrestrial water storage

Monitoring changes in the Earth’s gravity field for terrestrial water storage change (soil moisture, ground water, snow etc.) is a relatively new and novel concept that has been inspired by the Gravity Recovery And Climate Experiment (GRACE) satellite mission. GRACE will soon be providing time varying maps of the Earth’s gravity. However, there has been no field based study to demonstrate that such a signal can be extracted from either space or ground-based measurements of gravity. This paper seeks to show the potential to monitor changes in terrestrial water storage from measurements of changes in gravity, using a network of soil moisture, groundwater and gravity monitoring sites established in the Murrumbidgee catchment. Moreover, we assess the various factors that might contribute noise to the gravity measurement. As changes in the Earth’s gravity field due to changes in terrestrial water storage are expected to be small, this type of measurement is pushing the accuracy envelope of ground-based gravity measurement to orders of magnitude higher than usual applications (i.e. 1 μ Gal for soil moisture compared to a typical mineral search application of 1000 μ Gal). We use a portable relative gravimeter (Scintrex CG-3M) with quoted accuracy of < 5 μ Gal and reading resolution of 1 μ Gal. A measurement protocol that enables correction for the high drift rate of the CG-3M (≈400 μ Gal/day) involving repeat gravity readings at various sites, readings at a stable reference site, and at the superconducting gravimeter in Canberra is described. Sources of error and uncertainty in this approach are discussed in the context of the accuracy required to reliably estimate changes in the terrestrial water store.