Hydrology and Noise Affecting Land-Based Gravity Measurements

To observe secular land movements of the order of a few millimetres per year, a very precise instrument with long-term stability is required. This can be achieved using absolute gravimeters. Repeated absolute gravity (AG) measurements have now been performed at several sites to estimate tectonic deformation. It is often assumed that these time series contain only white noise. However, many geodetic data sets suffer from large temporal correlations into the data. One common statistical model for many types of geophysical signal (which may contribute to the noise) maybe described as a power-law process. Accounting for the type of noise is not only very important when estimating gravity variations and their uncertainties, but also to mitigate this noise. Generally speaking AG measurements still suffer from a sparse spatial and temporal resolution, and are not yet available in data centres. To better understand the structure of the noise affecting ground- based gravity measurement at different locations, we take profit of 19 superconducting gravimeter (SG) continuous time series provided by the GGP data centre. Presently, the quality of hydrological models is not sufficient to correct the ground-based gravity time series, but it is possible to study the spectral behavior of the hydrological effects down to the decadal time scale. Such study allows one to test whether the colored noise of the gravity signal can be attributed to hydrological causes. The spectra of the SG time series are compared with the predicted gravity effect calculated using water storage estimates from the Milly and Shmakin Land-Energy balance (LaD) model (1° X 1°, monthly). For most of the GGP stations, we evidence a good agreement for periods longer than 100 days, though they are located in quite different climatic regimes.