Groundwater monitoring and characterization by a vertical dipole of superconducting gravimeters in a karst aquifer, France

Paper in proceeding, 2021

The Earth's mass distribution is continuously changing due to physical processes taking place either beneath the subsurface or on the surface. Some of the primary sources for these mass variations are tides in the ocean and solid Earth, atmospheric disturbances and seasonal climate changes. The development of Superconducting Gravimeters (SGs) has made it possible to characterize and monitor such mass variations at micro scales. Our study focuses on the LSBB karst catchment's hydrodynamics using a unique configuration of two SGs located 520 m depth apart. The installation of a SG (iGrav-31) at the surface of the LSBB several years after the installation of the first (iOSG-24) inside the tunnel has provided several new insights into the understanding of hydrological processes occurring in the LSBB.

In this work, we compare differential and residual gravity time-series together with the ERA5 global hydrological loading model. In the subsequent section, we implement a rectangular prism method to compute forward gravity responses using input from the hydrological model. We also numerically evaluate and validate the already published hypotheses (Mouyen et al., 2019) about uncertainties related to groundwater storage's location in this catchment. Based on our observations, we find that most water-storage changes occur in the unsaturated karst zone between both SGs. The misfit between the residual gravity time-series and the local hydro-gravity effect computed from ERA5 model shows large lateral fluxes and rapid runoff occurring in the LSBB. Finally, we conclude this work by examining the radial and depth sensitivity of water masses' effect near the SGs, and enlisting some recommendations for further studies.