Monitoring aquifers and subsidence with the chronometric leveling

Subsidence poses a significant global threat to the viability and economic development of approximately one-fifth of the world's population, especially in coastal or heavily urbanized regions experiencing ground settlements ranging from centimeters per month to meters per decade. This phenomenon, driven by natural or anthropogenic factors, is intricately linked to hydrology, geology, tectonics, and the geotechnical properties of underlying formations. Multiple triggers, including underground cave collapses, organic soil oxidation, natural gas and oil extraction, and aquifer consolidation, contribute to subsidence, disrupting vital water reserves crucial for societal needs. Spatial and temporal limitations of conventional techniques like gravimetry, positioning, and spatial imaging in resolving the effects of extreme weather events and resource exploitation on subsidence prompt an exploration of their complementarity and the emergence of quantum sensors—specifically, atomic clocks sensitive to gravitational potential variations.Since 2021, the SYRTE, IPGP, IGN, and SHOM participate in the ANR ROYMAGE project (Optical Ytterbium Mobile Atomic Clock Applied to Geodetic Exploration). The project aims to develop a transportable atomic clock prototype with sufficient performance to determine altitude differences within the T-REFIMEVE fiber network, achieving a centimeter-level uncertainty in just a few hours across points separated by hundreds of kilometers for geodetic applications. As optical atomic clocks become integral to fieldwork, their sensitivity to mass anomalies and vertical displacement becomes a crucial consideration.Within the ANR framework, we have initiated digital tool implementation to model the signal generated by remote clock comparisons, mainly gravitationally and geometrically influenced by mass and altitude variations. The challenge lies in identifying and decorrelating signal sources to reveal the studied phenomenon. Geopotential differences, a novel geodetic observable, necessitate modeling considering gravitational signatures at the Earth's surface from buried mass anomalies (e.g., aquifers), the (in)elastic medium response to pressure changes causing vertical crust displacement, and other effects like solid Earth tides, ocean tide loading, polar motion, etc. Preliminary results suggest that clock comparisons with centimeter-level uncertainties could detect variations in regional groundwater levels. This work proposes to assess whether chronometric leveling, in France, can complement gravimetric, spatial imaging and levelling techniques to study vertical soil displacements resulting from disturbances in underground water resources due to climatic or anthropogenic origins. The authors acknowledge the support of the French Agence Nationale de la Recherche (ANR) under reference ANR-20-CE47-0006.