Monitoring groundwater dynamics in the shallow crust over the Upper Rhine Graben (France) using ambient seismic noise interferometry

Ambient seismic noise interferometry is a powerful tool to monitor changes in seismic velocities within the upper crust induced by various forcings. Several studies have shown that the dynamics of aquifers can generate significant seismic velocity variations, concealing more subtle variations linked to other phenomena. Here, we present a temporal and spatial analysis of subsurface seismic velocity variations over a portion of the Upper Rhine Graben in north-eastern France, hosting one of the largest watertable in Europe. We analyze 4 years of continuous seismic records between 2019 and 2023 from 144 permanent and temporary seismological stations, together with data from 195 piezometers from the APRONA observatory. Ambient seismic noise cross-correlations were calculated using horizontal and vertical components records, and we performed velocity variations in different frequency bands (ranging from 0.1 to 4 Hz) and lapse times. We systematically compared temporal variations in seismic velocities with ground water level variations. Overall, our results indicate a strong seasonality of seismic velocity changes above 1 Hz mainly in the ballistic surface waves time window and the beginning of the coda of correlations. This signature persists at lower frequencies, around 0.5 Hz, for longer times in the coda of correlations only. This suggests a possible influence of aquifer dynamics at greater depths. We spatially localised velocity changes above 1 Hz using coda waves sensitivity kernels and found patterns consistent with piezometric observations and the known limits of the water table.