Monitoring groundwater dynamics in the Crepieux-Charmy water catchment using DAS-based surface wave tomography

Ambient noise interferometry applied to Distributed Acoustic Sensing (DAS) arrays is an increasingly common approach to subsurface investigations. In this study, we show that analysis of urban seismic noise acquired on a DAS array can be used to track velocity variations caused by groundwater level changes in an alluvial aquifer.

We apply our methodology to the Crépieux-Charmy aquifer, a strategic site for the city of Lyon, France. This site provides more than 90% of the city's drinking water and uses a Managed Aquifer Recharge (MAR) system, which allows controlled recharge of the aquifer through infiltration basins. We analyzed four weeks of DAS ambient noise data recorded on a 3 km spiral DAS array surrounding an infiltration basin.  During this period, a controlled water infiltration experiment was conducted by the site operators. 

We used traffic noise in the 2-5 Hz frequency band and performed DAS-based time-lapse surface wave tomography of velocity variations in the area. We were then able to map relative changes in seismic velocity in the vadose zone. Comparison of point-scale water level measurements from piezometers and adjacent cells in the velocity variation maps shows good agreement between the two observables. These velocity variations are directly related to the water table variations and to residual water saturation changes within the unsaturated zone.

This pilot application demonstrates the potential for groundwater monitoring in aquifer systems using DAS combined with seismic interferometry.