Seismic velocity variations generated by controlled hydrological changes : field and laboratory studies based on seismic noise crosscorrelation

Continuous seismic noise recordings has demonstrated a remarkable ablitiy to monitor changes of the investigated media at various scales. In this study, we focused on the link between seismic velocity variations (dv/v) derived from seismic noise cross-correlations between pairs of stations and hydrological variations observed both at the field and lab scales. The field-scale experiment was performed at the water supply pumping site of Crépieux-Charmy (Lyon, France). 99 3-C velocimeters were deployed during 20 days around an infiltration basin operated for managed aquifer recharge and designed to generate an hydraulic barrier to prevent a potential contamination from the nearby river. This dense seismic network set-up allowed to dynamically image the seismic velocity variations during two filling/drainage cycles of the basin. Punctual values extracted from computed high resolution tomographies of the velocity variations were compared to local measurements of the water table level using piezometers. A remarkable agreement was found between the 2 observables in particular during the establishment of a 3D dome in the water table. During drainage phases, systematic response delays were observed which are most probably due to variations of water content in the unsaturated zone between the basin and the water table.
To better understand these effects occurring in the critical zone, we tried to reproduce a similar monitoring experiment at the laboratory scale. A tank filled with sand was designed in order to characterize controled hydrological variations (water table depth, water saturation). We used continuous seismic sources deployed on the edges of the tank. The seismic noise was recorded using 10 3-C accelerometers . The combination of these two approaches at different scales provides a better understanding of the links between seismic velocity and hydrological (water table level and water content in the vadose zone) variations.