Time-lapse seismic experiments to constrain hydrodynamic parameters at the stream-aquifer interface

VP/VS or Poisson's ratio estimated from active seismic methods recently proved to be efficient in the imaging ofthe critical zone and associated hydrosystems. We suggest here a time-lapse application of this approach to provideboth spatial and temporal constraints on the hydrodynamic model of the Avenelles experimental basin (Seine etMarne, France). The preliminary studies of this hydrosystem relied on typical combined interpretation of sparsegeological and hydrological data. Geophysical surveys, performed throughout the watershed, helped delineatingthe different compartments and identifying their connectivity with the stream network. Once a basin-scale globalhydrogeological model established, hotspots were targeted with local high frequency monitoring stations to inves-tigate its stream-aquifer exchanges. At these stations, recorded data (bank piezometers, stream water temperatureand level, temperature profiles in the hyporheic zone) clearly showed contrasts in the dynamic of the hydrosystemalong the stream network. However, the nature of the compartments and their associated properties, observed at thebasin-scale, would not explain the data observed at the local scale. It highlighted the need for detailed descriptionof the hydrosystem, at the stream-aquifer interface. One specific hotspot was thus selected to perform soundingsand geophysical measurements of higher resolutions. Thanks to electrical resistivity tomography, P-wave refrac-tion and surface-wave seismic imaging, we provided a description of the local heterogeneities both in terms oflithology and water content. The seismic experiments were then repeated with a two-month time step. At eachtime step, pseudo-2D sections of Poisson's ratio clearly showed strong spatial and temporal variations in satura-tion of the vadose zone. These results then helped providing updated constraints and boundary conditions to thehydrodynamic model