Expanded stream-aquifer interface model constrained by time-lapse seismic

International audienceEstimating evolution of exchanges within the stream-aquifer interface is frequently tackled with the help of numerical models. Yet, the definition of boundary conditions is generally based on poorly constrained assumptions and restrained to the location of piezometers. We suggest here to stretch the modeling domain and build stronger constraints, both in space and time, by using a multi-method approach. On a hotspot of the Orgeval Critical Zone observatory (France), we show how a thorough interpretation of high-resolution geophysical images, combined with geotechnical data, helps describing the spatial heterogeneities of the shallow aquifer. It provides a detailed distribution of hydrofacies, valuable prior information about the associated hydrodynamic properties and makes it possible to expand the modeling window in space. We show how the local temporal dynamic of the water table can be captured with high resolution time-lapse seismic acquisitions. Time-lapse variations in seismic data are discriminated from noise or measurement errors to be interpreted, regarding hydrological observations, as temporal changes in the saturated-unsaturated zone continuum. Each seismic snapshot is then thoroughly inverted to actually image spatial water content variations and delineate the water table outside the limits defined by the piezometers. This posterior geophysical information is then suggested as initial and boundary conditions of the expanded hydrogeological modeling domain. We finally calibrate and provide plausible ranges of hydraulic parameters to reproduce the water table and improve the estimation of stream-aquifer exchanges