Résumé
The unsaturated part of the critical zone (CZ) can host considerable amounts of water and plays a major role in regulating plant-available water, streamflow, and weathering. Characterizing fluid redistribution in the CZ is crucial for exploiting and protecting these resources. Karstic unsaturated zones are particularly difficult to characterize due to their complex hydrological behavior. Here we study a karstic unsaturated zone above a tunnel nearly equal 35-50 m deep which is part of the Low-Noise Underground Laboratory (LSBB) in Rustrel, France. It intersects faults, karst networks, and flow paths and offers a unique setup to study the unsaturated zone. For one year, we continuously acquired muon data with a scintillator-based detector located inside the tunnel. In the meantime, we performed 5 time-lapse acquisitions of surface seismic and electrical resistivity tomography along the same 2-D profile, with repetitions every nearly equal 2-3 months. This profile is oriented almost perpendicular to the axes-of-observation of our muon detector, such that there is a maximum overlap between regions imaged by the different methods. We calculate the 2-D average density radiography of the region imaged by the muon detector and compare it to the 2-D electrical resistivity and seismic velocity sections. Densities are relatively high with mean values of 2.6-2.7 g/cm (super 3) , which are in good agreement with typical values observed in the Urgonian limestone found in the area. We also study the temporal changes in density from the continuous muon measurements. No clear seasonal behavior is observed, which may be expected in an unsaturated karstic zone. However, we find that different parts of the scanned region evolve differently in time. Significant density increases are indeed inferred after extreme rainfall events and are located in well-defined zones. These zones seem to correspond to fractured regions hosting higher rainfall infiltration, as imaged by our time-lapse geophysical measurements.