Geophysical characterization of the vadose zone above an abandoned underground quarry of Chalk
Résumé
An abandoned underground quarry of Chalk located near Beauvais (France) is of particular interest to study infiltration and dissolution processes in the vadose zone. The presence of permanent underground lakes created by the outcroping water table allows to describe the spatial and temporal variability of groundwater geochemistry within the quarry. Geophysical measurements were carried out at the surface above the quarry to characterize the geometry and the physical properties of the vadose zone and study their influence on the groundwater quality variations. Electromagnetic induction (EMI) mapping performed at the surface provided a spatial description of the thicknesses of soil and clay-with-flints superficial formation covering the Chalk. Electrical resistivity tomography (ERT) emphasized deeper geological structures along a transect located directly above an underground lake. Finally, the combined use of pressure-wave traveltime tomography and surface-wave profiling along the same transect highlighted strong lateral variations of the Poisson's ratio corresponding to significant water content variations within the clay-with-flints and the Chalk formations.