Geophysical Imaging for the Petrophysical Properties Characterization of a Limestone Heterogeneous Vadose Zone - Beauce Aquifer (France)

International audienceThe O-ZNS observatory, under development at an agricultural site in Villamblain (Centre Val de Loire, France), offers a unique support for deciphering, at relevant scales (from nano- to metric scales), the interactions throughout the vadose zone. This observatory developed thanks to an exceptional well (depth - 20 m & diameter - 4m) is associated with boreholes to combine different geophysical imaging with high resolution and focused monitoring techniques. It allows the elaboration of innovative environmental sensors to quantify fluid and heat transfers within the vadose zone of a karstified/fissured limestone geological structure from the soil to the aquifer. Previous geophysical studies on the vadose zone of calcareous medium shows that multi geophysical imagery is of key importance to spatialize petrophysical information which are a prerequisite for the development of geological and hydrogeological models. Geophysical investigations have been conducted to characterize the initial state of the O-ZNS site. Surface measurements consisted in 3D electrical resistivity imaging (ERI) and 2D Magnetic Resonance Sounding (MRS). The 3D ERI gave information on the lithology of the vadose zone with three main geological groups: a few meter thick soil, a highly heterogeneous and degraded karstified limestone and the massive limestone. These results were improved by data issued directly from three boreholes, which allowed the collection of core samples as well as logging measurements that gave additional information on the properties of the vadose zone materials. Cross-hole radar measurements completed those acquisitions. Both 2D MRS and cross-hole radar investigations brings valuable information on the water content repartition in the vadose zone. These studies highlights a complex vadose zone that have a huge impact on the dynamics of this calcareous hydrosystem. Perspectives involve field geophysical imagery technique to highlight heterogeneities with measurements from the well itself in addition to the surface and surrounding boreholes, and laboratory experiments to identify the origins of the geophysical signals. Finally, joint geophysical procedures will take into consideration different sensitivity and reduce uncertainties in order to build in fine 3D multiphase reactive transport models