Hydrogeophysical Characterization of the Vadose Zone in the Beauce Aquifer: Integrating GPR and Geological Data at the O-ZNS Observatory

The vadose zone presents significant challenges for hydrological characterization, particularly in unraveling mass and heat transfer mechanisms influenced by medium heterogeneities and the interplay of geophysical and geochemical processes. Geophysical methods, in particular Ground Penetrating Radar (GPR), provide a high resolution, non-invasive imaging of the subsurface features and dynamic processes. In this context, the Observatory of Transfers in the Vadose Zone (O-ZNS) in Orléans, France, provides a unique framework for studying the Beauce limestone aquifer across various spatial and temporal scales. At the O-ZNS site, GPR data were acquired inside a central well (20 m deep, 4 m diameter) at multiple depths, to image key geological structures and diagenetic features. To maximize the interpretive value of this data, we developed an integrated workflow based on geophysical and geological approaches. The GPR profiles processing scheme includes time-zero corrections, DC removal, bandpass filtering, gain function, migration, and time-depth conversion, resulting in enhanced profiles for precise interpretation. GPR data are correlated with the geological and sedimentological information derived from optical log imaging, drill core pictures, and photogrammetry of the main O-ZNS well. 2D and 3D models of GPR profiles are used to identify radar facies and evaluate the impact of geological, diagenetic, and petrophysical features – such as fractures, karst, and porosity variations – on GPR signal responses. These findings establish a foundational framework for consistent and accurate radar data interpretation in the study area and contribute to better understanding of flow and transport mechanisms in the vadose zone. This study contributes to refining hydrological models and highlights the importance of integrating geophysical and geological data for characterizing heterogeneous aquifers.