3D near-surface geophysics and geostatistics for heterogeneities characterization and water table monitoring on the Orgeval critical zone observatory

Groundwater fluxes and interacting zones between groundwater and surface water are crucial for understanding the water dynamics of the critical zone. Groundwater within the critical zone plays a significant role in the ecosystem, biodiversity, and water supply. However, estimating these fluxes remains a key challenge because they are not directly measured in the field. Model calibration involves adjusting key parameters—such as saturated hydraulic conductivity and soil-water retention properties—using observed data like hydraulic head and river discharge, while initial and boundary conditions are prescribed to define the model l setup. That calibration is often done by comparing simulated soil water saturation and water table level to piezometers. Nevertheless, real flows occur in 3D in a complex medium containing heterogeneities with various lithologies, with different hydraulic parameters such as hydraulic conductivity and porosities.

Geophysical methods such as electrical resistivity tomography (ERT), seismic refraction tomography (SRT), and multichannel analysis of surface wave (MASW), which are sensitive to lithology,  content, and nature of fluid, represent helpful tools for hydrogeological modelling, both in terms of model parameterization and physical property characterization. ERT, which is particularly sensitive to lithology, allows us to identify and delineate heterogeneities, while seismic methods, which are sensitive to mechanical properties, will enable us to infer the water saturation and the piezometric surface in the near surface through the P-wave and S-wave velocities ratio (Poisson’s ratio, e.g. ) (Dangeard et al., 2021).

We propose a workflow combining geophysics and geostatistics to reconstruct the heterogeneities and the piezometric surface in an alluvial plain context. We implemented the workflow in a 30 x 30 m area at the Avenelles site of the Orgeval Critical Zone Observatory (CZO), which is part of the French network of CZOs OZCAR. ERT, SRT, and MASW surveys were carried out along 7 profiles to obtain 2D sections of electrical resistivity, P and S wave velocities (6 profiles of 72 electrodes/geophones and one profile of 48 electrodes/geophones, with 0.40 m spacing leading to 12,708 apparent resistivity data, 33,888 first wave arrival picks, and 277 dispersion curves). Geophysics allows us to pass from punctual piezometer data to 2D vertical sections. However, carrying out 3D geophysical acquisition is cumbersome. To overcome these limitations, we then use geostatistics to get a distribution of our geophysical parameters in the 3D volume delineated by the geophysical survey. Once the 3D interpolation is done by kriging methods, we can retrieve a view of the heterogeneities distribution in the near surface as well as the water table position to inform hydrogeological inversion. Furthermore, with the addition of petrophysical relationships, it is possible to estimate saturation and porosity distribution for a future 3D hydrogeological physics-based model run to better characterise groundwater fluxes. Finally, all these workflows, including complementary methods, could be performed on different dates for time-lapse monitoring of the water table.