Integrating Transient Hydrogeology Models for Enhanced Interpretation of Geophysical Data in the Vadose Zone

Quantifying water and heat fluxes at the interface between surface water (SW), groundwater (GW), and the vadose zone (VZ) is critical for sustainable water and energy resource management under global change. Direct field measurements are challenging because SW–GW exchanges depend on initial and boundary conditions and the spatial distribution of hydrofacies, which are often poorly constrained. Usually, these fluxes are estimated by calibrating models using classical data like hydraulic heads and river discharge. But it is well known that these data did not get enough information to constrain these fluxes. To overcome the lack of direct in situ data, de Marsily et al., (2005) and Schilling et al., (2019) suggested to couple the classical observations with unconventional data such as the geophysical surveys, for instance successfully applied in the context of SW-GW exchanges by Dangeard et al., (2021). Binley et al., (2015), in their comprehensive review, highlighted the robustness of geophysical methods for imaging subsurface structures and estimating saturation profiles, reinforcing their role as essential tools for characterizing vadose zone processes.
This study develops a transient, process-based hydrogeophysical forward model that integrates hydrological and geophysical processes. The geophysical methods used in this study are electrical resistivity tomography (ERT), seismic methods, and heat tracing, applied as complementary approaches to characterize vadose zone dynamics and link hydrological processes to geophysical data. The hydrological model (Rivière et al., 2020) rigorously solves Richards’ equation coupled with heat transport—simulating variably saturated water and thermal fluxes in porous media under transient conditions—and was validated with experimental and field data to explore the variability of saturated flow and heat fluxes. The seismic model, based on Solazzi et al., (2021) uses the Hertz-Mindlin contact theory combined with the Biot-Gassmann model and simulates the influence of capillary suction with a transient method. The electrical model uses the Waxman-Smits petrophysical law to quantify electrical conductivity of the soil. The outputs of the hydrological model are coupled with geophysical forward models to compute synthetic geophysical models (P and S wave velocity, electrical resistivity) and associated data (more particularly surface wave phase velocity, apparent electrical resistivity); as well as heat tracing signals). The synthetic case considered in this study is a 1D soil column, subjected to seasonal variations in precipitation and temperature, to analyze the resulting dynamics and their geophysical data.
Testing this integrated model under typical spring conditions in the Paris Basin demonstrates:

• The added value of transient modeling for interpreting geophysical data.
• Sensitivity of seismic and electrical responses to soil saturation and pressure changes, even without water table fluctuations.
• The influence of past infiltration events on geophysical survey interpretation.

This approach provides new insights into VZ functioning and strengthens the link between hydrological processes and geophysical signatures, paving the way for improved characterization of subsurface dynamics under global changes.