Advancing measurements and representations of subsurface heterogeneity and dynamic processes: towards 4D hydrogeology

Abstract

Abstract. Essentially all hydrogeological processes are strongly influenced by the subsurface spatial heterogeneity and the temporal variation of environmental conditions, hydraulic properties, and solute concentrations. This spatial and temporal variability generally leads to effective behaviors and emerging phenomena that cannot be predicted from conventional approaches based on homogeneous assumptions and models. However, it is not always clear when, why, how, and at what scale the 4D (3D + time) nature of the subsurface needs to be considered in hydrogeological monitoring, modeling, and applications. In this paper, we discuss the interest and potential for the monitoring and characterization of spatial and temporal variability, including 4D imaging, in a series of hydrogeological processes: (1) groundwater fluxes, (2) solute transport and reaction, (3) vadose zone dynamics, and (4) surface–subsurface water interactions. We first identify the main challenges related to the coupling of spatial and temporal fluctuations for these processes. We then highlight recent innovations that have led to significant breakthroughs in high-resolution space–time imaging and modeling the characterization, monitoring, and modeling of these spatial and temporal fluctuations. We finally propose a classification of processes and applications at different scales according to their need and potential for high-resolution space–time imaging. We thus advocate a more systematic characterization of the dynamic and 3D nature of the subsurface for a series of critical processes and emerging applications. This calls for the validation of 4D imaging techniques at highly instrumented observatories and the harmonization of open databases to share hydrogeological data sets in their 4D components.

Predicting the Electrical Conductivity of Partially Saturated Frozen Porous Media, a Fractal Model for Wide Ranges of Temperature and Salinity

“…The selection of appropriate experimental methods for subsurface characterization is of crucial importance. For this goal, the field of geophysics can provide a wide category of techniques that now constitute the well‐established field of hydrogeophysics (e.g., Binley et al., 2015; Hermans et al., 2023). Electrical and electromagnetic methods, including electrical resistivity tomography, self‐potential (SP), and (spectral) induced polarization (SIP), can be used to investigate the water distribution and solute concentration of porous media at various scales, from small‐scale (e.g., from micrometer to several meters) laboratory measurements to long‐term (e.g., over several months) large‐scale (e.g., from several meters to several kilometers) field monitoring.…”

Understanding the Fate of H2S Injected in Basalts by Means of Time‐Domain Induced Polarization Geophysical Logging

“…While many applications of geophysics focus on structural characterization using static geophysical data (Binley et al, 2015), time-lapse geophysics provides a promising avenue to characterize the spatial distribution and temporal evolution of transport and reaction processes (Hermans et al, 2023). So far, most of the field applications of time-lapse geophysics have focused on characterizing flow and transport processes: groundwatersurface water interaction (Slater et al, 2010), water absorption by roots (Jayawickreme et al, 2010;Mary et al, 2020), migration of methane plume (Steelman et al, 2017), migration of injected remediation agent at contaminated site (Flores Orozco et al, 2015;Lévy et al, 2024).…”

Modeling Water Flow and Solute Transport in Unsaturated Soils Using Physics‐Informed Neural Networks Trained With Geoelectrical Data

“…As opposed to intrusive subsurface characterization and monitoring techniques, geoelectrical methods can provide continuous, extensive, and non‐invasive information about the subsurface (Garré et al., 2022; Hermans et al., 2023). Out of several methods, Electrical Resistivity Tomography (ERT) is an appealing approach because of its simplicity, cost‐effectiveness, and the (relatively) large spatial extent that can be covered.…”