Ensemble Smoother with Multiple Data Assimilation of Tracer and Electrical Resistivity Tomography data for subsurface modelling: a laboratory test case

In groundwater modeling, the accurate characterization of heterogeneous media remains a critical challenge, crucial for a thorough and continuous understanding of subsurface dynamics. Hydrogeophysics, which combines hydrological variables with noninvasive geophysical techniques, provides a valuable tool to investigate the subsurface. This study evaluates the Ensemble Smoother with Multiple Data Assimilation (ES-MDA) as an inversion method for aquifer characterization. The goal is to estimate hydraulic conductivities by jointly assimilating tracer test data and Electrical Resistivity Tomography (ERT) measurements collected in a fully controlled laboratory setting. The experimental test involves a laboratory sandbox located in the hydraulic laboratory of the University of Parma, filled with heterogeneous media mimicking real-world subsurface conditions. ERT measurements taken in the laboratory sandbox produce spatially distributed apparent electrical resistivity data, providing indirect insights into subsurface heterogeneity. Additionally, a dye tracer is injected into the sandbox, and tracer data at monitoring points are recorded using photographic techniques. This study aims to contribute to subsurface characterization, enabling an increased understanding of hydrological processes. This procedure enhances the potential of integrating advanced data assimilation techniques with geophysical imaging methods across various hydrological and environmental applications. Further research is necessary to explore the applicability of the proposed framework in field-scale studies and real-world environmental settings.