Groundwater flow modelling and waterborne electromagnetics

Understanding how surface water and groundwater interact is vital for hydrological studies, especially in water-covered areas where direct data is scarce. Geophysical methods like electrical resistivity tomography (ERT) and electromagnetic (EM) techniques are commonly used, but interpreting these data can be uncertain without ground-truthing. This study highlights the use of the FloaTEM system—a waterborne version of the tTEM system—to map subsurface structures in freshwater lakes, including Ijsselmeer (Netherlands) and Iseo Lake (Italy). FloaTEM uses a boat-based EM setup combined with GPS and bathymetry data to generate detailed resistivity models of lakebeds. In Ijsselmeer, FloaTEM data helped identify clay layers and sandy aquifers, and a groundwater flow model was built using MODFLOW (via the FloPy tool) to test and validate the geological interpretations. These models simulate how freshwater and saltwater interact beneath the lake. Because clay and saltwater aquifers can appear similar in EM data, different simulations were run to compare outcomes. The same methodology is being applied to Iseo Lake, where the geology is more complex and requires advanced modeling approaches. FloaTEM proves to be a fast, effective way to map underwater geology. Combining its results with groundwater modeling helps interpret uncertain geophysical data and improves understanding of subsurface hydrology in challenging, data-poor environments.