Waterborne electromagnetics: two case studies

Understanding interactions between surface water and groundwater is essential for hydrological studies. Subsurface characterization beneath water bodies has traditionally been challenging, but recent advancements in geophysical methods, particularly electrical and electromagnetic (EM) techniques, have provided new opportunities. This study focuses on the waterborne application of the tTEM system, known as FloaTEM, which offers high-resolution subsurface imaging beneath lakes and rivers. Two case studies are presented: one in Iseo Lake (Italy) and the other in Ijsselmeer Lake (Netherlands). In Iseo Lake, 200 km of data were acquired to investigate the connection between the lake and underlying aquifers. Inversion results revealed distinct resistive features, such as a mountain dipping into the lake and the presence of a discontinuous aquifer. In the Ijsselmeer, FloaTEM was used to map the distribution of clay layers separating fresh and saline groundwater. Results were validated by traditional underwater electrical resistivity tomography (ERT), confirming the presence of freshwater intrusion through gaps in the clay. The adapted FloaTEM system, mounted on plastic boats with synchronized GPS and echo sounder, demonstrated excellent productivity and spatial resolution. Bathymetry-integrated inversions improved model accuracy by reducing near-surface resistivity errors. Overall, FloaTEM proved to be a fast, efficient, and reliable method for subsurface mapping in aquatic environments, capable of detecting small-scale geological heterogeneities critical for hydrological analysis.