Comparison of electrical resistivity tomography and frequency domain electromagnetic methods for mapping seawater intrusion in a shallow coastal aquifer (Northern Italy)

Abstract Electrical Resistivity Tomography (ERT) is a common method for assessing saltwater intrusion in hydrogeological studies, but its application is often limited to small areas due to the need for physical contact with the ground, resulting in long data acquisition times. In contrast, electromagnetic induction techniques (EMI) offer a promising alternative, but face ongoing debates regarding the robustness and calibration of their data. This study compares data from two different Frequency Domain Electromagnetic (FDEM) instruments - a multi-offset, constant frequency, constant induction number instrument and a single-offset, multi-frequency, variable induction number instrument - with ERT surveys. The comparison was made at four different sites of varying salinity and morphology, including both saline and freshwater aquifers. The results show that FDEM is effective for mapping shallow salinity when ERT data are used for calibration. In fact, while the qualitative conductivity trends obtained by FDEM are consistent, discrepancies in the range and values of electrical conductivity persist between different FDEM instruments and when compared to ERT data. Calibrations achieved through statistical analysis of cross plots can correct these discrepancies, but remain site- and season-dependent. The study highlights the potential of FDEM to detect seawater intrusion, even for long-distance profiles and 3-D spatial analysis, offering a faster, cost-effective alternative to traditional ERT approaches. The proposed technique is not limited to hydrogeological studies as it can be easily extended to other applications and geological backgrounds.