Comparison of electrical resistivity tomography and frequency domain electromagnetic methods for mapping seawater intrusion in shallow aquifers -

Seawater intrusion into coastal aquifers is an increasingly critical global issue, exacerbated by climate change, rising sea levels, and overexploitation of groundwater. Effective monitoring and assessment tools are crucial to ensure the long-term sustainability of freshwater resources. While conventional hydrogeological techniques such as borehole logging and Electrical Resistivity Tomography (ERT) provide detailed subsurface data, they are often constrained by limited spatial coverage and labor-intensive acquisition due to the need for direct ground contact. This study explores the use of Frequency Domain Electromagnetic (FDEM) methods as a rapid, non-invasive alternative for identifying shallow seawater intrusion. Field measurements were carried out at four distinct coastal sites in the Friuli Venezia Giulia region (N-E Italy), each presenting unique salinity gradients, geological settings, and seasonal variations. Two FDEM instruments were tested: a multioffset, constant-frequency device and a single-offset, multi-frequency system, both selected for their ability to efficiently acquire broad, spatially continuous datasets. ERT was used as the reference technique for evaluating subsurface conductivity. Apparent electrical conductivity (ECa) values from FDEM surveys were normalized and statistically compared with ERT-derived data to assess accuracy and correlation. Although differences in absolute conductivity values were observed between the FDEM devices and the ERT profiles, the overall spatial trends in salinity distribution were consistent. Statistical calibration via cross-plot analysis significantly improved data alignment, though the effectiveness of such remained site- and season-dependent. The results demonstrate that, when appropriately calibrated, FDEM systems can reliably map shallow salinity distributions and are well-suited for extended survey areas and three-dimensional investigations. While ERT continues to serve as a powerful tool for localized, high-resolution studies, FDEM offers a faster and more practical solution for large-scale monitoring. Furthermore, this calibration approach is adaptable to a wide range of geological and environmental applications beyond hydrogeological studies. In summary, the integration of ERT and FDEM methods advances our capacity to monitor coastal aquifer salinization and supports the design of efficient, wide-area assessment strategies essential for sustainable groundwater management in at-risk coastal environments.