Investigation of offshore freshened groundwater using marine controlled–source electromagnetic: Insights from Gozo, Maltese Islands

Carbonate margins form a significant portion of the world's coastlines, contain substantial freshwater resources, and play a vital role in global hydrogeological processes. These regions are promising reservoirs for offshore freshened groundwater (OFG), a potential resource for coastal and island populations. Mapping OFG within continental margins using marine controlled–source electromagnetic (CSEM) data relies on electrical resistivity as a proxy, requiring sophisticated inversion techniques. Given the ambiguity in deriving discrete resistivity distributions from CSEM data, understanding uncertainty is essential for reliable OFG inference. The conventionally used two–dimensional deterministic inversion provides a best–fit solution but does not assess resistivity uncertainties, limiting OFG characterization. To address this, we apply trans–dimensional Bayesian inversion on marine CSEM data from a semi–arid carbonate setting off eastern Gozo (Maltese Islands, Mediterranean Sea). Here, we integrate deterministic and trans–dimensional inversion results with seismic reflection data to identify two distinct, continuous resistivity anomalies within the Lower Coralline Limestone formation. The first, shallower resistive body starts ~4 km from the coast, appearing across all the CSEM profiles at 210–250 m below sea–level, with resistivity increasing landward. This anomaly may suggest an OFG body. The second, deeper anomaly starts at 350–400 m below sea–level and extends deeper. Whether it represents a second OFG unit or geological changes remains uncertain. Our findings offer new insights into resistivity distributions within carbonate margins, highlighting their OFG potential and the value of trans–dimensional sampling. This study augments CSEM research, underlining the need to extend coastal hydrogeological studies offshore for improved environmental conservation and resource management. • Bayesian inversion improves offshore freshened groundwater (OFG) characterization. • Two resistivity anomalies identified offshore Gozo suggest potential OFG. • Shallow resistive body shows landward rising resistivity, suggesting fresher pore water. • Deeper resistive anomaly indicates uncertainty in OFG presence or lithological changes. • Uncertainty analysis enhances CSEM data reliability for mapping OFG.