Specific features of marine electrotomographic studies of coastal areas of the gulf of Finland

Introduction This study investigates the effectiveness of marine electrical resistivity tomography (ERT) methods—specifically surface electrical resistivity tomography (SERT) and bottom electrical resistivity tomography (BERT)—for characterizing the geological structure of bottom sediments in the shallow coastal waters of the Gulf of Finland, where high and variable pore-water mineralization presents a significant challenge. Methods Experimental and methodological work was conducted to assess the geological efficiency of both ERT configurations in this complex setting. The study evaluated how hydro-physical properties, lithological composition, and the consistency of sandy–clayey soils influence geoelectric sections, while also identifying previously overlooked factors such as the liquidity index of clayey sediments, water saturation in sands, gas migration from underlying Mikulino clays, and fresh groundwater infiltration from the coast. Results A key finding is the empirical validation of a petrophysical “reversal point”, where the relative resistivity of sands and clays inverts with increasing water mineralization, fundamentally altering the geoelectric structure. Additional critical factors were quantified: gas migration from underlying clays increases resistivity in overlying deposits, while fresh groundwater infiltration elevates the resistivity of sandy layers. The BERT method proved particularly effective, accurately delineating the engineering boundary between fluid and plastic soils and mapping the roof of load-bearing semi-hard loams, including a buried paleovalley filled with sand and gravel deposits. Discussion These results refine and supplement existing concepts regarding the influence of hydro-physical and lithological factors on marine ERT data. The BERT method is demonstrated as a reliable tool for locating subaqueous paleovalleys and critical engineering interfaces, essential for coastal infrastructure design. Collectively, the research enhances the reliability and accuracy of interpreting marine electrical survey data, showing that these technologies significantly expand marine geophysics capabilities for high-resolution site characterization in heterogeneous, low-contrast subaqueous environments.