Geophysical Methods for Differentiating Hydrocarbon and Groundwater Signatures: A Narrative Review of Techniques, Diagnostic Criteria and Field Performance

The petroleum hydrocarbon contamination of shallow aquifers is a continuing problem in site characterisation because the geophysical signatures of hydrocarbon-bearing areas overlap with those of clean groundwater under certain geological and temporal conditions. This narrative review assesses the diagnostic capability of four classes of geophysical methods, electrical resistivity and induced polarisation, electromagnetic (ground penetrating radar, time-domain and frequency-domain electromagnetics), seismic, and combined multi-method methodologies, to distinguish between hydrocarbon and groundwater geophysical signatures at depths of 1-50 m onshore. Scopus, Web of Science, and Google Scholar were synthesised thematically between 2010 and 2025, with pre-2010 sources incorporated because no current substitute exists. Evidence was systematised by method class and evaluated for consistency, geographic coverage, and quality. The most field-proven combination was electrical resistivity imaging together with induced polarisation, but contamination age reverses the resistivity response between resistive and conductive conditions due to biodegradation. Ground penetrating radar eliminated thin layers of LNAPL that the resistivity techniques could not detect, but not where the conductivity of the subsurface was greater than 0.28 S/m. Seismic was used to map the structure rather than to perform outright fluid discrimination at low depths. Researchers obtained the most reliable discrimination results across different site conditions by using staged, multi-method workflows comprising structural characterisation, fluid-sensitive measurements, and geochemical validation. Practitioners examining contaminated sites should use ERT-IP as the primary screening tool, complement it with GPR when vadose-zone conditions are present, and verify the presence of the anomaly through targeted hydrochemical sampling.