Integrated Electrical Resistivity Tomography and Borehole Correlation for Characterizing Salt Formation Geometry and Groundwater Circulation at Slănic Prahova, Romania

Abstract

Salt (halite) massifs undergoing active dissolution are among the most hazard-prone geological settings for ground instability, and their monitoring requires a reliable geometric model of the salt body and its overburden. This study presents an integrated geoelectrical–geological characterization of the salt formation at Slănic Prahova (Romania), carried out in 2026 within the GEOMONITOR project. Six electrical resistivity tomography (ERT) profiles were acquired along three transects, each surveyed with both Wenner and Schlumberger electrode configurations (5 m electrode spacing), and correlated with lithological logs from three boreholes (F1, F2, F3) that intercepted the salt formation at 98.0 m, 44.1 m and 68.0 m below ground, corresponding to 301.0, 346.4 and 327.0 m a.s.l. respectively. Reading both datasets in absolute elevation is the key step of the workflow: it shows that the salt roof enters the imaged volume only in the Baia Roșie sector, where a body exceeding 1000 Ω·m occupies the 348–370 m a.s.l. interval—an elevation range that agrees, to within the ±15 m datum uncertainty of the sections, with the shallowest salt roof recorded by drilling (346.4 m a.s.l.)—whereas along the other two transects, the salt roof lies 35–80 m below the base of the inverted models. The sections delineate a strongly heterogeneous cover, predominantly conductive (1–10 Ω·m) and punctuated by transition zones with steep lateral resistivity gradients. Two consequences follow for hazard-oriented surveys of this kind: shallow resistive anomalies confined to the uppermost meters cannot be attributed to the salt roof, and the absence of a resistive anomaly along a line of this length is not evidence for the absence of salt. The conductive–resistive contact zones, rather than the resistive cores themselves, are identified as the priority targets for hydrogeological monitoring, since they can concentrate permeability contrasts and control dissolution. The survey is a single, non-repeated campaign without pore-fluid characterization, and is therefore presented as the baseline from which a monitoring network can be designed rather than as a record of temporal change. The elevation-based reconciliation of ERT and borehole data proposed here is transferable to other salt karst settings.