Detection of fossilised solution pipes and their hydrological function using electrical resistivity tomography in Apulia, Italy

Solution pipes are vertical, cylindrical karst features formed by focused vertical water flow in carbonate rocks with matrix porosity, often with well-cemented rims and filled with sediment. The objective of this study was to detect and characterise solution pipes with cemented rims along the Melendugno coast in Apulia, Italy, using Electrical Resistivity Tomography (ERT). We employed both 2D and 3D ERT profiles, using Schlumberger and Mixed Dipole-Gradient arrays, to identify cylindrical resistivity anomalies associated with solution pipes, even in sediment-covered areas where surface mapping is insufficient. The ERT results combined with stratigraphic analysis of the bedrock showed that the pipes are shallow, typically less than 2 m deep, and are associated with layers of highly porous calcarenites. Furthermore, the detected solution pipes have a higher electrical resistivity than the surrounding bedrock, indicating that the solution pipes are fossilised and no longer actively contributing to vertical water flow. These fossilised pipes now exhibit a reversed hydrological function compared to their active formation phase; presently, water preferentially infiltrates into the subsurface through the surrounding bedrock rather than through the pipes themselves. This study demonstrates the effectiveness of ERT in diagenetically immature rocks, providing insights into the geomorphological and hydrological evolution of coastal karst systems.