Electrical resistivity tomography (ERT) of a coastal carbonate aquifer (Port-Miou, SE France)

As part of a larger regional research program “KarstEAU”, the authors have applied electrical resistivity tomography (ERT) techniques to characterize heterogeneities in the Port-Miou coastal karst aquifer (Cassis, SE France). Field surveys were carried out on intensely fractured and karstified Urgonian carbonates. Extensive research has characterized macro- and micro-scale geology of the Port-Miou area and particularly underground water-filled conduits and fault/fracture and karst systems within a former quarry. The authors applied 2D ERT along two surface profiles of length 420 and 595 m to test capability for delineating subsurface conduits and possibly relationship between conduit and fault/fracture/karst orientation; and 3D ERT with a dense 120 electrode array at 1 m spacing (11 × 10 m) was applied over an area of the quarry that had been profiled using 3D georadar and which has had intensive nearby structural geological interpretation. The 2D profiling imaged several underground conduits at depths to >50 m below ground surface and below sea level, including possibly the main Port Miou submarine spring and smaller springs. The 2D profiling within the quarry provided a better understanding of the connectivity between major fractures and faults on the quarry walls and secondary springs along the coast supporting flow of the secondary springs along interpreted fracture orientations. In addition, 2D inversion-derived conductivity models indicate that high resistivity zones above sea-level are associated with non-saturated zones and low resistivity anomalies in the non-saturated zone are associated with residual clays in paleokarsts. A partitioned lower resistivity zone below sea-level can be associated with a higher porosity/permeability zone with fractures and karstic features. Inversion models of the dense 3D ERT data indicate a higher resistivity volume within the larger surveyed block. The survey characterized the non-saturated zone and the ERT resistivities are correlated with karst features interpreted by 3D georadar and visible in the inferior wall of the quarry.