Influence of measurement conditions on the resolution of electrical resistivity imaging: The example of abandoned mining dams in the La Carolina District (Southern Spain)

Abstract The La Carolina district, which is located in Jaen (Southern Spain), has experienced important mining activity for centuries. These long-standing mining activities, which have been associated with the presence of a philonian network of metallic sulfides, have generated vast accumulations of mine wastes—primarily tailings impoundments—that contain high levels of heavy metals from ore concentration plants. In this paper, the resolution of electrical resistivity imaging (ERI) is analyzed to determine the internal structure of these tailings dams and to assess the influence of the tailings over groundwater and surface water. The “La Manzana” dam, which is one of the largest sludge dams in the district, was selected to achieve this objective. First, old aerial photographs that were taken prior to the existence of the tailings dam were examined. Second, probing was performed using mechanical drilling, which enabled the identification of the features of the fill material, the thicknesses of the mine tailings and the contact zone between the mining structure and the basement. Third, the probe acts as a piezometer, which is an excellent tool to correlate the geophysical prospecting techniques. The characterization of the internal geometry of this structure has been performed by two ERI surveys, in which electric profiles were obtained under different moisture conditions. During the dry season, this methodology has been effective for identifying the morphology of the structures, contact with the substratum, variations in the vertical and horizontal distribution of the deposited material, and the fracture zones, as well as the contact between the tailings and the substratum on which they are deposited. Conversely, during the wet season, this contact was not distinct in the profile; however, variations in the saturation degree of these deposits, preferential infiltration areas and water flow through the interior of the tailings dam were observed. Last, the ERI survey confirmed the absence of waterproofing measures in the dump areas, which represents a high risk of soil and water pollution in the surrounding areas.