Geoelectrical imaging of a thick regolith developed on ultramafic rocks: groundwater influence

A thick regolith with supergene nickel ore deposits, developed from ultramafic rocks in New Caledonia (southwest Pacific), was investigated using electrical resistivity tomography (ERT), This paper presents the results of an electrical survey and monitoring of the lateritic weathering mantle of Tiebaghi plateau. Resistivity pseudosections, validated by many borehole logs, contributed to defining a four-layer geoelectric model corresponding to four main weathering horizons: (1) ferricrete with soft nodular horizon; (ii) red mottled zone and yellow fine saprolite; (iii) coarse saprolite; and (iv) bedrock. All geoelectric sections reveal undulated bedrock topography, with each high representing a resistive bedrock ridge and each swale corresponding to a conductive saprolite trough. When parallel pseudosections are correlated, most ridges and troughs hove a prominent northeast-southwest strike. Seasonal monitoring of a selected ERT profile shows that the geoelectric structure is well preserved through time. Local resistivity variations occur in the mottled zone-fine saprolite layer and along a major fracture zone, and are interpreted as being due to water content variations in groundwater pathways. On the basis of field and laboratory experiments, it is proposed that the resistivity variations are not due to dewatering but could result from fluctuations in groundwater chemistry. ERT appears to be a useful tool to document the geometry of the regolith and the groundwater flow pattern in weathering horizons and bedrock.