Correlating ERT with AEM in a rock slide mapping project, same shape but different quantities

Abstract

We investigate an active rock slide in Western Norway with ground- and airborne resistivity mapping to ultimately find weakness zones & sliding planes embedded in crystalline bedrock. The study area comprises phyllite, a low grade metamorphic rock type that tends to be reworked to clay in disturbed zones. Mapping these electrically conductive clay zones was the aim of the survey. GPS measurements over the last 5 years indicate that precipitation drives rock slide movements. The role of ground water is thus a crucial factor to investigate for risk assessment in the area. Based on a successful airborne electromagnetic (AEM) demonstration survey, we conducted a total of 1.600 profile meters of ground resistivity (ERT) measurements to confirm AEM anomalies, to gain precise 2D geometries and to link conductivity anomalies with geology. All resistivity results confirm AEM anomalies and refine their lateral extent. In the East we find consistency between a strong conductor, dipping sub horizontal SW with an outcropping thrust fault, separating phyllite and gneiss. In the West a conductor dipping steeply NNW seems to be fed by surface water and may represent a formerly unknown sliding plane. While ERT and AEM anomaly shapes generally agree within their mutual resolution limitations, the resistivity values significantly deviate. It remains unclear whether anisotropy or strong 3D artefacts cause this disagreement.