ERT and GPR survey at the Indre Nordnes unstable rock slope, Kåfjord municipality, Troms and Finnmark county

Abstract

An integrated geophysical investigation was conducted at Indre Nordnes, focusing on an unstable rock slope that has shown increased movement since 2021. The study utilized Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) to map subsurface structures and identify potential glide planes. ERT results revealed highly resistive zones at the surface, corresponding to fractured, well-drained bedrock, while low-resistivity layers beneath indicated water-saturated zones, often associated with slope instability. The most unstable terrace displayed extreme resistivity values, suggesting significant air-filled voids within fractured rock. Beneath this terrace, an arched low-resistivity zone was interpreted as a possible glide plane. Further down the slope, similar high-resistivity zones, though less pronounced, were observed, with underlying low-resistivity layers correlating with spring locations. GPR data complemented the ERT findings, confirming major structural lineaments and supporting the presence of a possible glide plane. The survey identified a staircase-like pattern of vertical and horizontal low-resistivity features, highlighting a sequence of weak zones facilitating downslope movement. Although local interference from power cables introduced some data artefacts, the combined methods provided a detailed assessment of subsurface conditions. These results contribute valuable insights into the ongoing monitoring and hazard assessment of the Indre Nordnes slope.