Airborne EM Mapping of Rock Slides and Tunneling Hazards

The inner Aurland fjord and the adjacent Flåm valley (Western Norway) are subject to potential rock slides comprised of creeping rock- and debris masses. Based on indications that precipitation drives the sliding movements, the local municipality and regional hydroelectricity company are evaluating the option to drain the unstable area with a more than 10 km long drainage tunnel to a nearby hydropower reservoir. Both rockslides and tunnel corridor encounter phyllite, a low grade metamorphic rock type that is potentially reworked to clay in disturbed zones. Water saturated clay is a strong conductor and thus an ideal target for an electromagnetic (EM) survey. We conducted an airborne EM mapping survey to find indications for the sliding planes and to assess the tunnel corridor for potential tunnelling hazard areas. Spatially constrained inversion of the 250 line km data set reveals extended conductive zones that we interpret as sliding planes and/or gneiss / phyllite interface. Detailed follow up of initial results is planed with limited percussion drilling and ground resistivity surveys.