Scanning for weakness zone along hydropower tunnel and rock sliding planes in Aurland.

Introduction

The inner Aurland Fjord with the adjacent Flåm Valley (in western Norway) are among Norway’s most famous tourist destinations with up to 450,000 visitors and more than 100 cruise ships a year visiting the area. The main road between Oslo and Bergen (E16) passes through Flåm, bypasses the fjord, and enters the 24.5-km Lærdalstunnelen in Aurland-svangen. Evidence of large rockslides in the geological past has been documented in the area with ground movements evident to the present day. The area is subject to potential rockslides composed of creeping rock and debris masses.

Based on repeat GPS measurements and anecdotal observations in the area, rock and debris movements are influenced by precipitation and snow melt. Based on this empirical evidence, the local municipality and regional hydroelectric company E-CO Vannkraft are evaluating the potential of draining the unstable area to a nearby hydropower reservoir (Viddalsmagasinet) with the aid of a 10-km tunnel. Initial interpretations of an airborne electromagnetic (AEM) mapping survey conducted in June 2009 reveal indications of the sliding planes and assess the tunnel corridor for potential tunneling hazard areas.

Conclusion

Based on the geophysical data and knowledge from geological pre-investigations, we can draw the following preliminary conclusions:

The known, outcropping phyllite/gneiss interface close to Viddalsdammen (Area A, Figure 3 and Figure 5 in the article) appears as a strong conductor dipping southwest, consistent with outcrop data. This indicates the presence of crushed phyllite (potentially with graphite infill), thereby representing a formerly unknown potential tunneling hazard. A similar feature appears over large areas on the west flank of the mountain plateau (Area B, Figure 3 and Figure 4 in the article), which may indicate a thin, 50–150 m layer of phyllite overlaying gneiss.

More complicated anomalies appear around Joasete (Area C, Figure 3 in the article), potentially indicative of the anticipated sliding plane response. Further down the slope a consistent, conductive layer most likely indicative of the base of a debris field filled with fines and thus the sliding plane for the creeping debris along the fjord and valley.

No final conclusions can be drawn from geophysical data alone, however. At this point, only limited drilling is necessary to transform the geophysical maps to a firm geological model. We are planning a ground follow-up survey with electrical resistivity tomography to gain 2D or 3D information on the structures. The intent is to further distinguish clay from phyllite containing graphite by virtue of the induced polarization response.

Acknowledgments

We acknowledge Bjørn Sture Rosenvold (Aurland municipality) for initiating this investigation and partial funding. We thank Rasmus Teilman (SkyTEM ApS) for excellent work during AEM data acquisition. AAP and EG received funding from the Norwegian Research Foundation through KMB project 182728. Some material in this article will be presented at EAGE's Near-Surface Conference.

References

The full paper can be requested through the download link above or found directly at www.libraryseg.org.

Pfaffhuber, A. A., Grimstad, E., Domaas, U., Auken, E., Halkjær, M., and Foged, N. 2010. Airborne EM Mapping of Rock Slides and Tunneling Hazards, The Leading Edge 29 (8).