Combined three-dimensional electric and seismic tomography study on the Åknes rockslide in western Norway |
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| Authors: | Björn Heincke Thomas Günther Einar Dalsegg Jan Steinar Rønning Guri Venvik Ganerød Harald Elvebakk |
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| Institution: | 1. School of Geosciences, University of Aberdeen, Aberdeen, Scotland, UK;4. Institute of Geophysics, Department of Earth Sciences, ETH Zurich, Zurich, Switzerland;5. Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia;1. British Geological Survey, Keyworth, Nottingham, UK;2. ETH Zurich, Institute of Geophysics, Zurich, Switzerland;1. Institute of Hydrogeology, Engineering Geology and Applied Geophysics, Faculty of Science, Charles University, Albertov 6, 128 43 Prague 2, Czech Republic;2. Institute of Rock Structure and Mechanics, Czech Academy of Sciences, V Hole?ovi?kách 94/41, 182 09 Prague 8, Czech Republic;3. Department of Physical Geography and Geoecology, Faculty of Science, University of Ostrava, Chittussiho 10, Ostrava 710 00, Czech Republic;1. Engineering Geology, Lund University, P.O. Box 118, SE-22100 Lund, Sweden;2. Department of Geoscience, Aarhus University, C.F.Møllers Allé 4, Building 1120, 8000 Aarhus C, Denmark |
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| Abstract: | We present a combined 3-D geoelectric and seismic tomography study conducted on the large Åknes rockslide in western Norway. Movements on the slope are strongly influenced by water infiltration, such that the hydrogeological regime is considered as a critical factor affecting the slope stability. The aim of our combined geophysical study was to identify and visualize the main shallow tension fractures and to determine their effect on hydraulic processes by comparing the geophysical results with information from borehole logging and tracer tests. To resolve the complex subsurface conditions of the highly fractured rock mass, a three-dimensional set-up was chosen for our seismic survey. To map the water distribution within the rock mass, a pattern of nine intersecting 2-D geoelectric profiles covered the complete unstable slope. Six of them that crossed the seismic survey area were considered as a single data set in a 3-D inversion. For both methods, smoothing-constraint inversion algorithms were used, and the forward calculations and parameterizations were based on unstructured triangular meshes. A pair of parallel shallow low-velocity anomalies (< 1400 m/s) observed in the final seismic tomogram was immediately underlain by two anomalies with resistivities <13 kΩm in the resistivity tomogram. In combination with borehole logging results, the low-velocity and resistivity anomalies could be associated with the drained and water-filled part of the tension fractures, respectively. There were indications from impeller flowmeter measurements and tracer tests that such tension fractures intersected several other water-filled fractures and were responsible for distinct changes of the main groundwater flow paths. |
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