Resistivity mapping as a tool for identification and characterisation of weakness zones in crystalline bedrock: definition and testing of an interpretational model
Jan Steinar Rnning
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Guri V. Ganerd
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Einar Dalsegg
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Fabienne Reiser
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J. S. Rnning Norwegian University of Science and Technology (NTNU)
, 7491 Trondheim,
Norway
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J. S. Rnning G. V. Ganerd (&) E. Dalsegg F. Reiser Geological Survey of Norway (NGU), Post-Box 6315, Sluppen, 7491 Trondheim,
Norway
In recent years, the focus on feasibility studies for tunnels has increased in Norway. Traditionally, the refraction seismic method and the very low-frequency electromagnetic method (VLF-EM) have been used. The Geological Survey of Norway introduced the electrical resistivity traversing method (ERT) in feasibility studies for tunnel construction purposes. Resistivity modelling shows that the method has the potential to characterise fracture zones geometrically; i.e., the thickness, dip, and depth extent. Based on previous studies, a model for mineralogical characterisation is proposed. This model, and the possibility for geometrical characterisation, is critically tested with success on three tunnel projects. The results of the comparison study, with regards to weakness zones, show that VLF-EM is a method that is capable of locating fracture zones, while refraction seismic is capable of locating and indicating the width of the zone, and can be used to imply the thickness of the soil cover above bedrock. The 2D resistivity method is able to locate the weakness zone, indicate the width, depth extent, and the dip of the zone, and in addition, characterise the zone with respect to stability or water problems. The crystalline bedrock characterisation is divided into three groups: resistivity values above 3,000 X m indicating good rock quality, values between 3,000 and 500 X m indicating bedrock with mainly water problems, while values \500 X m indicate clay-bearing, unstable rock with fewer water problems. From our investigations, we conclude that the 2D resistivity method is a very good supplement to traditional methods for feasibility studies on tunnelling purposes in crystalline rock.
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Resume En Norve`ge, au cours des dernie`res annees,
laccent a ete mis sur les etudes de faisabilite pour la
construction de tunnels. Traditionnellement, la methode de
sismique refraction et a` tre`s basse frequence
electromagnetique (VLF-EM, Very Low Frequency Electro
Magnetic Method) est utilisee. Le Bureau de Recherches
Geologiques de Norve`ge (NGU, Norwegian Geological
Survey) a presente la methode de resistivite electrique
traversante (ERT, Electrical Resistivity Traversing method)
dans des etudes de faisabilite. La modelisation de la
resistivite montre que la methode a le potentiel de
caracteriser la geometrie des zones de fracture, cest a` dire
lepaisseur, le pendage et lextension en profondeur des
failles. Sur la base de ces etudes, un mode`le pour la
caracterisation mineralogique est propose. Ce mode`le, ainsi que
la possibilite de caracterisation geometrique, a ete teste avec
succe`s sur trois projets de tunnels. En ce qui concerne les
zones de faiblesse, les resultats de letude comparative
montrent que la methode VLF-EM est capable de localiser
les zones de fracture, et que la methode de sismique
refraction est capable de localiser et dindiquer la largeur de
la zone. Cette approche peut etre utilisee pour determiner
lepaisseur de la couverture au-dessus du socle. La methode
de resistivite 2D est capable de localiser la zone de
faiblesse, dindiquer la largeur, la profondeur et le pendage de
la zone, et peut egalement caracteriser la zone en termes de
stabilite ou de proble`mes lies a leau. La caracterisation du
socle cristallin est divisee en trois groupes: les valeurs de
resistivite au-dessus de 3000 ohm.m indiquent une bonne
qualite de roche, des valeurs comprises entre 3000 et 500
ohm.m caracterisent un socle avec des proble`mes de leau,
tandis que des valeurs inferieures a` 500 ohm.m indiquent la
presence dune roche argileuse instable, avec moins de
proble`mes deau. Nos etudes montrent que la methode de
resistivite 2D est un tre`s bon complement aux methodes
traditionnelles pour les etudes de faisabilite de construction
de tunnel dans une roche cristalline.
Mots cles methode de resistivite (ERT) identification
et caracterisation zone de faiblesse dans la roche
cristalline cartographie geophysique etudes de
faisabilite tunnels
Each year, more than 30 km of road and railroad tunnels in
bedrock are constructed in Norway. Normally this is done
without any impact on the surrounding environment and
within economical budgets, however, over the years a
number of tunnelling projects have encountered significant
problems. Such problems include encountering zones with
major water leakage and substantial volumes of unstable
rocks, resulting in extensive delays and considerable
overspending during tunnel construction. Therefore, an
increased emphasis on site investigations for tunnel
projects has been critical in recent years. Studies such as the
research and development project Tunnels for the
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