Anisotropy of electrical conductivity of the excavation damaged zone in the Mont Terri Underground Rock Laboratory

• Published in: Geophysical journal international 2010-04, Vol.181 (1), p.303-320
• Main Authors: Nicollin, Florence, Gibert, Dominique, Lesparre, Nolwenn, Nussbaum, Christophe
• Format: Article
• Language: English
• Subjects: Earth Sciences; Electrical properties; Environmental Sciences; Geophysics; Global Changes; Inverse theory; Physics; Sciences of the Universe
• ISSN: 0956-540X; 1365-246X
• DOI: 10.1111/j.1365-246X.2010.04517.x

Electrical resistivity measurements were performed to characterize the anisotropy of electrical resistivity of the excavation damaged zone (EDZ) at the end-face of a gallery in the Opalinus clay of the Mont Terri Underground Rock Laboratory (URL). The data were acquired with a combination of square arrays in 18 zones on the gallery's face and in two series of four boreholes perpendicular to the face. Each data set is independently inverted using simulated annealing to recover the resistivity tensor. Both the stability and the non-uniqueness of the inverse problem are discussed with synthetic examples. The inversion of the data shows that the face is split in two domains separated by a tectonic fracture, with different resistivity values but with a common orientation. The direction of the maximum resistivity is found perpendicular to the bedding plane, and the direction of minimum resistivity is contained in the face's plane. These results show that the geo-electrical structure of the EDZ is controlled by a combination of effects due to tectonics, stratigraphy, and recent fracturing produced by the excavation of the gallery.