Geophysical and geological characterization of fractures within a granitic pluton

Abstract

Geophysical data have been acquired to characterize the fracture network of the surrounding volume within the FEBEX gallery (Grimsel Test Site, Aar granitic massif). The geophysical data include new borehole logging such as natural gamma and borehole televiewer and borehole ground-penetrating radar (GPR). After processing an integrated interpretation of these different data sets, a pseudo three-dimensional model of the distribution of fractures was constrained. Three main fracture planes nearly parallel to the axis of the FEBEX gallery have been interpreted. These features most probably provide the pathways for the water that contribute to the present hydrological flow regime. Presently, the tunnel is not sealed and acts as a water outflow point. The GPR images provided relative high amplitude events with a marked lateral correlation that was interpreted as two main fracture planes. The GPR is specially sensitive to the water content as it directly affects the electrical conductivity and the dielectric permittivity. Therefore, the fractures were considered as part of the conductive pathways through the crystalline rock. © 2010 European Association of Geoscientists & Engineers.