A methodology to interpret cross-hole tests in a granite block

Abstract

Water flow through low permeability fractured media is often concentrated in a small number of fractures. This fractures need to be characterized in order to simulate accurately the hydraulic behaviour of such media. We present a methodology to identify the most significant water conductive fractures. The method is based on the interpretation of cross-hole tests, and is supported by geology, geophysics and hydraulic data. The methodology has been applied to the hydrogeological characterization of a granitic block around the FEBEX experiment, at the Grimsel underground laboratory, Switzerland. Geology and geophysics are used for identifying candidates to flow controlling fractures. Single borehole hydraulic tests help in neglecting those with low transmissivity, but the only means to assess connectivity consists of performing cross-hole tests. The resulting geometry is later implemented into a 3D finite element mesh, where the fractures are simulated as 2D elements embedded into a 3D porous medium. The latter includes the effect of minor fracturing and can be heterogeneous and anisotropic. The resulting model is calibrated to obtain an integrated model. This methodology has proven capable of reproducing single-hole and cross-hole tests and steady-state heads, and also of quantifying groundwater flow to the experimental area of the FEBEX tunnel.