Fluid flow is directly controlled by the geomety of the fracture network. However, the exact relationship between fracture geometry and flow is unclear. It has been shown that only a small percentage of fractures within a rock mass represent true flow paths. Important factors such as permeability and fracture orientation with respect to the actual stress field must be evaluated.

        A 3D fracture characterization within a rock mass has been performed in order to provide a more realistic and representative geological fracture network as a basis for numerical fluid flow simulations.

         The studied rock mass is a 12x45m mine pillar in the CANMET Experimental Mine at Val d'Or, Québec, located within the Bourlamaque granodioritic batholith. All four vertical faces of the pillar were mapped and fractures were located and measured along the walls by multidirectional sampling. Structural analysis of the fractures shows two main conjugate sets, oriented at N95E and dipping between 55*S and 80*N. Fractures are compatible with the Cadillac-Larder Lake fault system and parallel to the regional Archean structural trend.

         Groung Penetrative Radar (GPR) profiles were performed along two 20m vertical faces with a 450MHz antenna in order to image penetration of the  fractures within the rock mass. Experimental conditions and electrical conductivity of the granite provided penetration depths of less than 2m. GPR profiles were processed using methods adapted from seismic. Linear anomalies, interpreted as fractures, were identified on the profiles and correlations were established between the measured fractures and fractures detected on GPR profiles. Some fractures, present on both sides of the pillar, were visible throughout the entire penetration depth of the profile so that face to face correlations within the rock mass were possible.

         Combined detailed mapping, structural characterization and GPR work helped reconstruct a geologically sound 3D representation of the fracture network by:

    1) recognizing physical fractures that cut through the entire rock mass and that probably correspond to major flow paths,
    2) discriminating between wet fractures that actively participate in flow and non-penetrative dry fractures,
    3) demonstrating that fractures that actively participate in flow are 2 Ga old structures, favorably oriented within present-day stress conditions,
    4) documenting that younger, much smaller, non penetrative fractures related to erosional uplift or excavation do not seem to play an important role in fluid flow.

    These conclusions will help provide important geological constraints towards an integrated model of fluid flow within a fractured rock mass.