Litho-structural characterization in granitic rocks using single-hole and crosshole radar techniques

An experiment was initiated to study the solute transport characteristics of a region of 'Moderately Fractured Rock' between the 130 m and the 240 m levels as part of the continuing geoscience research at ABCL's Underground Research Laboratory (URL). Two coplanar, subhorizontal characterization boreholes, MF-l and MF-2, each approximately 200 m long, were drilled into a region of moderately fractured granite. Detailed single-hole and crosshole geophysical surveys including reflection and transmission borehole radar were performed in these two boreholes. Boreholes MF-l and MF-2 are separated vertically by a domain of xenolithic granite, two domains of pink gneissic granite, and a discontinuous domain of grey gneissic granite. Two regional low-dipping, intrablock fracture zones (FZ-3 and FZ-2) with associated sub-vertical fracturing also straddle the region. Single-hole radar reflection and crosshole radar transmission tomography surveys using the RAMAC borehole radar system, with 22 and 60-MHz antennas, were used to investigate the litho-structural characteristics of the rock mass. Structural projections 'of FZ-3 and FZ-2 in the plane of the boreholes suggest a good correlation between radar reflectors and the fracture-zone interfaces at distances of 10 m to 60 m from the boreholes. The crosshole radar tomographic images show a decrease of the radar wave velocity of approximately 10% within the experiment region. Discrete regions of below and above average radar wave velocity have been interpreted to reveal the approximate positions of intense fracturing, mafic xenoliths and possibly zones of unfractured granite. Information obtained from these borehole radar data sets has been included in the construction of a litho-structural model of the experiment region.