Use of borehole radar techniques to characterize fractured granitic bedrock at AECL's Underground Research Laboratory

Single-hole radar reflection and cross-hole radar tomography surveys have been used to assist in characterizing a 105-m3 block of granite rock at AECL''s Underground Research Laboratory (URL) in southeast Manitoba, Canada. The surveys were conducted in a series of seven boreholes drilled in moderately fractured granite rock from the URL 240 meter level. The RAMAC borehole radar system with rated dipole antenna frequencies of 22 and 60 MHz was used for these surveys. Results of single-hole radar reflection surveys revealed several linear reflectors and hyperbolic diffractions events. Some of the linear reflectors were interpreted to be reflections from fracture planes; others were from boreholes near or within the survey area. The hyperbolic diffractions are from point reflectors related to discrete vertical fractures or inhomogeneities in the rock. The 60-MHz surveys provided high-resolution reflection records and detected reflectors up to 50 m away from the boreholes. Compared to 60-MHz surveys, the 22-MHz reflection data showed marked decrease in resolution but considerable increase in probing-range (∼100 m). Both the 22- and 60-MHz surveys were able to detect water-saturated discrete fractures and fracture zones a few centimeters thick. Reflections from the HQ size (96-mm diameter) boreholes were also detected in both the 22- and 60-MHz reflection surveys. The radar velocities in the Moderately Fractured Rock (MFR) study block varied from 105 to 125 m/μs, which translates to a total velocity variation of 8–10% in the URL granite (with average velocity 120 m/μs). Results from borehole radar surveys were compared with core log data and hydraulic test results from the boreholes. The single-hole reflection data correlate well with fractures and fracture zones observed in the core logs. Combined interpretation identified low dipping fracture zones (with 10–30° dip) and two sets of subvertical fractures trending northeast and southwest. In addition, the radar velocity images from tomographic surveys show good correlation with the geologic model reconstructed from core log data. Above-average radar velocities correlate with more competent rock and lower velocities with more fractured rock. The tomography interpretations are also consistent with transmissivity values from hydraulic tests in the boreholes. The regions of low radar velocity anomalies correspond to transmissivity values of 1×10−6–1×10−8 m2/s in the boreholes, and high radar velocities to transmissivity values of 10−12–10−13 m2/s. In addition, the lower radar velocities correlate with increase in permeability as observed from groundwater flow measurements (e.g. 22 l/min in borehole MF12) and higher radar velocities corresponding to lower groundwater flow rates (e.g. 0.5–0.8 l/min in borehole MF6). [Copyright &y& Elsevier]