DETECTION OF BEDROCK FRACTURES AND JOINTS BENEATH COVER: GEOPHYSICAL APPROACHES TO AN ENGINEERING GEOLOGY PROBLEM

This research used geophysical methods to detect the presence of sub-vertical failure planes, their direction(s), and their frequency in bedrock beneath soil or glacial drift overburden. Azimuthal measurements using the seismic, electrical resistivity (ER), electromagnetic conductivity and ground penetrating radar (GPR) were made to evaluate which techniques might provide the best indication of the bedrock joints, faults, or shear zones. Measurements were made at multiple sites near each of three locations: Jackson, Alpena, and Grand Ledge, Michigan. The first two are former limestone quarries and the third is underlain by sandstone. Seismic measurements began with one or two linear refraction spreads, used to establish thickness of overburden and identify the S waves. The azimuthal seismic was done by placing the 24 geophones around a 10 or 15m radius circle (15 degree intervals), with the shot point in the center, measuring arrival times of the P and S waves. The EM-31 conductivity was used to quickly determine that there were no wires or pipes in or near the survey circle, as well as to profile along the diameters of the circle at 12 different azimuths. The square array was used for the resistivity measurements in the same circle, with the diagonal of the square being the circle diameter. This array was rotated to 12 unique azimuths at 15° intervals. GPR was tested at one site using 100 MHz antennae. The Rx was at the center, and the Tx was moved around the circumference of the circle at 15° intervals, to measure the arrival times of the ground wave. Reference measurements of the strikes of failure planes were made using a Brunton compass and photographs at adjacent bedrock exposures. Verification of results was done by comparing the geophysical results and the measured strikes and dips of the nearby exposures of the joint systems. The preliminary analyses of the results showed coincidence of the tests results with the strike measurements in some areas, whereas others did not. Some of the methods are very sensitive to variations in thickness of the overburden. These effects, as well as lateral resistivity gradients in the bedrock and overburden can apparently cause false indications of fracture systems.