Use Of Directional Borehole Radar And Azimuthal Square-Array D.C. Resistivity Methods To Characterize A Crystalline-Bedrock Aquifer

Directional borehole radar and azimuthal square-array resistivity surveys were used to determine<br>orientations of fracture zones in a crystalline-rock aquifer in the Piedmont Province of northern Georgia. The aquifer<br>media is a sheared and highly jointed amphibolite that is overlain by about 25 feet (ft) of regolith. A production well<br>tapping fracture zones in this aquifer supplies ground water to the City of Lawrenceville, Ga., at an average rate of<br>about 230 gallons per minute (gal/min). Four bedrock observation wells near the production well yield more than<br>100 galimin each, and respond quickly when the production well is pumped. An integrated suite of borehole<br>geophysical logs, including caliper, focused resistivity, long- and short-normal resistivity, gamma, acoustic<br>televiewer and velocity, temperature, spontaneous potential, fluid resistivity, deviation, heat-pulse flowmeter, and<br>video camera, and single-hole directional radar, were collected in the four observation wells to delineate and<br>characterize zones of fracture permeability. Orientations of highly permeable zones were determined by correlating<br>results from directional borehole radar with surface azimuthal square-array resistivity, standard borehole geophysics<br>data, and detailed surface geologic structural mapping.<br>Single-hole borehole radar reflection surveys were conducted using a 60-megahertz (MHz) directional<br>antenna that allowed interpretation of the orientation and projected borehole intersection depth of reflectors as far as<br>115 ft from the borehole. The borehole radar data include many reflectors; however, only reflectors that could be<br>correlated with other geophysical anomalies and production zones reported in drilling logs were selected for further<br>reflector orientation analyses. Stikes of many reflectors near production zones were determined to be NlO-20W, N40-<br>5OW, and E-W (N80E to N80W), dipping 24-88 degrees. Depths of these fracture zones ranged from about 30 to 290<br>ft below land surface. Caliper, video camera, and acoustic televiewer logs indicate that productive fracture zones are<br>associated with zones of borehole enlargement and intense fracturing. Anomalies interpreted from focused resistivity<br>and acoustic velocity logs also were associated with borehole fracturing.<br>Surface azimuthal square-array resistivity surveys were used to provide estimates of the orientation of<br>areally extensive fractures. Results of azimuthal square-array resistivity soundings indicated fractures oriented at<br>N30-60W, N15E, and E-W. Apparent resistivities of the bedrock are low, ranging from about 55 to 450 ohm-meters.<br>Using calculated anisotropy ratios of 1.35 to 1.75, the interpreted secondary fracture porosity in the bedrock aquifer<br>ranges from 13 to 25 percent. These values of fracture porosity are extremely high and most likely reflect the<br>influence of foliation and (or) sulfide mineralization on bedrock anisotropy and apparent resistivity data.