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

Abstract

Directional borehole radar and azimuthal square-array resistivity surveys were used to determine orientations of fracture zones in a crystalline-rock aquifer in the Piedmont Province of northern Georgia. The aquifer media is a sheared and highly jointed amphibolite that is overlain by about 25 feet (ft) of regolith. A production well tapping fracture zones in this aquifer supplies ground water to the City of Lawrenceville, Ga., at an average rate of about 230 gallons per minute (gal/min). Four bedrock observation wells near the production well yield more than 100 galimin each, and respond quickly when the production well is pumped. An integrated suite of borehole geophysical logs, including caliper, focused resistivity, long- and short-normal resistivity, gamma, acoustic televiewer and velocity, temperature, spontaneous potential, fluid resistivity, deviation, heat-pulse flowmeter, and video camera, and single-hole directional radar, were collected in the four observation wells to delineate and characterize zones of fracture permeability. Orientations of highly permeable zones were determined by correlating results from directional borehole radar with surface azimuthal square-array resistivity, standard borehole geophysics data, and detailed surface geologic structural mapping. Single-hole borehole radar reflection surveys were conducted using a 60-megahertz (MHz) directional antenna that allowed interpretation of the orientation and projected borehole intersection depth of reflectors as far as 115 ft from the borehole. The borehole radar data include many reflectors; however, only reflectors that could be correlated with other geophysical anomalies and production zones reported in drilling logs were selected for further reflector orientation analyses. Stikes of many reflectors near production zones were determined to be NlO-20W, N405OW, and E-W (N80E to N80W), dipping 24-88 degrees. Depths of these fracture zones ranged from about 30 to 290 ft below land surface. Caliper, video camera, and acoustic televiewer logs indicate that productive fracture zones are associated with zones of borehole enlargement and intense fracturing. Anomalies interpreted from focused resistivity and acoustic velocity logs also were associated with borehole fracturing. Surface azimuthal square-array resistivity surveys were used to provide estimates of the orientation of areally extensive fractures. Results of azimuthal square-array resistivity soundings indicated fractures oriented at N30-60W, N15E, and E-W. Apparent resistivities of the bedrock are low, ranging from about 55 to 450 ohm-meters. Using calculated anisotropy ratios of 1.35 to 1.75, the interpreted secondary fracture porosity in the bedrock aquifer ranges from 13 to 25 percent. These values of fracture porosity are extremely high and most likely reflect the influence of foliation and (or) sulfide mineralization on bedrock anisotropy and apparent resistivity data.