CHARACTERIZATION OF FRACTURES USING BOREHOLE GEOPHYSICS IN A CRYSTALLINE-BEDROCK AQUIFER, LAWRENCEVILLE, GEORGIA

Borehole geophysical techniques were used to determine geohydrologic characteristics of fracture zones in a crystalline-bedrock aquifer in the Piedmont Province of northern Georgia near Lawrenceville, about 25 miles northeast of Atlanta. The bedrock aquifer is a sheared and highly-jointed amphibolite with biotite-gneiss and button schist overlain by 5 to 55 feet of regolith. Four test wells drilled to an average depth of 600 feet were logged. Well yields, estimated by the air-lift method, range from 50 to 250 gallons per minute. Estimated yields from individual fractures range from 3 to 35 gallons per minute. An integrated suite of borehole geophysical logs were collected, including caliper, focused resistivity, long- and short-normal resistivity, spontaneous potential, natural gamma, temperature, fluid resistivity, and optical and acoustic televiewer. Fracture zones were classified as major and minor; and the zones orientation (strike and dip) and relation to foliation was determined from the caliper, optical and acoustic televiewer, and other log data. Fluid resistivity and temperature logs were used to determine if the fracture zones are hydraulically active. Hydraulically active fracture zones were identified in each well. Depths to the fracture zones range from 80 to 200 feet and the distribution correlates well with driller records.