FRACTURE DETECTION IN CRYSTALLINE BEDROCK USING SQUARE-ARRAY RESISTIVITY METHODS, EASTERN CONNECTICUT

Fractures in crystalline bedrock in eastern Connecticut were detected using azimuthal square-array and crossed square-array direct-current resistivity methods. To determine anisotropy as a function of depth, the square arrays were expanded and rotated around a central point, with each array sampling a cube of earth with dimensions approximately equal to the length of the side of the square (i.e., a-spacing).

Two field sites in the footwall of the Honey-Hill Lake Char Fault, both underlain by porphyritic gneiss, yielded different fracture strikes despite being only ½ mile apart. Whereas graphical representations of azimuthal data from Site A indicate one principal fracture strike of 045°-060° for a-spacings of 5.00m to 40.00m (with apparent minor strike directions), data from site B indicate three recurring fracture strikes of 120°, 030°, and 150°. At Site B a fracture strike of 120° dominates at a-spacings of 5.00m to 14.14m; 030° dominates at 20.00m and 28.28 m; and 165° dominates at 40.00m. At least three directions of fracture strike are apparent at Site B at each a-spacing.

The fracture strikes (at both sites A and B) determined using the azimuthal square-array data are consistent with data collected by colleagues conducting a detailed structural analysis of the region during the same field season. Due to multiple fractures detected at both sites, the crossed-square array method was not successful in analytically determining fracture orientation. In this study, we attempt to resolve the complex azimuthal plots into their respective apparent resistivity ellipses in order to determine the coefficient of anisotropy and the secondary porosity of the fracture systems.