TWO-DIMENSIONAL DIRECT-CURRENT-RESISTIVITY PROFILING IN A CRYSTALLINE ROCK AQUIFER NEAR LAWRENCEVILLE GEORGIA

Two-dimensional direct-current resistivity (2D dc-resistivity) profiling was used to detect fracture zones in a crystalline-rock aquifer near Lawrenceville Georgia. This work, which is a component of a ground-water resource investigation, was conducted by the U.S. Geological Survey in cooperation with the City of Lawrenceville. Profiling using 2D dc-resistivity methods is conducted by laying out an array of electrodes along a straight line and measuring an apparent resistivity. Electrical current is injected into the ground through two current electrodes and the voltage difference is measured at two potential electrodes. A multi-electrode system was used to collect apparent resistivity readings along the linear arrays. A commercially available switching unit was used to automatically select four electrodes along the array; this allowed the collection of several hundred to several thousand measurements along a single profile, depending on the configuration of the array and the number of electrodes used.

Three types of linear arrays were used for profiling. A dipole-dipole and a pole-dipole array appeared to show good horizontal and vertical resolution, whereas a Schlumberger array had poorer resolution, but provided more rapid data acquisition. The pole-dipole had the greatest depth penetration of the three arrays. The 2D dc-resistivity profiles were first conducted at the Rhodes Jordan Wellfield where subsurface fracture zones have previously been characterized. Bedrock resistivity imaging was conducted to a depth of as much as 55-meters (180 feet) using a 4-meter dipole-dipole array and 100 meters (328 feet) using a 4-meter pole-dipole array of 83 electrodes. An electrode spacing of more than 4 meters allows for a greater depth of penetration but with less resolution.

Resistivity profiling was also conducted at a well site where the underlying crystalline-rock aquifer is relatively unfractured. The profile exhibited higher resistivities than those for the Rhodes Jordan Wellfield. Results from the Rhodes-Jordan Wellfield and the unfractured well site provided guidelines that were used to conduct resistivity profiling at other sites being evaluated for ground-water resources in the vicinity of Lawrenceville.