Detection of bedrock fractures and lithologic changes using borehole radar at selected sites

Borehole-radar methods were used to map fractures and lithologic changes in metamorphic and igneous rock at sites in New Hampshire and Massachusetts, and in sedimentary rock at a site in lllinois. Interpretations of single-hole and cross-hole surveys were correlated with other geophysical and hydrologic data. Single-hole borehole-radar surveys using directional antennas were conducted in New Hampshire and Massachusetts. The surveys detected reflectors, which are interpreted as individual fractures or fracture zones that extend laterally up to 40 m (meters) from wells. Average radar velocities at the sites are 0.113 mlns (meters per nanosecond) in gneiss and 0.123 mlns in granite. Single-hole directional surveys were also conducted in lllinois. Reflectors that are interpreted as fracture zones and lithologic changes in dolomite were detected up to 25 m from the wells, and reflectors that are interpreted as solution zones were detected up to 15 m from the wells. The radar velocity in rock at this site is 0.081 mlns. Changes in lithology and permeability appear to correlate with differences in the amplitude and arrival time of the radar pulse that travels through the bedrock directly between transmitter and receiver. Cross-hole radar tomography surveys were conducted at the Massachusetts and lllinois sites to determine velocity and attenuation changes in bedrock between wells. Low-velocitylhighattenuation zones identified can be interpreted as lithologic boundaries and (or) water-saturated fracture zones.