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IDENTIFYING FRACTURE ZONES IN CRYSTALLINE BEDROCK AQUIFERS OF NEW HAMPSHIRE USING SURFACE GEOPHYSICAL TECHNIQUES
National Ground Water Association
Ground Water Expo, December 7-9, 2001, Nashville, Tenn.
James R. Degnan, U.S. Geological Survey
Richard Bridge Moore, U.S. Geological Survey
Abstract
Bedrock-fracture zones near high-yield bedrock wells in southern New Hampshire were identified and located using a number of surface geophysical survey techniques, including ground penetrating radar (GPR), seismic refraction, magnetometer, electromagnetic (EM), very low frequency (VLF) electromagnetic, and two-dimensional (2D) and azimuthal-square-array direct current (dc) resistivity surveys. A variety of surficial and bedrock hydrogeologic settings were surveyed. Geophysical data, from line and array methods conducted at survey sites, were processed and interpreted. The use of several methods allowed for an integration and comparison of results.
Previous site characterizations that included geologic and lineament data were used to enhance geophysical interpretations. Geologic data collected as part of a regional-scale investigation included outcrop fracture measurements, ductile fabric, and contact information. Lineament data, developed as part of statewide and regional-scale investigations of bedrock aquifers, were available as indicators of the potential presence of near-vertical fracture zones.
Surface geophysical techniques helped determine location, orientation and other characteristics of fractured bedrock. Seismic refraction and GPR were used primarily to characterize the overburden materials, but in a few cases they were also indicative of bedrock-fracture zones. Magnetometer surveys were used to identify magnetic lows that may result from weathering of fractured rock. EM and VLF surveys were used to identify electrically conductive anomalies that indicate potential fracture zones. However, the low conductivity of crystalline bedrock often made it difficult to distinguish VLF and EM survey anomalies from atmospheric and cultural noise.
DC resistivity surveys were used to provide subsurface information about fracture depth and orientation. Two-dimensional dc-resistivity surveys using Dipole-Dipole and Schlumberger arrays were used to characterize the overburden, bedrock, and bedrock-fracture zones through analysis of field- and model-data inversions. Results of azimuthal-square-array dc-resistivity surveys indicated orientations of conductive, steeply dipping bedrock-fracture zones. In some cases, this method corroborated results from other geophysical survey methods, observed lineaments, and outcrop-fracture measurements. The azimuthal square array and 2D dc resistivity surveys required more time to complete than electromagnetic techniques. However, the detail of the data obtained was useful in interpreting the complex patterns of fracturing in crystalline bedrock that underlies a variety of glacial overburden materials in New Hampshire.
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