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INTEGRATED APPROACHES to WATER RESOURCES EXPLORATION IN FRACTURE-BEDROCK AQUIFERS USING SURFACE GEOPHYSICAL SURVEYS and REGIONAL STATISTICAL WELL-YIELD ANALYSES
National Ground Water Association
October 3-4, 2002, Burlington, VT
James R. Degnan, U.S. Geological Survey
Richard Bridge Moore, U.S. Geological Survey
Abstract
The U.S. Geological Survey has conducted investigations of New Hampshire's bedrock aquifer using statistical analysis of well yields (statewide and regionally) and by local surface-geophysical surveys . In the statistical analyses, variations in well-yield probabilities were correlated with well-location factors such as proximity to steep slopes, hilltops, surface-water bodies, valleys, large up-gradient drainage areas, and specific types of lineaments. Lineaments may have high well-yield probabilities associated with them if they are correlated with bedrock fractures.
Sites with a variety of features associated with well yields in the statistical analysis, as well as sites with existing high-yield (greater than 40 gal/min) wells, were targeted for surface geophysical surveys. Several surface-geophysical techniques were used to help determine the location and orientation of fractured bedrock at these sites . The sites included several hydrogeologic settings with different surficial materials and bedrock lithologies. At these sites, the computed probabilities of high yielding wells (based on the statistical analysis) ranged from 5 to 38 percent (12 to 38 percent at the location of the existing high-yield wells).
Surface geophysical techniques employed included: seismic refraction and ground penetrating radar (GPR), which were used primarily to characterize the overburden materials (but in a few cases, were indicative of bedrock-fracture zones); magnetometer surveys, which were used to identify magnetic lows that may result from weathering of fractured rock; and Electro-Magnetic (EM) and Very Low Frequency (VLF) surveys, which were used to identify electrically conductive anomalies that indicate potential fracture zones. The low conductivity of crystalline bedrock, however, often made it difficult to distinguish VLF and EM survey anomalies from atmospheric and cultural noise.
Direct Current (DC) resistivity surveys were also 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 electrically 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 2-dimensional 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. The use of several geophysical techniques allowed for a comparison and greater confidence in the results.
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