USGS Groundwater Information: Branch of Geophysics
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Bruce P. Hansen, U.S. Geological Survey, Northborough, MA
John W. Lane, Jr., U.S. Geological Survey, Storrs, CT
Four geophysical techniques were used to determine bedrock fracture orientation and other site characteristics that can be used to determine ground-water flow and contaminant transport at a study area underlain by fractured crystalline bedrock in Millville and Uxbridge, Massachusetts. In the study area, azimuthal seismic-refraction and azimuthal square-array direct-current resistivity surveys were conducted at three sites, borehole-radar surveys were conducted in a cluster of three wells, and ground-penetrating radar surveys were conducted along roads.
Azimuthal seismic-refraction data indicated a primary fracture strike ranging from 56 to 101 degrees at the three sites. Graphical and analytical analysis of azimuthal square-array resistivity data indicated a primary fracture strike ranging from 45 to 90 degrees at the same three sites, Directional borehole-radar data from three wells indicated 46 fractures or fracture zones located as far as 147 feet from the surveyed wells. Patterns of low radar-wave velocity and high radar-wave attenuation from cross-hole radar surveys of two well pairs were interpreted as a planai fracture zone that strikes 297 degrees and dips 55 degrees south. Ground-penetrating radar surveys with IOO-MHz antennas penetrated 5 to 50 feet of unconsolidated overburden and as much as 60 feet of bedrock where the bedrock surface was at or near land surface. Horizontal and subhorizontal fractures were observed on the ground-penetrating radar records at numerous locations. Comparison of results from the various geophysical techniques indicates good agreement and indicates primary high-angle fracturing striking east-northeast. Square-array resistivity data yielded an average secondary bedrock porosity of 0.0044 and an average aperture of 0.007 1 foot for high-angle fractures.
Final copy as submitted to Symposium on the Application of Geophysics to Engineering and Environmental Problems for publication as: Hansen, B.P., and Lane, J.W., Jr., 1996, Orientation and characteristics of fractures in crystalline bedrock determined by surface and borehole geophysical surveys, Millville and Uxbridge, Massachusetts, in Bell, R.S., and Cramer, M.H., eds., Symposium on the Application of Geophysics to Engineering and Environmental Problems, Keystone, Colo., April 28-May 2, 1996, Proceedings: Wheat Ridge, Colorado, Environmental and Engineering Geophysical Society, p. 927-940.
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