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Many bedrock units contain joint sets that commonly act as preferred paths for the movement of water, electrical charge, and possible contaminants associated with production or transit of crude oil or refined products. To facilitate the development of remediation programs, a need exists to reliably determine regional-scale properties of these joint sets: azimuth of transmissivity ellipse, dominant set, and trend(s). The surface azimuthal electrical resistivity survey method used for local in situ studies can be a noninvasive, reliable, efficient, and relatively cost-effective method for regional studies. The azimuthal resistivity survey method combines the use of standard resistivity equipment with a Wenner array rotated about a fixed center point, at selected degree intervals, which yields an apparent resistivity ellipse from which joint-set orientation can be determined.
Regional application of the azimuthal survey method was tested at 17 sites in an approximately 500 km2 (193 mi2) area around Milwaukee, Wisconsin, with less than 15 m (50 ft) overburden above the dolomite. Results of 26 azimuthal surveys were compared and determined to be consistent with the results of two other methods: direct observation of joint-set orientation and transmissivity ellipses from multiple-well-aquifer tests. The average of joint-set trend determined by azimuthal surveys is within 2.5° of the average of joint-set trend determined by direct observation of major joint sets at 24 sites. The average of maximum of transmissivity trend determined by azimuthal surveys is within 5.7° of the average of maximum of transmissivity trend determined for 14 multiple-well-aquifer tests.
Douglas Carlson received a Ph.D. in geosciences with a minor in water resources engineering from the University of Wisconsin-Milwaukee in 2001, an M.S. degree in geophysics from the New Mexico Institute of Mining and Technology, and B.S. degrees in geology and geophysics from the University of Minnesota. He was a physics and astronomy instructor at Ball State University and a physics lecturer at the University of Wisconsin-Stout in between M.S. and Ph.D. studies. He was a hydrologist for the U.S. Geological Survey, a geology lecturer and a geology associate lecturer for the University of Wisconsin-Milwaukee while completing his Ph.D. studies. For the past eight years, he has been working for the Louisiana Geological Survey as an assistant professor-research. His research interests include application and interpretation of near-surface and borehole geophysics for the determination of aquifer characteristics. He is also interested in analysis of spatial distribution of water quality and water supply and hydraulic properties within aquifers for the development of groundwater models for regional water resource management.