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Joints within unconsolidated material such as glacial till can be primary avenues for the flow of electrical charge, water, and contaminants. To facilitate the siting and design of remediation programs, a need exists to map anisotropic distribution of such pathways within glacial tills by determining the azimuth of the dominant joint set. The azimuthal survey method uses standard resistivity equipment with a Wenner array rotated about a fixed center point at selected degree intervals that yields an apparent resistivity ellipse. From this ellipse, joint set orientation can be determined.
Azimuthal surveys were conducted at 21 sites in a 500-km2 (193 mi2) area around Milwaukee, Wisconsin, and more specifically, at sites having more than 30 m (98 ft) of glacial till (to minimize the influence of underlying bedrock joints). The 26 azimuthal surveys revealed a systematic pattern to the trend of the dominant joint set within the tills, which is approximately parallel to ice flow direction during till deposition. The average orientation of the joint set parallel with the ice flow direction is N77°E and N37°E for the Oak Creek and Ozaukee tills, respectively. The mean difference between average direct observation of joint set orientations and average azimuthal resistivity results is 8°, which is one fifth of the difference of ice flow direction between the Ozaukee and Oak Creek tills. The results of this study suggest that the surface azimuthal electrical resistivity survey method used for local in situ studies can be a useful noninvasive method for delineating joint sets within shallow geologic material for regional studies.
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 8 yr, 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 the 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.