Azimuthal Electrical Resistivity As A Tool For Determination Of The Orientation Of Preferred Hydraulic Transmissivity For A Dolomite Aquifer In Southeastern Wisconsin

Abstract

Joints in rock act as avenues for the flow of both electrical charge and water. Therefore methods which can efficiently map anisotropic distribution of electrical pathways should also provide a good indication of hydraulic anisotropy. The azimuthal electrical resistivity survey method has been successfully used as a method for determining the trend(s) of generally local joint sets in a variety of rock and soil types. This study, which is a regional study, demonstrates that the azimuthal resistivity method can be a reliable method for determining the trend of regional joint sets within the Silurian-Devonian dolomite, a single rock type, in southeastern Wisconsin. In particular the study demonstrates on a regional scale the consistency of azimuthal resistivity survey results with results from direct observation of joints and from the analysis of numerous multiple-well-aquifer tests conducted nearby. For this study, 26 azimuthal resistivity surveys were conducted at 17 sites scattered throughout the approximately 500 km2 area around Milwaukee, Wisconsin. Direct observation by previous workers have shown two major sets of joints within the dolomite at 24 sites throughout southeastern Wisconsin. Azimuthal resistivity surveys for this study have been able to reproduce the average observed joint trend to within 3 degrees. In addition, the transmissivity ellipses from 14 multiple-well-aquifer tests in nearby Mequon, Wisconsin are compared with the apparent resistivity ellipse results from azimuthal resistivity surveys. The average tend of maximum transmissivity obtained from this study's azimuthal resistivity surveys is within 6 degrees of the average trend of maximum transmissivity determined by multiple-well-aquifer tests. In addition, the azimuthal resistivity surveys may be can differentiated between which of the joints sets should dominate the flow of water.