CHARACTERIZATION OF LARGE-SCALE GLACIOTECTONIC DEFORMATION IN THE LUDINGTON RIDGE, MICHIGAN, USING ELECTRICAL GEOPHYSICAL METHODS

A bluff along the eastern shore of Lake Michigan south of Ludington contains Late Wisconsin deformation structures. Differential loading associated with a glacial re-advance caused glaciolacustrine loamy material to deform into several narrow anticlinal structures that rise from below beach level to near the top of the nearly 50 m high bluff. The anticlines separate broad synclines that control local ground water flow and impact bluff stability. The objective of this study was to characterize the orientation and lateral extent of the structures beneath the NW-SE trending ridge. We used different galvanic electrical resistivity methods that exploit the large electrical contrast between the glaciolacustrine loams and overlying coarse sandy outwash material. Electrical resistivity methods have long been part of the geophysical tool set. Recent advances, including the availability of multi-electrode systems and advanced data processing software, have made electrical resistivity tomography (ERT) a popular tool to obtain 2D models of subsurface resistivity. In this study, vertical electrical soundings (VES) were combined with borehole logs and lab-derived petrophysical relationships to characterize the site stratigraphy. Constant-spread traverses (CST) and ERT data were then used to map the spatial extent of the deformation structures. Field, lab, and modeling results presented in this work identify various strengths and limitations of electrical resistivity methods for the characterization of deformation structures in general and glaciotectonic landforms in particular.