MAPPING BEDROCK TOPOGRAPHY UNDER GLACIAL DRIFT USING SHALLOW GEOPHYSICAL METHODS IN MICHIGAN, USA

Determining glacial drift thickness and bedrock topography is important for identifying glacial landforms and groundwater resources. Geophysical methods combined with geologic data from boring logs and published maps were used to estimate bedrock depths below sedimentary cover and infer previously unknown glacial features in two adjacent topographic quadrangles (Climax and East Leroy) in Michigan, USA. The study area contains mostly Mississippian Shale bedrock overlain by Wisconsin-aged glacial drift deposited during multiple glacial advances and retreats of the Saginaw Lobe of the Laurentide Ice Sheet. This glacial history created complex landform assemblages that have only recently been mapped in detail at and near the surface, but are generally poorly understood in the subsurface. Buried bedrock valleys typically contain coarse glacial sediment and thus are valuable aquifers, but these can be difficult to locate due to a lack of surface expression or bedrock topography information. The Horizontal to Vertical Spectral Ratio (HVSR) technique was used estimate the thickness of the upper of a two-layer case where a soft geologic layer (glacial sediment) is underlain by a harder substrate (bedrock). Over 400 HVSR station readings were obtained at approximately half-mile intervals across the two quadrangles to create a bedrock surface and drift thickness map. However, the study area has a low bedrock-drift acoustic impedance contrast due to weathered or unconsolidated bedrock, resulting in poor data and only about 250 usable stations. Limitations of a single method were overcome by employing additional techniques such as vertical resistivity soundings and active seismic refraction/reflection surveys that were interpreted and compared with five test borings, previous well logs, industry seismic data, and available remote sensing data for a better understanding of the subsurface and groundwater resources in SW Michigan. This study resulted in the discovery of several bedrock lows of different orientations that are probably bedrock valleys or tunnel valleys, which may contain previously unknown aquifers and provide insight on the Pleistocene glacial history. Topographic bedrock highs correspond to glacial terminal ice margins and also appear to have some control of post-glacial drainage systems.