GEOPHYSICAL INVESTIGATION OF GLACIAL LAKE SHORELINES IN LUCAS COUNTY, OHIO

Near-surface geophysical methods are providing important constraints on a surprisingly diverse stratigraphy associated with shorelines of glacial lakes in northwest Ohio. Seismic and resistivity surveys were conducted at various locations in the Oak Openings sand ridge to determine stratigraphy. Geophones and electrodes were spaced closer than normal in order to resolve the shallow details. A variety of stratigraphic units were found in the area, including silts, sands, clays, bedrock, and possibly a stony till in a wetland area. Stony till is in an area exhibiting an electrical resistivity higher than that of soft, water-saturated clay sampled in boreholes but significantly lower than that measured in bedrock. The seismic refraction method using first arrivals distinguished the sand, clay, and bedrock layers, but depth to bedrock did not correspond with results from the resistivity model. Using second arrivals in the seismic surveys indicate a masked layer which results in a depth to bedrock model consistent with the electrical resistivity model. These second arrivals are more easily identified and timed when high-frequency geophones are used. When compared to core data, both seismic refraction and electrical resistivity indicate that hand auger refusal does not necessarily indicate the top of carbonate bedrock. Coring, however, does provide more stratigraphic detail of the near surface sediment layers. When investigating glacial lake shorelines, these geophysical methods provide the basic stratigraphy of an area with minimal ground disturbance. These geophysical methods may then direct future coring locations.