USING GEOPHYSICAL METHODS TO DIFFERENTIATE BETWEEN TILL AND GLACIAL RHYTHMITE SEDIMENT AT WILDWOOD METROPARK, TOLEDO, OHIO

Geophysical investigations at Wildwood Metropark in Toledo, OH were used to determine whether geophysical techniques can differentiate between till and glaciolacustrine rhythmite sediments. The study site stratigraphy consists of carbonate bedrock overlain by Late Wisconsinan till, glaciolacustrine rhythmites, and aeolian sand dunes. With the upper and lower contacts of the rhythmites outcropping at only a few sites, the thickness and lateral continuity of the units are poorly known. Geophysical data was collected along two 50-m long parallel transects 5-m apart above a 9-m deep gully in which the rhythmite/till contact is exposed. Geophysical techniques included: seismic refraction (SR) using a 24 channel Field Geode Seismogram with a 2-m P-wave geophone (4.5 Hz) spacing; vertical electrical sounding (VES, 1D) and electrical resistivity tomography (ERT, 2D) using an AGI R1 SuperSting resistivity meter with a 2-m electrode spacing; and ground penetrating radar (GPR) using a Pulse EKKO, SmartCart system with 250 MHz antennae. The GPR penetrated through sand but not through the rhythmites, while the VES, ERT, and SR data recorded the stratigraphy well into bedrock. The SR surveys predicted the depth to till at 8.45 m and 8.89 m for the two surveys. The surveyed depth to the rhythmite/till contact exposed in the gully was 8.75 m revealing that SR can accurately differentiate between the till and rhythmite sediments. The 2D ERT survey could not effectively differentiate the rhythmites from the till sediment because of low contrasts in resistivity between the two layers. The VES results show that the sand, rhythmite, till sediment and bedrock have average resistivity values of 2,200, 42.5, 121, and 217 Ωm, respectively. The results suggest that GPR is the best methodology for determining the thickness of sand over clay-rich sediments, and SR is best to distinguish between the rhythmites and till units. Additional transects from nearby indicate that the sedimentary units are all laterally continuous.