STRATIGRAPHY OF FLUVIAL COMPOUND BARS AT HERITAGE PARK, COLERAIN TOWNSHIP, OHIO

This project aims to further understanding of fluvial compound bar stratigraphy along the Great Miami River in southwestern Ohio. We conducted Electromagnetic Induction (EMI) and Electrical Resistivity Tomography (ERT) surveys on a compound bar near Heritage Park in Hamilton County, Ohio. The complex depositional processes result in heterogeneous stratigraphy that is best investigated through non-invasive geophysical techniques.

The EMI device used was an EM-421 from DUALEM. EMI survey lines were positioned longitudinally along the compound bar at 5m intervals. Additionally, 5 perpendicular lines were analyzed to provide three-dimensional data for cross-sections. The ERT device used was a Syscal Kid Switch from IRIS instruments and was employed to calibrate the EMI data before inversion and determine water table depth. For this, a Wenner array was used with a-spacings of 1, 2, and 3m. Data processing involved removing outliers from the EMI data and contouring the results using Surfer. An inversion was performed on the ERT data and used to forward model the expected EMI response. The forward model response was then used to calibrate the field EMI data. Calibrated data were used to calculate the depth of investigation. Results were then integrated to build a 3D model of the surveyed area.

Results reveal a distinct pattern of low conductivity in the bar center with increasing conductivity toward the edges, suggesting grain-size variation from coarse-grained central deposits to finer-grained marginal deposits. The EMI survey identified three distinct layers: a resistive upper layer (0-2m), a transition zone (2-3.5m), and a conductive deeper layer (>3.5m), indicating the transition between surface and groundwater. The ERT data revealed discontinuities in the high-resistivity layer, indicating heterogeneous sediment distribution or pre-existing bar morphology control on deposition.

Compared to similar studies of fluvial bars, this site shows distinctive vertical stratification and clear lateral facies changes. The integration of EMI and ERT methods provides a more complete understanding of compound bar internal structure than either method alone, with implications for understanding fluvial depositional processes and groundwater flow patterns in similar settings.