Urban waterways are important transport pathways, as well as depositional sites for sediments and contaminants. Geophysical methods are one class of techniques that can be used to characterize sediments within urban impacted lake and river systems. A seismic reflection survey was performed in Summit Lake, located in downtown Akron, Ohio. Ground penetrating radar (GPR) surveys were performed in both Summit Lake and the Cuyahoga River Le Fever and Brecksville dam pools, located in Cuyahoga Falls, Ohio and Brecksville, Ohio, respectively. The methods were qualitatively assessed to determine their effectiveness and limitations within these urban settings. Seismic reflection profiles could image the lake bottom in all water depths, except in depths less than 3.6 m deep because transducer ringing overprinted all geologic reflectors. Much of the Summit Lake sediment was charged with methane gas, which completely attenuated the seismic signal and prevented subbottom penetration of the seismic energy. Fortunately, there were areas of less gassy sediment where subbottom penetration was achieved. Sediment cores were collected and the subsurface seismic reflectors were confirmed to correspond to pronounced lithologic changes. GPR surveys conducted from a rubber boat penetrated 3 m or less of lake/river water. The only subbottom signals recorded by the GPR system were point diffractors that were confirmed to be buried logs by probing.
From field observations it was evident that there was a relationship between the lake/river bottom reflector amplitudes and the sediment grain size as inferred from probing. Sediment samples ranging from mud to gravel were collected and compared to seismic or GPR data collected at the sample site. The sediment samples were measured for grain size, composition, porosity, organics, density, and P-wave velocity. The amplitude of the lake/river bottom reflector was extracted from the seismic and GPR profiles. Methods were developed to relate lake/river bottom reflection amplitude to sediment grain size. It was found that grain size could be approximated by the lake/river bottom reflector amplitude using both geophysical theory and empirical site-specific relationships from the sediment samples. Seismic and GPR amplitude analysis methods successfully classified all test samples into the correct Wentworth size class (i.e., mud, sand, gravel). However, remotely sensing sediment grain size is limited because seismic reflection and GPR methods were both site and equipment specific. In addition, not all water depths could be imaged by the seismic reflection and GPR equipment.
GPR surveys performed within the Brecksville Dam pool were also successful in locating and delineating a submerged historic crib dam. Identification of this submerged structure is now being used for the historical assessment component of the Brecksville Dam removal planning process.