Unconsolidated sediment thickness mapping by waterborne geophysics along the Lake Michigan shoreline

Mapping unconsolidated sediment transport at the Illinois Lake Michigan shoreline (ILMS) is complex, but also vital for sustainable management and use of this dynamic system that has undergone significant redistribution of sand in the littoral transport system in the past. To understand the erosion and accretion processes it is critical to map the ILMS sediments in high spatiotemporal resolution as the system is constantly changing over time. Here we used two geophysical methods, waterborne electrical resistivity imaging (wERI) and sub-bottom profiling, ground-truthed by hydraulic jet probing and historic borings, to map the thickness of unconsolidated sediments along two reaches at the western ILMS. At the two reaches, geophysical methods show that the sediments have not undergone folding and were generally horizontal, and the thickness of the unconsolidated material ranges between 4 to 5 m. Both methods are in agreement with jet probe results which provide direct evidence of loose sediments up to depths of 4 to 5 m below the lakebed. The wERI shows more detailed variation in the sediment and bedrock topography than the other methods. Overall, the geophysical methods, particularly the wERI, appear as effective tools to accurately map the sediment structure along the ILMS at high spatial resolution. Considering the relatively low cost of the operation of geophysical surveys and simplicity of data analyses, wERI and sub-bottom profiling show promising potential for comprehensive and frequent mapping of the ILMS. The methods supplement the limited extent of direct sampling and the lower spatial resolution but great extent of airborne geophysics and provide the information needed for better understanding of sediment transport mechanisms.