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

Mapping unconsolidated sediment at the Illinois Lake Michigan shoreline (ILMS) is complex but vital for sustainable management and use of this dynamic system which undergoes significant redistribution of sand in the littoral transport system over time. To understand erosion and accretion processes it is critical to map the ILMS sediments at high spatiotemporal resolution. Here we used two geophysical methods, waterborne electrical resistivity imaging (wERI) and sub-bottom profiling (SBP), ground-truthed by hydraulic jet probing and historic borings, to map the thickness of unconsolidated sediments along two reaches of the ILMS. These geophysical surveys show that the sediments have not undergone deformation, and the thickness of the unconsolidated sandy material ranges between 4 and 5 m over semiconsolidated clay and bedrock. Both geophysical methods agree with jet probe results which provide direct evidence of loose, sandy 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 to be effective tools to map the sediment structure along the ILMS at high spatial resolution. Considering the relatively low cost of the operation of geophysical surveys, simplicity of operation and data analyses, wERI and SBP show promising potential for comprehensive 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.