Airborne time-domain electromagnetic method was used to model the detailed distribution and thickness of near-shore sand deposits along the Illinois’s Lake Michigan shoreline. This is the first time airborne time-domain electromagnetic (TEM) method has been used in the Great Lakes of North America. Significant erosion along the Illinois coastline has highlighted the need for a large scale and cost effective means to acquire spatially-rich data to build models of sediment distribution along the entire shoreline. Thus, we implemented a high-resolution TEM method to determine sand distribution along the shore from the beach to ~1 km into the lake. We acquired 1049 line-kilometers of data extending from Kenosha, WI to Chicago, IL. Through inversion of the TEM data, models displaying variations in electrical resistivity from the lake surface (and land surface in some areas) through bedrock (~300 m total depth) were developed. We used existing lithology logs from the area to establish relationships between the measured resistivity and sand and clay units. Given the contrast in resistivity between sediments in the lake (including sand/gravel, clay and till) and the fresh lake water, we are able to map bathymetry, different geological units, and the top of the bedrock. We focused on mapping the spatial distribution and thickness of the uppermost sand unit, which ranges from 0 – ~10 meters thick (Figure 1). The sand unit is thickest (4.5 – ~10 m) in the Zion and Waukegan area, in the northern part of the study site. In southern Wisconsin and south of Waukegan to Chicago, the sand layer underneath the water column is very thin, <1 m (Figure 1). Recent (2019) nearshore probing in the northern part of the study area measured sand thicknesses ranging from 0.73 to 7.59 m. The new observations and older lithologic core logs correlate well with the interpreted sand thicknesses from the TEM survey, demonstrating that TEM can be an effective method for mapping large-scale sediment distribution in coastal settings. Results from this project will help inform coastal managers and improve management of shoreline erosion and accretion issues and better manage and protect coastal resources.
|Title of host publication
|AGU Fall Meeting 2019, 9-13 December 2019, San Francisco, California
|Published - 2019