GEOMAGNETISM OF THE SUBSURFACE ALONG THE NORTHERNMOST ILLINOIS COAST OF LAKE MICHIGAN

Coastal processes directly impact the lives of millions of Illinois residents along the Lake Michigan shoreline, yet very few studies have mapped the geologic framework of the geomorphically complex land-water interface region known as the “white-ribbon” in detail. This portion of the coast is highly dynamic and difficult to map. In northernmost Illinois it is the avenue for the southward littoral transport of approximately 80,000 cubic yards of sand per year. As part of efforts to monitor and quantify this sand volume estimate, we conducted a high-resolution drone-based aeromagnetic survey of both onshore and offshore portions of the coast from Waukegan, IL, to the Wisconsin state line. The primary objective of the survey was to develop a technique to map sand thickness across the highly dynamic region straddling the transition zone from the onshore to the offshore environment. We used a MagArrow magnetometer from Geometrics in October 2020 and June 2021 to map variations in Earth’s magnetic field intensity along the coast. The unit was deployed from a drone at heights ranging from 5 to 20 m. We interpret variations in magnetic field strength to signal the presence of magnetic bodies in the subsurface. These observations establish a baseline condition for further studies addressing coastal sand erosion and transport. Additional data applications include mapping buried subsurface infrastructure that may become exposed by erosion, contextualizing dune ridge forming processes and their eolian/anthropogenic modification, and identification of archeological sites. Ongoing work to ground truth these observations for calibration purposes will help provide high-resolution sand volume-change estimates, such as those associated with construction of a submerged ‘passive’ sand management structure. Geomagnetic data should provide a valuable tool for monitoring the effectiveness of such an engineered structure as well as offering insights into the presence of other subsurface hazards that may pose risks to future construction and/or other potential coastal management activities. This work complements ongoing efforts to monitor shoreline response to climate and lake-level change along the “white ribbon”.