GROUND PENETRATING RADAR STUDIES ALONG THE PRESENT AND PAST COASTLINES OF THE GREAT LAKES: AN OVERVIEW

Ground penetrating radar (GPR) provides a methodology that enables one to image the subsurface stratigraphy of many geomorphic environments, including coastal settings. GPR studies conducted along the present and past shorelines of large lake basins have provided detailed subsurface stratigraphic information that have aided in better understanding lake history. The Great Lakes contains one of the greatest concentrations of coastal depositional features in the world. This paper presents a summary of results from GPR studies conducted along modern and past coastlines of the Great Lakes including Lake Eric, Lake Michigan, Lake Ontario and Lake Superior.

 GPR is an electromagnetic geophysical tool that enables one to image (2D & 3D) the subsurface stratigraphy of coastal environments in a continuous, non-destructive, and non-invasive manner. The GPR acquisition systems used for the study were pulseEKKO 100 & 1000 with antennae frequencies ranging from 50 MHz to 900 MHz and transmitter voltages ranging from 200 volt to 1000 volt. Often coastal studies are conducted using lower frequency antennae (50-100 MHz) to better understand the complex stratigraphy that is revealed. As our understanding of coastal depositional systems in large lake basins improves, higher frequency antennae (200 - 900 MHz) are being utilized to provide a more detailed image of the subsurface layers. Step sizes and antennae separation varied depending on antennae frequency chosen and on the site conditions. The digital profiles were processed and plotted using pulseEKKO software. Depths of deposits were calculated using near surface velocity measurements from collected common midpoints.

Radar stratigraphic analysis on the collected data provided the framework to investigate both lateral and vertical geometry and stratigraphy of the coastal deposits. GPR profiles, with depths of penetration greater than 30 m, reveal patterns showing coastal growth by aggradation and/or progradation as well as features showing coastal erosion. Interpretations of the results suggest that several different coastal processes are at work within the Great Lakes including longshore transport, changes in sediment supply, and response to lake level changes.