GPR IMAGING OF COASTAL SEDIMENTARY DEPOSITS ALONG THE LAKE SUPERIOR SHORELINE OF THE HURON MOUNTAINS, MICHIGAN, USA
The GPR acquisition systems used for the study were pulseEKKO 100 and 1000 with 50, 100 and 225 MHz antennae frequencies. Step sizes and antennae separation varied from 0.1 m to 2.0 m depending on the site conditions. Each trace was vertically stacked with a sampling rate adjusted to antennae frequency. The digital profiles were processed and plotted using pulseEKKO software. Basic processing included automatic gain control (AGC), signal saturation correction, trace stacking (horizontal averaging) and point stacking (running average). To calculate depth, near surface velocity measurements were calculated from CMP surveys. Topographic surveys, using a Topcon laser leveler, of the GPR traverses were performed to complete terrain corrections, and to compare the position of the subaerial stratrigraphy. The application of radar stratigraphic analysis (distinct signature patterns) on the collected data provided the framework to investigate both lateral and vertical geometry and stratification of the coastal deposits.
The GPR transects represent the first shallow geophysical profiles collected along Lake Superior’s shoreline of the Huron Mountains. The GPR transects record the shallow subsurface stratigraphy (>10-15 m depth) of episodically accreting beaches and dune ridges in this active littoral cell. The observed reflections from the collected profiles are interpreted as beachface and upper shoreface deposits of an actively prograding shoreline. The images also document the location of prehistoric and historic shoreline positions (lake levels). The results demonstrate that coastal sedimentary deposits can be mapped and show the utility of this geophysical tool as a regional, geomorphic mapping tool of coastal environments.