Radar analysis of the Grand Island Tombolo, MI, USA: A case study for coastal landscapes

Subsurface stratigraphy provides insight into the genesis and evolution of coastal landscapes. Since ground penetrating radar (GPR) provides an ideal way to image subsurface stratigraphy it is commonly used to help understand how coastal landscapes evolve. The Lake Superior shoreline provides an excellent opportunity to study coastal environments. Grand Island, Michigan, USA is the largest island on the southern shore of Lake Superior and consists of several geomorphic features including bedrock highs connected by a sandy tombolo. A GPR transect of the Grand Island Tombolo utilized a pulseEKKO 100 GPR system with 100 MHz antennae, 1.0 m seperation and step size of 0.5 m. An average depth of penetration of 25 m was determined after analysis of a common midpoint survey. A Topcon RL-H3CL laser level was used to collect topographic data to adjust profiles for changes in relief. Radar stratigraphic analysis divides the profile into four radar facies. The deepest radar facies include packages of sigmoid and hummocky reflections. A second radar facies downlaps lower radar facies and consists of northward dipping inclined reflections. A third radar facies, concordant with lower radar facies, includes continuous subhorizontal to inclined reflections. The upper radar facies down laps lower radar facies with northward dipping inclined reflections. Our interpretations indicate radar facies relate to shoreface and beach progradational phases of the Grand Island Tombolo´s.