A side scan sonar signal projects a fan that sweeps large areas along the floor up to one hundred meters. The signal is constantly recorded producing a photo like image. Dark images indicate features unique to the floor bottom. Frequencies range from one hundred to five hundred hertz. Better resolution is possible with greater frequency; however, range is sacrificed. The device is towed by a ship so it is closer to the sea or lake bottom and at a lower view angle.
Thunder Bay National Marine Sanctuary and Underwater Preserve in Alpena , Michigan performed side scanning sonar imagery of western Lake Huron . The bedrock here is part of the Traverse and Bayport limestone formations. The survey recorded over a dozen sink holes in this karst region. This sinkhole is several hundred meters in diameter and about twenty meters in depth:
Photo is Courtesy of NOAA OceanExplorer: Lake Huron, Michigan sinkholehttp://oceanexplorer.noaa.gov/projects/thunderbay01/thunderbay01.html
It is interesting to note that Lake Huron 12500 years before present was one hundred meters below today's lake level. Bayport chert is very prevalent in this region. Therefore, there is potential to find 12000 year old Paleo Indian evidence at this site.