Sidescan sonar mapping
By Dr. Michael K. Faught
For years we have tried to describe the bottom of the Aucilla River. Terms like “highly variable, craggy, and variegated” come to mind. There are places with rocky shoals, there are sandy bottoms, sinkholes, and leafy sediment beds. There are dead fallen trees, branches and boulders. There are also a few old docks and boats. This highly variable bottom, in combination with the low visibility of the dark tannic waters, makes diving in the Aucilla treacherous and finding evidence for prehistoric peoples challenging, to say the least. For years we have desired a way to map the bottom of the river, to make sense of its morphology, and to predict where we might go to find additional sediments for coring, or excavations, to seek out more data about Paleoindian and other material culture (archaeology) in the river.
We now have the capability to do this because of a familiar piece of equipment that is being used in new ways. In the summer of 1996, while having dinner with Roger Smith of the Bureau of Archaeological Research, and Brett Phaneuf, graduate student with Texas A&M’s Nautical Program, and discussing the benefits of Marine Sonics’ Windows based high resolution sidescan sonar equipment, I was excited to think that this same device might reconstruct the channels in the geological features of the Woodville Karst Plain. (See “Wakulla Springs Karst Plain symposium”). Phaneuf’s relating how well the device worked in a rock quarry got me to thinking about how well it might work in the Aucilla. The usual targets for sidescan sonar devices are shipwrecks and other historic features protruding from usually flat sandy bottoms.
As one of the cooperative efforts between the University of Florida and Florida State University, several students and staff undertook sidescan sonar imaging of portions of the Aucilla River in April of 1998 as an experiment in its utility. The device, purchased by FSU’s Program in Underwater Archaeology, is a Marine Sonics splashproof model with Pentium CPU, 2 gigabyte hard drive, and Windows 3.1 environment. The towfish sends out pulses at 600 Khz, and the device is integrated with a Trimble NT200D differential GPS that determines position from satellites to an accuracy of about 12 feet (4 meters). It is very useful.
However, at the time of the Aucilla River survey in April our equipment was undergoing some growing pains. The GPS unit and the sidescan both performed admirably, but they were unable to communicate because of a burned out serial port. We were able to overcome the adversity by manually recording the locational data, but the post-processing time has been greatly increased because of it.
The data consists of about 200 megs of images on CD ROM, along with a GPS position data sheet taken every 30 seconds. We still have to lay out the track-lines and determine the placement of the images on local maps. The device read out straightens out the river track way, but the images can be saved as *.tif files and cropped to fit a GIS based map in proper order and alignment. The figure shown is a segment of the Aucilla, north of Williams Fish Camp, but south of Ward Island. In this image one can discern rocky portions and smooth sediment filled portions, some logs and evidence of erosion. The river flow is from top to bottom, the width of the image is 150 meters.
We have scheduled another trip this spring to re-do the data so that the locational fixes are integrated onto the images and to expand our coverage to the Page/Ladson site. The crew on the April 1998 cruise included: Michael Faught, Andy Hemmings, Binion Williams, Joe Latvis, Thadra Palmer, Grayal Farr, and John Davidson.