DISCOVRE 2008

Cruise Log: 10/21/2008

Mapping the Seafloor with Multibeam Sonar

Steve W. Ross, Chief Scientist

We left Pascagoula, MS on Sunday for the second leg of our DISCOVRE 2008 mission. Unlike the last leg, we basically have a single, but very important, objective, and that is to map the bottom at selected deep reef target sites. This objective calls for a smaller scientific crew of only four people. The ship's two survey technicians will be the main ship personnel helping us collect data. We will examine two areas: one at the head of DeSoto Canyon, about 80 nautical miles off Mobile Bay and a deep coral area off the West coast of Florida. The tool we use is called multibeam sonar, and the detailed bathymetric maps that result from this instrument have added tremendously to our ability to conduct our work in these rugged, deep reef ecosystems.

Specifically, the Nancy Foster has a Kongsberg EM 1002 multibeam sonar. It processes 111 individual sonar beams that paint a picture of the bottom that can be as wide as 1500 m, and it can sound the bottom to depths of 1000 m. Large portions of the ocean bottom can be mapped with complete coverage in a relatively short time. In addition to sounding bottom depths, we also obtain data on the strength of reflected signals (backscatter data), and we analyze this to help interpret general bottom type (hard versus soft). First, we make a CTD (conductivity, temperature, depth) cast and load these data into the multibeam system which helps the system make adjustments for sound velocity in the water. About every six hours we make new casts in case water conditions have changed. Then, the ship steams down pre-plotted, overlapping transect paths at a speed of about five to six knots (a knot is one nautical mile per hour). The survey technicians load the data into another computer and clean it, removing obviously bad sonar returns. Finally, we can make detailed (about 5 m resolution) two and three dimensional pictures of the bottom.

While the work is routine and less active than our other sampling, it is interesting. We have to keep in mind that we are mapping the deep seafloor in new ways and seeing it in detail never before presented. Much of the depth soundings in deep water may be hundreds of years old, and were not updated because as long as water was deep enough for navigation, then other details were not needed. Clearly this is inadequate for scientific investigations, especially of deep reef ecosystems. Last night we mapped the small target site off Alabama where some black corals had been collected. We did not find any indication of an elevated reef, so now we are steaming to our major mapping site off Ft. Myers, FL. This area is rugged, large and in about 500 m of depth. We look forward to producing some exciting pictures starting later tonight.

Above image from multibeam sonar is a cleaned two dimensional view of the coral mounds at Vioska Knoll 826, one of our major study areas in the Gulf of Mexico. Colors represent different depth contours (violet=deepest and tan=shallowest). - (image credit: Erik Cordes and Andrea Quattrini (Lophelia II, Reefs, Rigs, and Wrecks expedition, second leg). - click to enlarge