FRESH-WATER RUNOFF EFFECTS ON SHALLOW WATER MULTIBEAM SURVEYS

The effects of incorrect sound speed values on multibeam survey accuracy are well-documented. Typical artifacts appear in the logged data such as "ripples" when the Sound Velocity Profile (SVP) is overestimated (frowning) or undervalued (smiling). These events can be caused by a fresh-water runoff or upwelling; it is possible to re-compute bathymetric data in post-processing by generating an equivalent SVP that can be applied to the raw data. During the last ten years the Coastal Marine Environment Institute of the National Research Council (I.A.M.C. - C.N.R, Naples, Italy) has developed national and regional research programs on morpho-bathymetric mapping. The Regional Geological Cartography (CARG) Project was carried out to study the Campanian continental shelf, from 3 meters to 200 meters depth. Multibeam data collected in shallow water during the CARG were often suffering from smiling or frowning because of the fresh water run-off produced by rivers along the Campanian coasts. In this paper the method used to correct these data and the use of these fixes for tracking unreported underwater springs are described.