ACOUSTIC BACKSCATTER CHARACTERIZATION OF THE SEABED IN FRONT OF THE MARATEA COAST (BASILICATA, ITALY) USING MULTIBEAM SEABAT 8125

The Institute for coastal marine environment (IAMC) of Naples carried out a multidisciplinary study focused on the construction of subsurface lithological and bionomic maps along the Maratea coast (Basilicata region, south Tyrrhenian sea) which represent the raw data for the definition of marine protect area. For this purpose we collected data by acoustic swath mapping, sediment sampling and scuba investigation. In September of 2007 the entire margin of the Basilicata Tyrrhenian coast was mapped using the Multibeam Seabat 8125 which, apart from producing very high resolution bathymetric information, can record backscatter information with the so called "sidescan sonar option". In practice the Multibeam's two most external receivers work as the receivers of a standard sidescan sonar by recording the acoustic response of the seabed, in relation to the morphology, composition and fine scale roughness of the beds. The main difference between standard sidescan and the "multibeam sidescan" is that the first may be brought to the towing vessel to the wanted depth, while the "multibeam sidescan" is mounted to the pole and its resolution decreases with increasing depth. Nevertheless the high-resolution data of acoustic swath mapping with Seabat 8125 allowed to create a 20 cm per pixel sidescan sonar mosaic until to -50 m depth. The boundaries of the recognized acoustic facies were subsequently digitalized with the software Geomedia Professional so to create a first interpretation by eye. In June 2008, on the basis of acoustic facies recognized, forty-five samples along the coast (-10 to -30 meters) were collected by grab Van Veen which, combined with scuba investigation, have been used to validate the sidescan acoustic interpretation. In the end all data were collected making it possible to obtain two extremely detailed seabad lithological and bionomic maps. Furthermore the differences between the acoustic signals derived from different facies, in terms of pixel intensity of the sidescan image, have been tested using statistical methods.