GNGTS 2014 - Atti del 33° Convegno Nazionale

18 GNGTS 2014 S essione 3.1 knowledge of the Sorrento continental slope, the Salerno Valley and the northern and southern Sele highs. A fifth project deals with the morpho-bathymetric surveys committed to the CNR IAMC of Naples by the Authority of the Naples harbor for monitoring the marine pollution of sediments. Dedicated software as the PDS2000 (Thales), the NEPTUNE (Merlin) and the ISIS (Triton Elics) have been used for the cartographic restitution of bathymetric data. The bathymetric maps, both contour isobath maps and shaded-relief maps have been interpreted with the aim of reconstructing the main morphological lineaments occurring at the sea bottom, in particular for the Naples Bay canyons, the continental slope off the Sorrento Peninsula, the sedimentary basin of the Salerno Valley and the related depocenters. Bathymetric profiles have been also produced, allowing to distinguish erosional and depositional areas. The correlation of Multibeam data with some significant seismic profiles recorded in Naples and Salerno Gulfs has completed the geological interpretation. The Multibeam is a sonar allowing for the high resolution reconstruction of the sea bottom morphology. It is a highly technologic instrument having its development during the last fifteen years, since only the power of modern computers has allowed to manage the large amount of data and related operations (Veen and Buckley, 1988; Krim and Viberg, 1996; Van Trees, 2002 ). The principles of functioning of the instrument are here briefly recalled From the transducer (head) it starts a belt of acoustic signals (beams), perpendicular to the ship’s tracking. The times of arrival of each single beam are recorded and corrected based on the head’s starting angle and to the sound velocity profile, previously measured. For the known problems on the ship’s instability, the data are corrected through an accelerometer, measuring the heave, roll and pitch of the ship. The acquisition system is interfaced with a positioning system, usually a differential GPS. One fundamental advantage of the Multibeam system is that of surveying a belt of sea bottom varying from four to six times the water depth and allowing, due to the data redundancy, the creation of three-dimensional models, namely the Digital Terrain Models (DTM). The large number of points (x, y, z) gives the processing phase very long. The methods of automatic cleaning (statistical filters, gradient filters, beam intersection filters) may provoke the loss of the requested detail and are then applied with large parameters after a first cleaning of the spikes. It starts then a phase of detailed morphological control of the Multibeam data. The acquisition of Multibeambathymetric data in the frame of previouslymentioned research projects has been carried out by using Multibeam systems used in coastal marine environment and shallow waters (Multibeam Reson 8125, Multibeam Reson 8101) and Multibeam systems of intermediate and deep waters (Multibeam Reson 8111, Multibeam Reson 8160), depending on the finalities of the work. Moreover, the Multibeam Simrad EM3000 has been used for the acquisition of morpho-bathymetric data among water depths of 4 and 50 meters. Some principles of electroacoustic subaqueous methods and applications to the case studies of Naples and Salerno Gulfs are here resumed. Among all the forms of radiations, the sound is the only which better propagates in the sea. The electromagnetic waves are attenuated in the salt waters of the sea, in a major measurement than the mechanical ones. Due to this characteristic, the sound may be used for many applications in the water exploration and these applications are determined by the use of the sonar, defined as the system which uses the acoustic energy to carry out the measurements in an aquatic environment. Some typical sonar applications include the individuation of banks of fishes, the bathymetric measurements and the stratigraphy and the geomorphology of the sea bottoms. The sonar may be active or passive. An active sonar may use a proper acoustic source, while a passive sonar records the signal generated by the object of interest. The equation of the sonar has been developed during the Second World’s War as an aid in the determination of the maximum distance of functioning of the subaqueous acoustic systems. Starting from that period, the equation has been used to describe the functioning of a wide range of submarine measurement systems. The equation of sonar includes all the factors regarding the generation, propagation and attenuation of the sound in the water; for this reason, it constitutes