GNGTS 2015 - Atti del 34° Convegno Nazionale

The data, downloaded from ISPRA web site www.mareografico.it , have been imported in the acquisition software PDS2000, to obtain a full interpolated tidal curve. Bathymetry. Multibeam echo sounders (MBES) emit sound waves in the shape of a fan from directly beneath a ship’s hull. These systems measure and record the time it takes for the acoustic signal to travel from the transmitter (transducer) to the seafloor (or object) and back to the receiver; by the analysis in term of amplitude and phase of the received signal, with beam forming techniques, it is possible to detect the incidence angle and the distance. In this way, MBES produce a “swath” of soundings (i.e., depths) for broad coverage of a survey area. The coverage area on the seafloor depends on the depth of the water, typically three to five times the water depth. MBES is an useful tool to acquire water depth information, to determine least water depths over critical items such as wrecks, obstructions, and dangers to navigation and to detect objects in general. The morpho-bathymetric survey was carried out with multibeam echosounder, Teledyne- Reson Seabat 7125. This dual frequencies (200 and 400 kHz) instrument utilizes 512 equidistant beams with a maximum ping rate of 50 pulse per second. To correct the raw data, the system has to know precisely, for each pulse, the vessel attitude in terms of pitch, roll, heave, yaw; this is the task of the Motion reference unit model MAHRS, produced by TSS-TELEDYNE. The roll value is also used to steer the acoustic pulse so to insonify always the nadir area of the seafloor. Data acquisition was performed at a velocity of 4 knots along parallel lines allow to obtain an overlap of 20%. As the acoustic pulse can be oriented, we obtained images not only the seabed but also the pillars supporting the wharf. Time to depth conversion was made by real time data from Sound Velocity Probe, Reson SVP71, and sound velocity profiles using the YSI CastAway CTD. Side scan sonar and magnetometric survey. The Side Scan Sonar (SSS) is a technique based on the emissions conical or fan-shaped pulses down toward the seafloor across a wide angle perpendicular to the path of the sensor through the water. SSS produces images of the sea floor similar to a large-scale aerial photographs useful to detect seafloor installations (i.e., cables, pipelines, etc.), shipwreck and other obstructions that may be hazardous for navigation. Depending on the different acoustic response it is possible to obtain also indirect information about the nature of the sediment (sand, mud or their mixtures) or the presence of outcropping bedrock, metal bodies, etc. Fig. 1 – Position map. GNGTS 2015 S essione 3.2 65