GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.1 19 a subject of study of the submarine acoustics and is the basic component in order to project systems for geophysic and oceanographic applications. The sonar, due to its technical characteristics, is used in many applications, both scientific and technologic. It is used for conventional bathymetric measurements or Multibeam bathymetry. By resuming, the acoustic echosoundings measure the travel time of the acoustic pulses and the related echoes to survey the sea bottom. The conventional acoustic survey determines a depth value for each pulse, while the Multibeam system determinates many points of measurement in a cross section. The echo intensity should be used to estimate the nature of the sea bottom. The acoustic systems as the Subbottom profiler, the Sparker, the Airgun and the Watergun are used in order to carry out stratigraphic surveys on the sea bottom. These systems use the acoustic pulses of minor frequency, with respect to the acoustic echosounders, in order to use the characteristics of penetration, reflection and refraction of the signal. In this category of instruments we find the systems of shallow seismic (Subbottom Chirp, Sparker) and deep seismic (Watergun, Airgun). Lateral scanning sonar are usually used for the geomorphologic studies. These systems measure the shallow lithology of the sea bottom by using the interferometric principle to survey the cross sections and to record the echo intensity, as a function of the nature of the sea bottom. These instruments are also used in order to survey particular structures or objects at the sea bottom (antropic structures as pipelines or relicts). High frequency fish finders are used for the biological studies, with the function to survey fish banks, accompanied by software for the calculation of biological masses. Moreover, passive sonar are used to track the migratory tracks of the great Cetaceous and for studies of bioacoustics. The procedures of Multibeam processing applied to the Gulf of Salerno are here briefly mentioned. The treatment for the verification and the processing of the bathymetric data with a high resolution Multibeam technology has been carried out accordingly to several phases. The first phase includes the application of the tidal curve to the data, in order to correct the altimetric variations. This phase of elaboration is performed accordingly to one of the three methods, i.e. the correction through a single mareographic station, the correction through two or more pounded stations, based on the distance from the survey area, the correction through tables of tides for works at a regional scale and at high water depths. The second processing step includes the correction of the GPS navigation. This is carried out by visualizing the tracking with the fix in relationship to the diagram of the HDOP (horizontal dilution of precision), which is one of the more precise methods to estimate the horizontal accuracy of the GPS (latitude and longitude). Some mistakes in the navigation lines will correspond to discontinuities and abrupt increase of the values of HDOP, which will be corrected by manually shifting the mistaken fixes and interpolating the positions. The third processing step includes the applications of the profiles of sound velocity along the water column. Normally it is acquired a sound velocity profile each 4-8 hours, in the survey zone by using velocity profiles or CTD multiparametric probes to measure the temperature, the salinity and the pressure. The fourth processing step is represented by the visualization of the single beam and the manual and statistical elimination of the spikes. This cleaning is carried out through the visualization of 50 swaths for each time, equal to the half of the measured ones, in such a way to guarantee a notable precision and accuracy of the filtering. The processed beams have then been used for the creation of a DTM (Digital Terrain Model), which furnishes a three-dimensional representation of the sea bottom. In the DTM the knots are given from the average of the depth of the beams, which falls in the inner of a dimension cell established from the operator. The DTM is further processed through filters operating on the standard deviation or manually clean from the cells affected by the mistakes. The bathymetric data of the Salerno Gulf have been processed both according to the above described procedure and with a statistic method: Combined Uncertainty Bathymetric Estimator (CUBE), ideated from the University of New Hampshire (U.S.A.). With this second method a time advantage has