Swath bathymetry using phase difference: Theoretical analysis of acoustical measurement precision

The phase difference principle is widely applied nowadays to sonar systems used for sea floor bathymetry, The apparent angle of a target point is obtained from the phase difference measured between two close receiving arrays. Here we study the influence of the phase difference estimation errors caused by the physical structure of the backscattered signals. It is shown that, under certain current conditions, beyond the commonly considered effects of additive external noise and baseline decorrelation, the processing may be affected by the shifting footprint effect: this is due to the fact that the two interferometer receivers get simultaneous echo contributions coming from slightly shifted seabed parts, which results in a degradation of the signal coherence and, hence, of the phase difference measurement. This geometrical effect is described analytically and checked with numerical simulations, both for square- and sine-shaped signal envelopes. Its relative influence depends on the geometrical configuration and receiver spacing; it may be prevalent in practical cases associated with bathymetric sonars. The cases of square and smooth signal envelopes are both considered. The measurements close to nadir, which are known to be especially difficult with interferometry systems, are addressed in particular.

DORIS: Processing and management of Sound Velocity Profiles for echosounding applications

If a bathymetric echosounder is the essential device to carry on hydrographic surveys, other external sensors are absolutely also necessary (positioning system, motion unit or sound velocity profiler). And because sound doesn‛t go straight away into the whole bathymetric swath its measurement and processing are very sensitive for all the water column. DORIS is the very answer for an operational sound velocity profile processing.

Acoustic backscatter of small pelagic fish species in the Adriatic Sea

The aim of the present study is to apply a broad range of techniques to increase the knowledge of acoustic properties of Sprattus sprattus, Scomber colias and Trachurus mediterraneus in the Adriatic Sea. A novel study using tethered live fish but not involving hooks and anesthetic was tested on T. mediterraneus and S. colias through several ex situ experiments using a split-beam scientific echosounder operating at 38, 120, and 200 kHz. The mean TS was estimated for 29 live specimens, resulting in a conversion factor b20 value of -71.4 dB re 1 m2 and -71.6 dB re 1 m2 respectively which is ~3 dB lower than the current one in use in the Mediterranean Sea. Successively, two monospecific trawl hauls were analyzed through the application of in situ approach for the computation of TS values of S. sprattus which led to six b20 values for sprat (range, -68.8 dB re 1 m2 to -65.6 dB re 1 m2), all higher than the current known value of -71.7 dB re 1 m2. The high difference up to 4.2 dB compared to the current value translates in a significant decrease of absolute sprat biomass along the time series un to 20%. Finally, 149 specimens of the three species were collected for backscattering model application(i.e. Kirchhoff-ray mode model (KRM) and Finite Element Method (FEM)) from digital images of the fish body and swimbladder obtained from Computer Tomography (CT) and X-Ray scans. The values resulting from the application of KRM and FEM are in agreement with empirical results. In general terms the present work proposes the acoustic backscatter characterization of S. colias, S. sprattus and T. mediterraneus in the Mediterranean Sea.

Mappatura degli habitat bentonici dei canali di marea della Laguna di Venezia tramite strumenti acustici e tecniche di ground-truth

The Venice Lagoon is a complex, heterogeneous and highly dynamic system, subject to anthropogenic and natural pressures that deeply affect the functioning of this ecosystem. Thanks to the development of acoustic technologies, it is possible to obtain maps with a high resolution that describe the characteristics of the seabed. With this aim, a high resolution Multibeam Echosounder (MBES) bathymetry and backscatter survey was carried out in 2021 within the project Research Programme Venezia 2021. Ground-truthing samples were collected in 24 sampling sites to characterize the seafloor and validate the maps produced with the MBES acoustic data. Ground-truthing included the collection of sediment samples for particle size analysis and video footage of the seabed to describe the biological component. The backscatter data was analysed using the unsupervised Jenks classification. We created a map of the habitats integrating morphological, granulometric and biological data in a GIS environment. The results obtained in this study were compared to those collected in 2015 as part of the National Flagship Project RITMARE. Through the comparison of the repeated morpho-bathymetric surveys over time we highlighted the changes of the seafloor geomorphology, sediment, and habitat distribution. We observed different type of habitats and the presence of areas characterized by erosive processes and others in which deposition occurred. These effects led to changes in the benthic communities and in the type of sediment. The combination of the MBES surveys, the ground truth data and the GIS methodology, permitted to construct high-resolution maps of the seafloor and proved to be effective implement for monitoring an extremely dynamic area. This work can contribute not only to broaden the knowledge of transitional environments, but also to their monitor and protection.