GNGTS 2014 - Atti del 33° Convegno Nazionale

76 GNGTS 2014 S essione 3.1 since our target is shallow and ultra-shallow waters, we are able to control the echosounder pulse length from 50 to 500 µs via software. This would widen the use of the system to both shallow and ultra-shallow environments and to deeper areas, such as the coastal zones. Moreover, we implemented a robust bottom detection/depth of the echographic signal [see Gasperini (2005) for further details]. In this way, the shallow-depth limit was reduced to 0.1 m. In order to carry out estimates on the seafloor reflectivity, the echosounder signal is sampled with constant time windows, and the data (the echograms) stored in SEG-Y format (Barry et al. , 1995) files using a specific acquisition system based on a Raspberry-Pi board. Subbottom Profiler. A typical chirp-sonar subbottom profiling (SBP) system, operating with standard magnetostrictive transducers, at high-voltage (hundreds to thousands of volts) is not suitable for operating on board of SWAP, either for the heavy weight of the transducers and for the power consumption. For this reason, we developed an innovative SBP system based on electromagnetic resonators ( µChirp, ISMAR-CNR) that show interesting performances in the shallow-water environment, and, due to its lightweight, it is easily installed on the vehicle. The system is composed by: 1) a digital generator of frequency-modulated signals based on an Arduino Due board; 2) a 600W RMS power amplifier; 3) an array of waterproof magnetic resonators composed of four 4Ω MONACOR transducers; 4) an acquisition system based on a hydrophone array, a signal amplifier, and an Arduino Due board used as analog-to-digital- converter (ADC). A Raspberry-Pi is employed to store the digital data in SEG-Y format on a SD memory card. Side-Scan Sonar. We employed a Starfish 450F Side-Scan sonar system, interfaced to a PC board trough an USB port. The system is powered by a 450 kHz CHIRP transmission, to provide a wide range sonar coverage up to 100 m per channel (200 m total swathe) with good, clear image definition. All data, including echograms, seismic lines and side-scan sonar records were converted processed and interpreted using the open-source package SeisPrho (Gasperini and Stanghellini, 2009). Case studies. The Valli di Comacchio Coastal Lagoon. The Valli di Comacchio coastal lagoons are shallow, brackish water environments connected to the Adriatic Sea, which extends south of the Po river between Comacchio and the Reno River. These lagoons (Valli) formed around the tenth Century as a consequence of subsidence and were originally fresh water basins supplied by river floods (Bondesan, 1986). The hydrodynamics of the Valli di Comacchio is controlled by the inflow of freshwater from several sources, but the tidal cycle is a major controlling factor on water circulation in the lagoon, causing water-depth excursions of over 1 m. Because the average depth of the Valli is < 2 m, most of the lagoon areas are not accessible to geophysical surveys performed using conventional vehicles. For this reason, we carried out a combined high-resolution seismic reflection and morphobathymetric survey of the lagoon using the chirp-sonar system and the 200 kHz echosounder mounted onboard of SWAP. The acquisition of the entire echosounder sweep at each sounding point, rather than the simple depth estimate, gave us the opportunity to calculate the bottom reflectivity. Propagation and scattering of high frequency acoustic sound at or near the bottom is controlled by a number of factors, including biological, geological, biogeochemical and hydrodynamic processes operating at the benthic boundary layer (Richardson and Briggs, 1996). However, experimental measurements of compressional wave attenuation suggest that the single most important geotechnical property related to acoustic attenuation is the mean grain size of the insonified sediment (e.g., Shumway, 1960; McCann and McCann, 1969, 1985; Dunlop, 1992). Moreover, a quantitative ground- thruting of relationships between floor reflectivity and mean grain size has been carried out in Valle Fattibello, N of the main lagoon (Gasperini, 2005). Using the reflectivity pattern calculated at each surveyed point of the lagoon floor enabled us to map the distribution of sediments in the Valli di Comacchio lagoon. We note a different distribution of sediments E and W of the Poscoforte Peninsula, that cuts in the N-S direction the lagoon. This could be due to the fact that