Assessment of submerged aquatic vegetation abundance using multibeam sonar in very shallow and dynamic environment. The Lagoon of Venice (Italy) case study

Underwater acoustic devices are widely recognized as very effective tools to remotely map and characterize the seabed and overlying habitats. Multibeam echosounder systems (MBES), in particular, deliver high-resolution co-located bathymetry and acoustic backscatter. The processing of MBES signals is a complex problem requiring knowledge of the angular dependence of the reflected signal, the bottom morphology and the water column reverberation. Especially in shallow, tidal environment, such as the lagoon of Venice (Italy), the multipath reflections and the environmental conditions affect the signal quality. Our study aims to find a repeatable methodology for submerged aquatic vegetation (SAV) imaging and its abundance assessment in a very shallow and dynamic environment. Currents, salinity and turbidity influence the acoustic characteristic of water column and benthic backscatter and the SAV detection by multibeam sonar. Combining MBES and ground truth data allowed us to estimate SAV coverage with high accuracy taking into account environmental conditions. We present the preliminary results of a 12 hours experiment in one of the channels of the Venice lagoon partly covered by SAV. The channel was repeatedly surveyed about every hour, using a Kongsberg 2040c dual head multibeam system with two frequencies: 200 kHz and 320 kHz. At the same time CTD profiles were acquired to estimate the environmental conditions, while video sampling was used for ground-truthing. New algorithms were developed for signal and image analysis of MBES signals (including water column data) for efficient and reliable SAV detection and assessment in shallow subtidal environment.