Underwater acoustic communication is a key point for performance improvement in an autonomous underwater vehicle (AUV) swarm. The communication process is essential for improving the AUVs localization accuracy for navigation and is a convenient way for sharing information among the AUVs in a network. The objective of this work, which was developed in the COMET and NEMOSENS projects, is to address the communication process required in a mobile underwater wireless network, with a focus on the proposal of an adaptive physical layer methodology. We discuss about the employed channel access method, the frame structure, and we propose the usage of an adaptive guard interval in order to ameliorate the network usage rate. We explain the physical layer aspect of the communication: the data processing at the transmitter and receiver side. In addition to that, we propose the usage of smart communications among AUVs. We design a method for adapting some physical layer parameters. The proposed approach relies only on the knowledge of the transmission geometry, and it optimizes the number of subcarriers and the cyclic-prefix length of the Orthogonal Frequency-Division Multiplexing (OFDM) system. The obtained results show a performance improvement in terms of bit-error rate when compared with the case of random parameters selection. These results corroborate the benefits of our adaptive parameters approach.

This paper describes an end-to-end underwater acoustic transmission system based on MIMO (multiple input multiple output) principle. The described transmission system is experimented over various sea conditions and compared against conventional SIMO (single input multiple output) mode operating with a single transmission stream and multiple receive sensors. By analyzing the optimal performance of the proposed decoder, we select a MIMO and SIMO mode achieving similar robustness against underwater acoustic channel and demonstrate over longterm experiment, that, as foreseen by theory, MIMO technology provide a data rate gain at a same robustness level.

In order to study the geoacoustic relations for shallow water sediments, a project has started in 2014, bringing together French academic and industrial partners. One of this project objectives is to build a large database of acoustical and sedimentological parameters collected in-situ with specific devices. Therefore, an important task is to manage the data acquisition, storage, and exploitation. A two-step process has been designed to achieve this: first, a mobile application which helps the users during the measurements, and second a relational database system which gathers all the data and favors their exploitation. The first dataset come from extensive measurements on the French coastlines. The data acquisition, visualization, and exploitation have been considerably facilitated with this data management system.

Located in Corsica, the Bay of Calvi is a natural environment sheltering protected species such as Tursiops truncatus, fishes (Epinephelus marginatus, Sciaena umbra) and invertebrates. This ecosystem is subject to a higher anthropogenic pressure during summer and becomes a privileged area for leisure and recreational activities. A flotilla of vessels of different sizes appears: jet-skis, zodiacs, sailboats (mainly used as motorboats), yachts, and ferries. An experiment in the summer of 2015 allowed the creation of a database of underwater noise from 190 vessels and the proposal of a method based on the estimation of Sound Exposure Level (SEL) to describe the soundscape of the bay, where the average time corresponds to the mean duration of these vessels’ noises: 60 s. Due to high nocturnal fish activity, the mean SEL is similar during the day and night. The SEL between 7 am and 8.30 pm is higher than the maximum nocturnal fish noise during an average 873% for this period. Ship noise seems to have a limited effect on Tursiops and Sciaena umbra, according to the estimation of both M-weighted and dBht levels, and the biologists’ observations during the research of the STARESO station.

In the framework of the development of a MIMO (Multi-input Multi-output) capable underwater acoustic modem, this paper describes on the one hand a remotely operated experimental platform at sea in real conditions able to transmit and receive real-time multi-streams signals and the other hand an analysis of the experimental channel capacity gain brought by the MIMO approach with respect to traditional single transmitter system. The original remotely operated generation/acquisition system provides an extensive characterization of the MIMO underwater acoustic channel for various transmission ranges and sea states. The capacity analysis extracted from real conditions channel estimates confirm that MIMO approach in experimental scenario leads to a substantial capacity gain with respect to single transmitter even by taking account signal overhead. As foreseen by theory, the MIMO capacity gain is also shown to be dependent from transmission range and transducers arrays configuration.

The knowledge of geoacoustic parameters of the seafloor, especially the sound speed, can be obtained by geoacoustic inversion methods or by direct measurements done in-situ or on cores. The relation between geotechnical parameters such as grain size and acoustical parameters is either given by empirical relations or by theoretical models. The presented work is a part of CARASEDIM, an experimental project devoted to refine these geoacoustic relations in marine sediments, including coarse sands. In this paper, we focus on a velocimeter prototype that is developed for this purpose. It is designed for both laboratory and in-situ measurements, and is light enough to be used by a single diver. It is equipped with two emitting and two receiving transducers. The distance between emitters and receivers can be adjusted between 5 and 40 cm and their depth can vary from 0 to 15 cm. The frequency band is 40 kHz to 400 kHz and any type of signal can be used. We discuss about the processing techniques, the protocol of measurement, and the first results obtained. Some laboratory results are presented in both real coarse sands and artificial glass beads. They are compared with theoretical predictions based on grain-shearing model.