F. Zabel

Linking Acoustic Communications and Network Performance: Integration and Experimentation of an Underwater Acoustic Network

Underwater acoustic networks (UANs) are an emerging technology for a number of oceanic applications, ranging from oceanographic data collection to surveillance applications. However, their reliable usage in the field is still an open research problem, due to the challenges posed by the oceanic environment. The UAN project, a European-Union-funded initiative, moved along these lines, and it was one of the first cases of successful deployment of a mobile underwater sensor network integrated within a wide-area network, which included above water and underwater sensors. This contribution, together with a description of the underwater network, aims at evaluating the communication performance, and correlating the variation of the acoustic channel to the behavior of the entire network stack. Results are given based on the data collected during the UAN11 (May 2011, Trondheim Fjord area, Norway) sea trial. During the experimental activities, the network was in operation for five continuous days and was composed of up to four Fixed NOdes (FNOs), two autonomous underwater vehicles (AUVs), and one mobile node mounted on the supporting research vessel. Results from the experimentation at sea are reported in terms of channel impulse response (CIR) and signal-to-interference-plus-noise ratio (SINR) as measured by the acoustic modems during the sea tests. The performance of the upper network levels is measured in terms of round trip time (RTT) and probability of packet loss (PL). The analysis shows how the communication performance was dominated by variations in signal-to-noise ratio, and how this impacted the behavior of the whole network. Qualitative explanation of communication performance variations can be accounted, at least in the UAN11 experiment, by standard computation of the CIR and transmission loss estimate.

Using passive acoustics for monitoring seagrass beds

This paper discusses the ambient noise acquired during the period of one week, from May 8 to 15, 2013 over a Posidonia oceanica bed in the Bay of la Revellata, Calvi, Corsica. The acoustic receivers were moored at 3 locations with water depth ranging from 2 to 20 m. Simultaneously with acoustic measurements, the dissolved O2 was measured by an array of optodes. Preliminary results have shown that the acoustic noise power measured at various locations in the meadow is highly correlated with dissolved O2 (measured by other methods). Nevertheless, a close inspection of noise waveforms shows impulsive signals with shape similar to those produced by snapping shrimp and other shell fish. This work discusses the challenges faced on using these noise waveforms as ecosystem indicators and particularly to monitor the photosynthetic activity.

Variability of the ambient noise in a seagrass bed

This paper discusses the ambient noise acquired during the period of one week, from May 8 to 15, 2013 over a Posidonia oceanica bed in the Bay of la Revellata, Calvi, Corsica. The acoustic receivers were moored at 3 locations with water depth ranging from 2 to 20 m. Simultaneously with acoustic measurements, the dissolved O2 was measured by an array of optodes. The noise field in the band 2-7 kHz was dominant. The noise directionality and bottom reflectivity estimated using a short vertical array show a diurnal variability pattern. The mean noise power measured at all receivers shows also a diurnal variability pattern, which is negatively correlated with the O2 dissolved in the water. Changes in noise power between the night (highest noise power) and day period (lowest noise power) as high as 5 dB were observed at the various locations. These results suggest that the ambient noise power and directionality are correlated with the photosynthetic activity of the seagrass meadow and that a passive acoustic system can be used to monitor the O2 production of the meadow.

Shipping noise field calibration via source inversion

There is significant evidence that the low-frequency mean acoustic noise pressure level in the ocean has been constantly increasing in the last decades. The main noise sources responsible for this increase were identified as ship traffic, offshore construction and oil & gas surveying. The Portuguese Navy funded SUBECO project aims at deploying a network of multiparametric offshore buoys for environmental and acoustic monitoring. A preferred location for the individual buoys is within or close by the ship traffic separation lanes along the west coast of Portugal. It is therefore expected that the received acoustic field will be dominated by shipping noise as a mixture of both short and long range ships. Most ships will have an AIS so their position may be known at all times. One of the objectives of this network is to use the recorded sound to infer and calibrate model predicted noise in a wider area. This work proposes a relatively simple technique for estimating the radiated noise level of ship tracks within acoustic reach of a single (or a small number of closely localized) hydrophone(s). It is expected that the data constrained predicted field with the estimated source levels will provide a better fit to the actual wide area acoustic field. This work is a contribution to fulfill the requirements of Portugal to the European Union Marine Strategy Framework Directive (MSFD) aiming at a good environmental status.

Baseline measurement of underwater noise under the SURGE project

The Simple Underwater Renewable Generation of Electricity (SURGE) project aims at the demonstration of the Waveroller - a Wave Energy Device, to be installed off the West Coast of Portugal, near the City of Peniche. This project includes a task related to the measurement of ambient and operational noise of the Waveroller device. This paper reports on the analysis results of baseline underwater noise recordings performed in September 2010, before any activities related to the installation of the energy device took place.