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Measuring Underwater Noise With BASDA14 Buoy


Luc Simon

BASDA14 drifting in Audierne, France,
with two hydrophones.
Assessing noise in the offshore environment, e.g., monitoring noise levels, measuring ship noises, and detecting/studying mammals, is one of the marine sector's key concerns. Passive acoustics are widely used for this task, and data are gathered by various oceanographic actors, from cetacean researchers to piling companies. Measuring underwater noise is not an easy task. Indeed, these noises are so varied and hard to predict that they are quite difficult to record and analyze accurately. From nearshore construction noise to bioacoustic studies, the mission and type of noise to be studied, as well as the deployment modes and equipment, must be taken into account.

Currently, two standard processes are commonly used for in-situ offshore noise assessment: autonomous acoustic recorders and towed arrays. Autonomous loggers are deployed for a fixed period. Recording and resting periods are scheduled according to battery and storage capacity limitations of the equipment. These systems are preferred for long-term, fixed deployments. However, there is a risk of missing a key moment when equipment is not recording. To prevent this situation, several units can be deployed in order to obtain the best acoustic picture of an area. Besides the potential for recording gaps, autonomous recorders also do not retrieve information in real time.

In contrast, towed or remote systems allow for real-time surveying. They generally consist of one or two hydrophone sensors towed at some 100 meters from the ship, with information streamed and processed in real time. The use of towed systems is limited by cable length and recording capacity, and has the risk of being disrupted by ambient noises along cables.

For several years, RTsys (Caudan, France) has proposed solutions allowing both autonomous and towed recordings with a singular piece of equipment. From its experience, feedback from the field and consideration of improving the two common practices for in-situ offshore noise assessment, RTsys decided to develop a new solution to increase capabilities to measure underwater noise.

The BASDA14, a Wi-Fi remote buoy, is the achievement of two key objectives. The first was to have a flexible measurement station allowing monitoring of all types of noises, from explosions to dolphin clicks. The second was to provide a single-person deployable and recoverable remote system, accessible wirelessly in real time at more than 1 kilometer by any connected operator.

The BASDA14 weighs 34 kilograms, making it easily transportable. It is composed of a Wi-Fi antenna, GPS, four hydrophone inputs, serial digital inputs, embedded processing and data storage up to 2 terabytes, and a rechargeable lithium-ion battery pack.

Wi-Fi remote access provides access to a Web-embedded interface via any computer. The user can stream or download selected data and start or stop recordings. Embedded digital signal processing allows the implementation of real-time processing, e.g., for threshold detection and alarms.

The four hydrophone inputs accept any type of hydrophone (passive, preamplified) with any cable length. These four hydrophone channels are synchronized (less than 100 nanoseconds), and the digitization is broadband (3 hertz to more than 500 kilohertz on each channel) with great dynamic (more than 100 decibels). The frequency range allows monitoring of any type of noise, including porpoise clicks (130 to 150 kilohertz) and ship noise (3 hertz to 1 kilohertz). The great dynamic, which represents the difference between the highest and lowest level of noise to be monitored, is important, especially when looking at different source levels, from a discrete fish to very noisy ships to pile driving. Thanks to the four hydrophone channels, it is possible to ensure the collection of a variety of data, for example, combining cetacean-sensitive broadband hydrophones with low-sensitivity sensors for high-level noises or using multichannel acquisition for beamformings. To continue this article please click here.

Luc Simon is the RTsys international project manager. He has worked on the technical and business development of RTsys passive acoustic systems. Simon holds an undergraduate degree in embedded electronics and a master's degree in international business development.

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