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Towed Hydrophone Arrays

Towed hydrophone arrays are the most common Passive Acoustic Monitoring solution for large survey areas and real-time mitigation purposes.

In the last few decades, public interest in marine mammals has risen and awareness of conservation issues has become widespread.

Marine mammals are most often studied using visual methods, and while this is still effective and good practice, in some cases this is unsuitable considering cetaceans (whales, dolphins and porpoises) spend most of their time below the water surface. Poor weather conditions and limited visibility also reduces the effectiveness of visual observations.

To combat these shortcomings, visual observations by Marine Mammal Observers (MMOs; www.marinemammalobserver.co.uk) are often used in conjunction with Passive Acoustic Monitoring (PAM) systems (www.passiveacousticmonitoringsystem.com) which are employed to detect vocalising marine mammals. Passive Acoustic Monitoring is not recommended as an effective mitigation measure for pinnipeds (seals, sea lions and walruses), which do not echolocate. These animals do vocalise underwater, but visual detection is a more appropriate mitigation measure.

In recent years, underwater Passive Acoustic Monitoring systems (www.pamsystems.co.uk) have become more reliable and cost-effective, facilitating mainstream research. Increased global awareness of marine mammal conservation issues has ensured the use of towed arrays as Environmental Impact Assessment (EIA) tools or real-time mitigation instruments.

Currently, towed hydrophone arrays are the most common Passive Acoustic Monitoring (www.passiveacousticmonitoring.co.uk) solution for large survey areas and real-time mitigation purposes.

Towed hydrophone arrays

Towed hydrophone arrays use multiple sets of hydrophones (underwater microphone) elements that are encased in oil or resin and towed astern vessels. At the top-end of the system, it is possible to receive process and analyse the wide spectrum of sounds produced by marine mammals covering large survey areas.

The Passive Acoustic Monitoring Operator using the towed array must be capable of recognising the signals from marine mammals, ranging from low frequencies (required to detect some baleen whale vocalisations as low as 10 Hz), to high frequency sounds that are common in porpoise echolocation clicks (reaching up to 150 kHz). Trained Ocean Science Consulting Ltd (OSC) personnel are able to identify animals (and when possible species) emitting such sounds, by careful recognition of their frequencies and sound patterns.

In some cases, by combining these features with stereo hydrophone sets monitoring in the same spectrum, it is possible to obtain, via sound triangulation, the source emitter's range from the system. Triangulation is not often required for common mitigation purposes, especially when the mitigation zone (exclusion zone around a source) exceeds the distance at which it is possible physically to hear the vocalisations of higher frequency animals such as harbour porpoises (Phocoena phocoena). Clients should be aware of false claims as to the “detection distances” of some Passive Acoustic Monitoring systems on the market. All detections depend on various factors such as oceanographic/bathymetric features, the loudness of the sound, its frequency and whether the animal is on-axis to the hydrophone or not. Moreover, cetaceans do not vocalise all of the time.

OSC towed arrays

Ocean Science Consulting provides two non-invasive, passive acoustic towed array options.

For use inshore, a new generation of high quality towed array systems have been developed by OSC to fit the demands of research, industry and regulators. Using these systems, OSC often provides services together with the 8 m research vessel R/V Porpoise that can be used for surveys up to 20 miles from shore in any European location (see www.osc.co.uk for more details).

The second towed array system is a commercial towed hydrophone array solution for use during seismic surveys or other offshore operations.

These systems are operated by our Passive Acoustic Monitoring operators who have the high level of training and experience required to operate and retrieve this equipment accurately and efficiently.

All systems commercialised by OSC have proven track records in the field. While many factors influence detection rates, both towed array systems have detected various cetacean species successfully in waters world-wide.

Complementary systems

Towed hydrophone arrays are not the best solution when static or temporal acoustic surveys are required; however, OSC also provides inexpensive and easy to recover echolocation click detectors known as T-PODs (www.t-podporpoiseclickdetector.co.uk). Multiple T-PODs can be widely deployed in order to obtain data on odontocete (toothed whales, dolphins and porpoises) habits and interactions. We are also experts in the use of the T-PODs successor, the digitally-based C-POD (www.cpodclickdetector.com) that provides extended capabilities and higher quality results. These smaller, fixed Passive Acoustic Monitoring Systems are more appropriate for long-term studies of cetacean echolocation, especially before, during and after industrial activities. Ocean Science Consulting’s scientists were the first in the world to detect porpoises feeding around oil and gas installations in the North Sea (www.osc.co.uk).

Why choose Ocean Science Consulting?

Ocean Science Consulting assures operability, even in the most extreme conditions. Our towed hydrophone arrays have been designed for reliability; their solid construction avoids delays during poor weather conditions and during deployment. All OSC towed arrays are fully tested before field mobilisation. While we have never lost an array or experienced any operational downtime since company incorporation in 2004, our systems are supplied with 100% redundancy, and benefit from 24-hour technical support.

Since all our towed arrays are operated exclusively by our own Passive Acoustic Monitoring operators (www.pamoperators.com) and all parts are interchangeable between sets, we have the capability to repair malfunctioning equipment immediately.

Company Experience

Ocean Science Consulting is a Scottish-based company which has fulfilled world-wide business and research requirements since 2004. Our highly qualified employees have worked for clients such as Shell, BBC, SFF, National Geographic, the militaries of several countries, Noble (drilling), Noble Denton, PGS, Fugro and Gardline. Please consult our references at www.osc.co.uk for a full list of clients. Client testimonials are also available upon request. We are contracted both directly or subcontracted and adhere strictly to confidentiality agreements.