Towed hydrophone arrays are often used for scientific research and mitigation purposes to detect vocalising marine wildlife, especially mammals. Towed hydrophone arrays comprise multiple hydrophone elements (underwater microphones) drawn behind a vessel. When connected to a Data Acquisition Unit (DAU) and laptops with specialised software, Passive Acoustic Monitoring (PAM) operators are able to monitor for vocalising marine mammals in real-time. See www.pamoperator.co.uk for more details about the role of Passive Acoustic Monitoring operators.
Advantages of towed hydrophone arrays
Towed hydrophone arrays are useful for monitoring marine mammals at night and during poor weather conditions, when visibility is reduced. The use of towed hydrophone arrays can be combined easily with visual observations by Marine Mammal Observers, MMOs (www.marinemammalobserver.com) allowing both methods to complement each other for effective marine mammal monitoring during industrial anthropogenic noise activities.
Towed hydrophone arrays are useful for detecting marine mammals that are difficult to observe visually. For example, harbour porpoises (Phocoena phocoena) are very small marine mammals observed only in calm conditions, and beaked whales (Ziphiidae) are inconspicuous deep divers, seen only during brief surface intervals. The distinctive echolocation clicks produced by these animals adds to the ease of their identification during acoustic monitoring with towed hydrophone arrays.
Data from towed hydrophone arrays can be recorded and stored easily, allowing for further inspection of detections. This allows PAM operators to confirm initial detections, minimising uncertainty.
How towed hydrophone arrays work
A hydrophone converts a sound signal (pressure wave), into an electrical signal which is processed and viewed on a computer screen and/or heard by a PAM operator. Specialised software is used to assist in the detection of marine mammal vocalisations and echolocation clicks. This is essential when dealing with sounds that are outside our hearing range (ca. < 20 Hz – >20 kHz). Either calls can be analysed in real-time, or automatic detection software can inform the user of species identity. PAMguard (www.pamguard.org) is the prevalent software used throughout the seismic industry, but other signal processing software is available such as Raven (www.birds.cornell.edu/brp/raven/ravenoverview.html). For playback of sound in spectrogram format, with time on the horizontal axis and frequency on the vertical axis, Ishmael (www.bioacoustics.us/ishmael.html) is a useful tool.
When multiple hydrophones are placed in an array, the bearing to the vocalising marine mammal can, when conditions are right, be calculated based on the Time of Arrival (TOA) difference of the received signal on two different hydrophones in the array.
Towed hydrophone arrays from OSC
Ocean Science Consulting Ltd. (OSC) offers top of the range, towed hydrophone arrays for research and mitigation. With in-depth knowledge and experience in underwater acoustics, marine mammals, and industry application, OSC is able to provide towed hydrophone arrays capable of performing at the highest standard. All of our towed hydrophone array systems are quality-controlled at our warehouse facilities by our PAM technicians, and are dispatched with 100% redundancy, ensuring full back up in the field.
Ocean Science Consulting is a Scottish-based company that has been providing towed hydrophone arrays since 2004. With a strong focus on R&D, OSC offers custom design services whilst executing a wide range of research and monitoring projects with towed hydrophone arrays. Ocean Science Consulting also offers autonomous static acoustic monitoring systems (www.staticacousticmonitoringsystems.com), such as T-PODs (www.tpodclickdetector.com) and C-PODs (www.cpodclickdetector.com), but also real-time telemetered monitoring devices, capable of delivering data to the user anywhere in the world. Ocean Science consulting also offer acoustic noise measurement and noise field propagation modelling services (www.underwatersoundmeasurement.com).
OSC towed hydrophone array operators and vessel
Ocean Science Consulting’s towed hydrophone arrays are used by a variety of companies throughout the world (www.osc.co.uk/clients.php). All towed arrays are installed by our qualified and experienced engineers, and operated by our highly trained and experienced full-time employees. All OSC staff are degree qualified in a science discipline, and have been chosen through a competitive screening process. We subcontract only when full-time staff are unavailable, when we then draw from our database of >1500 contractors. All OSC Passive Acoustic Monitoring operators benefit from 24-hour land-based technical support throughout the duration of the survey.
All OSC Passive Acoustic Monitoring operators are also trained and experienced Marine Mammal Observers, allowing for increased flexibility when working in the field and offshore. See www.marinemammalobserver.co.uk for more information on our Marine Mammal Observers.
Ocean Science Consulting owns and operates a trailerable Invader 772 research vessel, Orca, fully equipped and rigged with two towed arrays and an MMO platform. Orca is a category 3 vessel licenced for six (plus skipper) and is available for private charter for inshore survey work or for use with our towed hydrophone arrays, MMOs and PAM operators.