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Underwater Sound Measurement

Underwater sound travels five times faster than in air and under certain propagation conditions, can travel great distances across ocean basins.

Underwater sound travels five times faster than in air and under certain propagation conditions, can travel great distances across ocean basins. When anthropogenic (human-induced) sound is produced either incidentally or intentionally during industrial operations such as oil and gas exploration (seismic, drilling), oil and gas production, dredging operations, military and scientific research, there is a possibility that aquatic wildlife – in particular marine mammals (whales, dolphins, porpoises, seals, sea lions, sea cows, otters and polar bears) – may be impacted by these sounds either positively or negatively.

Marine species that are most likely to be affected by anthropogenic noise are those that have sensitive hearing at the frequencies the sound produces and which have important habitat in areas where industrial operations are being undertaken. Although noise effects are likely to be restricted to species or groups of species, research into the acoustic sensitivity of marine wildlife suggests that odontocete cetaceans (toothed whales, dolphins and porpoises), will be the group most vulnerable to any effects (if any), followed by pinnipeds (seals, sea lions and the walrus). Results into the effects of sound on other marine mammals and their prey species are currently also being reported in the literature.

Understanding dB

The volume of sound is expressed in units of decibels (dB), which provide a measure of the intensity of sound; however, a common misconception is that the decibel scale is linear i.e. 100 dB would be twice as loud as 50 dB. Sound is a log scale. For every 6 db decrease, sound pressure is halved i.e. it’s actually only necessary to drop from 150 to 144 db in order to half the magnitude. In general, small cetaceans have lower hearing thresholds (by about 15 dB at their frequencies of best sensitivity) than seals, and their hearing sensitivity is more skewed towards higher frequencies.

Legal safe underwater sound limits

Numerous countries have issued individual standards (www.marinemammalmitigation.co.uk) for offshore noise monitoring before, during and after construction, such as the employment of Marine Mammal Observers (www.marinemammalobserver.com); however, because of the complexity of underwater environments, few standards specify actual noise thresholds. In the UK, activities that produce high levels of noise in the underwater environment typically require a Food and Environmental Protections Act (FEPA) licence. In Germany, the German Federal Environment Agency (UBA) set a legally binding, threshold consisting of a dual criterion of 160 dB re 1 μPa²s Sound Exposure Level (SEL) and 190 dB re 1 μPa² (peak to peak Sound Pressure Level, SPL), which should not be exceeded at a distance of 750 m around the anthropogenic noise source. This threshold is based on a Temporary Threshold Shift (TTS) found in a harbour porpoise (Phocoena phocoena) at 164 dB re 1 μPa²s SEL and 199 dB re 1 μPa² (peak to peak SPL); thus, the chosen values include some safety adjustment.

Measurement of underwater sound

Understanding and measuring underwater sound accurately is critical during industrial operations in areas where marine mammals are prevalent. Prior to operations, the potential effects of sound are often assessed via desk-top studies that employ sound propagation modelling. Real-time in-field attended noise validation monitoring of these operations may also be undertaken where results indicate dangerous (cumulative) noise levels.

Passive acoustic monitoring (PAM) systems are underwater hydrophones which comprise myriad forms including towed arrays (www.pamsystems.co.uk), over-the side dipping or static moored or pop-up systems, in conjunction with top-end processing units and software that detect, in some cases classify and quantify underwater sound. Ocean Science Consulting Ltd. uses highly experienced acoustic engineers (qualified up to PhD) level in conjunction with state of the art passive acoustic monitoring systems to make sound level and frequency measurements of anthropogenic industrial noise operations such as exploration drilling, pile-driving or noise from Floating Production and Storage Offtake vessels (FPSOs). An example of one of our drilling papers can be downloaded here (Download PDF).

How does loud noise affect marine mammals?

The exact mechanism by which underwater sound can attract or displace marine mammals is poorly understood. Impacts can be both positive and negative. For example, Ocean Science Consulting’s research indicates that porpoises may be attracted to drilling rigs by recognising “rig noise signatures” alerting them to a predictable feeding location between the rigs’ legs (http://icesjms.oxfordjournals.org/content/66/4/734), also known as the “dinner bell effect”. Potential negative effects may also include physical injury, changes in behaviour and vocalisations, interference with acoustic communication, loss of directional hearing capabilities, shifting their acoustic output in frequency and volume to avoid excessive background noise levels or indirectly by affecting thy prey species. Temporary threshold shifts have also been measured in marine mammals and repeated exposure to TTS-causing noise is thought to cause permanent hearing damage or permanent threshold shifts (PTS).

Zones of acoustic impact

A crucial part of sound estimates is the ability to analyse noise in a way that interprets the environmental effect in a biologically significant way. Ocean Science Consulting can, using standard off-the-shelf models, estimate the zones of the levels of noise which can potentially cause harm to animals.

Based on a combination of the hearing characteristics of marine mammals, the masking effects of ambient noise levels, the sound transmission characteristics in the region, and the sound levels of the source, four zones have been identified: (1) The zone of audibility in which the marine mammal can hear the sound; (2) the zone of responsiveness in which the marine mammal reacts behaviourally or physiologically to the sound; (3) the zone of masking in which the sound interferes with the animal’s ability to detect useful sounds or the calls of conspecifics and (4) the zone of hearing loss, in which the sound levels are high enough to cause discomfort or damage to the animal’s hearing system.

About Ocean Science Consulting

Trading since 2004, OSC has gone from strength-to-strength. We specialise exclusively in the field of marine mammals and underwater acoustics and work world-wide for a whole range of clients. Please don’t hesitate to contact us for information.