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).