Knowledge Centre
Underwater Sound
Source of emissions
Sources of underwater sound offshore are diverse. For some sources such as air gun shots, the mechanisms of sound creation and propagation are well understood, but for other sources such as vessels and drilling operations, multiple origins and paths are possible.
Current seismic exploration techniques commonly use air guns which create an impulsive sound via a rapid release of compressed air in a metal cylinder. The impulse is largely broadband in nature but much of the sound energy is directionally focused towards the seabed.
Pile driving generates sound as the driving hammer strikes the pile. The sound then propagates into the water column and seabed through the pile itself.
Explosives are used by the oil and gas industry for several applications primarily for decommissioning of offshore structures. Explosives create a sharp pressure impulse which is multidirectional by nature but is controlled by using shaped charges that direct the energy into a narrow band to form a cut.
Sound levels originating from offshore installations are dependant on the platform type. Semi-submersible installations may generate more radiated sound than fixed installations when using thrusters to maintain position. Little information has been published on which sources and propagation processes are the most significant in generating sound from installations although sound generated during drilling operations, by the drill bit itself or the drill string or riser does not appear to be a significant source. [NCE, 2007].
Vessels used to support offshore operations are also a source with sound radiated from propellers/thrusters and internal machinery. The characteristics of the sound generated by shipping are governed by the ship size, mode of propulsion, operational characteristics, speed and other factors [NCE, 2007].
Potential environmental impacts
Sound propagation underwater exceeds that in air and absorption is less compared to air. In the marine environment, perception of sound is a significant sense for many organisms including marine mammals, fish and some invertebrates. These animals are relying on acoustic functions for communication, orientation, predator avoidance and in finding food.
Impact assessments of underwater sound on marine life are generally concerned with anthropogenic sounds that overlap in frequencies with the hearing range of marine organisms, except for very loud sound where the peak sound pressure is the important factor. A number of impacts, ranging from interference with the detection of biologically relevant sound signals, to behavioural disturbance, hearing loss and in the worst cases injury and death have been attributed to underwater sound.
It is generally accepted that anthropogenic sound has the potential to adversely affect marine organisms but the significance of these effects is the subject of a large number of research studies, notably on marine mammal behavioural responses. While many of these studies have reported changes in behaviour, no universal conclusions have been drawn as to whether they are biologically important [OSPAR Commission, 2009]. Similarly, no conclusive evidence of a link between sounds of seismic surveys and the mortality of any marine mammals has been established. Studies investigating sound-induced effects on other species are relatively scarce. Fish can be particularly vulnerable especially in their early, larval, stages but it appears that seismic-induced effects do not hinder recruitment to fish populations [OSPAR Commission, 2009].
The difficulties in observing marine organisms in their environment, together with the differing interpretations that can be applied to behavioural responses, makes it difficult to draw definitive conclusions about the impact of anthropogenic sound. As a result the precautionary principle is invoked and control measures are applied to minimise potential impacts.
Key control and mitigation measures
In European waters the, EIA Directive (Directive 25/377/EEC 1985; amended 1997/2003) requires that new projects and activities are subject to an Environmental Impact Assessment (EIA). The Offshore Petroleum Production and Pipelines (Assessment of Environmental Effects) Regulations 1999 (amended in 2007) implement this requirement in the UK and apply it to seismic surveys. The EIA would identify, for example, any geographical or seasonal restrictions that would need to be observed.
In addition, the Offshore Marine Conservation (Natural Habitats, & c.) Regulations 2007 implement the European Habitats Directive 92/43/EC and the Wild Birds Directive 79/409/EEC into UK law and give provisions for the protection of a number of marine species including whales, dolphins, porpoises, marine turtles and the common sturgeon (see European protected species listed in the Annex IV of the Habitats Directive) and wild birds from harmful activities.
The Offshore Petroleum Activities (Conservation of Habitats) Regulations 2001 (amended in 2007) require oil and gas operators to comply with JNCC guidelines for seismic surveys (JNCC 2009) which include several mitigation techniques. Firstly, a survey cannot commence if marine mammals are detected within 500m of the sound source and when it begins the power of the air guns is ‘ramped up, through a ‘soft start’ procedure to give mammals the opportunity to leave the area.are to establish an exclusion zone of 500m for the start of the seismic survey. During surveys in potentially sensitive areas there may be a requirement for the survey vessel to carry specialist Marine Mammal Observers to record the presence of marine mammals and to provide reports at the end of the survey.
References
- OSPAR Commission (2009) Overview of the impacts of anthropogenic underwater sound in the marine environment pp133
- NCE (2007) Review of Existing and Future Potential Treatments for Reducing Underwater Sound from Oil and Gas Industry Activities, Report 07-001, prepared for JIP on E&P Sound and Marine Life, pp185
- JNCC (2009) Guidelines for minimising the risk of disturbance and injury to marine mammals from seismic surveys
Updated: November 2009
