This report has been prepared by Subacoustech Ltd., for the UK Department of Trade and Industry under programme RDCZ/011/00018 entitled ‘A feasibility and demonstration study; active and passive detection of marine mammals’. This is the first report delivered as part of this programme aimed at ‘estimating typical lethal range for marine mammals from anthropometric sources’. The overall aim of the project is to identify the limits of performance of methods of acoustically detecting marine mammals during offshore activity such as construction and seismic survey.
The value of the seas and seabed as a natural resource has increased greatly over recent years, and the number and scale of offshore activities has increased in proportion. Many of man’s offshore activities cause underwater noise, from the noise created by ship movements through to the extreme levels of sound generated during the use of explosives underwater, for instance for decommissioning of unwanted oil and gas installations.
The noise from offshore activity has the capacity to directly cause disturbance to marine mammals such as seals, whales, dolphins and porpoises. The effects of noise can include death or lethal injury, physical injuries that can have longer term consequences for the animal such as deafness, and sub-lethal behavioural effects such as the avoidance of an area. All of these may have significant consequences for individuals or stocks of a species. In addition, secondary effects can occur, for instance by disturbance of the fish that are their food. Hence, it is generally a condition of consents issued for offshore activity that
- the likely level of noise created by various activities is estimated prior to an operation being undertaken,
- where the noise component of an activity may be significant, the noise levels are kept at the lowest reasonable level,
- and that where the noise of an activity is sufficient to create an adverse effect, mitigation measures are introduced.
Of the mitigation measures that might be used, a primary measure where sensitive species inhabit or may enter a proposed area of activity is the monitoring of the area for the presence of the species. This enables the activity to be terminated if there are marine animals present. Generally, use is made of Marine Mammal Observers (MMOs) in an attempt to visually detect marine mammals. However, this approach is ineffective, and in darkness or poor visibility detection is impossible. Under these circumstances acoustic detection offers significant advantages. There are three approaches for an Acoustic Detection System (ADS) that might be used, comprising
Passive Acoustic Monitoring (PAM). In this approach, a sonar-type system monitors for vocalisations or echolocation signals from the animals. The systems that have been fielded to date are of generally poor quality, have left-right ambiguity (i.e. cannot determine which side the signal is from) and have no range-finding ability.
Active Acoustic Monitoring (AAM). In this, a sonar “ping” is broadcast in the water and the system looks for a returning signal when it encounters a target. There are systems that have been well developed for military and other purposes, but they all suffer from the fact that they use a “beam” of sound and hence the area of water that is covered at any one time is small. In addition, the sonar may itself cause environmental effects, albeit at a low level.
Acoustic Daylight Monitoring (ADM). This class of sonar is new, and relies on detecting existing background noise being scattered from a target. It has significant potential advantages, including broad area coverage, lack of need for insonification and the ability to also act in PAM mode, but is “cutting edge” technology at the limits of achievability.
In order to specify the required performance of an acoustic detection system, it is necessary to know the range at which an animal may be affected by the noise. Most commonly, it is necessary to find the range at which injury will be caused. In order to calculate this, it is necessary to know the level of sound as a function of range that might be created, and the level of sound at which injury occurs. This report provides a technical review of current information related to the parameters that determine the lethal and physical injury range for marine mammals from acoustic energy in the underwater environment.