UK: Importance of Acoustic Surveys in Offshore Wind Industry

It is important to understand the status of marine mammals in an area prior to any development and understand potential impacts during and after development.

Offshore wind developments may adversely affect species, including those protected by European and domestic law. Mitigation of such impacts forms an intrinsic part of the Environmental Impact Assessment (EIA) process required as part of the consenting process for all offshore wind farms.

The value of marine mammal surveys is ever increasing as developments move into the offshore areas of seas. These development sites further offshore, have sparse baseline information available and as such have the additional requirement to ensure this information is gathered.

The collection of information is required during site characterisation to determine the species that are present. From that, an appropriate MMMP (Marine Mammal Mitigation Plan) is developed and implemented against the effects of noise from the construction phase.

Cetaceans (whales, dolphins and porpoise) utilise a variety of sounds underwater to sense environment, find food and communicate. The excellent acoustic propagation characteristics of water permit the sounds of many cetaceans to be detected at greater distances than can be visually sighted, allowing enhanced monitoring of cetaceans during hours of darkness and at times of poor viability.

Baseline surveys for marine mammals can greatly benefit from combining visual and acoustic monitoring methods to detect animals both vocalising and surfacing. These techniques should be combined in a line transect survey utilising distance sampling methods, using experienced personnel and proven equipment.

There are two main types of acoustic recording systems used during site characterisation surveys; mobile and static devices. A mobile device consists of hydrophones being towed behind a ship or other platform. Alternatively, the hydrophones may be left in one place for longer time periods in the form of a CPOD or Autonomous Recording Unit (ARU).

Marine mammals continue to be monitored carefully during the construction phase to ensure that mitigation is efficient and that marine mammal presence correlates to that identified in the baseline survey.

Mitigation measures in the MMMP will include the use of MMOs, Passive Acoustic Monitoring (PAM), soft start and delay procedures and mitigation reporting.

In addition to the mitigation at construction stage, a development’s Marine Licence requires the measurement of noise generated by the installation of the first few foundation pieces to ensure that the output levels do not exceed those predicted and the calculated impact from underwater noise on those species found to be present in the area.

Underwater noise data is collected using calibrated hydrophones and specialist acquisition software. Analysis and modelling allows calculation of metrics such as sound pressure level (SPL), estimates of transmission loss and source levels, as well as sound exposure levels (SELs) and impacts for marine mammals.

Marine mammal surveys continue once construction is complete. This data is then compared to that collected pre-construction in order to identify any changes in species presence, distribution or abundance.

Done correctly, high quality and comprehensive surveys throughout all phases of the wind farm development, will illustrate effective and valuable mitigation.

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Source: Gardline Marine Sciences, September 26, 2013; Image: Rachael Barber, Gardline