Airguns - an underestimated disruptive factor

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Airguns in the Antarctic – Underwater noise in one of the last refuges of rare marine mammals
Source: Sebastian French / Fotolia.com

The oceans surrounding Antarctica are among the few regions in the world where there is little underwater noise caused by human activities. An UBA study shows that the scientific use of airguns can emit noise in to this sanctuary at distances of 2000 kilometres which may reduce the naturally available communication range of blue and fin whales to as little as 1%.

Underwater sound disturbance

Whales and seals rely heavily on their sense of hearing and their ability to hear their environment is essential for their survival. Man-made underwater noise is now virtually unceasing in all of the world's oceans and is causing changes in the natural background noise to which marine mammals have been exposed. Shipping is one source of chronic noise with high masking potential. Much louder, although far briefer noise is emitted by airguns which are used to explore the seafloor or to drive the foundation pillars of wind power plants. Their sound bursts can be 1000 louder than a ship and have long been suspected of damaging marine mammals' hearing. Underwater noise can also disturb communication among a species and disrupt perception of the other ambient signals which whales, for example, rely on to find food or a mate.

A new study by the Federal Environment Agency shows that the potential long-distance impact of underwater noise is significant. The study analysed the impact of airgun signals on the communication ranges of marine mammals and determined that short, low-frequency sound signals may result in a continuous received sound at long range with a high disturbance potential for blue and fin whales. The study modelled the sound propagation of airgun signals at distances up to 2000 km from the source. The modelled airgun noise signals were then overlaid with vocalisations of the fin whale, blue whale and Weddell seal to assess the distances over which their communication could potentially be disturbed or prevented.

The model demonstrated that airgun signals can have an impact at distances of at least 2000 km, thus possibly affecting animals within the specially protected region of the Antarctic south of 60°S even when research vessels are operating north of 60°S. Even at medium-range distances between 500 and 1000 km, airgun signals are stretched and become intermittent noise that has a high masking potential. At distances of 1000 km and more, airgun signals are stretched and become continuous sound, and the natural communication range of blue and fin whales in the Antarctic can be reduced to 1% of the range without masking.

Background

The airguns used to explore the seafloor are basically metal cylinders in which air is compressed under high pressure and then released in explosive bursts. The shot of air creates a gas bubble that makes a very short but very loud sound when it bursts. Most of the sound waves produced by airguns are in the low-frequency range up to 300 Hz, making an overlap with the sounds and songs of whales and seals likely. The baleen, blue and fin whales of the Southern Ocean in particular communicate mostly in this frequency range.

Detailed results of modelling

The UBA study modelled the sound propagation of airgun signals across distances of 100, 500, 1000 and 2000 km. The modelled wave forms were overlaid with the song and calls of the fin whale, blue whale and Weddell seal to assess the distances over which masking of the communication signals could potentially occur. The signals were analysed using a leaky integrator within the bandwidth of the modelled vocalisations.

Although a number of questions remain unanswered, the study reaches the following conclusions based on the assumptions made for propagation and masking modelling:

  • A model to predict potential masking by intermittent sounds was established. This model is based on a leaky integrator and level detector in combination with a simple model for sound propagation of animal vocalisations.
  • The masking potential of airguns is greatest for low frequencies below 300 Hz. It follows that baleen whales (e.g. fin and blue whales), which communicate in the low-frequency range, are probably the most strongly affected species.
  • Airgun signals have a long-distance impact across distances of at least 2000 km, thus potentially affecting animals south of 60°S even when research vessels are operating north of 60°S.
  • Even at medium-range distances (500 - 1000 km), airgun signals are stretched and become intermittent noise that has a high potential to reduce the communication range of marine mammals which vocalise in the low-frequency range.
  • The modelled communication ranges of the blue and fin whales are reduced significantly by airgun signals, most of which are less than 5% of the natural potential communication range.
  • Seals are most likely less affected but their far-ranging, low-frequency vocalisations are also significantly masked: the modelled communication ranges are mainly less than 3% of the natural potential communication range. 

The developed model of sound propagation enables a reliable estimation of signal stretching and can be adapted to other environmental conditions. It must be noted, however, that the knowledge about hearing in baleen whales is very limited and there is a lack of empirical data with which to evaluate sound propagation.

The study outcome indicates that masking effects of airgun signals are likely and that significant impacts on the vocalisation behaviour of animals is possible across long distances. The masking model suggests that a population effect cannot be ruled out and should be included in the consideration of the environmental effects of impulse noise such as from airguns. The model will be further developed in a follow-up project to enable its applicability to other marine regions. One such region is the Arctic, where the use of airguns to explore the ocean floor for mineral resources is expected to be high in the next few years.

The German Ministry of Environment developed a sound protection concept for the North Sea with the aim of protecting the native harbour porpoise from the harmful effects of noise due to injury or other disturbance. The UBA is developing a corresponding concept for the Antarctic to protect the native species of whales and seals and is seeking to put the matter on the agenda for discussion at the annual meetings of Consultative Parties to the Antarctic Treaty (ATCM).

Diagram of scientific use of airguns
Diagram of scientific use of airguns
Source: Hannes Grobe / Alfred Wegener Institut