Seismic exploration surveys are carried out in order to gain detailed information on the geological features of the seabed. This is done by using seismic airguns which rapidly release a bubble of compressed air downwards in the water column. The signal, which penetrates the seabed, is reflected by different layers of rock and then received by a series of hydrophones towed behind the survey vessel.
Airguns produce high-energy, low frequency sounds. Source levels depend upon the size and number of airguns within the array. Sound is directed mostly downwards; however, a considerable amount of acoustic energy is distributed throughout the water column. Under the right propagation conditions, noise is capable of being heard by marine life kilometres from the survey area. For this reason, acoustic pollution caused from seismic exploration activities is of concern.
Marine mammals and seismic exploration surveys
Marine mammals rely on sound to carry out everyday activities such as hunting, navigation, exploration and communication. Any interference with their ability to detect sounds within their environment has the potential to impact on individuals or populations.
Seismic airguns are thought to impact marine mammals to some extent, but effects are largely unknown and difficult to determine. Attempts have been made to study the physiological and behavioural reactions of marine mammals both in captivity and by using visual and acoustic methods in the field. Results are still inconclusive and reactions appear to vary in accordance with auditory sensitivity of the species, the position of the animal with respect to the sound source, propagation conditions and background noise levels.
Since seismic operations are predominantly low frequency, baleen whales, which are most sensitive to low frequency sound, are possibly more likely to react than mid and high frequency marine mammals such as dolphins and porpoises respectively; however, results are mixed: adverse reactions have been observed in high frequency marine mammals, and not all baleen whales appear to be disturbed by active airguns.
Research on humpback whales (Megaptera novaeangliae) has found that some animals demonstrate localised avoidance of active airguns, whilst other individuals have been observed spending greater proportions of time close to the surface, where sound levels are likely to be reduced. This effect has also been noted in deep diving odontocetes such sperm whales (Physeter macrocephalus). Secondary to the avoidance reaction, increased time spent at the surface could disrupt feeding activity as the amount of time spent feeding at depth is reduced potentially.
Seismic airguns have also been correlated with course alterations in short-finned pilot whales (Globicephala macrorhynchus) and disruption of blue whale (Balaenoptera musculus) calling patterns. The continued presence of airgun noise within the environment has the potential to mask biologically important sounds, possibly affecting reproductive success of populations.
Obvious behavioural reactions are not prominent in all studies, with many reporting that marine mammals are seemingly unaffected by seismic noise. One hypothesis is habituation: seismic surveys can last for months at a time and it is possible that marine mammals could become accustomed to increased sound levels. Alternatively, if food is present within the area, motivation to tolerate increased noise levels could be raised, causing animals to remain in the area despite increased exposure risk.
Marine mammals, seismic exploration and mitigation measures
In order to minimise the risk to marine mammals several mitigation measures (www.marinemammalmitigation.co.uk) can and, in some cases are, required to be implemented during seismic operations. Guidelines vary depending on location but commonly, visual watches by Marine Mammal Observers (www.marinemammalobserver.co.uk) and acoustic monitoring by Passive Acoustic Monitoring operators (www.pamoperator.co.uk) is carried out prior to operations to ensure that marine mammals are absent from the immediate area. Other measures include careful planning to avoid ecologically sensitive periods such as migrations and increasing the power of the air guns gradually to allow any marine mammals in the area to leave before full power is reached.
Ocean Science Consulting
Ocean Science Consulting (OSC) has been providing high quality marine mammal mitigation services since 2004. Ocean Science Consulting Marine Mammal Observers (MMOs) (www.marinemammalobserver.com) and Passive Acoustic Monitoring (PAM) operators (www.passiveacousticmonitoring.com) are a minimum of degree level educated, have completed JNCC approved Marine Mammal Observer courses and have participated in a rigorous in-house OSC training scheme. By ensuring our staff keep up-to-date with all relevant guidelines and procedures, our Marine Mammal Observers and Passive Acoustic Monitoring operators are able to provide informative guidance to the seismic team throughout all stages of the operation. To date, there has been no operational downtime on any of our projects and all our equipment has 100 % redundancy. For more information on our Marine Mammal Observers, Passive Acoustic Monitoring equipment (www.towedarray.co.uk and www.t-pod.co.uk) or operators, to view our prior achievements or to enquire about the services we can offer you please see our website www.osc.co.uk.