Seismic Surveys & Marine Mammals

Sound is important to marine mammals. Even low levels of sound, made by humans or from other sources, has been shown to elicit a wide variety of responses as might be expected for intelligent animals with excellent hearing. The variability of reactions seems to be dependent on factors such as their species and hearing ability (e.g., whales, dolphins, etc.), demographics (e.g., age (Houser et al., 2013), sex (Symons et al.,2014)), and context (e.g., if they are feeding, migrating or breeding). (Ellison et al., 2011) Whether or not their responses to sounds from human activities have the potential to adversely affect an individual marine mammal or its ecological community is still a subject of ongoing research. However, there have been no scientifically documented instances of injury, mortality, or population level effects on marine mammals from exposure to the sounds of seismic surveys. (National Academies of Sciences, Engineering, and Medicine. 2016)

Factors that influence response to sound

Species hearing ability

Demographics

Context

There have been no scientifically documented instances of injury, mortality, or population level effects on marine mammals from exposure to the sounds of seismic surveys.

Sound in the ocean

The ocean environment is filled with sound. (Miksis-Olds et al 2016) Sources of sound include wind, rain, waves, marine mammal vocalizations and the sounds made by other marine fauna. Natural sounds have distinctive acoustic properties. For example, physical events such as volcanic eruptions, earthquakes and lightning strikes produce short-lived high intensity sounds bursts underwater.  In addition, there are many sounds generated by human activities in the ocean.  These include shipping, fishing, sonar (used for navigation, fishing and defense), construction, dredging, military activities, seismic surveys and other industrial activities.  Each of these sounds is unique and has different frequencies and intensities.

Natural sound sources

include wind, rain, waves, marine mammal vocalizations, sounds made by other marine fauna, volcanic eruptions, earthquakes and lightning strikes

Human sound sources

include shipping, fishing, sonar (used for navigation, fishing and defense), construction, dredging, military activities, seismic surveys and other industrial activities

These images of the seabed can be used to explore for and monitor the production of oil and gas, study the earth’s geological history, identify and define national maritime zones and boundaries offshore, and assess the suitability of the seabed for offshore construction and accurate placement of offshore infrastructure.

Sound from marine seismic surveying

A common acquisition technique is the "ractrack" pattern
A common acquisition technique is the “ractrack” pattern

Marine seismic surveys collect data produced using compressed air sound sources and kilometers long listening streamers. The data are processed to create images of the geology beneath the seabed, rather like the use of ultrasound in medicine. These images of the seabed can be used to explore for and monitor the production of oil and gas, as well as study the earth’s geological history, identify and define national maritime zones and boundaries offshore, and assess the suitability of the seabed for offshore construction and accurate placement of offshore infrastructure

The sound needed to create these images is generated by releasing high pressure air into the water creating a short duration pulse of sound when the air bubble collapses. This initial sound pulse lasts less than 0.1 second and is repeated approximately every 10 to15 seconds while the seismic vessel moves along a straight line at a speed of about 5 knots (2.5 m/s), usually for many kilometers. The sound produced contains primarily low frequencies (between 10-500 Hz).

Seismic surveys are conducted along pre-determined survey lines, typically arranged in a grid pattern. At the end of each line, the vessel makes a broad turn and begins to survey along the next line (usually parallel and offset by several hundred meters). The sound sources are usually silent during line changes and while the survey vessel is in transit between survey locations. This pattern of data collection may return the sound sources near the same general area many hours or days later. Since the survey vessel is continuously moving, the effect of sounds on any given area even during surveying will be temporary.

How marine mammals hear

Since sound is common in the ocean, marine mammals have adapted to hear well underwater and live successfully in this sound-filled environment. They use sound to find prey, avoid predators, communicate with each other and navigate through the seas. (Hawkins and Popper 2014) Marine mammals have the most diverse range of hearing capabilities of any mammals.  Whales are low frequency specialists that hear infrasound (sounds at frequencies below the limits of human hearing).  Porpoises are high frequency specialists, capable of hearing sounds up to 160 kHz, several octaves above the upper limit of human hearing (dogs, by comparison hear up to 40-60 kHz).

Researchers customarily divide marine mammals into five hearing groups based on their range of best underwater hearing. (Ketten, 1998)

Five hearing groups

7 Hz – 35 kHz

Low-frequency baleen whales
(e.g. blue whales)

150 Hz – 160 kH

Mid-frequency toothed whales
(e.g. most dolphins and sperm whales)

275 Hz – 160 kHz

High-frequency toothed whales
(e.g. some dolphins and porpoises)

50 Hz – 86 kHz

Seals

60 Hz – 39 kHz

Fur seals and sea lions

 Seismic has been used there for decades yet the marine mammal populations show no sign of decline nor changes in composition and the fisheries in the Gulf of Mexico are among the most productive in the U.S.

Impact of the sound from seismic surveys on marine mammals

For over 40 years, scientists worldwide have been studying potential impacts of seismic surveying on marine mammal populations. Global consensus has not yet emerged, but U.S. government agencies have concluded that any potential impacts are insignificant when surveys are collected using standard measures that reduce risk of impacts. The empirical evidence that seismic has little effect on marine animals is best seen in the Gulf of Mexico.  Seismic has been used there for decades yet the marine mammal populations show no sign of decline nor changes in composition and the fisheries in the Gulf of Mexico are among the most productive in the U.S. “To date, there has been no documented scientific evidence of noise from air guns used in geological and geophysical (G&G) seismic activities adversely affecting marine animal populations or coastal communities.” (BOEM, Science Notes) The U.S. National Academy of Sciences concurs and has stated that ‘no scientific studies have conclusively demonstrated a link between exposure to sound and adverse effects on a marine mammal population.’ Reference (page 15)

The potential for a loud sound to injure a marine mammal depends on the intensity and duration of the sound exposure and the hearing capability of the animal. Both the intensity and duration are dependent on sound level at its source and distance between source and marine mammal. Some of the most accurate records of the presence of marine mammals during seismic surveys in the North Sea are contained in the reports collected by the Joint Nature Conservation Committee (JNCC). Their analysis of surveys from 1994 through 2010 indicate marine mammals routinely move further away when seismic sound sources are active. (Stone, 2015)

 How Marine Mammals behave near seismic surveys

Even when a sound is not intense enough to cause physical injury, animals may react to the sound that may be audible to their ears. Observed responses of marine mammals to a variety of sounds, including the sound from seismic surveys, tend to be small, temporary movements away from the source of the sound, temporary modification in how deep or how long they dive, or short-term changes to their vocal behavior, such as how loudly they call. (Dunlop et al 2017). Scientists theorize that their varied reactions depend on where they are and what they are doing when they hear the sound (i.e. feeding, migrating, etc.). (Dunlop 2016).

Animals like humans can learn to ignore non-threatening sounds. Many marine animals, from whales to jellyfish, rapidly stop responding to a repeated sound such as seismic when no negative or harmful consequences accompany the exposure.  (DOSITS) The process is termed habituation.  This reaction is widespread in animal groups, occurs rapidly, and is measurable.

Industry experience with marine seismic surveying worldwide, combined with several decades of international scientific research, indicate that there is an extremely low likelihood of significant harm to marine populations from seismic surveying, if any. To date, there is no substantive evidence showing that serious injury, death or stranding of marine animals has occurred from exposure to seismic sound.

With appropriate planning and mitigation measures, seismic surveys can be conducted safely and without significant impacts to marine mammal populations.

Practices to avoid disturbance

Protective measures are employed to address site-specific environmental conditions of each operation to ensure that sound exposure and vessel traffic do not harm marine mammals. Surveys are planned to avoid known sensitive areas and time periods, such as breeding and feeding areas. Exclusion zones are typically established around the seismic source to further protect marine fauna from any potentially detrimental effects of sound. The exclusion zone is typically a circle with a radius of at least 500 meters around the sound source. Trained observers and listening devices are used to visually and acoustically monitor that zone for marine mammals and other protected species before any sound-producing operations begin. These observers help ensure adherence to the protective practices during operations and their detailed reports provide information on the biodiversity of the survey area to the local governments.

Sound production typically begins with a “soft-start” or “ramp-up” that involves a gradual increase of the sound level from the air gun source from a very low level to full operational levels at the beginning of the seismic lines – usually over 20 to 40 minutes. This soft-start procedure is intended to allow time for any animal that may be close to the sound source to move away as the sound grows louder.

With appropriate planning and mitigation measures (IOGP Report 579), seismic surveys can be conducted safely and without significant impacts to marine mammal populations.

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References

  • Dunlop, R.A. ‘Changes in vocal parameters with social context in humpback whales: considering the effect of bystanders’, Behav Ecol Sociobiol (2016) 70: 857. https://doi.org/10.1007/s00265-016-2108-0
  • Dunlop, Rebecca A., Michael J. Noad, Robert D. McCauley, Lindsay Scott-Hayward, Eric Kniest, Robert Slade, David Paton and Douglas H. Cato,  ‘Determining the behavioural dose–response relationship of marine mammals to air gun noise and source proximity’, Journal of Experimental Biology (2017) 220, 2878-2886 doi:10.1242/jeb.160192
  • Ellison. W. T., B. L. Southall, C. W. Clark, A. S. Frankel, 2011, A new context-based approach to assess marine mammal behavioral responses to anthropogenic sounds, Conservation Biology, DOI: 10.1111/j.1523-1739.2011.01803.x
  • Hawkins, AD. and Popper, A. 2014. ‘Assessing the impacts of underwater sounds on fishes and other forms of marine life.’ Acoust Today 10(2): 30-41.
  • Houser, D.S., S.W. Martin, J.J. Finneran, Exposure amplitude and repetition affect bottlenose dolphin behavioral responses to simulated mid-frequency sonar signals, Journal of Experimental Marine Biology and Ecology 443 (2013b) 123-133.
  • Ketten, Darlene R., 1998, ‘Marine mammal auditory systems: a summary of audiometric and anatomical data and its implications for underwater acoustic impacts’, NOAA-TM-N M FS-SW FSC-256
  • National Academies of Sciences, Engineering, and Medicine. 2016.
  • Approaches to Understanding the Cumulative Effects of Stressors on Marine Mammals.
  • Washington, DC: The National Academies Press. doi: 10.17226/23479.
  • Miksis-Olds, Jennifer L., and Stephen M. Nichols Citation: ‘Is low frequency ocean sound increasing globally?’, The Journal of the Acoustical Society of America 139, 501 (2016); doi: 10.1121/1.4938237 View online: http://dx.doi.org/10.1121/1.4938237
  • Southall BL Bowles AE Ellison WT Finneran JJGentry RL Greene Jr CR Kastak D Ketten DR Miller JH. and Nachtigall PE. 2007. ‘Marine mammal noise exposure criteria: initial scientific recommendations’, Bioacoustics, 17 (1-3), 273-75.
  • Stone, C.J. 2015. Marine mammal observations during seismic surveys from 1994-2010. JNCC report, No. 463a.
  • Symons, J., E. Pirotta, D. Lusseau, Sex differences in risk perception in deep-diving bottlenose dolphins leads to decreased foraging efficiency when exposed to human disturbance, Journal of Applied Ecology 51(6) (2014) 1584-1592.
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