Why are MMOs important during Seismic survey?

Mis à jour : 30 oct. 2019

The universe of cetaceans is above all a world of sound, because the view extends to a few metres underwater, while the sounds extend to tens of kilometres. For this reason, cetaceans are acoustic creatures: they have developed faculties that are hardly found in other animals and can perceive sounds that are impossible to perceive by human ear.

When their ears or echolocation systems are damaged, they are no longer able to hunt and feed themselves and die slowly or feed on plastics.

The work of Marine Mammal Observers (MMOs) and PAMs (passive acoustic monitors) are to monitor the area before the work begins and to stop the airguns if an animal is detected in a predefined area called the exclusion zone (area close to the sound source) (e.g. air guns used in seismic survey). In case an animal is seen, it allows the animal to escape.

Generally MMOs observe the area for 30 minutes to an hour before the start of the work and then if no animal are presents, a ramp up is started (airguns increase in noise until full power is reached).

Here is a video explaining the increasingly polluted sound environment of the oceans and the possible consequences of uncontrolled seismic activity (2min46) Oceancare video

Below you can read more explanations from the GREC (Cetacean Research Group) on the sound-related trauma that cetaceans can experience if they are exposed to an intense sound:

When confronted with intense sounds, cetaceans can suffer different consequences depending on the frequency of the waves, the intensity propagated, and the duration of exposure. The context of exposure is a parameter that is often overlooked but can also be important.

The consequences of sound waves on cetaceans are assessed on a gravity scale with 5 ranges, from simple perception (without disturbance) to death. These consequences are studied through experiments carried out in basins or in the natural environment, or during events observed in situ on different cetacean families.

Areas of sound wave impact, with increasing gravity closer to the source. The sketch is not to scale!

If the noise levels received are extremely high (zone Z1 below), cetaceans can be killed by an acute traumatic injury or suffer permanent damage to their hearing instruments.

If the perceived levels are very intense (zone Z2 below), cetaceans suffer a temporary damage to their hearing aid, which will take hours or days to resolve.

If the perceived levels are high (zone Z3 below), cetaceans are forced to flee, sometimes with panic, to escape noise pollution.

If the perceived levels are quite intense (zone Z4 below), cetaceans are disturbed and interrupt their current activity (predation, or rest, for example).

For moderate perceived levels (zone Z5 below), cetaceans perceive noise but continue their activities.

The effects of sound waves derive from the sound levels received, which in turn depend directly on the source level and the propagation losses between the source and the cetacean, or from the perceived levels, i.e. the sounds actually heard by cetaceans, which are equal to the difference between the levels received and the cetacean hearing threshold.

The hearing threshold of cetaceans is extremely variable according to frequency, and each species has its own sensitivity curve, the audiogram. At best, the hearing sensitivity of a cetacean is about 40 dB (re 1 microPa), but for lower or higher frequencies the hearing sensitivity is much lower (100 dB or more).

Auditory effects, such as temporary (TTS) or permanent (PTS) hearing loss, also depend on perceived levels, as well as the duration of exposure. Therefore, predicting the effects of sound waves requires knowledge of species audiograms, which is far from certain. If the cetacean is suffering from prolonged or permanent deafness, its vital prognosis is engaged in the short or long term, because the perception of environmental sounds is essential to the life of these species in the marine environment.

The physiological effects of intense sound waves affect different organs through several mechanisms not all well known. These physiological non-auditory effects depend on the sound levels received. Sound waves of more than 200 dB received can cause resonance of organs, such as mandibles or average ears, or resonance of air cavities such as lungs or sinuses. High amplitude resonances cause lesions of varying severity.

Gas bubbles observed in a cardiac vein during the stranding of Almeria in 2006

When cetacean tissues are oversaturated with nitrogen after deep and prolonged dives, intense sound waves of more than 180 dB received tend to cause a gaseous diffusion of nitrogen in the blood: gas microbubbles increase in diameter. This mechanism leads to gas and fat embolisms with lethal consequences. During accidental strandings of ziphiidae, this mechanism has been demonstrated several times by pathologists.

Sound waves have behavioural effects (disturbance, interruption of activity, escape, panic) that depend on perceived levels. Depending on the context, perceived levels that are not extremely intense can have fatal consequences. For example, it has been observed that groups of dolphins may move away from powerful sonars, but find themselves in a closed bay at risk of stranding. Or, dolphins isolated from their group (infants) and unable to escape in a good direction can then be subjected to extremely high levels that lead to their death.

During ASM exercises using high-powered low or medium frequency sonars, cetaceans are exposed to all the consequences described above. If the planning or execution of noise emissions does not take into account the presence and biology of cetaceans, severe consequences are to be expected. There are many cases of serious accidents caused by naval manoeuvres.

During petroleum seismic surveys, cetaceans are subjected to high intensities for days or weeks at low and very low frequencies. They develop escape reactions and may in some cases suffer irreversible damage.

This is why it is very important to conduct impact studies before, during and after the research.

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Virginie Wyss

Biologiste, assistante de projet et observatrice des mammifères marins




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