A seismic geophysical survey measures the earth’s (geo) properties by means of physical principles such as magnetic, electric, gravitational, thermal, and elastic theories. Seismic surveys determine elastic properties of subsurface materials by measuring their response to disturbances called seismic ‘elastic’ waves. Oil and gas companies use seismic surveys as a method to locate and estimate the size of potential offshore oil and gas reserves.
Vessel-based seismic surveys are carried out using towed single or multiple arrays of airguns, which release compressed air every 10-15 seconds to produce an impulsive signal. These signals produce loud explosive sounds of short duration and broad frequency, which are directed downwards through the water column and into the seabed. Returned signals are received through in-water hydrophones (underwater microphones, also called geophones), and data are processed to yield profiles of the rocks beneath the seafloor. Vertical Seismic Profiling (VSP) surveys are also performed from offshore oil and gas installations. Airguns are designed to direct a high proportion of energy downwards; however, energy is also projected horizontally into the water column and can therefore be heard at great distances from the source.
Marine mammals and seismic
Anthropogenic sound stemming from shipping, industrial and military activities and many other anthropogenic (man-made) and natural sources, has led to a substantial increase in underwater background noise in the oceans over the past decades. A wealth of literature is available on the topic (e.g. http://dx.doi.org/10.1121/1.2216565, doi: 10.3354/meps08353).
Seismic exploration is just one of many anthropogenic noise sources that have been found to cause behavioural reactions in marine mammals. Noise generated from seismic exploration is of high intensity (190-250 dB re 1 µPa peak to peak), with most energy below 200 Hz. Marine mammals, particularly cetaceans (whales, dolphins and porpoises) have developed a sophisticated auditory systems and rely on sound to navigate, communicate, find food and for whole host of activities within the wider social group such as cohesive actions, warnings and maternal relationships. The increase in background noise produced by seismic exploration and other anthropogenic noise outputs therefore may impact marine mammals at an individual, and potentially population level. Peak frequencies of the sounds produced during seismic surveys overlap acoustic signals and estimated hearing ranges of baleen whales (mysticetes). The hearing range of toothed whales and dolphins (odontocetes) is most sensitive over the frequency range 10-150 kHz, one hundred times higher than the frequencies of baleen whales, and outside the peak frequency range of seismic airguns. It is generally assumed that baleen whales are more susceptible to seismic sound than toothed whales and dolphins; however, it has recently been discovered that seismic airguns can still produce sound at frequencies ranging from 1-20 kHz, which would overlap the hearing range of odontocetes, and could potentially cause behavioural reactions. Some studies state that high frequency cetaceans, such as porpoises, increase their hearing threshold when exposed to airgun noise (doi: 10.1121/1.3117443).
Behavioural changes of cetaceans as a result of seismic surveys and other anthropogenic noise have included; cessation of feeding, resting, or social interaction; changes in surfacing, respiration and diving cycles; and direct avoidance by diving and/or swimming away. All reactions appear to be short-term with normal behaviour resuming after cessation of the sound source. The longer term wellbeing of an individual or population remains unknown and is of concern.
In one particular study investigating the impact of seismic surveys on blue whale (Balaenoptera musculus) vocal behaviour, found that whales called consistently more on days when seismic exploration activities were taking place, compared to days with no seismic exploration. It is assumed that, an increase in call production compensates for the masking of information by the anthropogenic noise source. The study concluded that, by increasing call rate, animals increased the probability that their signals were received by conspecifics (doi: 10.1098/rsbl.2009.0651).
In other studies, sighting rates of all small odontocetes have been shown to be reduced significantly during airgun firing periods, whereas baleen whales show a more localised spatial avoidance. Long-finned pilot whales (Globicephala melas) have been shown to respond with a change in orientation, while other odontocetes such as sperm whales (Physeter macrocephalus) showed no significant effect. It is assumed from these studies, that species of marine mammal elicit different responses to acoustic disturbances, which is to be expected.
Marine mammal mitigation measures
In order to reduce the risk of disturbance to marine mammals, mitigation measures (www.marinemammalmitigation.co.uk) have been designed and adopted for use during seismic surveys and other offshore industrial activities. Cetacean monitoring is a requirement on-board seismic survey vessels working in most oceans (Atlantic, North Sea and Irish waters, Gulf of México, New Zealand etc.). Marine Mammal Observers (MMO, www.marinemammalobserver.co.uk) are present to carry out searches for marine mammals before commencement of, and during, seismic operations. A pre-watch for marine mammals is undertaken. If no marine mammals are present within the set exclusion zone (e.g. 500 m in UK waters, 1000 m in Irish waters, up to 1500 m in German waters) then the ramp-up (soft start) procedure can commence. The ramp-up is a general build-up of the sound source over a 20 minute period, and is designed to allow marine mammals to leave the immediate area around the sound source.
The majority of MMO regulations stipulate daylight observation hours only; however, in more sensitive areas (or where Governments are more environmentally minded, such as Germany), there is an occasional requirement to observe at night. Conventional night vision binoculars operate in near darkness by intensifying existing visible (or near–visible) external radiation (from moonlight, starlight, sky-glow, etc.). Night vision binoculars are more common, and operate very well if there is sufficient external illumination, but they cease to operate altogether in absolute darkness or in deep shadows. Furthermore, night-vision binoculars do not work in seaspray, smoke, dust, haze, etc. and are thus, useless at sea in anything other than the calmest conditions. Therefore, while Passive Acoustic Monitoring (PAM, www.passiveacousticmonitoring.com) is used as a secondary mitigation measure during daylight hours, PAM is the sole mitigation measure during hours of darkness. Research from studies using PAM have shown that this additional mitigation method can enhance the detection rate of marine mammals e.g. http://www2.dmu.dk. Passive Acoustic Monitoring around offshore oil and gas installations using autonomous click detectorsT-PODs (www.t-pod.co.uk) and C-PODs (www.c-podclickdetector.com) is also extremely effective at detecting threatened harbour porpoises, Phocoena phocoena (http://icesjms.oxfordjournals.org).