Marine Seismic Sources Part IV

Author Lasse Amundsen and Martin Landrø

The 1956 documentary The Silent World by Jacques-Yves Cousteau wowed audiences with its vibrant depiction of aquatic life. But the Silent World is far from silent.

Sound is signal or noise. A listener will define sounds of interest as signals and everything else that might interfere with those signals as noise, especially if disturbing or unpleasant. For example, seismic operators consider their airgun array to produce a signal, while we may guess that marine mammals are likely to consider it to be noise.

Marine mammals use sound to communicate with one another, sense their environment, and find food. They obtain information about the environment by listening to sounds from natural sources, such as surf noise, which indicates the presence and direction of a shoreline or shoal, ice noise, and sounds from predators such as killer whales. Further, toothed whales use echo location sounds to sense the presence and location of objects, like prey.

For similar reasons, humans use the advantages of sound in the oceans for communication, for navigation, and to search for food using fish finding sonar. Marine mammals also use sound for communication, navigation, and localizing and catching prey and for many years we have wondered how man-made sounds in the sea could affect marine life. It enhances background noise levels and thus may prevent detection of other sounds important to the health and behaviour of marine mammals. Also, man-made noise could interfere negatively with the mammal’s calls and echolocation pulses.

Anthropogenic sounds which set acoustic “footprints” in the oceans come from many human activities. Ships are a major source of noise in the ocean, and industrial activity, including platform construction and drilling, also contributes to the noise levels. The anthropogenic sounds fill the whole range of frequencies of the natural sounds, and thus have a possible masking effect on the natural sounds, which may limit the ability of marine mammals to detect sound cues in their environment.

The King Island, Tasmania, whale stranding in 2009. Photo: John Nievaart. www.naracoopaholidayunits.com.au

Whale stranding

Every year thousands of whales beach themselves. Multiple strandings in one place are rare but when they happen they often attract media coverage as well as rescue efforts.

The whales that most frequently mass strand are toothed whales, which normally inhabit deep waters and live in large, tightly knit groups. Their social organization appears to be a key factor influencing their chances of mass stranding, as, if one gets into trouble, its distress calls may prompt the rest to follow and beach themselves alongside. However, many other, sometimes controversial, theories have been proposed to explain stranding, but the question as to why they do remains unresolved.

Naval activities, such as live ammunition training, vessel noise and explosions, introduce noise into the oceans. However, the naval activity that has been subject to the most scrutiny is mid-frequency sonar, which can produce sound at levels of up to 237 dB re 1uPa @ 1m at frequencies between 2-8 kHz (see GEO ExPro Vol 7 No 4 for an explanation of decibels).

A controversial issue is the extent of the relationship between sonar operations in nearshore areas and the relatively rare stranding of beaked whales. Sonar operations have been correlated temporally and spatially with strandings of 14 beaked whales in the Bahamas in 2000, but why these whales actually swam onto the beach is not understood.  

Can seismic effect marine mammels?

Can seismic air-gun arrays have harmful impacts on marine mammals? Could air-gun noise mask communication, or hamper their ability to identify and catch prey? Could air-gun activity cause populations to move from preferred habitats, feeding grounds, breeding and resting areas during movements along migratory pathways?

Biologists, acousticians, geophysicists, and government regulatory technical managers do not agree on a simple and definitive answer. Therefore, in 2005, an international E&P Sound and Marine Life Joint Industry Programme was formed with members from the International Association of Oil and Gas Producers, with the goal of obtaining scientifically valid data on the effects of sounds produced by the E&P industry on marine life. The research, carried out by prominent marine mammal researchers and institutions from around the word, is now in its third phase.

However, most experts in the field believe that there is a very low probability of physical damage to the hearing of any species of marine mammals from seismic surveys.

Sperm whale diving beneath the surface of the water. In large male sperm whales, one third of the body length is dedicated to the huge nose, the world’s largest and most powerful biological sound generator. Every second, the whale emits a directional click from its nose to create an acoustic picture of its surroundings, allowing it to navigate and locate prey – a process called echolocation. Sperm whales also make distinct patterns of clicks for conversation. The actual clicking mechanism is not yet fully understood. Photo: FRANCOIS GOHIER/SCIENCE PHOTO LIBRARY

Sound from airgun arrays

Let us look at some facts.

An airgun array consists of many single airguns distributed in a pattern. When geophysicists report the pressure sound level from an airgun array, they refer the level to a hypothetical point source that would radiate the same pressure sound level in the far field as the physical array. The far field, where the acoustic output appears to be coming from a single point source, is typically 150-200m beneath the center of the array. Therefore, when geophysicists state that airgun source arrays produce sound levels equivalent to 260-240 dB re 1 μPa @1m (peak-to-peak, or p-p), they are referring to the levels one metre away from the hypothetical point source, but since this is not actually a point source, the sound level 260-240 dB will never be realized in the water. No marine mammal can possibly be exposed to the pressure levels quoted a metre from the theoretical point source.

Thus, the point source model is convenient, but only has validity in the far-field of the array. If the point-source sound pressure level is stated to be 260 dB re 1 μPa @1m (p-p), we can predict using the model of geometrical spreading that the pressure measure in the far-field at 100m is (260-20 log 100) dB, or 220 dB (p-p).

The peak-to-peak level measures the entire height of the airgun array signal. When characterizing noise, it is common to measure the root-mean-square (rms) level, which gives the average of the pressure signal over a given duration. For airgun signals, subtract 18 dB from the p-p level to obtain an estimate of the rms level, so the above point-source pressure level 260 dB re 1 μPa @1m (p-p) corresponds to 242 dB re 1 μPa @1m (rms).

Most of the sound energy from seismic sources is at frequencies below 200 Hz. Single air-guns generate signals in the frequency range of 5-200 Hz, while the combined signal from air-gun arrays is in the order of 5-150 Hz. The sound pressure for individual frequencies, or bands of frequencies, varies, but the maximum sound level falls between 10-80 Hz.

But airguns, depending upon their method of operation, may emit acoustic amplitude well above the frequencies of interest for seismic exploration. Broad-band noise, up to 15 kHz or more, although small in amplitude, is detectable at relatively close range.

Sperm whale clicks, recorded presumably on axis (top) and about 20° off axis (bottom). Adapted from Møhl et al (2003).

Sperm whale clicks are intense!

The clicks generated by sperm whales are used for both echolocation and communication. From the mid-1960’s, the clicks were described as multi-pulsed, long duration, non-directional signals of moderate intensity levels about 170-180 dB re 1 μPa (rms), and with a spectrum peaking in the 2-8 kHz range.

However, during the last decade, studies by Møhl and co-authors, working in the Bleik Canyon offshore Norway, give new insights. The clicks recorded on large aperture hydrophone arrays are mono-pulsed, lasting 100 μs, and have pronounced directionality, with extremely intense (on-axis) source levels up to 236 dB re 1 μPa (rms), with spectral emphasis at 10 kHz. The measured sound levels make sperm whale clicks by far the loudest of sounds recorded from any biological source.

The fact that sperm whales themselves generate sounds at rms levels only 6 dB lower than the loudest levels generated by seismic surveys (although their peak levels will be about 10 kHz rather than the 10-80 Hz range of seismic surveys) have lead experts to believe that seismic operations probably do not cause any problems for sperm whales. This position is further backed by sperm whale studies in the Gulf of Mexico.

Marine seismic acquisition will probably always be under intense scrutiny by environmental groups. However, one should not jump to conclusions, and discussions should continue on exactly how much sound is acceptable, while seismic operators must comply with regulations. Scientific research that will increase our knowledge base is ongoing. It is critical that stringent mitigation measures on marine seismic exploration companies are based on sound science, not speculation.

Whales and Dolphins Whales and dolphins belong to the mammalian order Cetacea, which is subdivided into two suborders, Odontoceti and Mysticeti. Odontocetes are the toothed whales and dolphins, the largest being the sperm whale, followed by Baird's Beaked whale and the killer whale, called such because it feeds on warm blooded prey, and occasionally even hunts whales. Mysticetes, by contrast are “toothless”. In place of teeth they have rigid brushlike whalebone plate material called baleen, used to strain shrimp, krill and zooplankton, hanging from their upper jaw. All the great whales are mysticetes or baleen whales. Underwater, visibility is limited, and acoustics play an important role in the life of all cetaceans. Whales and dolphins emit a wide variety of of signals, utilizing a frequency range from about 15 Hz (blue and fin whales) to over 100 kHz, used by a number of odontocetes when echolocating and emitting burst pulses. The rule of thumb is that larger animals tend to emit lower frequency sounds and smaller animals emit higher frequency sounds. The use of a particular frequency band has implications as to the distance other animals can hear the sound. Acoustic propagation losses are related to geometrical spreading and absorption. Geometric spreading loss is frequency independent, while sea water absorption loss varies with frequency. But absorption is important only for very high frequency sound, with absorption loss negligible for frequencies below 5 kHz over ranges of more than 100 km. Thus, the sounds of baleen whales, which are mostly below 1 kHz, will not experience much absorption loss. Large whales are known to communicate over very large distances, even several hundred kilometers. These sounds appear to serve predominantly social functions, including reproduction and maintaining contact, but they may also play some role in spatial orientation. The sounds of dolphins, on the other hand, are generally above 20 kHz which limits their communication distance. They also emit a variety of sounds like whistles, burst pulses, and echolocation clicks, extending over a wide high-frequency range from about 5 kHz to over 135 kHz. This pretty pattern is a visual representations of songs sung by the Northern minke whale. The image was produced by engineer Mark Fischer, who converted the sound frequencies of the whale song into a graph using a mathematical process known as wavelets. Minke whale songs are produced by the male during the mating season and can travel long distances underwater. Image: AGUASONIC ACOUSTICS/SCIENCE PHOTO LIBRARY

References J Caldwell 2002 Does air-gun noise harm marine mammals? The Leading Edge, January.   W W L Au, A N Popper and R R Fay 2000 Hearing by whales and dolphins: Springer. W J Richardson, C R Greene Jr, C I Malme and D H Thomson 1995 Marine mammals and noise: Academic Press. B Møhl, M Wahlberg, P T Madsen, A Heerfordt and A Lund 2003 The monopulsed nature of sperm whale clicks: J. Acoust. Soc. Am. 114:1143-1154. W M X Zimmer, M P Johnson, P T Madsen and P L Tyack 2005 Echolocation clicks of free-ranging Cuvier’s beaked whales: J. Acoust. Soc. Am. 117:3919-3997.

Martin Landrø is a professor in Applied Geophysics at the Norwegian University of Science and Technology (NTNU), Department of Petroleum Engineering and Applied Geophysics, Trondheim, Norway.   Lasse Amundsen is Chief Scientist Exploration Technology at Statoil. He is adjunct professor at the Norwegian University of Science and Technology (NTNU) and at the University of Houston, Texas.