(Redirected from Seismic surveys)
Seismic Surveys (English)
Estudio sísmico (Español)
Seismic surveys are used to locate and estimate the size of offshore oil and gas reserves. To carry out such surveys, ships tow multiple airgun arrays that emit thousands of high-decibel explosive impulses to map the seafloor. The auditory assault from seismic surveys has been found to damage or kill fish eggs and larvae and to impair the hearing and health of fish and marine mammals, making them vulnerable to predators and leaving them unable to locate prey or mates or communicate with each other. These disturbances can disrupt and displace important migratory patterns, pushing marine life away from suitable habitats like nurseries and foraging,
mating, spawning, and migratory corridors. In addition, seismic surveys have been implicated in whale beaching and stranding incidents.
What are Airguns?
Seismic surveys utilize arrays of airguns to produce powerful sound waves. Sudden releases of pressurized air bubbles create the sound source, with up to 20 guns fired at the same time, while “streamers” of hydrophones listen for echoes. Using sophisticated acoustic processing, these echoes can provide information about geological structures up to 40 kilometers below the sea floor. Seismic surveys are used by the oil and gas industry in its search for new hydrocarbon deposits and the monitoring of reservoirs as they are emptied. The “source level” of most airgun arrays can be 200 to 240 decibels (dB) (in water). There is a difference of about 60 dB when converting the sound level from water to air, so in air, the airgun sound level would be about 140 to 180 dB. For comparison, a loud rock concert is about 120dB and a jet engine at 100 feet is about 140dB. A typical seismic air gun array pulled by a ship might fire its compressed air bubbles into the ocean five or six times a minute — more than 7,000 shots in 24 hours.
Because sound can travel hundreds or even thousands of miles under water, it’s not surprising that seismic airguns can be heard at great distances. In 2004, bioacousticians began reporting that airgun noise from distant surveys along the coast of South America (and perhaps Africa) can be the dominant sounds in some mid-Atlantic study sites, at times making it difficult or impossible to hear the whales or seaquakes they are trying to study. Airgun noise is over 200dB (often 230db) at the source, drops quickly to under 180dB (usually within 50-500 meters, depending on source level and local conditions), and continues to drop more gradually over the next few kilometers, until leveling off at somewhere near 100dB. At this level, the sound can still travel for hundreds or thousands of kilometers. In many or most locations, 100dB is significantly louder than the existing ambient background noise, so when the airguns raise the background noise to this level, it potentially masks local biological calls and signals. Such effects have been noted at ranges from 1,300 to 3,000 km from active surveys. These sounds are primarily low frequency, so at long distances, the effects are most pronounced for larger species such as the great whales and some fish that use low-frequency sounds.
At the International Whaling Commission 2004 meetings and at 2004 meetings of the US Marine Mammal Commission’s Advisory Committee on Sound, research was presented that suggests human noise can shrink the area in which whales can communicate with each other by two to four orders of magnitude (that is, when the sea is especially loud, their effective communication area is one hundredth to one ten-thousandth the size that it would be in the absence of human noise). See Resolutions Adopted during the 56th Annual Meeting of the International Whaling Commission and Marine Mammals and Noise - A Sound Approach to Research And Management from the US Marine Mammal Commission.
Why the Push for Seismic Surveys?
While seismic surveys have been taking place for decades, changes in industry practices (including exploration on the continental slopes where sound may bounce into long-range sound channels, and increasing use of 4D (repeat) surveys to monitor reservoir depletion during the life of active reservoirs) are increasing their use and the resultant concern about harm to sea life. The North Sea has been the site of many 4D surveys, and the industry considers the Gulf of Mexico fields now “mature” enough to “benefit” from surveys there.
The passage of an energy bill in the United States in the summer of 2005 increased attention on airguns, as the bill called for a comprehensive inventory to be made of oil and gas reserves on the Outer Continental Shelf. The Minerals Management Service (MMS) (now Bureau of Ocean Energy Management (BOEM)) plans to review existing data and research, then to conduct new surveys as needed to increase geologic knowledge in areas of high reservoir potential. Each year there are about 20 permitted 3-D seismic surveys in the U.S. Gulf of Mexico. That work is facilitated by a disputed 2004 Environmental Assessment (pdf) that concluded “geological and geophysical activities evaluated in the EA will not significantly affect the quality of the human environment.” The document does set a standard 500 m safety radius for seismic surveys. More recently, plans been announced for an environmental review followed by seismic surveys in the Chukchi and Beaufort Seas, Alaska and off the Atlantic coast. There will now be a supplemental EIS for the Arctic region.
Public hearings were conducted in April 2012 to receive public input on seismic surveys for the mid-Atlantic and south-Atlantic regions. Click here for more info and to hear what seismic testing sounds like underwater. According to a report issued by Oceana in April 2013, seismic testing in the mid-Atlantic and south-Atlantic regions would cause:
- 138,000 injuries to whales and dolphins and 13.5 million disturbances to vital behaviors in marine mammals such as breathing, feeding, mating, and communicating
- Injuries to nine critically endangered North Atlantic right whales and disruption of their critical habitat, when only roughly 500 right whales remain
- Widespread displacement of whales
- Disruption of loggerhead sea turtles as they head to nesting beaches that are soon to be designated as critical habitats
- Death of fish eggs and larvae
- Potential disruption of fish migration and spawning
As mentioned above, there is growing concern that sound introduced into the sea by human activities has detrimental effects on marine mammals. For example, mounting evidence suggests that high-intensity anthropogenic sound from sonar and airguns leads to strandings and
subsequent mortality of beaked whales. Although the mechanisms of injury in these events are unclear, the species affected and the implicated sound levels follow a consistent pattern.
A well-established body of research indicates that marine mammals try to avoid active seismic survey vessels, often exhibiting avoidance behavior at ranges of 5-30km; however, it is not uncommon for whales or dolphins to approach closer to operating airguns, whether out of curiosity or because of a biological need to be where they are. In 2002, two beaked whales (the species of whales that has proven most susceptible to sonar impacts) were found dead along a shoreline near where an academic seismic survey was underway. The whales were too decomposed to determine a cause of death, but the incident became the first case of a survey being stopped by the courts due to animal safety concerns. Since then, seismic surveys have continued to be implicated in whale and dolphin beaching and stranding incidents.     Beyond marine mammals, a 1996 study in Norway showed that airguns lowered fish catch rates over a 2,000 square mile area, a 2004 study provided the first suggestions that airguns can cause long-term injuries in snow crabs, and surveys in 2001 and 2003 were implicated in the beachings of squid that died from unexplained internal injuries.
Exposure to very high intensity noise can cause direct physiological damage, such as tissue or cell ruptures. Temporary threshold shift can occur at longer ranges (i.e., lower received levels of sound); this is basically a temporary loss of hearing, so that a sound must be louder than normal in order to be heard or understood. Permanent threshold shift, which is in effect permanent partial hearing loss, is also possible after incidental exposure to extremely loud sound or chronic exposure to moderately loud sounds, though this has not well studied, especially in the wild.
At least twice (once in the Gallapagos, and once in the Sea of Cortez) whale beachings have occurred coincidentally with seismic surveys. Although the biological effects of displacement/harassment by noise are not well studied, there is some evidence of long-term hearing damage in cetaceans (based on studies of beached dolphins, both living and dead, about half of which show signs of compromised auditory systems—likely caused by a combination of age, toxins, and chronic exposure to airgun or ship noise, or incidental exposure to louder noises such as explosives).
There is also evidence that seismic testing can scare fish away from the reefs that are their normal habitat.
And it isn't just large marine mammals that are killed or injured by seismic testing. A study published in journal Nature Ecology and Evolution, adds to the body of evidence that the loud noises produced during oil and gas exploration can disrupt marine life. For this study, researchers wanted to see what the effects are on the sea’s base of the food chain, the zooplankton. The researchers blasted airguns in the ocean off southern Tasmania, and checked zooplankton populations before and after by using sonar and nets. The abundance of these tiny creatures dropped by 64 percent within one hour of the blast, the study says. Two to three times as many zooplankton were also found dead — and the impacts were recorded as far away as 0.7 miles. More.
Underlying much of the concern among both scientists and the public are biological and ethical questions about frequent harassment by human noise; does our “need” for new oil supplies trump ocean creatures’ needs for acoustic space in the seas? Though the effects of behavioral changes caused by animals avoiding sound are difficult to separate from other population-level stressors on sea life (toxins, etc.), given the tenuous recovery of cetaceans, and the global decline in most fish species, calls are increasing to limit additional stressors on these fragile populations of sea life.
Regulation and Monitoring of Seismic Surveys
Some countries have begun to take a harder look at airgun noise. During 2004, Mexico rejected some permits for both academic and industrial surveys, and Brazil is prohibiting surveys near a key marine reserve. Still, worldwide awareness of the long-range acoustic effects of surveys is only beginning to develop. To date, mitigation measures and operational standards for seismic surveys have been largely aimed at assuring that no marine mammals or sea turtles are directly exposed to airguns at close range.
Most seismic surveys begin with a “ramp up” period, typically 30 minutes, during which the airguns are turned on a few at a time, so that any marine mammals or large fish in the area will be forewarned and theoretically have time to move away (smaller fish and turtles may need more time, and of course slow-moving bottom creatures are unlikely to flee). Similarly, as the ships move along their survey lines, their slow approach may allow time for animals to move.
In the US, Europe, and Australia, safety zones are routinely established around operating seismic survey vessels, with on-board observers watching for animals entering this zone, which ranges from 150m to 3km, depending on the intensity of the airgun arrays and local sound propagation properties. Most commonly, the safety radius is 500m to 1km; outside of this zone, sounds are generally considered to be less than 180dB, the threshold where physical damage is considered likely. (Thresholds are 160db for some species; the danger zone is different for each species and for each type of damage being considered; these have become the de facto standards for avoiding catastrophic tissue damage.)
The Bureau of Ocean Energy Management, Regulation and Enforcement, the federal agency that regulates oil and gas exploration and production in U.S. federal waters, began circulating drafts of rules in 2002 to protect whales. The rules included barring seismic tests if sperm whales were observed in the area, hiring government-approved staff to watch for whales and requiring seismic companies to perform a ramp-up, starting with one gun before undertaking a full test. The proposal drew swift protest from the International Association of Geophysical Contractors (IAGC), which represents 140 companies. At a lengthy meeting with the Minerals Management Service, IAGC persuaded the agency to make a number of changes. The biggest involved decreasing the size of the exclusion zone, the area that must be “whale-proofed” prior to testing.
The original proposal required a computation to determine the size of the zone, which IAGC claimed could theoretically create a five-mile exclusionary zone. Such a massive zone would have barred nighttime testing and required seismic companies to use fixed-wing aircraft or chase boats to look for whales. The agency ultimately decided to enforce a 500-meter radius zone, the same area used to protect mammals in the North Sea.
Since ocean noise by its nature is a trans-national problem, initiatives are underway to formally include noise as a “pollutant” under international treaties such as the UN Law of the Sea. Toward this, other regional, national, and state ocean policy agencies have begun to address ocean noise questions, driven by concerns about sonar and shipping, to which seismic surveys are beginning to be added. Examples of this include the International Whaling Commission (IWC) recommendations (.doc), California’s Ocean Resources Management: A Strategy for Action (pdf), and resolutions passed in the European Union parliament and ACCOBAMS (a consortium of 16 Mediterranean countries).
What More Can be Done?
Responses to the increasing concerns about the effects of seismic surveys range across the entire spectrum of possible actions. At one end of the spectrum is the “business as usual” response, which relies on the long history of airguns with little dramatic evidence of problems. For example, the Bureau of Ocean Energy Management, Regulation and Enforcement has released a Programmatic Environmental Assessment (pdf) for survey activity in the Gulf of Mexico, and is apparently content with the current 500m exclusion zones in that area. At the other end of the spectrum are calls for a moratorium on surveys and legal challenges that have stopped several surveys in their tracks.
Modest proactive steps have taken place within some permitting agencies, including the US National Oceanic and Atmospheric Administration (NOAA) and the UK Department of Trade and Industry (DTI), both of which have begun calling for use of “passive acoustic monitoring" (listening for whales, rather than relying solely on visual monitoring, which is known to spot only a small fraction of whales present), and at times enforcing larger exclusion zones (up to 3km in certain situations).
Additional suggestions have been made by the IWC (doc). In addition to calls for public information about industrial surveys (so that agencies can better consider the cumulative impacts of many surveys in one area), the IWC has strongly recommended that surveys be accompanied by “continuous” biological effects monitoring, extending before, during, and after all surveys. The industry has funded some of the most important studies to date and there remains a pressing need for more comprehensive research. Given that seismic surveys are the most common extreme noise source in the sea, it’s reasonable to ask commercial survey companies to fund the research needed to determine their long-term and long-range effects on sea life.
In 2009 a Workshop on Alternative Technologies to Seismic Airgun Surveys for Oil and Gas Exploration and their Potential for Reducing Impacts on Marine Mammals was held in Monterey, California. Here is the abstract from the consensus summary of the Workshop:
Past experience shows us that a fraction of the airgun sound that has potential to impact marine mammals (either physically or behaviorally) comes from "waste sound" that is either too high frequency and filtered out before recording or propagates laterally away from receivers and is also never recorded. The Okeanos Seismic Airgun Alternatives workshop panelists identified several ways in which unwanted sound or noise from seismic airguns might be reduced with little or no effect on the quality of data acquired. In addition to eliminating this noise or unused signal, peak sound levels required for exploration might also be reduced by spreading the source energy out over time, and/or moving sources and receivers closer to the seafloor. Panelists also discussed promising new imaging technologies that are either completely silent (e.g. controlled source electromagnetics) or that can lessen the amount of seismic sound required to gather seismic data (e.g. increasing the density of more sensitive receivers, such as fiber optics or through the use of passive seismic technology) thereby still allowing for a reduction of the economic risk of hydrocarbon recovery. Workshop panelists acknowledged that these technologies are purpose driven and do not work in all circumstances. Many of these technologies may be either available now or in the next 1-5 years, depending on funding and technology advancements.
In January 2015 scientists at Stanford University announced that a new technique had been developed that has several advantages over seismic surveys. The technique, called ambient seismic field noise-correlation tomography, or ASNT, uses sensors embedded in the seafloor. The sensors, which are typically installed by robotic submersibles, are connected to one another by cables and arranged into parallel rows that can span several kilometers of the seafloor. Another cable connects the sensor array to a platform in order to collect data in real time. The sensors record ambient seismic waves traveling through Earth's crust. The waves are ubiquitous, continuously generated and traveling in every direction, but using careful signal-processing schemes they developed, the scientists can digitally isolate only those waves that are passing through one sensor and then another one downstream. When this is done repeatedly, and for multiple sensors in the network, what emerges is a "virtual" seismic wave pattern that is remarkably similar to the kind generated by air guns. Because the ASNT technique is entirely passive, meaning it does not require a controlled explosion or a loud air gun blast to create a seismic wave signature, it can be performed for a fraction of the cost of an active-reflection-seismology survey and should be far less disruptive to marine life, the scientists say. More on this.
In April 2016 a group of marine scientists called on President Barack Obama to block seismic testing for oil and natural gas deposits along the Atlantic Outer Continental Shelf, saying the airgun blasts associated with the surveys could wipe out endangered North Atlantic right whales. Seismic surveys have been referred to as the "gateway drug" for offshore oil drilling. When you add the damage to marine life caused by seismic surveys to the inevitable environmental and recreational impacts caused by oil spills from offshore oil drilling, it is even more apparent that offshore oil drilling is Not the Answer.
Renewable Energy Alternatives
Renewable energy surveys use non-airgun seismic technologies that also produce high energy acoustics, including side-scan sonar, boomer sub-bottom profilers, chirp sub-bottom profilers, and multi-beam depth sounders. G&G PEIS Vol. 2, Table 3-11. All of these technologies produce sounds within hearing frequencies and above decibel safety thresholds for marine mammals. Compare G&G PEIS Vol. 2, Tables 3-11, D-23, and 4-6. However, seismic testing for renewable energy is much more limited in distance (G&G PEIS Vol. 2, Tables D-21 and D22) and depth (G&G PEIS Vol. 2, Table D-13). As a result, its adverse impacts on marine mammals and other creatures is also much more limited. For example, renewable energy technologies can have an adverse impact radius of approximately 40-200 meters; oil and gas technologies can have an adverse impact radius of roughly 1,000 to 10,000 meters. Compare G&G PEIS Vol. 2, Tables D-21 and D22.
The federal government modeled the adverse impacts on marine mammals of oil and gas seismic testing in the Atlantic, compared to seismic testing for renewable energy in the Atlantic. It concluded that seismic testing for oil and gas would directly injure approximately twenty thousand marine mammals annually; seismic testing for renewable energy, on the other hand, would injure only a handful of marine mammals. See G&G PEIS Vol. 2 Table 4-10 and 4-13 and this graphic. Oil and gas exploration would harass nearly two million marine mammals annually; renewable energy exploration would harass approximately a thousand annually. See G&G PEIS Vol. 2 Table 4-11 and 4-12. These figures suggest that although seismic testing for renewable energy is not without adverse impacts, these impacts are significantly lower than the impacts of seismic testing for oil and gas.
These comparisons speak only to the impacts of seismic testing, and not other noise-polluting activities (or other adverse environmental impacts, for that matter). For example, pile driving during construction of wind farms is also associated with noise pollution at decibels comparable to seismic testing for marine mammals in close proximity. BOEM 2014-061. Oil and gas development is associated with noise from tankers and construction. Effective policies for seismic testing involve understanding all of the environmental impacts, selecting the most sustainable options, and continuously pursuing mitigation options to reduce unavoidable impacts as much as possible.
Selected Research on the Effects of Airgun Noise
Squid Show Signs of Acoustic Trauma - Several beachings of giant squid along the coast of Spain have raised concerns that their deaths may have been caused by exposure to loud sounds, possibly seismic survey airguns. Unusual numbers of stranded squid appeared during seismic surveys in both 2001 and 2003. None had signs of superficial damage, but all had internal injuries. Ear damage was present in all specimens, with further organ and tissue damage in some. Local fishermen also reported seeing large numbers of dead fish floating at sea during the surveys. Source: New Scientist, 9/22/04
Snow Crabs Show Damage From Airguns - The first controlled study of snow crabs exposed to an active seismic survey revealed substantial physiological damage. Crabs, which were caged on the seafloor as airguns passed 40 meters above them, exhibited tissue and organ damage, slightly poorer reproduction, and an increased number of lost legs. Canadian Department of Fisheries and Oceans researchers, who did the study, noted that there was no significant change in mortality or feeding patterns in crabs exposed to airguns, but that hemorrhaging and membrane detachment in the crabs' ovaries was noted, and that the condition intensified between December (when the crabs were exposed to the airguns) and May. Similarly long-lasting and worsening effects were also detected in the hepatopancreas, which functions like a liver in a crab, with abnormal cell structure, swelling and stress detected. Environmental groups expressed shock at the results and called for consideration of an immediate moratorium on seismic testing. Sources: Halifax Herald, 10/2/04, 10/7/04
Sound from seismic survey airguns increased the measured ambient noise levels of a blue and fin whale feeding area in the North Atlantic by two orders of magnitude (a 100-fold increase). (Christopher Clark, Cornell) This increase, observed throughout a nearly hundred thousand square kilometer study area (200x400 nautical miles), was nearly continuous for days at a time; such long-range effects contrast with typical effects modeling, which focus on areas very near the survey vessels and consider the effects of a single seismic shot lasting only a fraction of a second.
Low-frequency whale and seismic airgun sounds recorded in the mid-Atlantic Ocean (Nieukirk, Stafford, Mellenger, Dziak, Fox. J. Acoust. Soc. Am., Vol. 115, No. 4, April 2004. P. 1832-1843) (From Discussion section):
Since this hydrophone array was deployed, the periodic impulses produced by seismic exploration vessels operating around the Atlantic basin were the dominant signal detected. Concern over the potential effects of anthropogenic noise on marine life has
been such that the National Research Council of the (U.S.) National Academy of Science has commissioned three studies on this topic to date (NRC 1994, 2000, 2003). Although seismic airgun arrays are designed to direct the majority of emitted energy downward through the seafloor, their sound emission horizontally is also significant (NRC, 2003). Airgun survey vessels were often located 3000 km or more from our array, yet airgun pulses were still clearly recorded on each hydrophone. The broadband frequency range and repeated firing of these guns make them a major contributor to the low-frequency sound field in the North Atlantic.
Airgun activity in shallow water has been shown to significantly damage the ears of fish (McCauley et al., 2000) and has been implicated in the stranding of beaked whales (Malakoff 2002; NRC 2003). Its effect on the baleen whales studied here is unknown; possible effects include masking of conspecific sounds, increased stress levels, changing vocalizations, and ear damage (Richardson et al., 1995). Most of the seismic vessels we located were operating in marine mammal habitat, including that of the critically endangered northern right whale.
Airgun pulses were recorded year-round but were most common from late spring through fall. This pattern is the opposite of the peak occurrences for all baleen whale calls. It is possible that the seasonal patterns seen in baleen whale calls are due to airgun interference: that is, the calls are produced in the summer months but obscured by airguns. However, because calls are detected during some months of frequent airgun occurrence in the fall, because the repetition rate of airguns is such that most whale sounds can be detected between pulses, and because the data were visually inspected, we don’t believe that many calls were missed due to interference (cf. Clark and Charif, 1998).
(From Results section):
Sounds associated with seismic airguns were recorded routinely on all hydrophones, and trends were similar in the two years. Typically airguns were heard every 10–20s. Although airgun sounds tended to dominate recordings during the summer months, loud whale vocalizations could still be detected during intense airgun activity. Occasionally the array recorded airguns from more than one location, masking cetacean sounds and on four occasions making the spectrogram data impossible to use. The high received level of these impulses on multiple hydrophones made it possible to estimate the locations of the ships conducting the airgun surveys. During the summer months, airguns operated off Nova Scotia, Canada, probably in support of exploration in the Sable Island region. From spring through fall seismic vessels, presumably commercial, were located working off the coast of western Africa and northeast of Brazil. Seismic vessels operating in other areas of active exploration, such as the North Sea and the Gulf of Mexico, were not observed by this array due to bathymetric blockage.
- Protecting Our Ocean and Coastal Economies: Avoid Unnecessary Risks from Offshore Drilling
A Deaf Whale is a Dead Whale: Seismic Airgun Testing for Oil and Gas Threatens Marine Life and Coastal Economies, Oceana, April 2013
Biers, John M., Scientists Worry Seismic Testing Could Harm Whales, 2002
Clayton, Mark, New Energy Probe May Harm Sea Life, Christian Science Monitor, July 2005
Does Seismic Exploration Harm Whales and Fish? Fisheries and Oceans Canada, 2009
Hildebrand, John A., “Impacts of Anthropogenic Sound” in J.E. Reynolds et al. (eds), Marine Mammal Research: Conservation beyond Crisis. 2005. The Johns Hopkins University Press. p.106, p.117. http://cetus.ucsd.edu/Publications/Publications/HildebrandJHU2005.pdf.
Interaction Between Offshore Seismic Exploration and Whales, Australian Government Department of the Environment, Water, Heritage and the Arts, September 2008
MMS. 2008. Beaufort Sea and Chukchi Sea Planning Areas: Oil and Gas Lease Sales 209, 212, 217, and 221, Draft Environmental Impact Statement. Vol. II, Chapter 4.4, pp. 4-41-4-42.
Offshore Seismic Surveys May Impair Hearing and Cause Ear Damage in Marine Fish and Mammals, Friends of the Earth Norway, 2007
Seismic Exploration and Drilling: Surveys and Leases Being Planned for the Near Future, The Acoustic Ecology Institute
Seismic Survey, Joint Nature Conservation Committee (UK)
Siebert, Charles. Watching Whales Watching Us. New York Times, 12 Jul. 2009.
International Whaling Commission Scientific Committee, 2004 meeting
Full Report on the Mini-symposium on Anthropogenic Noise: http://acousticecology.org/docs/IWC56-noisesymposium.doc
Section of Scientific Committee Report addressing the Noise Symposium and Recommendations (which was unanimously adopted by the full IWC membership): http://acousticecology.org/docs/IWC56-SCReportNoiseSymposium.doc
Reports on Ocean Noise:
Seismic Surveys: What We Don't Know Can Hurt - A research overview by Acoustic Ecology Institute Executive Director Jim Cummings, commissioned by Greenpeace, 2004. http://acousticecology.org/oceanairgunexecsumm.html
Impacts of Anthropogenic Sound on Marine Environments - Paper by Michael Stocker, commissioned by Earth Island Institute, with a special focus on the use of sound by fish and mollusks and a brief overview of natural and biological noise in the oceans. http://www.msa-design.com/FishEars.html
Oceans of Noise - The UK-based Whale and Dolphin Conservation Society has released a 165 page report, available on its website; its primary focus is on taking action to protect sea life from damaging impacts. http://www.wdcs.org/submissions_bin/OceansofNoise.pdf
Marine Mammals and Noise - A Sound Approach to Research And Management, A Report to Congress from the Marine Mammal Commission. March 2007.