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Environmental Impact of Air-Gun Seismic Surveys: Testing the Waters

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Offshore Energy Clash Over Undersea Sound

A diagram shows a survey ship gathering seismic data.

A survey ship sails back and forth to collect seismic data, towing both an array of air guns and receivers to pick up the signals of the sound penetrating the ocean floor.

Illustration by Davis Meltzer, National Geographic Stock

Before any drills descend into the Atlantic Ocean under the Obama administration’s plan to expand offshore U.S. fuel production, there is the considerable matter of testing the waters—or rather, what lies beneath them.

The first stage of what is sure to be a contentious approval process is a debate over the environmental impact of seismic surveys that would be conducted along a coast that hasn’t been mapped for oil and gas potential in at least 25 years.

At least 11 geological and geophysical research companies have applied for federal permits to survey the Atlantic’s outer continental shelf, using high-pressure air guns to create the sound-pressure waves capable of penetrating the sea floor. Because the noise levels from air-gun seismic surveys are louder than the noise of oil and gas production itself, the effect of the studies on marine life will be a top issue in public meetings on the environmental assessment scheduled for later this month.

The federal government’s current estimates of oil and gas resources don’t foresee a windfall for U.S. energy independence beneath the sea floor from Delaware to central Florida—the areas that the Obama administration proposes to open to drilling. The Department of the Interior calculates the production yield from new development to be as much as 1.15 billion barrels of oil and 11.7 trillion cubic feet of natural gas for the Mid- and South Atlantic planning areas.

At the current U.S. rate of consumption, that’s about 61 days’ worth of oil and two years’ worth of natural gas. For the area 50 miles off Virginia’s coast, which would be the first lease sale sometime before 2012, the estimate amounts to seven days’ worth of oil and 18 days’ worth of gas.

A New Look At Resources

But Richard Ranger, senior policy adviser at the American Petroleum Institute, says those numbers could be conservative. “There really has not been any methodological look at the geologic structures that might contain oil and natural gas in almost two generations,” he says. He says early resource projections often prove to be underestimates. For example, estimates done in the 1970s to gauge  the potential of North America’s largest oil field, Prudhoe Bay, Alaska, suggested that it held 9 billion barrels of oil. With the discovery of additional fields, he says the industry has since produced nearly 16 billion barrels from the region, and reserve estimates for the currently producing North Slope fields have also been revised upward.

Over the past three decades, the most significant improvement in exploration technology, says Ranger, has been vastly increased computer processing power. With the help of seismic surveying, the industry can create not only 2-D maps but 3-D or even 4-D images of the underground formations. To obtain those pictures of where oil or gas is likely to be found, arrays of air guns are towed behind large vessels that weave back and forth over exploration areas. Sound receivers, called hydrophones, are on streamer cables towed farther behind.

According to a 2004 federal environmental assessment of seismic surveys in the Gulf of Mexico, the air guns fire every seven to 16 seconds for hours, days, weeks or even months. The Interior Department’s Minerals Management Service (MMS) concluded that the sound levels, though strong, fall significantly at a distance from the air guns. Beyond 300 meters, or a little more than the length of a football field, “sound from a seismic survey is of a similar nature to other commercial vessel activity,” the agency said.

The MMS acknowledged that there were “potentially adverse” effects, such as hearing loss in marine mammals or disruption of commercial and recreational fishing. But it concluded that these did not rise to the level of “significant environmental impact” that would prompt limitations on the estimated 20 oil and gas seismic surveys that take place each year in the Gulf.

The MMS noted that there had been no mass strandings of whales associated with oil and gas activity in the Gulf even though seismic surveys have occurred there for decades. (In contrast, there have been mass whale strandings associated with high-energy military sonar.) “While there remains much discussion and debate, it’s the industry’s belief—and we believe the track record shows—that with the precautions we’ve taken, there are no threats at a population level,” says Ranger.

The Impact of Sound

But the MMS also noted that the scientific study of acoustic-based impacts on marine life began only recently. And Michael Stocker, director of the nonprofit advocacy group Ocean Conservation Research in Lagunitas, California, says that strandings are merely the most obvious sign of trauma exhibited by marine mammals. It has been harder to prove the harm done out of sight of humans, as when marine mammals develop compromised hearing and become easy prey for predators at sea, he says. Also, a number of studies published in just the past three years show that underwater noise, including from seismic air guns, disrupts the feeding and foraging behaviors of many different species.

“Most animals in the ocean use sound the way animals on land use eyesight,” Stocker says. “And when we talk about saturating their environment with noise, it’s going to have some impact, regardless of whether we know what that impact is.”

Ranger notes that the industry takes steps to mitigate potential impact. Seismic testing operations are required to create a marine mammal exclusion zone by having a trained observer aboard to call a halt to the activity if a whale or dolphin is spotted within a certain radius. Also, the sound is gradually ramped up from a low level to give marine mammals warning and a chance to leave the area.

But the MMS said the sound ramp-up “remains unproven as a mitigation measure, although recognized as a common sense measure.” And Stocker says that many “locally habituated,” or resident fish and marine mammals typically won’t leave their habitat, even if their hearing is being harmed by the sound: “It’s foolish to think if these animals don’t like something they’re bothered by, they’ll go away.” As for the exclusion zone, Stocker says, “Having somebody sitting on a watch stand looking for whales at 1,000 meters is a token gesture that’s fairly meaningless.”

Among the more recent evidence of the reach of seismic testing sound—sure to be cited at the upcoming meetings—is a 2004 study by U.S. scientists, including from the National Oceanic and Atmospheric Administration. They found that hydrophones placed in the middle of the Atlantic Ocean were able to record the sounds of seismic air guns from sites more than 3,000 kilometers (more than 1,860 miles) away—off Nova Scotia, the coast of western Africa, and northeast of Brazil. The hydrophones, intended to monitor low-frequency earthquakes, also recorded whale calls. But the most dominant signal the investigators detected—sometimes masking the whale sounds—was of the distant seismic air guns.

The MMS will be gathering public comment on its Atlantic seismic testing plan through May 17, with an eye to completing the environmental impact study by 2012.

National Geographic

 

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