Mermaids File: Air guns and JIP

http://www.earthmagazine.org/article/sounds-and-cetaceans-quieting-noisy-underwater-world

Air guns are a particularly thorny issue — so much so that JIP has already spent $7 million measuring their sound output in the ocean, according to Gentry. Air guns shoot low-frequency shock waves from an array towed behind a ship down into the water. These shock waves penetrate the seafloor and help locate pockets of oil, gas or mineral deposits. But to get the seismic profiles researchers need, the air guns release large quantities of highly compressed air at 15-second intervals. These guns can produce noise levels as high as 250 decibels, twice as loud as a jet engine and well over the 200-decibel threshold for permanent hearing damage in fish and whales.

Since 2005, researchers in Norway have been looking at how sound from air guns propagates through the water. To gauge how sound waves from air guns might affect whales or other marine life, researchers first need a better understanding of how sound actually travels underwater. Ideally, most of the energy produced by air guns is directed down toward the seafloor, but the seafloor’s bathymetry has a significant effect on how much sound energy escapes outward. “A big part of the air gun project is modeling how sound moves in different underwater terrains,” Gentry says. “The air gun arrays produce and propagate sound very differently depending on whether the environment is shallow water or deepwater, if the bottom is sloping or flat, and whether the seafloor is mud or rock.”

Another major focus of the 2010 program will be to modify existing technologies like hydrophones and radar to detect whales near a sound source like an air gun array — with the goal of avoiding whale-human interactions. “Right now the gold standard for detecting whales is to have somebody standing watch on the bridge of a boat looking out with big binoculars,” says James Eckman, a marine biologist at the Office of Naval Research in Arlington, Va. “But that method doesn’t work in bad weather or at night.” Fortunately, he says, the key to solving this problem might lie in already existing technology. “Most of these animals make noise, so just dropping a microphone in the water could work.” Radar, which is already required on all seagoing ships, could also be used to watch for large whales nearby, he says. The Supreme Court’s 2008 decision stated that sonar operations must cease if a whale or other marine mammal is spotted within two kilometers of a ship. “Right now seismic operations are shut down at night, when we can’t see the whales,” Young says. “Developing better ways to detect them would double our exploration efficiency at sea.”