Can Sonar Detect Undersea Oil Plumes?

Among the biggest questions about the Deepwater Horizon spill is how much oil remains underwater and where it is going. Figuring it out has been frustratingly slow with existing techniques. Now marine scientists who recently returned from a research cruise in the Gulf of Mexico say they have preliminary evidence that a new technique using sonar may be able to track undersea oil plumes far faster and better, although they caution that it is too soon to say whether the technique will succeed.

To study the plume, scientists have been lowering instruments from research ships to take fluorescence measurements of the water at different depths and to measure the amount of dissolved oxygen, among other things. At the same time, researchers try to correlate those findings by a chemical analysis of water samples that are sent ashore. But because lowering and raising the instrument panel and taking such samples can take up to 2 hours each, researchers' ability to track the full extent of possible underwater plumes is severely limited.

Larry Mayer, a marine geologist at the University of New Hampshire in Durham is trying to use sonar to speed that up. If Mayer and his colleagues can find a reliable acoustic signature of underwater oil, they say researchers will be able to map the spread of plumes using sonar aboard ships cruising at 10 knots. "If we can map this acoustically, we'll be able to cover a lot more ground," Mayer says.

"We have intriguing evidence it may work," Mayer adds. On their recent 8-day cruise, Mayer and his colleagues from the National Oceanic and Atmospheric Administration, the Woods Hole Oceanographic Institution, and the Environmental Protection Agency lowered their instrument panel at nine different sites. Twice they observed high fluorescence and reduced dissolved oxygen, indicating a possible plume at 1100 meters depth and 7.5 nautical miles southwest of the wellhead. At both sites, they also observed a subtle acoustic "anomaly," a faint reflection of something in the water that follows the contours of the seafloor below. Mayer says that it may be a sign a significant amount of oil is concentrated in a deep-water layer that is being pushed southwest of the wellhead by a deep-water current. Whether this will prove true as more data comes in, "The jury is still out on that," Mayer says. Mayer adds that one of his students is currently aboard another ship in the gulf taking more sonar readings.

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