Listening-for-ocean-spills

Listening-for-ocean-spills

Scientists who study acoustics (the “science of sound”) have over the years developed a variety of techniques to probe the hidden depths of oceans. This week, many of these acoustic researchers will come together to discuss how these technologies were used to monitor April’s Deepwater Horizon oil spill, to present new data on the gusher’s ecological impacts, and to highlight new techniques under development that could improve our ability to detect oil in ocean water. This special session will take place on November 17, 2010 at the 2nd Pan-American/Iberian Meeting on Acoustics, a major conference on the science and technology of acoustics held in Cancun, Mexico.

Some of these researchers will be presenting original data on the spill itself and its environmental impact.

When the deep wellhead burst, Natalia Sidorovskaia of the University of Louisiana at Lafayette found herself in a unique position. As a member of the Littoral Acoustics Demonstration Center of the University of Southern Mississippi, she had been part of a team listening to the waters near this rig for 9 years — using underwater microphones (hydrophone) to count sperm whales and beaked whales in the area.

“One of our sites was only 9 miles away from the Deepwater Horizon site, said Sidorovskaia.

Thanks to a rapid response award from the National Science Foundation and ship time donated by Greenpeace, Sidorovskaia and two mathematicians were able to revisit to these waters in September and spend a week collecting post-spill data.

By comparing this data to their pre-spill studies, the scientists hope to get an idea of whether the number of whales in the area has changed. They plan to present this new analysis in their talk on November 17.

“Our estimations agree with NOAA’s — about 1,655 sperm whales before the spill,” said Sidorovskaia. “If we killed 3 animals in the gulf, it might affect the population in the growth.”

Other researchers at the session will present new — though largely unproven — ideas for adapting technologies now used to study the structure of the ocean to detect the presence of oil in water as well.

At the University of Southern Mississippi in Hattiesburg, Michael Vera is exploring a technique commonly used to measure temperature gradients in the ocean (ocean-acoustic thermometry) which detects changes in the speed of a sound broadcast through water. Like a lens bending light, ocean water can distort sound in measurable ways that reveal various properties of the ocean.

Vera’s computer models, which simulate a Deepwater Horizon-like stream of oil, indicate that the presence of concentrated crude oil should also reveal itself by changing the speed of sound propagation.

“The model suggests that the oil should be detectable near the wellhead,” said Vera.

Mohsen Badiey of the University of Delaware in Newark, Boris Katsnelson of Voronezh University in Russia, and Jim Lynch of the Woods Hole Oceanographic Institute in Massachusetts are investigating another technique used to study the ocean’s three-dimensional structure.

Badiey and this team have spent years studying the structure of the ocean near the continental shelf using sound sources and receivers — ships and moored points that broadcast sound and arrays of sensors in shallow waters that detect the refraction patterns of this sound. Their recent findings reveal that the three-dimensional nature of sound propagation can be used to detect the boundaries of fluids with different indexes of sound refraction.

“This is a rather unusual approach for ocean acoustics,” said Lynch. “For almost a century, almost all work has been 2-D slices of sound.”

Any kind of stratified front — gradients of temperature or density — can cause interferences and perturbations in this sound. Measurements of these interference patterns have recently revealed information about propagating internal waves on the New Jersey continental shelf.

Their preliminary theoretical calculations give them hope that this technique could be adapted to detect oil spills and other liquids in large-scale areas of water up to few thousand square kilometers on the oceanic shelf.

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