Listening for Oil Spills

When the Deepwater Horizon oil spill erupted into the Gulf of Mexico last April, the only view researchers and citizens had of the gushing oil was the video feed controlled by BP. A team of scientists says it has now found a better way to track oil spills: sonar.

The researchers, from the University of New Hampshire's (UNH's) Center for Coastal and Ocean Mapping (CCOM) in Durham and the National Oceanic and Atmospheric Administration (NOAA), wanted to try sonar because its wide view can look at entire swaths of ocean at the same time. But no one had shown how to use the technology to map or track oil spills. "We were really doing crisis science. ... There were no proven methods for doing this," says team member Thomas Weber, an acoustician at CCOM.

Sonar works by bouncing sound waves off objects. When sound hits a droplet of oil, the waves scatter in identifiable patterns. The team found that frequencies near 200 kilohertz were best for tracking small oil droplets like those in the Deepwater Horizon spill. Using sonar devices on the NOAA ships Gordon Gunter, Thomas Jefferson, and Pisces, Weber and colleagues could tell that the oil plume was not spreading out more than a few kilometers underwater. Unfortunately, the frequencies they used to spot the oil peter out quickly in water and do not penetrate deeper than 150 meters, preventing the researchers from detecting oil in lower layers of water. The BP blowout was 1500 meters below the surface. Lower-frequency sonar could not detect oil directly, but it revealed density changes in deeper water that were probably caused by the oil, giving researchers another way to track the spill.

Other methods to track spills use light to look for oil, track oil on the surface, or take samples of the ocean water at various depths. But acoustic techniques can survey vast sections of water in the same amount of time. Weber will present the research tomorrow at the Pan-American/Iberian Meeting on Acoustics in Cancun, Mexico.

Another sign of the oil spill was natural gas, which often forms with oil and bubbles out of the same wells. Gas bubbles are resonant—they ring like a bell when sound waves hit them. This strong response made it easy to detect them, says Weber, even in deep water.

The team's detection of natural gas came in handy when BP's gushing well was capped in July. The UNH-NOAA team alerted mission control when its members spied what looked like gas escaping from the well. Small, remotely operated submersible vehicles with sonar were also looking for leaks, but the UNH-NOAA team surveyed the entire area at one time. The leak proved to be small, but the sonar techniques detected it quickly and monitored it until the well was sealed in August.

One difficulty is that the researchers saw natural seeps of methane gas almost everywhere they looked. The team discovered more than 20 previously unknown gas seeps near the oil well. Researchers had known that natural gas bubbled up from the floor of the gulf, but they had no idea it was this common, Weber says. Oil might also ooze from some of these gas leaks. The natural seeps of gas and oil could contribute to the overall levels in the ocean water, possibly complicating scientists' attempts to understand how much oil spilled and what the effects will be on the gulf.

The team's acoustic monitoring was "extremely significant" while the wellhead was being closed, says Kathryn Moran, an oceanographer at the University of Rhode Island, Kingston. The techniques the UNH-NOAA team developed during the Deepwater Horizon incident have already led to discoveries of natural gas seeps in other areas, she says. If another massive oil spill occurs, responders will have a suite of new acoustic techniques with which to monitor the spill. The UNH-NOAA team is already planning a sonar system that could be lowered on a cable hundreds of meters into the ocean and towed beneath the boat, allowing it to better track oil in the depths.

Correction: The original version of this article incorrectly implied that the UNH-NOAA team was first to detect a gas leak in August at the Macondo wellhead. Detection of the leak was a group effort within the entire spill response team.

For more on the gulf oil spill, see our full coverage.