How to Detect an Oil, Ice Sandwich

Wednesday, May 7, 2014 @ 02:05 PM gHale

Decreasing ice cover is currently spurring an increase in hydrocarbon extraction and shipping in the Arctic.

When you hear about hydrocarbon extraction, that means there is the possibility of some type of spill in this environmentally sensitive area and that is why Woods Hole Oceanographic Institution researchers are evaluating the effectiveness of emerging broadband active acoustic techniques to remotely detect oil spills under sea ice.

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That is where Christopher Bassett, a postdoctoral scholar in Woods Hole Oceanographic Institution’s Applied Ocean Physics and Engineering Department comes in. He demonstrated that acoustic scattering can work in detecting crude oil under ice. He presented his findings at the 167th meeting of the Acoustical Society of America in Providence, Rhode Island.

There’s a need to identify technologies capable of detecting oil under ice “because traditional air- and surface-based methods are of limited practical value,” Bassett said. “Our goal is to evaluate the effectiveness of emerging broadband active acoustic techniques for remote detection of oil spills under sea ice. Currently, not much research has been performed to identify instruments that can detect under-ice oil spills.”

The emerging broadband acoustic techniques Bassett and colleagues are evaluating operate at high frequencies — greater than 100 kHz — to detect oil spills under sea ice grown in the laboratory.

How are broadband acoustic techniques different than more traditional acoustic techniques? “Rather than transmitting a narrowband (single frequency) signal, we transmit a frequency modulated ‘chirp’ signal. Using signal processing, these ‘chirps’ can improve the temporal range resolution over narrowband methods and be used to evaluate frequency spectra of scattered signals,” Bassett said.

The goal is to exploit the improved range resolution and frequency spectra to identify thin layers of oil “spilled” under sea ice.

“Our work demonstrates that high-frequency broadband techniques can be used to identify layers of crude oil under sea ice in a controlled laboratory setting,” he said.

The researchers consider this “to be a critical step toward identifying one technology that could be used to remotely detect oil spills under sea ice,” Bassett said. “Ultimately, active acoustics may be just one component of a suite of instruments that could be used for oil detection.”

“Sea ice is a complex medium, and there are many challenges associated with oil detection under sea ice in a natural environment — many important research questions must be addressed before using broadband active acoustics to detect oil in situ,” he said.

Some of these unanswered questions include identifying the ideal frequencies for the application, evaluating the role of the size of the acoustic footprint, determining the limitations of the technology in detecting oil layers of variable thicknesses and improving our understanding of the physics of scattering from oil and sea ice, according to Bassett.

When broadband acoustics gain the thumbs up as a tool for detecting oil spills under ice, such a system would need to integrate into an autonomous package that could consist of a suite of instruments designed specifically for this application, Bassett said.

“Following an oil spill, ice growth can encapsulate the oil to effectively form an ‘oil sandwich’ within the ice — so identifying instruments capable of detection under these circumstances will also be important for a successful instrumentation package,” he said.

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