An Ultrasound for the Ocean:
Seeing the Sound of Marine Life
Researcher:
Dr. Jules Jaffe
Marine Physical Laboratory
Scripps Institution of Oceanography
UC San Diego
E.: jules@mpl.ucsd.edu
Dr. Paul Roberts
Marine Physical Laboratory
Scripps Institution of Oceanography
UC San Diego
E.:plrobert@ucsd.edu
Relevant Links:
Tools:
Revised:
May 18, 2009
A top view of the line of receivers used to collect multi-angle scattering data. Credit: UC San Diego/J. Jaffe
May 18, 2009
Contact: Christina S. Johnson, csjohnson@ucsd.edu, 858-822-5334
In the relative quiet of the laboratory, acousticians can now do what has eluded oceanographers to date: classify zooplankton taxa based on their reflected sound patterns.
The innovation, reported in the Journal of the Acoustical Society of America, uses multiple receivers to peer at zooplankton from multiple angles. Differences in the resulting echograms, in turn, allow scientists to do things like differentiate krill from copepods, at least in the laboratory.
The idea of using sound to penetrate the ocean is nothing new. Sonar is the standard technique for mapping ocean bathymetry and has also been employed to monitor benthic life such as oyster beds and invasive plants.
“The difference is, in the past, almost all acoustic projects have looked at scattering from a single receiver,” says Paul Roberts, a recently minted electrical engineering Ph.D., who conducted his experiments at UC San Diego’s Scripps Institution of Oceanography. “What we are doing is looking at scattering from multiple views. That is what is new.”
This fall, Roberts will lead a field experiment to test whether the multi-view apparatus is functional at sea and could therefore, potentially, be deployed to map fine-scale distributions of zooplankton taxa.
“Our results have the potential to make sonar a standard technique for surveying zooplankton in the ocean,” says Jules Jaffe, the lead investigator of the Sea Grant-funded project and a researcher at the Marine Physical Laboratory at Scripps Institution of Oceanography.
A technical sketch of the multi-view scattering apparatus.(Click to enlarge.) Credit: UC San Diego/J. Jaffe
Besides classifying zooplankton, the scientists have promising and as yet unpublished results showing the ability to classify fish taxa acoustically. In particular, multi-view laboratory experiments show it is possible to tell whether a fish in a tank is a sardine, anchovy or damselfish.
Part and parcel with the advance in acoustic translation has been the newfound ability to infer the size of a fish, the direction it’s swimming and its shape, all via the magnitude, angle and frequency of echoes collected by multiple receivers. The work builds on previous research led by Jaffe, in which fish swim bladder size was estimated from multi-view scattering data.
Norwegian biologists are currently testing the multi-view apparatus as a potential upgrade of the current single-view acoustic techniques for deriving stock assessments. The system under study produces two echograms of a school of fish. Differences are supposed to shed light on the swimming orientation of the school relative to the sound source, thus improving estimates of fish biomass.
Imagine trying to guess the size of the Empire State Building based on its shadow. Acoustic imaging poses a similar problem. “You don’t know what you are seeing,” says Roberts. “You are just getting energy back. What we are trying to do is measure the orientation of a school of fish explicitly by listening at multiple angles.”
Roberts plans to spend the next six months as a postdoctoral researcher with Jaffe to continue the fish classification research and publish his findings. After this, he may return to the field of computer science.
“I’d like to apply the algorithms from this project to computer vision,” Roberts says. “That is the problem of making machines do tasks that people do easily, like recognizing other people.”
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