Sounding the undersea world

Imagine living next to a busy highway operating 24 hours a day for 365 days per year.

That’s what life is like for ocean animals living next to busy shipping lanes. They must cope with the engine noise of ferries, pleasure boats, passenger ships, container ships, fishing fleets — and also with sonar, offshore construction, and the sounds of oil exploration.

Not only is such noise pollution stressful, but it hinders the ability of sea life to communicate, find food, avoid obstacles and detect predators. Chronic background noise creates conditions akin to those we’d experience living in a constant fog, says Chris Clark, director of Cornell University’s Bioacoustics Research Program. Although often overlooked by us land-dwellers, the danger of noise pollution to marine animals is on a par with that of ocean acidification, over-fishing and chemical waste.

However, in order to reduce noise pollution, researchers must first understand the use of sound by ocean-dwelling animals. To do so, scientists are turning to bioacoustics and computing. Along the way, they are finding some unexpected linkages to underwater physics experiments such as the Neutrino Mediterranean Observatory/Ocean Noise Detection Experiment, and the ESFRI-funded KM3Net underwater detector — projects designed to find neutrinos coming in from outer space.

How undersea animals see, er, hear the world

Some scientists think that cetaceans — marine mammals such as whales and dolphins — use sound to see their environments, in much the same way that humans use light. Gianni Pavan, a professor of Terrestrial and Marine Bioacoustics at the University of Pavia, Italy, says that marine mammals use sound to create a visual 3D image in their minds, by integrating and combining echoes produced in their underwater environments. 

Pavan, who collaborates with the US Office of Naval Research, NATO, Woods Hole Oceanographic Institute, the Italian Navy and other worldwide institutions to study marine mammals, has developed and maintained the SeaPro software package for real-time sound analysis and spectral display; he also designed various types of underwater equipment used for marine mammal surveys for small boats and oceanographic ships. (Currently, analysis is conducted using specialized software on a laptop or desktop, but as data volume increases he is considering parallel processing systems and grid computing.)

The system relies upon the placement of an array of acoustic sensors in a body of water. As animals pass through this acoustic ‘sensor grid,’ information about their size and speed is recorded. Using it, scientists can identify basic anatomical characteristics; because some marine species generate unusual acoustics which can be easily differentiated from other species, researchers can identify an animal as a whale or dolphin (but not whether it is of the Bottlenose or Spotted species of dolphin). Analyzing sound underwater is more practical than capturing video: in clear waters, good light visibility is limited to 10-20 meters, whereas sound can travel tens of kilometers. 

Click to hear a rare recording of a sperm whale “trumpet.” For the sound of an orca, click here. Sound files courtesy CIBRA.  

A multitude of different sensors are used to monitor marine life; from hydrophones and sonobuoys near the surface, to fixed platforms on the sea bed. One example is a towed array: a long tube filled with oil, inserted with hydrophones. This tube is connected to a long cable and towed by a ship. This helps reduce noise generated by the ship’s engines as it travels up to 10 knots (about 12 mph, or 19kmh), enabling a ‘sound map’ to be generated over a large area.

The data is then collected and stored for further analysis on the surface. Pavan uses computational bioacoustics, the use of algorithms and computer programs, to assist him in decoding sounds and identifying species. Intriguingly, hunters are the best people at recognizing species by sound, says Pavan.

Is it a particle or a whale?

A few years ago, the deepsea Neutrino Mediterranean Observatory-Ocean Noise Detection Experiment (NEMO-ONDE), found something unexpected in the western Ionian Sea, under the ‘heel’ of Italy. While using this two-kilometer acoustic platform to search for subatomic particles, researchers found sperm whales instead. In a Nature story Underwater acoustics: The neutrino and the whale, Pavan, who was on-site as an invited researcher, said that the finding was a great surprise: “Sperm whales are considered very rare in the region. Published data had hinted at a very sparse population.”

Following up on this experience, Pavan is now collaborating closely with KM3NeT, a one-cubic kilometer underwater detector that will search for neutrinos striking the Mediterranean from space; to obtain a clear signal, it is important that the KM3NeT detector can distinguish the activity of high-energy neutrino signals from the background noise of marine life. “Cetaceans produce signals in the same frequency range in which we expect to find neutrinos, so collaboration with bio-acoustics experts like Gianni is essential,” says Giorgio Riccobene, a physicist working on the KM3NeT project.

Pavan says, “Our studies help to expand awareness about the environment in which we live with other creatures, and at finding a balance for our need to exploit underwater resources and marine life preservation for future generations.

—Adrian Giordani and Dan Drollette, iSGTW. For more on bioacoustics, see the Sound of Science.

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