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Sound Solutions

By Jennifer Berglund

Whales remain one of marine biology’s most elusive subjects. The unpredictability of their migratory patterns has long frustrated biologists, who lament the lack of adequate technology to track them. Collecting data requires an agile toolset, but one tool in particular offers biologists their best chance to track whales reliably: sound.

Pinpointing underwater objects with sound is nothing new. Since an iceberg sank the Titanic in 1912, sonar technology has been a mainstay in the scientific toolbox. But active sonar, which locates objects by listening for echoes of emitted sound, is hazardous to whales and can lead to beachings.

For whale tracking, passive sonar is a better option. This technique is imperfect and expensive, but a team from Associate Professor of Mechanical Engineering Glynn Holt’s physical acoustics lab is meeting the challenge to improve the technology. Working with a team of scientists from the National Oceanic and Atmospheric Administration (NOAA) Northeast Fisheries, PhD candidate Robert Valtierra and senior Kara Silver are developing a device suitable for a wider range of marine mammalogists. With the help of a small team of undergraduates, the pair is doing much of the work on their own. “This has been the world’s easiest project for me,” says Holt, who has been impressed by the students’ initiative.

Two types of devices dominate the current passive sonar field: towed arrays and autonomous acoustic recording units (AARUs). Arrays, while known for quality, only collect data when towed behind a boat, and can weigh as much as a small car. Pricing can begin at several thousand dollars, and the boats to haul them can cost tens of thousands of dollars per day and mean long days at sea for the crew.

AARUs record acoustical signals from the sea floor. They can log several months’ worth of data, but they are costly. Ones used for similar research on the West Coast cost around $60,000 each, and to get useful location data, several are needed. With this equipment, localization is accomplished by comparing time signals between devices, requiring all clocks to be perfectly synchronized. Without a $2,000 clock, imperfect technology means inevitable drift. Time inaccuracies of mere milliseconds might position a whale in a different nautical zip code, which isn’t useful for scientific analysis. These difficulties present major roadblocks in deciphering the biological riddles of whales. To confront them, the BU/NOAA team stayed true to their engineering roots, first turning a whale call into a math problem.

Each whale species has a different call. Right whales make “up calls,” which resemble the sound of a heavy door opening, others “chirp” like birds. Some calls are inaudibly low or high. Each call is modeled differently, so to effectively use their AARU, the team had to boil each one down to a mathematical formula. Calculating the difference between a direct call and its reflection from the ocean’s surface, Valtierra developed an algorithm to generate the whale’s approximate location. Comparing overlapping results across an array of AARUs, the team can accurately pinpoint a whale’s location within tens of meters without synchronized clocks.

The BU team then tackled the machine itself. With a budget of $2,000, Silver and a team of undergraduates identified high-quality yet low-cost materials, and Valtierra designed a small instrument with a memory card to record data. Altogether, the new device weighs just 30 pounds and can be easily dropped into the ocean with a few sandbags to weigh it down. The most expensive part? The $400 batteries. The device is USB compatible, so it can be programmed for infinite uses on any computer, and it can measure anything with a sensor. “If you wanted to measure gas flux in a forest, light penetration, sounds, or temperatures, you could do all of this remotely,” Valtierra says.

Danielle Cholewiak, a NOAA bioacoustician, envisions the work will make underwater acoustic research accessible to a larger pool of scientists, and provide greater flexibility for research. It will allow a new cadre of marine biologists to “better evaluate the impacts of anthropogenic activities on the behavior, call migration patterns, and distributions of whales and other marine animals,” she says. Though the allure may never dwindle, this new technology may bring scientists one step closer to unlocking the biological mysteries of whales.

Note: Kara Silver graduated in 2012 and is now pursuing a graduate degree in marine science at the University of Hawaii.

 

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