Some marine animals can be seriously harmed by military sonar. Could natural noises made by sealife be used to detect underwater threats?
Whale skeletons guard the coast of Fuerteventura in the Canary Islands, a stark reminder of the devastation caused by military sonar. Sonar from ships and submarines is thought to be one of the factors contributing to whale strandings, confusing the whales’ own sonar and causing them to beach themselves.
However, this whale-killing technology may soon face competition. Lori Adornato, a project manager at the US military research agency Darpa, believes we can detect submarines by focusing on natural sound rather than blasting out sonar pulses.
“Right now, we treat all of this natural sound as background noise or interference, which we try to remove,” Adornato explains. “Why don’t we make use of these sounds?” “Can we see if we can find a signal?”
Persistent Aquatic Living Sensors (Pals), her project, listens in on marine animals to detect underwater threats. Due to limited battery life, current air-dropped sonar buoys used by the military to detect enemy underwater activity only work for a few hours over a small area. Instead, the Pals system could cover a large area for months. It could provide a near-constant monitoring system for coastlines and underwater channels. Reef-dwelling species that can be relied on to stay in one place, according to Adornato, are likely to be the best sentinels.
“You want to make sure your organism always is going to be there,” says Adornato.
Pals is sponsoring several teams looking at different approaches using very different reef species.
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Laurent Cherubin is the Grouper Guard team’s lead researcher at Florida Atlantic University, where he works with goliath groupers. These fish, which can weigh up to 300kg (660lb) and produce loud calls to deter intruders, are common in US waters.
“It’s a loud, low-frequency boom,” Cherubin says. “They are territorial, and any intruder on their territory will be boomed at.”
A booming grouper can be heard from 800m (2,640ft), but not every boom indicates a contact. In addition to specific alert calls for intruders and predators, the grouper repertoire includes courtship sounds to attract mates, threatening calls when staking a territory, and other sounds whose function is unknown.
The team is concentrating on alert calls. Cherubin compares it to listening for a guard dog barking at intruders. Because distinguishing these calls from others is difficult, the team has assigned machine-learning algorithms to the task. These have been trained to distinguish and classify different grouper calls by listening to a catalog of thousands of recordings.
The algorithm can then be turned into software which runs on a small but powerful processor built into an underwater microphone or hydrophone. An array of these hydrophones can cover a reef, listening to grouper calls and following them as the cause moves from one grouper territory to another.
Tapping fish conversations may seem outlandish; by contrast the work of Pals team at defence contractor Raytheon looks much more like traditional anti-submarine sonar. It does, however, have a twist.
“We are trying to detect the echoes that are created when shrimp snaps reflect off of the vehicles,” says Raytheon scientist Alison Laferriere. “In much the same way that a traditional sonar system detects echoes from the sound that its source generates.”
A bed of snapping shrimp emits a steady roar which Laferriere compares to bacon frying
In other words, it functions similarly to other types of sonar, but it uses shrimp noise rather than artificial pings. Snapping shrimp, also known as pistol shrimp, have been dubbed the world’s loudest creatures. They produce their distinctive snap by rapidly closing their pincers, creating a vacuum bubble that collapses in a burst of plasma measuring thousands of degrees. This causes a flash of light and a powerful enough shockwave to stun prey.
Shrimp use snapping to communicate with one another. Laferriere compares the sound of a bed of snapping shrimp to bacon frying.
“The signal produced by a pistol shrimp is extremely short in duration and extremely broad,” says Laferriere. “While a single shrimp snap is much quieter than a traditional sonar source, thousands of snaps can occur per minute.”
According to Laferriere, the sound changes depending on the time of day and the temperature of the water, but a shrimp colony is never quiet.
“One of the most difficult challenges we’ve had is dealing with the enormous amount of noise produced by the shrimp themselves, as well as the reflections of all of those sounds off of the surrounding area,” Laferriere says.
The interpretation of these reflections is particularly difficult because, unlike traditional sonar, the location of the sound source is unknown. Again, the solution includes cutting-edge software. Laferriere’s team created smart algorithms to analyze the sound and isolate a single snap, first calculating the location of the shrimp, then calculating the path taken by the reflected sound, and finally determining where it was reflected.
Laferriere’s team had to create computer models to determine which echoes came from stationary background objects and could be ignored in order to make sense of the returning sound. Subtracting these highlights moving objects in the environment, which could be fish, submarines, or unmanned underwater vehicles.
