From the albatross to the zebra, zoologists have long been able to track land animals using GPS trackers that send data back to them via satellite. For marine biologists, however, it’s more complicated – seawater remains fiercely opaque to electromagnetic radio signals. Unable to receive a GPS signal or send any data back to a receiver.
However, this does not prevent any tracing of marine fauna. Data collected and stored in a tracker can be sent in packets when the animal (if that is the behavior of its species) comes to the surface; Alternatively, the tracker can be designed to come to the surface itself after a given time, or be retrieved if the animal is caught by a fishing boat – fishermen are paid several hundred dollars by tracer sent back to its laboratory of departure.
Problem, none of these methods allows precise monitoring of the movements of the animal carrying the tracer. Hence the interest in finding an equivalent of GPS adapted to the marine environment. This is precisely what the Woods Hole Oceanographic Institution (WHOI) is doing. This oceanographic research center established in Massachusetts hopes to populate the oceans with sound beacons which, like GPS satellites, will send signals making it possible, by triangulation, to locate a sensor when it is under water.
The ocean can be broken down into layers of different water, temperature and salinity. As early as the Second World War, American experts in naval science showed that some of these ocean strata could behave like acoustic waveguides: this is what they called “Sofar channels”, to sound fixing and ranging [un nom construit sur le même modèle que sonar : sound navigation and ranging], or deep sound channels. A sound emitted in this zone is reverberated between the lower and upper aquatic layers, and therefore trapped – a bit like the light in the optical fiber. Thus channeled, a sound wave can travel hundreds of kilometers without attenuating, and remain detectable.
Beacons and pongs
The Sofar transmitters deployed by WHOI are one-ton buoys anchored at a depth appropriate to the channel concerned. Every twelve hours, they send a thirty-two-second location signal called a “pong” – by analogy with the “ping”, the impulse
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