Piggybacking

If you happen to have a spare fiber in your undersea fiber-optic cable, marine scientists might like to have a chat. They’re using the cables to listen to the sounds of the oceans—from the rumble of underwater earthquakes to the low moans of blue whales.

Scientists typically listen in with special undersea microphones. But they’re expensive, and their range is limited. Fiber-optic cables stretch across hundreds of thousands of miles of ocean floor, so they offer greater coverage at lower cost.

The technique is known as D-A-S—distributed acoustic sensing. A laser fires regular pulses through the cable. Any disturbance introduces a “strain” on the cable. That causes some of the light to reflect back to the source. Analysis of this reflection tells scientists when and where it happened. It can also tell them the cause of the change.

Early experiments tested the technique as a way to listen for earthquakes and landslides. More recently, biologists have been checking out D-A-S as well. They’ve done tests with dedicated cables, and with existing cables that are used for telecommunications. Scientists can piggyback on those cables—using fibers that aren’t otherwise in service.

The cables have detected the vocalizations of blue whales and other large whale species. The technique could help biologists count the number of whales, monitor their movements, and look at how they’re impacted by shipping—a new type of communication for undersea fibers.