An anonymous reader quotes a report from Wired: Stretching thousands upon thousands of miles under your feet, a web of fibrous ears is listening. Whether you walk over buried fiber optics or drive a car across them, above-ground activity creates a characteristic vibration that ever-so-slightly disturbs the way light travels through the cables. With the right equipment, scientists can parse that disturbance to identify what the source was and when exactly it was roaming there. This quickly proliferating technique is known as distributed acoustic sensing, or DAS, and it's so sensitive that researchers recently used it to monitor the cacophony of a mass cicada emergence. Others are using the cables as an ultra-sensitive instrument for detecting volcanic eruptions and earthquakes: Unlike a traditional seismometer stuck in one place, a web of fiber optic cables can cover a whole landscape, providing unprecedented detail of Earth's rumblings at different locations. Now scientists are experimenting with bringing DAS to a railroad near you. When a train runs along a section of track, it creates vibrations that analysts can monitor over time -- if that signal suddenly changes, it might indicate a problem with the rail, like a crack, or a snapped tie. Or if on a mountain pass a rockslide blasts across the track, DAS might "hear" that too, warning railroad operators of a problem that human eyes hadn't yet glimpsed. More gradual changes in the signal might betray the development of faults in track alignment. It just so happens that fiber optic cables already run along many railways to connect all the signaling equipment or for telecommunications. "You're utilizing the already available facilities and infrastructure for that, which can reduce the cost," says engineer Hossein Taheri, who is studying DAS for railroads at Georgia Southern University. "There could be some railroads where they don't have the fiber, and you need to lay down. But yes, most of them, usually they do already have it." To tap into that fiber, you need a device called an interrogator, which fires laser pulses down the cables and analyzes the tiny bits of light that bounce back. So, say a rock hits the track 20 miles away from the interrogator. That creates a characteristic ground vibration that disturbs the fiber optics near the track, which shows up in the light signal. Because scientists know the speed of light, they can precisely measure the time it took for that signal to travel back to their interrogator, pinpointing the distance to the disturbance to within 10 meters, or about 30 feet. For a given stretch of track, you'd have already analyzed the DAS signals for a length of time, building a vibration profile for a normal, healthy railway. When the DAS data suddenly starts showing something different, you might have an issue, which shows up like an EKG picking up a problem with a human heartbeat. "What we're doing is profiling the track, looking for changes in the acoustic signature," says Daniel Pyke, a rail expert and spokesperson for Sensonic, which develops DAS technology for railroads. "We know what track should sound like, we know what a train should sound like. And we know that if it's changing -- so let's say this joint is coming loose -- that needs someone to go and fix it before it becomes a problem."

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