In April, Neyoarlinan, an Icelandic telecom company, extended a fibre optic line to an Icelandic volcano, providing internet access to a region lacking cell service. For the tourists, the fiber is a digital necessity. For the researchers, it enables them to take the volcanos very pulse.
Solvi Thrastarson, a geophysics graduate student at ETH Zurich, and his colleagues, tapped the fiber with an “interrogator” – a box that fired laser pulses along the cable and recorded the response. The researchers then sifted through the data for clues to the interior fluctuations driving the restive volcano.
Fiber optic cables are bundles of glass fibers, each no thicker than a human hair, that carry information encoded in light. Small random defects within the fibers act like tiny mirrors, scattering the light. The interrogators – or boxes – work much like radars. They fire a laser pulse into an unused fiber and record the pattern of reflections returning from defects along the length of the cable. When an external pressure wave crosses a section of fiber, such as an earthquake, it stretches and squeezes the defects.
Unlike traditional seismometers, which are spread many kilometers apart, fiber offers the equivalent of a seismometer every one or two meters along the cable. This density, combined with the low cost and ruggedness of fiber, has prompted researchers to lay cables along glaciers, volcanoes, permafrost, and earthquake fault zones – any place the earth might crack or crunch, grind or grate. They have also tapped into unused fibers in existing telecom cables to pick up vibrations from sources as faint as pedestrians and cars.
Fiber is revealing previously unknown earthquake faults, the hidden mechanics of glaciers and avalanches, and volcanic gurglings that could help in predicting eruptions.