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28.003 Fibre optic cables as seismographs


Der Tagesspiegel
Directly put through to the earthquake, 11.02.2019
tagesspiegel.de, Fibre optic cables as seismographs



Fibre optic cables as seismographs

Directly put through to the earthquake

Jan Berndorff


Long since laid, but quite differently usable: Fibre optic cables offer seismologists new possibilities to research the danger of earthquakes.

Exactly that seismologist now intend: They want to measure tremors in the ground by sending laser signals through fibre optic wires wires, as they are already laid for telecommunication over umpteen millions of kilometres, particularly densely in urban spaces, but also in the periphery and even under the sea.

Already for just under ten years fibre optic cables are used in the oil exploration as well as for surveillance of pipelines or in the aeroplane industry, by sending laser instead of telephone signals through. Experts call this DTS and DAS - "Distributed Temperature Sensing" and "Distributed Acoustic Sensing." The cables from glass, fine as a hair, are for example laid together with a pipeline. If it is broken, light in the wire comes across a resistance and is getting reflected. With the help of its run-time one can calculate exactly to the metre, where the defect has occurred. With drilling with the help of changes in the material expansion the temperature in the deep can be inferred from.

Lumps of the fibre optic cables can be helpful

For research of earthquakes the seismologists have now still gotten a complete different idea to apply DAS: Fibre optics always also show small, individual thickened sections, which are completely insignificant for their normal functioning. But these lumps throw back parts of the laser signal, which is shot into the fibre optic. When through tremors on the basis of earthquakes, trucks thundering over roads or other disturbances the fibre optic is shifted, compressed or stretched, this also changes the light reflection.

"Even shifts by few nanometres we can read in the signal", says Charlotte Krawczyk, head of the section Surface Near Geophysics at the Deutschen Geoforschungszentrum Potsdam (GFZ). The seismologists send pulsed laser light into the fibre optics. At different places along the fibre the light gets reflected. When this reflection now changes at one of these places from one laser pulse to the other, the researchers know that the wire has moved there as a result of a ground tremor. With the help of the characteristic of the change they can read in which direction and how strong it has moved.

In the fibre optic cable therefore are at once several measuring sensors. A group about Kraczyk tried last year at a telephone line laid 1994 on the Icelandic peninsular Reykjanes to also really use it. "We were absolutely fascinated by the results", said the project leader Philippe Jousset, also geophysicist at the GFZ. In a 16 kilometre long cable the researchers had at their disposal one measuring point every four metre. "We had feared that the pipes in which the fibre optic wires are laid, possibly cushion the ground movements and the fibre itself possibly no longer reproduces it so very well at all", said Krawczyk. "But that was not so; we could wonderfully recognize in our signals both local, small quakes as well as also a faraway quake in Japan."


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