Ear to the ground
(appeared in July 2018)

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Telecom networks could monitor the earth’s crust to warn of earthquakes, says S.Ananthanarayanan.

The revolution in communications in recent decades has created a worldwide network of underground and undersea optical fibre cable. Optical fibre has many times the voice and data capability of traditional copper wire. Copper wire networks have hence all but disappeared.

Philippe Jousset, Thomas Reinsch, Trond Ryberg, Hanna Blanck, Andy Clarke, Rufat Aghayev, Gyl? P. Hersir, Jan Henninges, Michael Weber and Charlotte M. Krawczyk, from geoscience institutes in Potsdam, Berlin and Reykjavik and a UK based industry describe in the journal, Nature Communications, an application of optical fibre networks in earthquake detection or prediction. This would be a great saving in the costs of installing dedicated sensors in earthquake prone regions.

The earth’s crust and its rocky plates are understood to have condensed out of the melt that is still there deeper within the earth. While the continents can be considered to have finally formed, the process is not completely over and some slow movement and shifting of position of landmasses continues. The motion is sometimes in spurts and starts, which are sensed as tremors and earthquakes. There are, in fact, thousands of small tremors, and it is estimated that major events occur, somewhere in the world, about once a month. These major events are usually in remote regions, but when they occur in populated areas, there is great damage, cost and loss of life.

Earthquakes, when they occur, create compression waves within the earth’s crust. A part of the waves moves over the surface of the earth and a large part moves downwards. The part that moves downwards reflects off discontinuities in the rocky material within the earth and rises to the surface. Devices placed by seismologists sense both sets of waves, or tremors, and this helps ascertain the location and severity of earthquakes. Discontinuities or faults in the rocky structure also get located and mapped. As faults like this are usually the sources of earthquakes, monitoring the activity at these locations in areas that are prone to earthquake can provide advance warning.

A meaningful monitoring arrangement, however, is difficult to install. The electronic sensors that are required need to be manufactured just for this application and they are expensive. They also need to be securely housed underground and provided with power supply. And then, they need to be connected by a communication line to a central control station, so that their data can be collected and processed. Providing such facilities in sufficient numbers to obtain useful information is expensive and time consuming. It is in this context that the team writing in Nature Communications reports a trial where the existing optical fibre network was used to collect seismic data in south-west Iceland, a region with known seismic and volcanic activity. The trial was able to collect data at a very fine, 4-metre spacing and continuously monitor structural features with very high resolution.

The optical fibre used for communications is a thin glass capillary, which guides laser beams along its length. The Laser beams traverse the fibre by reflecting off its sides at a glancing angle. Rapid, electronic switching of the beam off and on allows digital data to be transmitted with great speed and accuracy. By using different glancing angles, many beams of light can be sent at the same time and a cable with a bundle of fibres can carry huge data. As no electric current is involved, the information in the cable is not affected by external fields, like power lines, and this is an added advantage. The bundles of fibres are packed into tubes of made of plastics and steel mesh, for strength. The cables are then laid in channels buried in the ground, to carry digital data for hundreds of kilometers.

Another advantage of optical fibres is that there is negligible loss of signal strength. There is, however, some loss, and this occurs because there is a small back reflection when the laser beam bounces off the sides of the fibres. This loss is made good by inserting repeaters, which pick up weakened signals and pump them back in with full strength.

While back reflection thus needs to be compensated, the same feature is useful in detecting seismic disturbances along the route where the optical cable is laid. Instrumentation at the transmitting end of the cable can detect this back reflected signal and from the timing of detection, the spot of reflection can be located. In fact, the location can be extremely exact, as close as correct to 1 meter to 10 meter.

Now, when a seismic wave passes, the earth that the wave passes through is compressed and stretched for an instant. As the optical fibre cable that is laid in earth is also stretched or compressed, this affects the back reflection of the laser beam that is passing through the cable. The movement is exceedingly slight, but the wavelength of the laser beam in the cable is so small that change is easily detected. Monitoring the back signal is thus a way of continuously watching for seismic disturbances. As optical fibres are now extensively laid, back reflection monitoring can be a sensitive medium for detecting and measuring seismic activity over wide areas.

The authors of the paper report that they tried the method out over a 15 km optical fibre cable length. The data from back scattering records enables segregation of the low frequency waves and the high frequency waves. Man-made disturbances, like traffic rumble or impacts due to power stations, pipelines, and so on give rise to high frequency seismic waves, while waves arising from earthquakes are low frequency. This is also the case because waves from earthquakes come from long distances. High frequency components are thus lost to scattering and only the low frequency part remains.

The data picked up by the 15 km piece of cable was able to make out the different source of seismic signals, including local earthquakes, quakes at intermediate distances and signals from very large distances. The team then verified the detections against the signals picked up by conventional sensors in the same area as the stretch of cable. The signals in the conventional sensors were also used to validate and calibrate the signals detected by the cable.

The trials carried out “point to the extraordinary potential of this technology for new applications in earth hazard assessment and exploration all over the world,” the team says in the paper.

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