Optical fibre could become earth’s sensory organ, says S.Ananthanarayanan.

Thanks to the explosion in communications and transfer of data, worldwide, vast surfaces are crisscrossed by optical fibre cable. As optical fibre uses laser beams to transmit digital signals, fluctuations of the laser beam help detect even minute disturbances of the ground where the fibre is laid.

This quality has enabled optical fibre networks, on land and in the sea, to keep watch over tremors in the surface of the earth. Apart from great sensitivity and accuracy, the use of fibre is easier and cheaper than arrays of expensive mechanical sensors. Tieyuan Zhu and David J. Stensrud, from the departments of Geosciences and of Meteorology and Atmospheric Sciences, University of Pennsylvania, write in the American Geophysical Union’s Journal of Geophysical Research: Atmospheres, of an extension of the application to detect seismic effects that are induced by the weather, to watch over events in the atmosphere as well. “Fiber-optics cable …….. will provide a high resolution monitoring tool, which is really potential,” says author, Tieyuan Zhu, in a communication.

The optical fibre used for communications is a thin glass capillary that guides laser beams along its length. The laser beams traverse the fibre by reflecting off its sides at a glancing angle. Rapid, electronic, off-and-on switching of the beam 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

Although transmission is highly efficient, there is an element of ‘back reflection’ at each reflection off the sides of the fibre, and this causes some loss of signal strength. For use in communications, the loss is compensated by boosters and it has no other consequence. For the seismologist, however, this back reflection turns out to be of value. This is because the composition of the back reflection changes quite perceptibly with even a minute change in the position or angle of reflection. Monitoring changes in back signals, and their timing, hence provides a means of getting the location of movements of the optical fibre cable, over its entire length, if a seismic wave should pass through. As the wavelength of laser beams is measured in nanometers, even very small movement of the fibre cable would show up as distinct, and measureable, change in the back signal.

As a large number of back-reflection signals are picked from all along the length of the cable, rich data is returned, as if from a large number of individual seismic sensors, and detailed information of seismic or to other disturbances becomes available. Groups of scientists in the US and in Europe have shown that data picked up by a 15 - 20 km piece of cable can identify different sources of seismic signals, including local earthquakes, quakes at intermediate distances and signals from very large distances. The data from the cable tallies with the signals picked up by conventional sensors in the same area, which helps calibrate the signals detected by the cable.

More recently, a paper published in the journal, Science, describes using a stretch of undersea cable in Monterey Bay, off the coast of California for the same purpose. Over a four-day trial, the group of researchers “turned 20 kilometers of undersea fiber-optic cable into the equivalent of 10,000 seismic stations along the ocean floor,” a news report says. During the short period of observation, the report says, they could record a 3.5 magnitude quake and reflected signals from fault zones along the seabed.

Monitoring of seismic activity is important in earthquake prone regions. If tremors are detected near the site of an event and conveyed electronically, surrounding populated areas could be warned even minutes before the seismic disturbance strikes. Sensors of earth tremors are hence installed over vast areas, and connected to a control centre, where signals are processed, to assess the location and magnitude of events. Such arrangements, which are found on land, are not equally practicable at sea. Although there are no urban centres located in the sea, not covering the seabed is to ignore 70% of the earth’s surface. This shortcoming, too, may be set right by the use of undersea optical fibre cables, which are now proliferating.

Watching the weather

The work of the researchers at Pennsylvania, writing in the AGU journal, is not about using optical fibre networks to detect seismic activity, but to detect the reaction of the earth’s surface to events in the atmosphere. As extreme weather events, like thunderstorms and hurricanes, can cause serious damage and loss, it is important to be able to detect such events, on land and in the high seas, well in time. At the same time, as these events affect overall climate, their monitoring is important for understanding atmospheric dynamics. There is evidence that such events do transfer energy to the earth’s surface, which can allow seismologists, in addition to meteorologists, to detect, track and assess large hurricanes, the authors say. The manner in which atmospheric events transfer energy to the earth, however, is complex and needs to be studied in detail, the authors say.

The authors say that detecting, ranging and assessing thunderstorms and lightening using arrays of microphones and instruments that measure minute variations of air pressure have led to understanding of the characteristics of acoustic pressures that arise when these events occur. And, it has been found that the pressure exerted by sound wave can induce motion of the near surface of the earth, to generate elastic waves that propagate. Seismometers, which detect such movement of the surface of the earth, have then identified these waves. The possibility, however, has been of limited value, says, as a great many seismometers are needed for results that are reasonably accurate.

This is where the use of optical fibre cables comes to the rescue. In the case of detecting ground waves arising from earthquakes, landslide, or even vehicular movement, a few kilometers of optic cable amounts to the equivalent of thousands of seismometers. The authors hence made use of an existing optical fibre array in the Penn State campus and recorded half an hour’s data during a thunderstorm in the region. Thanks to the high density of sensors that the fibre cable represents, it was possible to pinpoint the source of the thunder and ground vibration and its timing, with accuracy and sensitivity. Analysis of the frequency of the ground waves helps identify the nature of the thunderstorm or the lightening strokes and whether it is a cloud to ground lightening stroke or one from cloud to cloud. And, it becomes possible to trace the path the storm follows, as it moves.

The frequency of extreme whether events are expected to rise in the decades to come. The network of optical fibre cable, that covers large parts of the globe, could be the foundation of a system of painless and automated weather surveillance.

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