Acoustic sensing: The ear underground; How fibre-optic cables can work like microphones

Acoustic sensing: The ear underground; How fibre-optic cables can work like microphones

Jan 4th 2014 | From the print edition

EARLY in the morning in Texas a vehicle slows and turns onto a side road. Some 7,600km (4,700 miles) away in Farnborough, south-west of London, a series of red lines zigzag across a computer screen and raise an alarm. That the lines resemble those on a seismograph picking up a small earth tremor is entirely appropriate: these lines represent tiny ground vibrations made by sounds from above. More lines appear as the vehicle rumbles across a cattle grid, identifying it as a two-axle vehicle, probably a car. Then it stops. Subsequent lines are interpreted as footsteps followed by the sound of digging. A security camera swings round to take a look—and a man with a spade cheerfully waves back. This time it is just a demonstration.The security system being used here is based on a technology known as distributed acoustic sensing (DAS). This allows underground fibre-optic cables, like those used by telecoms companies, to be turned into a giant string of microphones. They can then be used to monitor all sorts of sensitive locations, from oil and gas pipelines to railway tracks, military bases and international borders. In its latest guise, DAS is even being used to help make hydraulic fracturing, or “fracking” as it is known, more efficient at releasing natural gas and oil trapped in rocks.

The technique relies on a phenomenon known as Rayleigh scattering, named after Lord Rayleigh, a 19th-century British physicist, who discovered it and in the process explained why the sky is blue. Molecules in the atmosphere scatter the sun’s light, especially the blue end of the spectrum, because its shorter wavelengths are closer to the size of the molecules. An observer looking at the sky (rather than directly at the sun) sees light waves that have been scattered and are thus mainly blue.

A similar light-scattering can occur inside a fibre-optic cable, caused by minute fluctuations in its molecular structure. The cables are engineered to keep such scattering to a minimum. Even so, OptaSense, a subsidiary of QinetiQ, a British technology group based in Farnborough, has been able to exploit the effect to build highly sensitive DAS systems. These can detect not just footsteps ten metres or so either side of a buried cable, but also a low-flying drone passing overhead, vandals damaging a railway line, a terrorist burying a bomb beside a roadway or someone probing the ground trying to find a pipeline from which to steal oil. Existing cables can be used, or new ones can be laid especially for the purpose.

The system works by sending sharply defined pulses of laser light down the cable. When sounds from above are transmitted through the ground, they cause minuscule vibrations in the cable, which change its light-scattering properties in tiny ways. This can be detected when the pulses are received farther down the cable. High-speed signal-processing techniques—which QinetiQ originally developed for submarines’ acoustic-detection systems—are used to analyse instantly what the sounds are likely to be.

There are some limitations to the technology. Its powers of hearing are not sufficiently acute to pick up a conversation, for example. And since the cables inside buildings are typically a tangle of short lengths interrupted by junction-boxes, it is unlikely to work there. However, a long cable buried outdoors can provide the equivalent of a microphone every ten metres.

Algorithms are used to establish acoustic “fingerprints” for the sounds that are detected; and depending where and when they occur, each is assigned a level of risk, says Magnus McEwen-King, OptaSense’s managing director. Footsteps around a guarded facility at midday may not be unusual, but at 2am they would be.

OptaSense is also using the system to monitor sounds coming from below ground, in particular those produced by the water, sand and chemicals pumped under high pressure to fracture rock during fracking. There is concern about exactly what is going on underground, and in particular if the process might contaminate aquifers. Various seismic sensors can be used to monitor the fracking process, sometimes from test bores drilled nearby. But it is a costly and tricky process.

Shell and other oil companies are using a DAS system, which OptaSense calls vertical seismic profiling, to monitor their fracking. It uses a fibre-optic cable inserted into a well bore to build up an acoustic picture of the fracking fluid going into the rock at multiple levels. This means that potential problems, such as blockages, or leaks from one layer of rock to another, can be spotted before they become serious. And by having a clearer idea of how much fluid is going where, the fracking process can be constantly adjusted so that it runs in the most efficient way.

Listening for intruders and monitoring the efficiency of fracking are just two of the potentially lucrative applications of DAS technology. No doubt there will be others in the pipeline.

About bambooinnovator
Kee Koon Boon (“KB”) is the co-founder and director of HERO Investment Management which provides specialized fund management and investment advisory services to the ARCHEA Asia HERO Innovators Fund (, the only Asian SMID-cap tech-focused fund in the industry. KB is an internationally featured investor rooted in the principles of value investing for over a decade as a fund manager and analyst in the Asian capital markets who started his career at a boutique hedge fund in Singapore where he was with the firm since 2002 and was also part of the core investment committee in significantly outperforming the index in the 10-year-plus-old flagship Asian fund. He was also the portfolio manager for Asia-Pacific equities at Korea’s largest mutual fund company. Prior to setting up the H.E.R.O. Innovators Fund, KB was the Chief Investment Officer & CEO of a Singapore Registered Fund Management Company (RFMC) where he is responsible for listed Asian equity investments. KB had taught accounting at the Singapore Management University (SMU) as a faculty member and also pioneered the 15-week course on Accounting Fraud in Asia as an official module at SMU. KB remains grateful and honored to be invited by Singapore’s financial regulator Monetary Authority of Singapore (MAS) to present to their top management team about implementing a world’s first fact-based forward-looking fraud detection framework to bring about benefits for the capital markets in Singapore and for the public and investment community. KB also served the community in sharing his insights in writing articles about value investing and corporate governance in the media that include Business Times, Straits Times, Jakarta Post, Manual of Ideas, Investopedia, TedXWallStreet. He had also presented in top investment, banking and finance conferences in America, Italy, Sydney, Cape Town, HK, China. He has trained CEOs, entrepreneurs, CFOs, management executives in business strategy & business model innovation in Singapore, HK and China.

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