[T]he technology known as distributed acoustic sensing (DAS)… 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.
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.
Acoustic sensing: The ear underground, Economist, January 4, 2014, at 62