Offline telecom cables called “dark fibres” can be used to sense underground seismic waves. Geophysicists are increasingly using such signals to study aspects of Earth’s subsurface, including hidden sources of geothermal energy and earthquake hazards.

“If a large earthquake happens on the fibre on which we are speaking, the frequencies in my voice would be slightly distorted,” Andreas Fichtner at ETH Zurich in Switzerland told New Scientist on a video call.

He is referring to how earthquakes or even mild ambient seismic activity can subtly stretch or deform the networks of fibre optic cables that criss-cross the planet, carrying the internet and most of our telecommunication data.

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By firing pulses of lasers through such cables, and measuring the backscatter reflections, researchers have recorded seismic activity that would be impractical to capture with conventional sensors, including undersea earthquakes and strain within volcanic dykes. “It’s a huge logistical operation to get high-density measurements across large sections of the Earth,” says Jonathan Ajo-Franklin at Rice University in Texas.

Such measurements can’t be taken through most active fibre optic cables, which are already in use transmitting data, without interrupting service. But telecom companies often build extra fibres into their networks that sit offline until someone leases them. A growing set of researchers are harnessing these “dark fibres” to record seismic activity and map the subsurface in detail.

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Ajo-Franklin and his colleagues recently completed data collection on one such project to demonstrate they could use a dark fibre to find hidden sources of geothermal energy. Over several years, they used a 28-kilometre-long dark fibre to collect ambient seismic data in California’s Imperial valley.

They then used the variations in velocity among the seismic waves they recorded to create a detailed cross-section of the 3 kilometres of ground beneath the cable.

The resulting map revealed a previously uncharted fault. It also identified an area of low-velocity waves, which the researchers linked to the presence of a geothermal energy source hidden beneath the surface. This source had previously been identified by boreholes drilled for oil and gas exploration. Finding it using the fibre shows the approach can be “a tool which can show signatures of these types of systems in the right place”, says Ajo-Franklin.

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Fichtner, who wasn’t involved with the project, says the approach may not lead to new geothermal discoveries in many other places. For all our extensive fibre optic infrastructure, the cables tend are often clustered where people live, not in remote areas where new exploration tends to occur. “In some ways, they have been lucky that the fibre optic cable happens to be on top of a geothermal reservoir,” he says.

But he says dark fibres could provide a powerful way to create a high-resolution picture of the subsurface beneath cities, where fibre optic networks are most dense, and where detailed maps could be useful to understand earthquake hazards.

Just this month, Fichtner and his colleagues will attempt to monitor such earthquakes using dark fibres that connect two seismically active islands in Greece. That project only came about thanks to a serendipitous partnership with a Greek telecom company who agreed to let them use the fibre. “At the moment, it’s a matter of having the right connections,” he says.