A breakthrough in fiber optics turned an undersea cable into 12 seismographs
There’s a 3600-mile-long (5860 km) cable that extends from eastern Canada to the west coast of the United Kingdom. The undersea cable is part of the internet’s vast hidden infrastructure that carries data across oceans in far less time than it takes to blink. But that particular cable does something else.
According to a paper published Thursday in the peer-reviewed journal Science, the cable can double as a state-of-the-art array of more than 100 sensors that detect seismic activity. In tests last November, the cable registered a magnitude 7.5 earthquake that shook northern Peru, thousands of miles away.
“The technique we present has the potential to transform our Earth-monitoring capabilities,” according to the authors. They say the “seafloor could be instrumented with thousands of [such] sensors without modifying the existing submarine telecommunication infrastructure.”
These findings advance a nascent technique
The first cable enabling fast communication between North America and Europe — via telegraph — was laid before the Civil War. In the 1990s, governments and companies began laying undersea fiber optic cables that use light to transmit gargantuan amounts of internet data. Three decades after the first fiber optic cables were installed, roughly 750,000 miles (1.2 million km) of cable crisscross Earth’s seas and oceans.
Five years ago, a group of researchers came up with the idea to use this network to monitor subtle changes on the seafloor, including the telltale signs of earthquakes and tsunamis. Early versions of the technology borrowed techniques that engineers in the oil and gas industry use to study deposits of fossil fuels. They measured backscatter from signals that bounced off molecules in the cables. Those signals contained valuable information about the physical state of the cable, including seismic activity that was occurring in the ground where it was laid. Other researchers figured out how to make similar calculations by measuring the polarization of light waves as they travel along the cable.
These methods share a huge drawback: the entire cable acts as a single sensor. That doesn’t make it impossible to garner useful data, but it introduces a couple of problems. For one, there’s a huge potential for noise so loud that finding the signal — the tremors of an earthquake, for instance — can be extremely difficult. It also limits the amount of data that can be collected at any particular moment. No matter how long the cable is, it can only create one data point at a time. Anyone wanting to extrapolate useful information from that kind of system would need to triangulate data from multiple cables to figure out where a seismic disturbance came from.
The new method solves those problems.
The method exploits two key aspects of cable design
Undersea fiber optic cables aren’t simple, one-way streets for photons. Modern cables are outfitted with devices called repeaters that detect the incoming signal and then amplify it for the next leg of its journey. Repeaters are located every 28 to 56 miles (45 to 90 km) along the length of a cable, separating the long distance into dozens of shorter “spans.”
Instead of using the entire cable as one sensor, the technique described in the new paper uses each span as its own sensor. This approach reduces the amount of noise per sensor while dramatically increasing the total number of sensors. And instead of relying on backscatter or polarization to sense the subtle physical changes in the cable, the method described in Thursday’s paper takes advantage of special fibers that connect the repeaters.
“These return paths are used by the cable operator to periodically check the health of the optical amplifiers,” the researchers explain. “These checks are typically performed on a schedule or if there is a malfunction, leaving these channels unused most of the time,” they say. The researchers managed to detect earthquakes by carefully measuring how light changed as it traveled along these pathways.
Undersea cables could monitor earthquakes and shine light on climate change
The researchers tested the new technique using just 12 spans of the transatlantic cable connecting Canada and the UK. Even with that limited array, they managed to detect two earthquakes and signals from hurricane Larry. With any luck, this pilot is just the beginning.
“By converting submarine cables into arrays of environmental sensors, a large network of hundreds or thousands of permanent and real-time seafloor sensors could be implemented without modification of the existing subsea infrastructure,” they write. An array of that size could make it possible to detect earthquakes and tsunamis with better accuracy and resolution than current methods.
The array could also enable scientific research that’s far too expensive with today’s technology. “ The ability to record seismic phases in the middle of ocean basins could enable the imaging of previously obscured structures such as mid-ocean ridges and oceanic fault zones,” they write.
The technique could also contribute to more accurate climate models. “The cable sensitivity to water currents could be explored to improve our understanding of deepwater flows.. including the proposed slowing down of ocean currents due to rising global temperatures,” they write.
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