The 1.2 million km of fiber-optic cables that stretch across the globe are utilized to transmit phone calls, internet signals, and data. However, researchers recently discovered something new – they were able to pick up the eerie sounds of blue and fin whales via a fiber-optic cable in the west coast of Svalbard. Now, the scientists aim to listen in on something even larger – the Earth itself. By combining the existing fiber-optic network with remote-sensing systems like satellites, a low-cost, real-time global monitoring network could be created, according to Martin Landrø, a professor at the Norwegian University of Science and Technology’s Department of Electronic Systems, and head of the Center for Geophysical Forecasting.
“This could be a game-changing global observatory for ocean-Earth sciences,” he said. Landrø was lead author for an article about how such a system could work, published in Scientific Reports.
Small Changes in a Fiber the Size of Human Hair
Fiber-optic cables have been around for some time and are commonly used to transmit information. They are likely responsible for the data that your computer is currently processing to allow you to read this text. However, what has evolved are the techniques used to extract information from these networks. One such tool, known as an interrogator, has been developed.
The interrogator can be connected to a fiber-optic cable network and used to send a pulse of light through the cable. When a sound wave or physical wave impacts the underwater cable, the fiber slightly flexes. This movement is detected by the interrogator and can be used to extract information about the wave, such as its frequency and intensity.
“And we can measure the relative stretch of the fiber extremely precisely,” Landrø said. “It has been around for a long time, this technology. But it has made a huge step forward in the last past five years. So now we are able to use this to monitor and measure acoustic signals over distances up to 100 to 200 kilometers. So that’s the new thing.”
The team led by Landrø, which includes researchers from Sikt, the Norwegian Agency for Shared Services in Education and Research and Alcatel Submarine Networks Norway, AS (which supplied the interrogators), employed a 120 km long fiber-optic cable running between Longyearbyen, Svalbard’s largest settlement, and Ny-Ålesund, a research station on the southwest coast of the archipelago’s largest island. They monitored the cable for 44 days in 2020, and were able to detect over 800 whale vocalizations.
“The fiber cable between Longyearbyen and Ny-Ålesund, which was put in production in 2015 after 5 years of planning and prework, and mainly funded by our ministry, was intended to serve the research community and the geodetic station in Ny Ålesund with high and resilient communication capacity,” Olaf Schjelderup, head of Sikt’s national R&E network, said in an earlier article about the monitoring project. Schjelderup was also a co-author on the new paper.
“The DAS sensing and whale observation experiment shows a completely new use of this kind of fiber optic infrastructure, resulting in excellent, unique science,” he said.
What Else Can the Researchers Detect – Ships, Earthquakes?
During the process of identifying whale vocalizations, the researchers also stumbled upon other occurrences such as ships passing near or over the cable, earthquakes, and an unusual pattern of waves, which they later determined to be caused by distant storms. The precision of their measurements was such that they could match them with specific events, including a significant earthquake in Alaska, according to Landrø.
The team provided an example of how the system can detect ships by describing the detection of the Norbjørn, a general cargo ship that was identified crossing the fiber-optic cable approximately 86.5 km from Longyearbyen. By analyzing the ship’s track across the cable, the researchers were able to estimate its speed, which they then confirmed using the ship’s Automatic Identification System (AIS) data.
More on Earthquakes
Geologists currently use a network of sensors known as seismometers to monitor and measure earthquakes. These instruments are precise and provide a wealth of detailed information, according to Landrø. However, seismometers are costly and not as widely distributed as the world’s fiber-optic cable network. While the fiber-optic network has a lower signal-to-noise ratio, meaning there is more background noise and the signal from the earthquake is not as distinct, it does have the advantage of being more widely available and already in place. The idea is not to replace the current system but to enhance it by providing additional information from the fiber-optic network.
“The question then is, what can we learn from a method that has lower signal-to-noise ratio, but has better spatial coverage? How could we use that extra information, even though it is lower quality, to learn more about the earthquake and its properties?” Landrø said.
Monitoring Undersea Pipelines
Additionally, researchers are considering the potential use of existing fiber-optic networks to monitor underwater pipelines, a crucial task following the explosion that damaged the Nord Stream 1 and 2 pipelines in September. “An important question is whether we can use this fiber-optic technology to monitor and safeguard infrastructure on the seabed,” Landrø said.
A challenge with pipelines is that they produce noise as gas flows through them. “We must first understand the natural variability of background noise, and then determine what threshold to use for detecting and responding to something approaching the pipeline. We don’t know yet, so our plan is to conduct specific tests on this,” he stated. The ultimate goal could be to have real-time monitoring of pipelines for safety, using the real-time stream of acoustic data from the Svalbard fiber network.
To learn more: Martin Landrø et al, Sensing whales, storms, ships and earthquakes using an Arctic fibre optic cable, Scientific Reports (2022). DOI: 10.1038/s41598-022-23606-x