FEATURE ENVIRONMENT
‘We’re not trying to beat dedicated sensors, we’re trying to create a complementary technique that works well with telecommunications’
gtransceiver sensing data, enabling early- warning applications.”
Subsea monitoring In the subsea realm, there has been more development in using telecommunication networks for environmental sensing purposes.
In 2022, a team of scientists at the
National Physical Laboratory (NPL) demonstrated a new technique to transform undersea power and telecom cables into arrays of environmental sensors. Previous work by NPL in 2018 showed
that submarine cables could be repurposed as sensors for the detection of underwater earthquakes by using ultra-stable interferometric techniques. However, one cable could act only as a single sensor, and measurements were limited only to the integrated changes over the entire length of the cable. The new research demonstrates how some cables can be converted into an array of sensors rather than just a single sensor. The NPL-led team, including researchers
from the University of Edinburgh, the British Geological Survey, the Istituto Nazionale di Ricerca Metrologica (INRiM), and Google, tested the technique on a 5,860km intercontinental submarine optical fibre link between the UK and Canada. They demonstrated the detection of earthquakes and ocean signals, such as waves and currents, on individual spans between repeaters spread across the entire transatlantic connection. The optical fibre in each span acted as a sensor, with up to 12 sensors implemented along the cable. Future upgrades will increase this number to 129. Crucially, the data from these sensors can be recorded continuously and in real time. The cable-based array of sensors can identify the epicentral area of earthquakes in the same way as land-based seismometers. It is thought that by applying this
new method to an existing network of submarine cables, huge and currently unmonitored areas of the ocean could potentially be instrumented with thousands of permanent real-time environmental sensors. It could effectively transform underwater telecoms infrastructure into a giant array of geophysical sensors.
22 Electro Optics May 2023
A map of Europe’s vast subsea cable network
Integrating this approach with current seismometer-based networks could provide the potential to substantially expand the global earthquake monitoring infrastructure from land to the seafloor where only a handful of permanent seismometers are currently installed. Google has also partnered with the
California Institute of Technology (Caltech) to develop a method to use existing subsea cables to detect earthquakes. The team at Caltech devised a way to analyse the light travelling through existing and functioning submarine cables to detect earthquakes and ocean waves without the need for any additional equipment. The researchers focused on the Curie
Cable, which stretches more than 10,000km along the eastern edge of the Pacific Ocean from Los Angeles to Valparaiso, Chile. On land, many factors, such as temperature fluctuations and lightning strikes, can change the polarisation of light travelling through cables. Because the temperature in the ocean remains nearly constant and there are fewer disturbances, the team found that the change in polarisation from one end of the Curie Cable to the other remains quite stable over time. However, during earthquakes and when storms produce large ocean waves, this changes suddenly and dramatically, allowing the researchers to easily identify such events in the data. Using the new technique, the entire length of a submarine cable acts as a
single sensor in a hard-to-monitor location. Polarisation can be measured as often as 20 times per second, so if an earthquake strikes close to a particular area, a warning could be delivered to the potentially affected areas within a matter of seconds. During the nine months of testing
reported in the study (between December 2019 and September 2020), the researchers detected about 20 moderate-to-large earthquakes along the Curie Cable, including the magnitude-7.7 earthquake that took place off of Jamaica on January 28, 2020. Although no tsunamis were detected during the study, the researchers were able to detect changes in polarisation produced by ocean swells that originated in the Southern Ocean. They believe the changes in polarisation observed during those events were caused by pressure changes along the seafloor as powerful waves travelled past the cable. ‘This means we can detect ocean waves, so it is plausible that one day we will be able to detect tsunami waves,’ explained assistant professor of geophysics, Zhongwen Zhan. Merging optical fibre networks could not
only improve reliability and coverage for the telecoms industry, but help provide a better understanding of phenomena happening across vast areas of the Earth’s surface. In the future, it is likely that the optical telecommunications network will play an even bigger role in our society than it does today. EO
@electrooptics |
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