Wednesday, February 28

Submarine cables will contribute to the detection of earthquakes and tsunamis

A 2015 United Nations report estimated that, each year, an average of 60,000 people and $4 billion in assets are exposed to the threat of tsunamis on a global scale, which can be triggered by certain types of underwater earthquakes or eruptions. volcanic. Over the decades, several countries have deployed tsunami-specific warning systems, such as floating buoys, to complement ground-based seismic detectors.

Seismic waves travel between 20 and 30 times faster through the earth’s crust than a tsunami wave, so their detection and localization should make it possible to determine which coastal areas are at risk and send early warnings. We need detectors in as many places as possible because, for every 200 km of distance between the epicenter of the earthquake and the point of detection, there is an additional delay of one minute for a possible alert.

This is where submarine communication cables can play a role. The Joint Task Force for Telecommunications Reliability and Scientific Monitoring (JTF SMART) has launched initiatives to promote the incorporation of sensors dedicated to seismic detection and environmental monitoring in the next generation of repeaters of submarine cables. But new cables are installed at a relatively low rate, perhaps 20 to 30 a year. Instead, there are hundreds of long-distance submarine cables already deployed around the world: could something be done retroactively with these cables to turn them into seismic detectors?

As the diagram shows, seismic events cause measurable effects on the state of polarization (SOP) for a given data wavelength on the cable. Modern transponders are specifically designed to eliminate this “background noise” but could also be programmed to identify unexpected SOP disturbances. Also, submarine cables can be many thousands of kilometers long, so how can we determine where in the cable run the seismic effect is best felt?

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Every 50 to 100 km along a submarine cable there are optical amplifiers, which are often configured with a passive filter device called a Bragg grating that reflects about 1% of a specific wavelength – typically 1561nm. By adding a measurement transponder at the ends of the cable operating at this wavelength, it is possible to isolate where the seismic effect is most strongly felt to the granularity of the amplifier spacing – the bottom left of the diagram shows the peaks of the “ wave” at amplifier 104. For this system to provide maximum benefit, responses from as many leads as possible could be correlated to allow triangulation of the epicenter to provide data for early warning.

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This exciting new approach is in the works, with Google playing a special role by offering its cables as testbeds and developing open source software for signal analysis. As a leader in submarine optical transmission infrastructure, Infinera is an active partner in this field. At OFC 2022, Infinera and Google will present the results of work on the Curie cable system in the Pacific Ocean, while additional work on seismic detection is underway.

By Geoff Bennett, Director, Solutions and Technology, Infinera Corporation

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