An astounding amount of information is becoming available now due to new monitoring methods that provide extraordinary visibility that will improve operations at no additional cost.
Detection and forecasting the impact of undersea seismic events has been an area of research for many years. Early detection of such events has tremendous implications in minimizing loss of life and property. The National Ocean and Atmospheric Administration’s (NOAA) Deep Ocean Assessment and Reporting of Tsunamis1 (DART) system of underwater tsunameter pressure sensors in strategic locations has been providing warnings successfully for tsunamis over the last 20 years. Distributed Acoustic Sensing (DAS) using undersea fiber has also been a promising approach with accurate detection of seismic events and characterization of submarine structures2. Satellites have also been shown to be able to detect earthquakes from space by monitoring tiny variations in gravity over the earth’s surface using accelerometers3.
DART and other similar tsunami warning systems don’t provide coverage in all places across the globe. DAS approaches require dedicated access to undersea fiber that is not readily available, and DAS is also usually limited to about 150km. Satellite-based detection at about 30 minutes after a seismic event is slower than desirable for early warning systems. While launch costs are improving with new generations of reusable rockets from Space-X and others, satellites are still expensive to design, build, and launch. This is where seismic detection – based on coherent optics over subsea telecom fiber – is a real breakthrough.
Telegeography’s 2021 submarine cable map has 464 cables across the globe. Deployment of new subsea cables for telecom applications continues at a rapid pace as cloud infrastructure connectivity expands. As per Telegeography, there is $8B USD of investment across 60 new cables that are set to be deployed over the next three years. These telecom cables can be used for seismic activity sensing using coherent transponders without disrupting service, deploying special sensor equipment, or allocating dedicated fiber.
The digital signal processors in coherent optical transponders collect lots of information on optical signal impairments, which then uses the data to compensate for the impairments to generate error-free signals. One such impairment is the State of Polarization (SOP). Light is an electromagnetic wave with electric and magnetic fields that twist and turn in an orthogonal way to the direction of the propagation. This vibration of the electric field is called polarization. Transponders can even compensate for extremely rapid rotations in the state of polarization to the tune of 10 million radians per second, which is useful in situations where aerial fiber is deployed along power lines and is often exposed to lightning strikes.
SOP is sensitive to mechanical disturbance and can track the disturbances caused by low frequency seismic waves, and a subsea cable at the bottom of an ocean floor provides a very stable environment with extremely low level of disturbance. This capability lies within the DSP of the optical transponder deployed.
The result is better operational efficiencies. If an early warning can be recorded across multiple fiber cables, early detection of an earthquake or tsunami could result in the advantage of extra minutes to react, potentially saving lives and/or resources. When deployed over hundreds of subsea cables around the world, this capability on the existing infrastructure can even create a network-wide mesh that can deliver seismic reporting. This is an added beneficial feature in addition to the bandwidth carrying capacity of the cables deployed. This feature comes with no incremental cost to the cable owners.
While this seismic and subsea application is currently being used today with the Cisco NCS 1004 platform, other applications – if tied to big data analysis – might be able to perform more reliable and predictable operations in the long run. This results in fewer optical protection and/or restoration resources required, thus lowering capital expenditures. The SOP tracking and monitoring feature is available and working today.
References:
- https://nctr.pmel.noaa.gov/Dart/Pdf/DART_II_Description_6_4_05.pdf
- https://www.nature.com/articles/s41598-021-84845-y
- https://www.nature.com/articles/nature.2013.12545
- https://cloud.google.com/blog/products/infrastructure/using-subsea-cables-to-detect-earthquakes
- https://ieeexplore.ieee.org/abstract/document/9333176
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