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Using submarine cables for deep-ocean monitoring
By Drs Ben Loveday and Juliet Hermes, SAEON Egagasini Marine Offshore Node
SAEON researchers at the Egagasini Node are part of a new, NASA-funded, initiative to incorporate bottom temperature and pressure sensors into the global array of telecommunications cables, providing a new tool for climate change monitoring and disaster warning.
The 2014 undersea-cable science workshop, organised by the University of Hawaii, was held at Jet Propulsion Lab in Pasadena, California and brought together a global team of oceanographers (see picture below) to discuss the scientific value of the proposed measurements, the associated technological challenges, and the utility of the data in satellite and model validation.
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Sampling the depths: from seconds to decades
While the observations gathered from in situ and remote satellite sources are starting to give us a well-described picture of the upper ocean, the deep ocean remains sparsely monitored. The vast majority of global telecommunication traffic is carried across the deep ocean by a series of undersea cables that lie on the ocean floor but remain ignorant of their environment. Every 50 km or so each cable has a repeater, a device designed to amplify any incoming communication signal such that it can reach from continent to continent.
By incorporating bottom temperature and pressure instruments into these repeaters, the cables can be made ‘aware’ of their surroundings, relaying high-frequency oceanographic data along the pre-existing cable infrastructure. As cables have a typical maintenance cycle of ~20 years, they would produce a decadal time series at measurements at the highest possible temporal resolutions, spanning an entire ocean basin.
Ocean monitoring and disaster management
Sparse observations have begun to paint a picture of warming in the deep ocean basins. However, this picture is subject to poor spatial coverage, temporal aliasing and a lack of long-term monitoring. An instrumented cable array could address all of these issues, providing a far more detailed picture of the effects of climate change at depth and, with the addition of IES (Ion and Electron Sensor) instruments, offer the possibility to calculate total mass change associated and stearic contributions to sea level rise.
Recent satellite missions such as GRACE have been successful is refining our picture of the geoid, allowing us to infer ocean mass movement from minute gravitational changes. However, monitoring the long-term signal is hampered by drift in the inferred bottom pressure, and aliasing at the sub-monthly scale is still a substantial source of error. Independent confirmation of bottom pressure from an instrumented cable array would provide a necessary validation source for gravimetry, substantially reducing these errors.
Much of the world’s population lives along the coast, in many cases in regions that are threatened by the possibility of the occurrence of tsunami. Deviations in bottom pressure associated with such events would be immediately evident in the cable array’s instruments, allowing them to operate as an invaluable, pan-basin early-warning system.
Expanding capability …
Currently, array coverage is far from global, and there is a notable focus on the northern hemisphere (see picture below). However, cables are already in place on both the east and west coast of South Africa and planned expansion of the network may soon see the nation connected to South American and Asian BRICS economies across the South Atlantic Ocean and South Indian Ocean, respectively. In the interim, continued development of sensor technology will see the engineering of new, low-power, self-calibrating instruments that are equal to requirements of long-term, deep-ocean, deployment.
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The next science meeting is scheduled for mid-2015, and SAEON's continuing involvement in this programme will see that a Southern Hemisphere perspective is represented as the cable network expands.
