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20/12/2006
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Tom Shelley reports on some of the technology now being deployed to protect against a repetition of the Indian Ocean tsunami and other hazards
Buoys are now being deployed that can detect changes in water pressure corresponding to sea level changes of a few mm at depths of several km, distinguishing the effects of an earthquake from tidal surges of much greater magnitude.
But they are only part of a complete ocean monitoring system that will constantly monitor the weather, pollution, and the effects of global climate change to an extreme degree of precision.
Stars of the system have to be the ‘Seawatch’ Buoys and Deep Sea Modules made by Fugro Oceanor in Trondheim. The sea bottom modules contain a pressure sensor, processor, batteries and an acoustic modem. Water pressure is constantly monitored and the depth above the sensors calculated to an accuracy of 10mm or better even at depths of up to 4km. The effects of waves average out at depth but built-in software has to be used to separate unexpected water depth variations that might be caused by a tsunami or other event, from tidal variations, which occur at regular intervals and can thus be allowed for, and sensor drift.
If the deep sea module detects a difference between measured and forecast depth, greater than a pre-set limit, it transmits depth, water temperature and battery voltage measurements over an acoustic link every 15s to a moored surface buoy that can pass messages to shore via a satellite link. Average power consumption is 0.02W and the unit will run for 2 years on alkaline batteries. When a service is required, an acoustic signal from the surface tells the sea bottom module to release itself from its anchoring platform and surface, lift being provided by flotation spheres.
Three Fugro Oceanor Wavescan buoys with Deep Sea Modules have been delivered to the Malaysian Meteorological Service. One has been deployed in the sea near Pulai Rondo, 60km North of Banda Aceh and a second has been deployed near Pulau Layang-Layang.
But these are only a very small part of the Global Ocean Observing System or GOOS, now 55 per cent complete, which also includes 160 tide gauges (101 in place), 1250 surface drifting buoys (all in place), 119 tropical moored buoys (84 in place), 51 participants in the ship of opportunity program (39 at present), 3000 Argo profiling floats (2300 at present), 89 OceanSITES reference stations (42 presently) and 37 repeat sections completed by the Ocean Carbon Network s (15 so far).
The Argo floats came out of an idea independently arrived at in 1955 by Henry Stommel in the US and John Swallow in the UK to measure sub surface ocean currents by floats that would go down to depth and stay there. The first floats were made by John Swallow using pressure cases made from aluminium scaffold tube. The present generation, mostly US-French, although some have UK instruments, use a gear motor, hydraulic piston and external inflatable bladder so they alter their buoyancy, go down to a preset depth and then slowly come up again, recording temperature and salinity along the way.
This has everything to do with learning about the ocean and detecting the effects of global climate change, and nothing about tsunamis except that these functions are both part of the GOOS mission. The reason for combining two functions, according to Keith Alverson, director of GOOS and head of the Intergovernmental Oceanographic Commission (IOC) Ocean Observations and Services section of UNESCO, is that if a system were to be set up purely to monitor tsunamis, it would hardly be likely to be fully efficient if required to function after a period of nothing to report over several decades. If, on the other hand, it is combined with the monitoring of other parameters, it is going to be continually maintained, and since the same buoys and monitoring stations can be inexpensively extended with a few extra sensors to be multi-functional, is likely to be much better value for money.
Hopefully, while we can only hopefully speculate as to the cause and effect relationship, the effect of deploying a high performance world wide ocean monitoring will be similar to that resulting from the deployment of the tsunami monitoring system in the Pacific in 1968, where no major events have occurred since, despite tsunamis occurring relatively frequently in prior years, with particularly large events during the period 1946 to 1965.
Global Ocean Observing System
Fugro Oceanor
Pointers
* Tsunami detection is just one of the functions of the new GOOS system that will monitor the world’s oceans for hazards, whether natural or man made
* The most advanced of the monitoring buoy systems can detect non tidal surges of a 10mm or better at several km 6000m depth
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Author
Tom Shelley
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