But it’s not just a matter of buying the buoys and dropping them in the ocean. Ensuring they are assembled correctly, working faultlessly and go in the right place has been the job of NIWA oceanographic engineer Mike Brewer. Brewer says the buoys will not only be useful to New Zealand, but also to Pacific nations. “The buoys will detect tsunamis but also provide verification one is actually happening which can avoid false evacuation warnings.”
Planning an ocean observation network
The buoys will become an integral part of the tsunami early warning network, which includes sea level monitoring as well as other buoys deployed by a range of countries to monitor tsunami.
The closest buoy to New Zealand is about 150 nautical miles offshore.Three trips aboard NIWA’s flagship research vessel Tangaroa are needed to deploy the buoys with two already completed taking in Tonga and Niue. The third will head towards New Caledonia.
The buoys are fourth generation technology manufactured in the US and together they will comprise the most advanced monitoring system in the world.
“It’s like a giant Lego set,” says Brewer. You put everything together and put the units through some rigorous testing so that when they’re deployed we have complete faith in the system.”
The buoys comprise two main components—the Bottom Pressure Recorder (BPR) that sits on the sea floor, and the surface float. Deploying them is a mission in precision that requires hi-tech equipment.
A bathymetric survey of the seafloor is done on site using Tangaroa’s multibeam echosounder to check the gradient is less than 5ᵒ with no big dips that might interfere with signals travelling through the ocean. Current and wave tests are also undertaken before both parts of the buoy are lifted off the ship and attached to separate moorings. The two parts communicate with each other through the water column. Every 15 seconds the BPR records the height of the ocean above it and sends that information to the surface buoy once an hour. From there the data goes via satellite to the server based at GNS Science back in Wellington.
The measurements are so sensitive that they record height differences in fractions of millimetres. Then comes the really clever bit. If the BPR records a measurement it thinks is outside normal parameters, it will send a special signal to the surface float. This is when it flicks into “event mode”—a trigger that means it will update the server every few minutes rather than once an hour. Event mode can also be triggered remotely if it is suspected that a tsunami may have occurred.“Tsunami are very long period waves, with some having wave lengths of up to an hour. Wind and swell waves are short period waves, so by measuring every 15 seconds we are cutting out those short waves and concentrating on the big differences,” says Brewer.
Several organisations have been involved in bringing the project to fruition including GNS Science and the New Zealand National Emergency Management Agency.