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You've probably heard it said that only 10 percent of an iceberg is visible above the ocean's surface. It's true, and the other 90 percent is of great interest to the people who run offshore installations such as Canada's Hibernia and Terra Nova oil fields, where a drifting iceberg could pose a serious risk.
|Dr. Ralf Bachmayer travelled to the Jakobshavn glacier in Greenland to practice sailing an unmanned glider underneath an iceberg.|
Software programs can predict an iceberg's drift in time for a tow ship to pull it off a dangerous course. But programs need data to make accurate predictions, and in this case they need to know the shape of the iceberg below the water. Capturing this underwater "profile" with ship-based equipment is expensive, but Dr. Ralf Bachmayer of the NRC Institute for Ocean Technology is working on a more economical way to profile icebergs using a mini-submarine called a Slocum underwater glider.
"There are much more sophisticated instruments out there than a glider, but they usually have to be launched from a ship, which can easily cost $20,000 a day," says Dr. Bachmayer. "We can launch and recover a glider from a Zodiac if the weather allows." The slim glider carries instruments that can map the underwater shape of an iceberg and collect data on the surrounding currents. "These are two pieces of the puzzle that scientists can use to predict how the iceberg is moving," he says.
Last June, in the iceberg-strewn waters around Greenland 's Jakobshavn glacier, Dr. Bachmayer performed a first-ever experiment: sailing an unmanned glider underneath an iceberg to profile its shape. The glider test was an offshoot of an international project to investigate the melting of Jakobshavn (see Glacier holds clues to rising sea levels.)
|Searching for the perfect iceberg for the underwater test.|
On the drive into the Greenland settlement of Ilulissat, Dr. Bachmayer got his first view of the massive glacier. "It was incredible – like the Alps made of ice," he says. A local tourist boat took him and several other researchers to the waters outside the mouth of the Ilulissat Fjord, where hundreds of icebergs that have calved off from Jakobshavn float in ghostly silence. "There are no waves, because you're sailing between all these ice masses," says Dr. Bachmayer. "It's dead quiet. Then you hear what you think are shots, but it's actually the ice cracking."
|"Profiling" an iceberg involves sending a sensor-equipped vehicle beneath the iceberg to map its underwater shape.|
As the ship nosed past icebergs the size of skyscrapers, Dr. Bachmayer was looking for something very specific – an iceberg surrounded by hundreds of metres of open space where the glider could dive and resurface safely without risk from rapidly shifting "brash ice". They eventually found a suitable area around a medium-sized iceberg and the glider began its descent. Once launched, it was completely on its own. "You don't have any way to communicate with it at that point," says Dr. Bachmayer.
He had calculated that the glider would return to the surface after 25 or 30 minutes. But after 35 minutes, there was no sign of it. "I'm walking around the deck of the ship thinking 'how am I going to explain this?'" he says. The glider was programmed to send a signal to a beeper that would alert Dr. Bachmayer as soon as it resurfaced. "My ears were tuned to hear the sound of this beeper," he says. The beeper finally went off, and afterwards the rest of the team joked that "they heard me give this big relieved breath."
|Recovering the glider after the test.|
The data showed that the glider had briefly gotten stuck under the iceberg – information that will help Dr. Bachmayer to modify how the glider navigates under the ice. "The biggest challenge is to judge how the currents will affect the glider, since the flow around icebergs is very complex," he says. Another challenge will be finding a way to determine the glider's location while it's underwater, since the radio waves from the glider's GPS don't penetrate seawater. "In future we're going to be working on ways to get actual navigation while the glider is submerged, such as with low frequency sound sources."
With the test complete, Dr. Bachmayer is starting to apply what he has learned. The glider has also been equipped with a specialized sonar for ice profiling, which could be used not only to profile icebergs, but also in future studies of glacial melt and thinning ice in places like Greenland and Antarctica, where under-ice research can require an icebreaker that costs up to $50,000 per day.
"Now we've shown that you can fly a glider in this environment, and we know what the issues are," he says. "In the future, we can reduce the cost for people to do this kind of research."