A specialist mine-discovery team is using underwater sonar to comb Tauranga's harbour for shipping containers which may have sunk to the sea floor.
The navy's mine counter-measures team has been charged with making sure the harbour, particularly the "choke point" at its entrance, remains clear of debris from the stricken container ship Rena. Its work has allowed the harbour to remain open for salvage operations and shipping.
One of the team, petty officer combat systems underwater specialist Damian Olson, has worked for 22 years in the navy. "Standby to deploy the fish," Olson yelled aboard the HMNZS Manawanui yesterday before two men pushed the long cylinder device, called a c-max, into the water.
The c-max is pulled behind the ship and can provide scans of more than 150m in radius and detect submerged debris. If a container is detected on the scan then c-max's superior, the bright yellow Remus, is deployed. Remus, a large, torpedo-shaped device, can navigate depths of up to 100m. Operating autonomously, the torpedo can send and receive text messages from its operator and provide high resolution images of submerged objects. "We can just programme it up and send it out for a couple of hours," Olson said.
The device - which comes with a $100,000 price tag - is the same one used by the team to find the wreck of the Princess Ashika which sank in Tonga in 2009. It took two days of searching by the Remus to discover the wreck lying 112m below. The Remus had been travelling 97m deep - near its maximum range - when the boat was discovered with many bodies still inside. "It was an eerie feeling when we found it. But it was nice to provide some closure and help to those people's families," Olson said.
The high-powered underwater sonar had also been used in searches across the country this year to find the bodies of divers, jet-skiers and fishermen, and was deployed to check the safety of wharves in Lyttelton Harbour after the Christchurch earthquake.
Lieutenant Commander Deane Ingram said being able to interpret the images and recognise what debris was from the Rena was a difficult task. "The fact is that as the containers are coming off the ship, they are splitting and sometimes sinking to the bottom. After that they don't necessarily look like a container any more. But finding shipping containers is certainly easier than finding mines." On finding a container, the team drops a buoy to mark the spot, alerts port authorities and informs the salvage team which then has 24 hours to clear it.
The navy's other mine counter-measure team is looking for WWII mines in Papua New Guinea.