New 3-D sonar puts ocean floor in sharper relief
Larry Mayer will always remember the day he showed Nova Scotia scallop fishermen what his high-tech sonar mapping technology could do.
Dr. Mayer, then at the University of New Brunswick in Fredricton, displayed his team's new high-resolution, three- dimensional model of Browns Bank - a key scallop fishing ground - on the 12-foot glass screen in his lab.
Using a joystick, the fishermen were able to fly through seascapes they had fished their whole lives, seeing rocks and crannies as small as seven or 10 feet across, and finding likely scallop habitats hidden on the hilly terrain more than 300 feet below the surface.
Then one burly 6-foot, 6-inch scalloper walked up from the back of the room, tears in his eyes, and touched the screen where a rocky crag was displayed. "I always thought it looked like this," he said. "Now I know."
Amid the ongoing crisis in the North Atlantic's fisheries, a quiet revolution has been under way in research labs in the US and Canada, one that offers a powerful new tool in the exploration and management of the oceans.
In recent years, ocean-mapping technologies have undergone advances that not only allow a more detailed view of the bottom, they even allow people to identify likely seafloor habitats from ships on the surface.
Researchers like Mayer, now the head of the University of New Hampshire's Center for Coastal Mapping, scan the bottom with multibeam sonar. Conventional depth sounders send a single beam of sound down to the bottom, calculating the depth on the basis of how long it takes the echo to return. But multibeam systems send dozens, even hundreds of beams simultaneously, allowing mappers to build detailed 3-D maps of structures on the bottom.
The technology has been used to find sunken tanks off the beaches of Normandy and sturgeon habitats in Canadian rivers. UNH's new computerized multibeam chart of the harbor of nearby Portsmouth, N.H., is so detailed that individual lobster traps are clearly visible on the seafloor in 60 feet of water.
Researchers have used the systems to detect herring schools at sea and to determine the difference between soft and hard bottoms based on the qualities of returning sonar beams.
"We can't see individual animals - say, a lobster - but we can map the types of seabed they like to live on," says John Hughes Clark, head of the University of New Brunswick's Ocean Mapping Center.
"That has some dramatic applications," he adds.
For scientists who study bottom-dwelling creatures, it's a difference between night and day. "When we drop our cameras and other instruments over the side, we're really running blind most of the time," says Les Watling of the University of Maine's Darling Marine Center in Walpole, who studies the corals, worms, sponges, and other creatures that live on the deep ocean bottom. "Multibeam mapping allows us to pinpoint places and things we might want to study."
The resolution of the maps is directly related to depth. In 20 feet of water, the maps reveal rocks as small as basketballs, while in 4,000 feet of water, the smallest perceivable objects might be the size of a modest home.
Scientists hope the technology will help them detect ecologically important features that might best be protected from disturbances, while opening less sensitive areas to bottom-trawling gear.
"This lets us look at the ocean bottom in a way that lets us zone it for different kinds of activities," says Mr. Watling, a proponent of these so-called "marine protected areas."
Fishermen also stand to gain.
Nova Scotia scallopers were so impressed with Mayer's maps that one of the larger companies purchased a million-dollar multibeam system of its own and has used it to pinpoint the gravelly bottoms that scallops like. This has allowed the firm to fill its scallop quotas in one-quarter of the time it used to take, while dragging its dredges over one-third as much ocean bottom, reducing collateral damage to other organisms.
"With good management this is a win-win situation," says Mayer. "But if you did this without firm quotas, it would be a disaster."