Sonar, GPS, Robots: The Science of Underwater Recovery

Sonar scans, Global Positioning Systems and robotics could play primary roles as Minnesota and federal authorities continue recovery efforts today after the fatal bridge collapse over the Mississippi River.

The collapse of the Minneapolis bridge left as many as 30 people missing and cars and debris jutting out from the water below, making efforts to retrieve the dead very tricky.

In this morning's news conference, Hennepin County Sheriff Rich Stanek said the recovery would include the use of sonar, heavy barges and trained diving teams. Those efforts were initially stymied Wednesday evening and this morning by safety concerns for divers and other recovery workers.

Diving Is Time-Consuming, Tiring

Traditionally in recovery missions, divers were used for both the searching and recovering, but this method can be time-consuming and exhausting for authorities and divers.

According to John L. Saunders, the director of the National Underwater Rescue Recovery Institute, the first step in any underwater recovery situation is to use sonar scanning technology. Founded in 1970, the institute trains divers around the country in underwater rescue and recovery. Institute members have collectively recovered 617 drowning victims.

"Before, everything would have been done by hand, but now the technology is so good we just put the instruments in the water and we know everything that's down there," Saunders said. "We don't have to put the diver in the water. It makes it 100 percent safer and a lot quicker."

With side-scan sonar machines, authorities can look under and around bridges to scan the wreckage for submerged cars and people and also look out for any debris — in this case concrete and rebar — that might be dangerous to diving teams, Saunders said.

"Safety is the No. 1 thing. All these things will make this thing a safe recovery and make it safe for the divers," he said. "You don't want to put anyone else's life in danger."

The next step is to create a map from that sonar scan for divers who can then retrieve victims.

In recent years, sonar technology has gotten even better and is commonly combined with GPS to pinpoint the exact locations, within "a couple of feet," of any debris, according to Saunders.

Send in the Robots

The newest in search-and-recovery technology is the ROV, or remotely operated vehicle. These underwater-ready robots, equipped with sonar and GPS, are controlled from above the water's surface and can send back video, photos and sonar scans to rescuers.

Tom Glebas, vice president of a Pennsylvania-based company called VideoRay that makes sonar-equipped ROVs, describes them as "swimming cameras." The company's ROVs have been successfully used to recover vehicles and bodies.

In addition to video, the machines can use two types of sonar: scanning sonar, which resembles air traffic control radars — think of a wand going around in a circle on the screen — and multibeam, or imaging sonar, which provides a live-action image that resembles an ultrasound.

The biggest advantage of ROVs, especially in the Minneapolis situation, is increased visibility in river water made murkier by the falling bridge and cars, according to Glebas.

"Typically, in a diver situation, [a diver] is going to crawl hand over hand and is limited to close proximity searches," he said. "With sonar, you can extend your range of visibility … up to 150 feet."

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