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Device 'listens' for sounds of life at WTC
by Bruce Wiebusch, Regional Editor -- Design News, November 5, 2001
New York City — "This makes Oklahoma City look like child's play," says Cynthia Fajardo, a police officer and Federal Emergency Management Administration (FEMA) search manager at the World Trade Center. She and her urban search and rescue task force use a combination of technologies to try to locate victims in one small section of the 14-acre pile of debris left behind after both towers collapsed on September 11.
A major challenge is the chaotic structure of the seven-story deep pile of rubble. Fajardo explains that the building collapse was different than has been described in popular media—instead of the Trade Centers' 110 stories each stacking neatly like an accordion, they slumped sideways into overlapping layers. "It's more like several rows of dominos piled on top of one another than it is a pancake stack," she says. "The layers upon layers makes the search for victims very difficult."
Human and canine members of eight FEMA teams work 12-hour shifts in the huge pile of rubble, estimated to weigh more than 100 million tons.
But the rubble pile's jagged structure is difficult for search dogs to traverse. Fajardo explains that search dogs are normally taught to bark when they find a live victim and lie down when they find a corpse. Yet the teams have found that the dogs didn't behave like they normally do, because they were uncomfortable on the jagged rubble. "They wouldn't lie down anywhere, but we knew there were supposed to be more than 5,000 bodies in there," says Fajardo.
So FEMA teams considered using cameras on the dogs. They decided against the idea, fearing the camera straps would snag, trapping the animals in the pockets and voids they explored.
Rescue workers also considered thermal imaging cameras that detect the heat of a living person. Unfortunately, the thermal imager had limited value at Ground Zero, especially during the first week after the collapse when numerous fires created hot spots throughout the debris.
Another tool is a listening device called the Life Detector, from Delsar Inc. (Chapel Hill, NC). The device uses sensors for "hearing" the minute vibrations in concrete, wood, soil, and other materials caused when victims knock, or dig.
"Layers of solid concrete sometimes block any airborne sounds," says Engineer Uwe Beckman, Delsar's founder. "However, if the victims can tap, scratch, or move, the collapsed structure carries these sounds effectively."
The human ear can't hear sounds below 20 Hz, but the Life Detector can pick them up. It covers a range of frequencies—from 1 Hz seismic sound up to 4 kHz audio sound—and displays the data as a bar chart on an LED display, much like the equalizer on a home stereo. To locate the source of sounds, each Life Detector uses six sensors on daisy-chained cables. Rescuers search between the two loudest channels.
Cables used for transmitting signals meet several extraordinary requirements. "It has to be extremely flexible," says Beckman. "It constantly bends and twists when used around rubble and debris like you have at the World Trade Center disaster."
This cable is custom-built by Northwire Inc. (Osceola, WI). "We designed a 24-gauge, 14-conductor cable that is contra-hellically wound for flexibility," says Del Marty, a manufacturing engineer at Northwire. "The outside ten wires are wrapped in the opposite direction of the four wires inside."
Its outer covering is a mixture of thermoplastic and PVC. "Thermoplastic urethane provides the durability the cable needs, and the PVC makes it slippery for sliding in cracks and crevasses," Marty says. The cable's requirements also include EMI shielding of up to a dozen pairs of wires used with the six sensors, Beckman says. And it withstands temperatures below 0F, for avalanche use.
But the dozen Life Detectors used in New York have run into additional hurdles because of the size of the rescue operation. They usually filter out low-frequency rumble and high-frequency hiss sounds on either end of its spectrum. However, filtering has limitations if sounds in the surrounding area are too loud, or if the surrounding structures are shaking or vibrating.
"We didn't get to use the listening devices as much as we thought we would," says Fajardo. "With 14 acres of debris, we had to constantly use heavy machinery for making any kind of headway." Excavators, front-end loaders, and trucks flooded the area with vibrations and noise that prohibited extensive use of all listening devices much of the time.
"Sometimes all the technology in the world can't help you," says Fajardo. "There were times when we just had to get in there and dig with our hands." At press time, her FEMA team had recovered three firemen and one police officer. None was found alive.
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