15 September 2010—On 5 August, the collapse at a Chilean gold and copper mine left 33 miners trapped in a chamber nearly 700 meters below the surface. They're still down there, while officials coordinate their rescue. As bad as their situation is, it could have been much worse: They could have been mining coal.
The Chilean miners have been able to talk to their families by phone, and video images of the men have been broadcast around the world. But if they'd been in a coal mine, their television debut would likely have been impossible. The slightest spark from electronic equipment can ignite methane gas and coal dust, touching off a deadly explosion.
In the aftermath of a lightning-induced explosion at a West Virginia coal mine in January 2006, everything went wrong for the 13 miners trapped inside. It took rescuers nearly two days to find them. By then, all but one had died—victims of the explosion and subsequent carbon monoxide poisoning. The lone survivor's account highlighted the conspicuous absence of technology for communicating with the surface. "[We] attempted to signal our location to the surface by beating on the mine bolts and plates" with a sledgehammer, said Randal McCloy Jr. after he was rescued. "We never heard a responsive blast or shot from the surface."
The outrage from that accident was heightened by the deaths later that month of two miners who were lost in a haze of thick smoke from an underground blaze. According to David Chirdon, new technology program manager at the U.S. Department of Labor's Mine Safety and Health Administration (MSHA), people began to ask, " 'If we can put a man on the moon, why can't we talk to miners underground?' I always responded to that by reminding people that we could talk to the astronauts across 240 000 miles of empty space, but when they were on the dark side of the moon, that big rock between us and them presented the same challenge as the earth between miners and the surface."
By June 2006, the Mine Improvement and New Emergency Response, or MINER, Act was signed into law by the U.S. Congress. Among its provisions was the creation of the Office of Mine Safety and Health within the National Institute for Occupational Safety and Health (NIOSH). The new office is responsible for spurring the development of mining communications technology. NIOSH was given just three years to commercialize wireless two-way communication and electronic tracking systems.
According to David P. Snyder, senior mine electrical engineer at the newly created NIOSH unit, the team focused its efforts on getting the systems to be, in mining industry parlance, "permissible." A component is permissible when MSHA certifies that its circuits are incapable of releasing enough energy—during normal operation or if they're damaged in an accident—to trigger an explosion by igniting methane gas or coal dust. Among the rigors a component has to withstand is the "two-fault test." Testers at the agency use an electrical schematic of a device to determine the two worst-case faults it could experience. Even when the testers create short circuits at those points, the device must avoid creating an arc.
MSHA is uncompromising about these tests, says Snyder, because "a methane-air mixture requires less than a half millijoule of energy to ignite." Consequently, a miner trapped underground is going to be severely limited in the amount of power he can transmit.
NIOSH eventually came up with two competitive solicitations for primary communication systems, which it defines as those designed to operate in the conventional radio frequency band (from a few hundred kilohertz to a few gigahertz). Miners either carry handheld radios or wear radio tags.
NIOSH awarded a contract to Pillar Innovations of Grantsville, Md., for its rugged leaky-feeder system. It consists of a coaxial cable with openings in the insulation along its length, which allow it to give off and pick up signals. The cable loops down into the mine and back to the surface. And unlike standard leaky feeders, it is powered from both ends, and the signals it transmits run in both directions. So even if the cable is severed in a collapse, signals from miners' radios can still reach the surface.
The other contract went to L-3 Communications, in Chantilly, Va., and Innovative Wireless Technologies, in Forest, Va., which together commercialized a wireless mesh communication network they call Accolade. The system operates at 900 megahertz, which is within a range of frequencies that offer the best propagation down the twists and turns of a coal mine tunnel.
But at best, conventional radio signals only propagate perhaps 300 to 600 meters down a coal mine tunnel, says Snyder. So if all the infrastructure within a thousand meters or so is destroyed by a catastrophic event, "you would be out of business." To ensure the survivability the MINER Act calls for, NIOSH also began surveying the landscape for technologies that would best serve as secondary communication systems. Secondary systems operate at frequencies well below the part of the AM band (148.5 to 283.5 kilohertz) that in North America is reserved for airplane radio beacons. "We've worked with systems that transmit as low as 2 to 8 hertz," Snyder reports. NIOSH is currently evaluating two approaches: transmitting through the earth and a technology called parasitic coupling.
Snyder notes that past research showed that it is possible to transmit signals through solid rock at frequencies below 10 kHz, but those studies were conducted with high-powered military transmitters. "We're now talking about doing the same thing on a watt or two of power," he says. Complicating matters is the fact that devices must be able to detect transmissions amid the cacophony of environmental and man-made noise in this band—including that from thunderstorms whose electromagnetic effects can be "heard" in the United States from as far away as the equator.
NIOSH has contracts for earth transmission technology with five firms, among them Lockheed Martin, which is leveraging its expertise in advanced signal processing and noise cancellation techniques gained from its work with sonar and radar. The company has a product called MagneLink that is up for MSHA approval.
Parasitic coupling systems take advantage of the fact that radio signals in the frequency band between 300 kHz and 3 MHz propagate through any metallic objects in a mine, such as rails or pipes, creating non-line-of-sight communications links that can stretch for kilometers. These systems compress digitized voice signals so that the systems' 2.4-kilobit-per-second bandwidth can support up to three voice channels. One such system, developed by Kutta Technologies, in Phoenix, recently received MSHA permissibility approval.
From the perspective of product development, it's amazing that there are any technologies available for MSHA to certify. "I've done engineering development my whole career, and this was the first time I walked into a situation where there was this very high-level requirement but no specific requirements at the system level," says Snyder. He notes that the team had to formulate technical requirements—engineering details that a designer might want regarding battery life, redundancy, and such—as systems were being built.
He also notes that time wasn't on the engineers' side. The 2006 MINER Act required NIOSH to draw conclusions about what wireless communication technologies would work and get them to the point where they could be handed off for commercialization by June 2009. "We had to look at technology that was available or on the cusp of availability to have any shot of meeting the deadline," says Snyder. "Maybe if we had 10 years, we would have approached things differently."