Officials began drilling a fifth hole in the Crandall Canyon mine on Sunday, in an effort to locate the six men who became trapped inside nearly two weeks ago. Why don’t miners carry GPS-tracking devices to help rescue workers find them?
Because those gadgets don’t work underground. The wireless signals that we use on the surface to make phone calls, send e-mails, or find our GPS coordinates have a hard time traveling through the earth. All of these signals can be obstructed by the concrete, mineral water, and coal in a mine. To send a signal through a tunnel that runs for miles, you might need to place wireless receivers in a relay at each bend or turn, which would be very expensive.
On the surface, engineers can use high-powered transmitters to boost a signal and help it to get through physical obstructions like walls or doors. But this would be impossible in a methane-filled mine, where communication devices need to operate on very little power—say, 1,000 to 2,000 times less energy than an average cell phone. Otherwise, an antenna could put out enough energy to ignite a gas explosion. For now, miners have to rely on rudimentary locating means. They might call a dispatcher each time they move to a different part of the mine, using phone lines that are strung along the tunnel ceiling. A more high-tech version of this same system uses radio frequency sensors-RFID tags-on each person to track them when they reach certain areas. Placed at key tunnel intersections or work areas, RFID reader pick up each worker’s tag and note when a person is in a particular place. All these data go back to the surface through a physical communication line, like the fiber optic cables that connect office workers to the Internet.
The men in Utah did have one-way, wireless communications devices, called PEDs, that can receive short text messages from the surface. These gadgets use low-frequency waves that can travel through the earth and deliver data at very slow rates. * But it would take a lot of power to send one of these signals (not to mention a device the size of a small TV), so miners can’t use them to pass messages back to the outside world.
Accidents in mines often destroy parts of the main data lines, making phone communication or RFID tracking impossible. To address this danger, scientists are developing technologies to send wireless signals through hundreds of feet of coal, water, limestone, sandstone, and other types of rocks, which all have different transmission properties. (Sending radio waves through a layer of limestone turns out to be pretty easy, for example; coal and water tend to absorb the signals.)
The National Institute for Occupational Safety and Health, an agency charged with encouraging mine safety technologies, tested about 50 wireless systems after the accident in West Virginia’s Sago mine, but none were up to snuff (PDF). Officials there now hope a hybrid wired-and-wireless system might be the answer. They want to take advantage of medium-frequency waves, which can travel through air as well as metal structures. In theory, a medium-frequency signal could jump onto a metal pipe to skip the damaged portions of a physical line.
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Explainer thanks Chris Hamilton of the West Virginia Coal Association, Jeffrey Kohler of the National Institute for Occupational Safety and Health, Joe Miller of Vital Alert Communications, David Reagor of Los Alamos National Laboratory, and Wesley Shumaker of the Mine Safety and Health Administration.
Correction, Aug. 22, 2007: The original version described low-frequency waves that “travel very slowly” through the earth. They travel at the speed of light, but they transmit data at a slow rate. (Return to the corrected sentence.)