LOUISVILLE, Ky. (AP) -- Federal regulators have approved a new wireless technology developed to help underground coal mines meet safety standards set after a deadly 2006 blast in West Virginia.The Mine Safety and Health Administration said Wednesday it had approved Lockheed Martin's MagneLink system, the first device that can send signals through the earth, unlike traditional radio systems that depend on wires that can be damaged in a disaster. Lockheed said the technology uses a low-power battery that won't cause an underground spark."It's important that any kind of equipment that's supposed to operate when there's methane gas present, it doesn't put out enough energy to spark off an explosion," said Dave LeVan, the engineer who developed the system.Coal operators had been slow to meet the communication standards outlined in safety laws passed after the Sago Mine explosion that killed 12 workers in January 2006. Mine safety officials said in April that nearly two-thirds of the nation's 529 underground mines had not yet fully installed new communications and tracking equipment.Dave Chirdon, MSHA's new technology program manager, said Wednesday that he believed the mines are now nearing full compliance with the law. He says a survey in May found that more than 50 percent of underground mines had upgraded communications equipment installed.Chirdon said the MagneLink system is the first underground technology approved by MSHA that can send signals through the ground, and is ideal for use in a disaster because of minimal equipment. Other radio-based systems need underground access points and wiring, he said."There's no infrastructure there to get destroyed in an accident," Chirdon said. "It does have a much greater likelihood of being available after an explosion."LeVan said Lockheed has performed successful range tests in an underground mine from 1,550 feet below the surface. The system's underground unit has a low power setting that's about 100 times weaker than the electric igniter on a gas grill.The trapped miners in the Sago explosion were vertically about 280 feet below the surface.Chirdon said Lockheed's system has distance limitations that would keep it from being used in deeper mines, and miners can only use it if an underground station is accessible after a disaster. He said it would be ideal for use in a mine's refuge shelter.The MagneLink underground units can send and receive voice and text signals to a surface unit, which is portable, so it can be moved around to find a stronger signal. LeVan said the magnetic signal can penetrate thick layers of earth, a limitation of radio."If there's rock and coal between (trapped miners) and anything else, a regular two-way radio just doesn't go anywhere," he said.LeVan said he began developing the system after the Sago explosion when a retired company engineer with roots in West Virginia asked if the company could use its technology to develop a better communication system.Lockheed said Wednesday it is still developing a price for the unit.