I don't know much about geology but couldn't some beacon be created that is able to be tracked underground? Obviously satellite is not going to work but maybe some sort of sonar or radar or simply wireless? Not that a signal could be sent 1,500-2,000 ft. up to the surface but they have air lines and power running down there so why not a fiber optic or whatever with sensors all the way down...then each person wears a transmittor that is picked up by the nearest sensor. Computer/software attached to the line with the sensors is able to determine which sensor is picking up which transmittor...unless the cable gets broken it seems like a relatively simple way to track who is where.