Constraints affecting through-the-earth electromagnetic signalling and location techniques

Over the past several years much effort has been expended in the development of through-the-earth communication links. The development of such links would improve the chances of survival following coal mine disasters by allowing miners to make their circumstances known to rescue teams. In order to implement effective electromagnetic communication links for signalling between mine workings and the surface, the electrical properties of the rock overlying mine workings and ambient background noise levels in several mining provinces are studied. The results of both CW and pulse field transmission tests show good agreement with theoretical considerations, and several techniques for locating either a buried finite line current source or a buried finite vertical-axis loop are demonstrated. The effect of loop size on the latter location criteria is considered, as well as the relative merits of a pulse or CW system.