Through-the-earth radio offers a possible means of communication with trapped miners after a mining disaster. Much of the original work was the outgrowth of war research. Since the 1920's the efforts to develop through-the-earth radio have paralleled developing electromagnetic geophysical techniques. The electrical characteristics of the earth medium as well as the ambient electromagnetic noise have been determined and theoretical models of electromagnetic propagation have been derived. Since 1969, under U.S. Bureau of Mines leadership, numerous researchers have tried to develop deployable systems. Despite some successes the basic problem remains unresolved. Low frequency signals must be used to penetrate the earth to a sufficient depth; but low frequency equipment is bulky, heavy, expensive and requires large power supplies which are not available to trapped miners. New techniques utilizing miniaturized circuitry and computers shows promise of providing a technology from which workable through-the-earth communication systems may develop in the near future.

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