Abstract: A system is disclosed for investigating subterranean formations by analyzing the propagation of low-frequency radio transmissions. Commensurate signals of different frequencies are received from a transmitting station and are processed into a phase-comparable form. The phase-comparable signals are then compared in phase to provide a representative signal indicative of a subterranean formation. A system utilizing at least three transmitted signals is disclosed to provide specific information related to the depth of formations. A system utilizing signals from a plurality of transmitting stations is disclosed to afford improved formation illumination. Also, a mobile form of the system is disclosed which incorporates a position-information system for combined use.