Features of a modification of the controlled source audio-frequency magnetotellurics (CSAMT) sounding method with a powerful source and the long-range action of thousands kilometres are considered. Characteristics of electromagnetic field of a cable as a source in this CSAMT modification are analyzed and a waveguide zone at great distance from the source is allocated. Differences of the electromagnetic field structure in the waveguide zone are analyzed in comparison with the usually allocated near-field, intermediate and far-field zones. Technique of CSAMT data correction in the intermediate zone and an experimental example of its using are described.