The theory of electromagnetic sounding of the earth, based on interference effects in the spatial structure of a pair of coupled waves, propagating along the earth's surface with the same frequency but different wavelengths, is proposed. A hitherto unknown heterogeneous structure of surface impedance, the conductivity being constant, is analysed by means of new exact analytical solutions of Maxwellian equations, described by the propagation of coupled waves from the source along the boundary between two media, the complex dielectric permittivities of both media being arbitrary. The classical formulae for Biot-Savart law and VLF impedance of conducted surface prove to be the private causes of obtained interference structure, related to the limit values of parameters. The perspectives of use of amplitude-phase effects of interference pattern, generated by direct and lateral waves, for earth sounding, especially for undersea sounding, are illustrated.