The present paper deals with a new inverse method for quantitatively interpreting magnetotelluric apparent resistivity and phase-lag sounding curves over horizontally stratified earth sections.The recurrent character of the general formula relating the wave impedance of an (n-1)-layered medium to that of an n-layered medium suggests the use of the method of reduction to a lower boundary plane, as originally termed by Koefoed in the case of dc resistivity soundings. The layering parameters are so directly derived by a simple iterative procedure.The method is applicable for any number of layers but only when both apparent resistivity and phase-lag sounding curves are jointly available. Moreover no sophisticated algorithm is required: a simple desk electronic calculator together with a sheet of two-layer apparent resistivity and phase-lag master curves are sufficient to reproduce earth sections which, in the range of equivalence, are all consistent with field data.

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