The fundamental model of magnetotelluric sounding

It is widely believed that the Tikhonov-Cagniard model can be applied in the magnetotelluric method of exploration geophysics if the horizontal field changes are sufficiently slow. Actually, this is overly restrictive and may give rise to unjustified doubts about the validity of the Tikhonov-Cagniard model. This paper shows that the condition determining the applicability of the model is the linearity of the horizontal field changes, rather than the slowness of these changes. This considerably extends the sphere of practical applications of the Tikhonov-Cagniard model.