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E l e c t r o m a g n e t i c I n d u c t i o n i n N o n -u n i f o r m C o n d u c t o r s , a n d t h e D e t e r m i n a t i o n o f t h e C o n d u c t i v i t y o f t h e E a r t h f r o mT e r r e s t r i a l m a g n e t i c V a r i a t i o n s

The theory of the relationship between the tangential components of E and It for geomagnetic fluctuations over a stratified earth is extended to take account of the distribution of the ionospheric inducing field. It is shown that Cagniard's simple formulas on which magnetotelluric methods are generally based need modification to take account of the dimensions of this field. This is so even when the inducing field is much more extensive than the region under consideration and when the depth of the probe is quite moderate. It is further shown that, for deep probing, magnetotelluric methods can be satisfactorily applied only if an analysis of the field over a region having dimensions comparable with those of the inducing field is first made. The relation between these methods and the earlier methods of determining the conductivity distribution from analyses of the components of the surface magnetic field is discussed. The evaluation of the amplitude and phase relations of E and It over the oceans is also discussed, and it is shown that some results obtained recently by Fonarev need extending and amending.

The two-dimensional problems of interest in studying the perturbation of alternating electric current by a sharp discontinuity of conductivity in a conductor are considered, and their applicability to geophysical problems discussed. A numerical method has been developed for solving the appropriate differential equations and boundary conditions. The method has been applied to a vertical discontinuity in conductivity such as at a continental-oceanic interface. The two polarization cases are solved, and the fields and current distributions are determined in detail.