A Generalized Representation of the Electromagnetic Fields In A Layered Earth

A generalized representation of the electromagnetic (EM) fields due to an arbitrary source in a layered earth is developed in terms of toroidal and poloidal modes. This modal potential representation is examined in detail and it is shown that EM Green's functions for stratified earth can always be factored into ‘geometrical’ operators and ‘electrical’ kernels. the electrical kernels represent the physical expansion of the EM field in the layered earth and are independent of the survey geometry. Conversely, the geometrical operator is independent of the earth model and represents the source/receiver configuration. the geometrical operators are essentially frequency-independent weighting functions which act on the electric kernels. Thus, changing the source-receiver geometry only serves to change the ‘window’ through which we observe the fundamental physics of the interaction of the EM fields with the layered earth. the factored representation of the EM fields is illustrated with examples from surface electrical prospecting methods.