Electromagnetics for Geophysical Exploration: Part I

Abstract

Electromagnetic exploration methods suffer seriously from the fact that many users have difficulty in visualizing the physics which is taking place in the ground to produce the survey results. In this respect, EM is unlike the gravity and magnetic methods, where most users can sketch for themselves the potential field configurations around various sources of attraction, or seismic methods where simple Snell's law ray theory offers a reasonable first approximation. Furthermore, we have not yet reached the stage where EM induction processes can be routinely simulated by computed models, so a certain amount of trained intuition is definitely required by an interpreter. The object of these notes is to assist you in visualizing EM induction processes in a qualitative manner. The quantitative mathematical theory of EM fields can be developed in a parallel manner, but examining the mathematical niceties too carefully can distract one from the essential physics. Also, several very convenient mathematical analogies with wave theory can obscure the fact that EM induction at very low frequencies has more in common with diffusive potential field processes such as thermal diffusion and with electric circuit theory than with wave processes as epitomized by optical ray theory.