Abstract

The electromagnetic fields inside the earth are calculated by continuation downward of the electromagnetic (EM) solutions at the location of a frequency-domain helicopter electromagnetic (HEM) sensor. Models examined using the continuation approach include a layered isotropic and anisotropic earth. The finite-element approach is used to model 2D structures of a dipping contact or a dipping dike. By incorporating a time factor, we display the EM diffusion in the earth (change in direction and amplitude of the EM field through time) as 3D animated vectors or contours. The propagation of the EM smoke ring, influenced by the resistivity and structure of the earth, is apparent from the dynamic presentations. The current propagates downward and outward with time, becoming wider and more diffuse, and the phase varies with time, depth, and outward distance. The downward propagation of EM fields is slower in more conductive geology. In a layered isotropic earth, the current ring is symmetric with no vertical current flow for both vertical and horizontal dipole transmitters. In anisotropic or 2D structures; however, the current flow is significantly distorted resulting in vertical current flow and nonsymmetric smoke rings.

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