A general solution for a spherical conductor in a magnetic dipole field

Abstract

In electromagnetic (EM) prospecting for volcanogenic massive sulfide ore deposits, a significant number of the responses are associated with compact conductors. As a first approximation, these bodies are studied using a conducting sphere model. An exact solution is given for a spherical conductor excited by a magnetic dipole field in free space for arbitrary transmitter‐receiver (T-R) configurations with receiver positions inside or outside the conductor. In this general approach, it is possible to investigate the lateral attenuation of EM systems. In particular, the effects of flight‐line displacement from the center of the spherical conductor on several airborne EM responses are presented. For example, at normal flying heights, the standard Dighem system has a lateral attenuation 50 times larger than the EM-30 system (for a sphere of 100 m radius). Field results from the Clearwater deposit in New Brunswick are compared to the spherical model attenuations for the Dighem, Otter, and F-500 systems. The behavior of the total magnetic fields [Formula: see text] and [Formula: see text] inside the conductor are presented in the form of magnitude and phase contours. The [Formula: see text] amplitude was found to be approximately the same inside and outside the sphere; the [Formula: see text] amplitude, however, differs significantly in these two regions. Observations such as these may provide some guidance in subdividing anomalous inhomogeneities in future numerical modeling.