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

Abstract

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 H xx and H zz inside the conductor are presented in the form of magnitude and phase contours. The H xx amplitude was found to be approximately the same inside and outside the sphere; the H zz amplitude, however, differs significantly in these two regions. Observations such as these may provide some guidance in subdividing anomalous inhomogeneities in future numerical modeling.