Insertion loss due to a magnetically permeable spherical shell covering a concentric conducting target

Abstract

A layered section in a uniform electromagnetic field is assumed to have two concentric spherical shells (in nongalvanic contact). Such targets may occur in native copper deposits viz., conducting ore bodies surrounded by a halo of altered rocks. It is found that (a) changes in characteristic properties of overburden and/or ore formations cause significant distortions in the geoelectromagnetic parameters and may change the detection sensitivity in some bands of frequencies; (b) multifrequency measurements afford the possibility of seeing beneath highly conducting layers; and (c) the variation of the shielding parameter with frequency exhibits, qualitatively, different patterns for conductive and resistive covers and may help distinguishing EM anomalies due to conductive and resistive overburden formations. The results may be useful in (1) understanding the shielding phenomenon in complex geological situations, and (2) providing background information (i.e. shielding behaviour of overlying or surrounding formations) against which geoelectromagnetic experiments can be planned.