I. Introduction
The investigation of electromagnetic induction (EMI) problems involving highly conducting and permeable bodies has been a subject of intensive study and research for many years, with new momentum in the present provided by subsurface sensing applications. Researchers are currently investigating the application of low-frequency broad-band induction (30 Hz to 25 kHz) for detection and identification of buried unexploded ordnance (UXO) (e.g., [1]). This has driven the development of new analyses and analytical tools for EMI scattering [2], [3]. The broad-band EMI response of a conducting and permeable sphere is well established [4]. A quasi-magnetostatic solution has been developed only recently for the conducting and permeable spheroid [5], [6]. Numerical methods have arisen to address the kind of problem we consider here, using 3-D Method of Moments (MoM) based on Magnetic Field Integral Equations (MFIE), which incorporated an impedance boundary condition (IBC) [7], MoM for body of revolution (BOR) [2], and MoM-BOR with hybrid finite element–boundary element formulations (FEM–BEM) [8], [9]. In the first case, the authors combined the MFIE with IBC, and solved for surface electric current. This approach has a limited and largely untested range of validity, in terms of (frequency-dependent) skin depth relative to object dimensions. Further, the method in [7] does not solve for the internal fields and the method is limited in this respect. We are quite interested in internal fields and currents in order to understand the underlying mechanisms of response and their relation to object shape and composition. This is at the heart of the progress needed for advances in discrimination, and can readily be investigated by the Method of Auxiliary Sources (MAS).