Analysis of EMI scattering to support UXO discrimination: heterogeneous and multiple objects

Near field ( ~ 1 m) electromagnetic induction (EMI) sensing, from 10's of Hz up to 100's of kHz, has been successful in detecting subsurface metallic targets. However, the discrimination of buried unexploded ordinance (UXO) from innocuous objects still remains a challenging problem. The EM fields radiated by both antenna and target fall off very sharply as function ~1/R3, for a combined decay rate of ~ 1/R6. Therefore EMI sensors affect different materials and sections of the target differently, and signals depend very strongly on what parts of the target are closest to the sensor. Taking into account proximity effects is particularly important for identification and discrimination of actual UXO. The classification of unseen, buried objects, which in general is an inverse problem, requires very fast and accurate representation of the target response. To address these critical issues and to enhance of UXO identification, this paper presents very fast, rigorous ways to compute EMI scattering from a composite target. The method is based on the hybrid full method of auxiliary source (MAS) and MAS-thin skin depth approximation technique (MAS-TSA), together with modal decomposition and reduced source set techniques. For general excitation, a primary field is decomposed into the fundamental spheroidal modes on a fictious spheroid surrounding a real target. Then the problem is solved for each spheroidal mode, taking advantage of axial symmetry. Finally the total response from the target is reproduced using only a few auxiliary magnetic charges. The numerical results are given and compared with experimental data.