Subsurface discrimination using electromagnetic induction sensors

This paper reviews the problem of subsurface discrimination using electromagnetic induction (EMI) sensors. Typically, discrimination is based on differences in the multiaxis magnetic polarizability between different objects. We review work on frequency and time domain systems, and their interrelationship. We present the results of comprehensive measurements of the multiaxis EMI response of a variety of inert ordnance items, ordnance fragments, and scrap metal pieces recovered from firing ranges, The extent to which the distributions of the eigenvalues of magnetic polarizability for the different classes of objects do not overlap establishes an upper bound on discrimination. For various reasons, the eigenvalues cannot always be accurately determined using data collected above a buried target. This tends to increase the overlap of the distributions, and hence degrade discrimination performance.