Time and Frequency Domain Electromagnetic Induction Signatures of Unexploded Ordnance

This paper documents some of the progress that has been made in recent years in the application of electromagnetic induction (EMI) technology to the problem of discriminating between buried unexploded ordnance and clutter items such as exploded ordnance fragments or scrap metal. EMI classification of an unknown subsurface target is based on estimating the target's magnetic polarizability tensor from data collected above the ground. One can discriminate between different targets only if their polarizability tensors are sufficiently different. In this paper, we review the relationship between the time and frequency dependence of the polarizability tensor, evaluate the relative information content of the time and frequency domain signatures, and discuss how the sensor response affects signature measurement fidelity. Our analysis centers on simple parametric representations for the time and frequency domain EMI signatures that have been synthesized from extensive empirical studies of the EMI response of ordnance and clutter items. There are three basic parameters (amplitude factor, time constant and demagnetization factor) for each of the target's principal axes. They are related to the physical characteristics of the target and can be used for classification and discrimination.