Accurate Measurement of Characteristic Response for Unexploded Ordnance With Transient Electromagnetic System

Characteristic response parameterized in time-dependent target intrinsic polarizabilities has been widely used to create feature libraries for unexploded ordnance (UXO) discrimination. This paper discusses the accurate measurement of characteristic response for UXOs with transient electromagnetic (TEM) system. First, a specially designed TEM system, which utilizes a modified solenoid and a pair of rectangular coils as transmitting coils to ensure the uniformity of primary field up to 96% throughout the target, is described. Then, the characteristic responses of 12 UXOs are measured from directions vertical or parallel to the axis at distances from 40 to 120 cm. The measured characteristic responses show a power law decay function at an early time and an exponential decay function at a later time. The characteristic responses parallel to the target axis are larger, and the decay is considerably slower than those vertical to the axis, leading to one to two orders of magnitude difference at a later time. Results also show that the characteristic response measured from the direction vertical to the axis of the target is more precise than that from the direction parallel to the axis at the same distance. The portion of the target that is close to the system accounts for a large proportion of the response. A target can be approximated as a single dipole only when it is located no less than twice its length from the system. Finally, the characteristic responses of 12 targets are parameterized with the Pasion-Oldenburg expression for feature library creation.