Combined differential evolution and surface magnetic charge model algorithm for discrimination of UXO from non-UXO items: simple and general inversions

Abstract

This paper presents an application of a combined differential evolution (DE) and surface magnetic charge (SMC) model to discriminate objects of interest, such as unexploded ordnance (UXO), from innocuous items. In entire electromagnetic induction (EMI) sensing considered here (tens of Hertz up to several hundreds of kHz), the scattered magnetic field outside the object can be represented in terms of scalar magnetic potential, from which one can obtain all scattered magnetic fields. Such fields are appropriately and readily produced mathematically by equivalent magnetic charges. The amplitudes of these charges are determined from measurement data. The surface magnetic charge model takes into account the scatterer's heterogeneity and near- and far-field effects. It is very fast and simple to implement in EMI inverse scattering algorithms. For simplification of discrimination algorithms, the frequency spectrum of the total normalized equivalent charge is investigated here as a discriminant. Two inversions scenarios are discussed: 1. Simple, when we assume that a buried object's location and orientation are known but its identity is not; and 2. General when both identity and all positional parameters are unknown. In the first case, because the task is only to identify the object, only the SMC model is required and this serves as a test of it alone. In the second case the combined DE and SMC model approach is required for identifying the object as well as its location and orientation. In this case an iterative two-step inversion procedure is used together with measured data. One step calculates an object's location and orientation, and the other calculates the amplitudes of the responding fictitious magnetic charges. Once the object's location, orientation, and spectrum of total magnetic charge are all determined, then that spectrum is compared to cataloged library data for UXO's of interest. To illustrate the applicability of the combined DE and SMC algorithm for UXO discrimination, first a simple inversion methodology is given for an actual UXO and then a general inversion approach is tested for a single object.