This paper presents an application of a simple surface magnetic charge model to discriminate between objects of interest such as unexploded ordnance (UXO) and innocuous items, in cases when signals from buried objects are a mixture of responses from two or more items. In the low frequency (ten’s of Hertz up to several hundred’s of kHz) electromagnetic induction (EMI) sensing considered here, both conduction and displacement currents may be neglected within the medium surrounding a metallic object. Therefore, the scattered magnetic field outside the object is represented in terms of scalar potential fields, from which one can obtain all scattered magnetic fields. While these are time dependent by virtue of forcing functions or boundary conditions, they correspond in structure to static fields. Such fields are appropriately, and readily, produced mathematically by equivalent elementary magnetic charges placed on a convenient fictitious closed surface [1, 2, 3]. This forward model is physically complete in the sense that all heterogeneity, near and far field, and internal interaction effects within the object are included. It is very fast; in particular it can be implemented in inversion calculations on a PC. According to the Gauss’s law,∇⋅B=ρeq , the net flux of magnetic field through any closed surface equals the total (equivalent) magnetic charge ρeq inside the surface. The frequency spectrum of this total induced equivalent charge is used here as a discriminant. Based on measured data from two scatterers together, an iterative two step procedure is used in conjunction with the differential evolution (DE) algorithm [4, 5]. One step determines of each object’s location and orientation and the other determines the amplitudes of the responding fictitious magnetic charges. Once the objects are isolated, the total magnetic charge for each is calculated as a function of frequency and compared to cataloged/library data. Finally, blind classification analyses are performed for a single object as well as for multiple subsurface scatterers, when two objects appear simultaneously within the field of view of the sensor.


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