Numerical modeling of scattering from targets with small but non-negligible skin depths

In the urgent quest for adequate discrimination of metallic targets such as unexploded ordnance (UXO), considerable progress is being made by resorting to the electromagnetic induction (EMI) frequency range, between about 100 Hz and 300 kHz. In this frequency range, the fields may penetrate the metal target to some significant degree. Thus the authors must model electromagnetic activity within the target as well, complicated by the fact that penetration of the target will be very slight, though non-negligible, over much of the relevant parameter range. It is often grossly impractical to discretize the entire surface or volume of a target in space increments corresponding to this scale of electromagnetic activity. To deal with this problem they retain a simple integral equation formulation in scalar potential for the region outside the target, applying ∇·H=0 within the target and bringing to bear an approximation reflecting the particular nature of the fields within the shallow layer below the target surface. Examination of analytical solutions for simple geometrical shapes shows the utility of the method. They also show scaling or similarity relations that apply in the limiting case of negligible penetration of the target