Time - Domain Measurements and Modeling of Uxo

A commercial finite-element engineering-analysis code was used to model pulsed-induction (time-domain) sensing of UXO for comparison with measurements made with the Geonics EM61-3D. Late-time decay constants are observed to be independent of shape and orientation in solid nonpermeable (aluminum) objects with the same volume, in agreement with theory, except for flattened objects on end with respect to the primary field. Solid permeable (steel) objects are strongly influenced by the amount of metal inline with the primary field. Hollow interiors are manifested as a straight-line semilog decay, indicating that the eddy-current system has collapsed to a single mode. The time until collapse is given by the diffusion time through the wall thickness. Complexly shaped, material-composite UXO show complex eddy-current patterns, but the measured magnetic field or voltage is dominated by polarization in steel portions of the target. In an 81-mm projectile, for example, strong eddy currents circulate in the fin shroud in early time and decay rapidly, but a model without the fin shroud showed little difference in receiver voltage. Large magnetic inductions are present in the permeable steel body in both models and decay slowly, supported only by small eddy currents. Although there are some shifts in the loci of maximum induction, the generally smooth penetration and decay of magnetic fields in the target accounts for the relative success of simple analytic and empirical models in reproducing EM measurements of ferrous UXO.