Response Characteristics of Transient Electromagnetic Methods for Unexploded Ordnances Considering Metal Shell Thickness and Shell Fragments

Unexploded ordnances (UXOs) present a substantial and enduring threat to the human society in their vicinity. It is therefore imperative to explore effective and efficient methods for detecting UXOs. The coincident-loop transient electromagnetic (TEM) method is commonly used as a nondestructive technique for detecting UXOs. To gain a deeper understanding of TEM responses, a finite-element time-domain (FETD) solver has been developed. This solver utilizes unstructured tetrahedral grids, enabling precise discretization of the real, complex shapes of UXOs. The FETD forward-modeling solver has been validated using the analytical method for the coincident-loop configuration. The numerical results from the landmine models show a significant impact of the metal shell thickness on the electromagnetic force (EMF), especially for a metal shell conductivity of
$10^{5}$
S/m and shell thicknesses under 5 mm. In models incorporating both the projectile and fragments, the numerical findings indicate that a fragment, when positioned 1 m away from the projectile, can generate a similar EMF response to that of the projectile during the initial stages. Consequently, the clutter generated by fragments can have adverse effects on detecting the projectile. However, if a fragment is directly above or near the projectile, the fragment may not have a detrimental impact on detecting projectiles.