A Discrimination Algorithm for UXO Using Time Domain Electromagnetics

Abstract

An assumption is made that the Time Domain Electromagnetic (TEM) response of a buried axisymmetric metallic object can be modelled as the sum of two dipoles centered at the midpoint of the body. The strength of the dipoles depends upon the relative orientation between the object and the source field, and also upon the shape and physical properties of the body. Upon termination of the source field, each dipole is assumed to decay as k(t+α)−βe−t∕γ. The parameters k, α, β and γ depend upon the conductivity, permeability, size and shape of the object, and these can be extracted from the measurements by using a nonlinear parametric inversion algorithm. Investigations carried out using an analytic solution for a sphere and laboratory measurements of steel and aluminum rectangular prisms, suggest the following two-step methodology: (1) The value of β is first used as a diagnostic to assess whether the metallic object is non-magnetic or magnetic, (2) the ratios of k1∕k2 and β1∕β2 are then diagnostic indicators as to whether the geometry is plate-like (uninteresting) or rod-like (a high candidate for being a UXO). Results from the application of this algorithm to a TEM field data set acquired at the United States Army Corps of Engineers Research and Development Centre (ERDC) UXO Test Site have successfully identified a UXO to be magnetic and rod-like.