Model-Based UXO Classification Based On Static 3-Component TEM Measurements

The dipole model characterized by an anisotropic polarizability tensor is well accepted as a valid model describing the EM behavior of electrically small, highly conductive metal objects such as UXO. The model has been used successfully to characterize and classify EM anomalies acquired with single-gate TEM metal detectors such as the EM-61. Successful inversion of EM-61 anomalies using this dipole model depends on measurements at many points within a small radius of the target. Since these measurements are acquired dynamically, survey specifications in terms of line-spacing and survey speed must be tight in order to be assured of acquiring a sufficient number of independent data to robustly parameterize the target for classification.

An alternate method of characterizing an anomaly is to reacquire it and to take more precise data by locating the antenna array at a few discrete stations in a pattern referenced to the center of the observed anomaly. In this paper, we describe experiments in UXO characterization using a multi-gate 3-component fast TEM (NanoTEM) system. With this system, three orthogonal receiver antennas simultaneously acquire 31-gate TEM transients. The 3-component data triple the number of independent data measurements supplied at each field point. Using this system, we have acquired data sets using two methodologies. In the first methodology, we take measurements with a 3-component cart system at 5 locations centered on the anomaly peak, thus acquiring 15 31-gate transients for use in the dipole inversion. In the second methodology, we use an array of flat-lying loops arranged to illuminate in 3 orthogonal directions and measure the target's polarization response over a range of angles. Both data sets assure that the UXO has been polarized in its 3 principal directions. The dipole model simultaneously models both time and spatial components of the measured fields and reports a three-dimensional target position, spatial attitude, and polarizability parameters (i.e., the "beta" parameters) as a function of time. Results from characterization of various UXO and non-UXO targets models buried locally in Tucson and from the NRL Baseline Ordnance Classification Test site at Blossom Pt will be used to illustrate the technique.