Inversion-Based UXO Discrimination: Utilizing the Magnetic Multipole Response

We investigate the feasibility of using information about quadrupole and higher-order magnetic moments in UXO discrimination based on total-field magnetic data. Current state-of-practice only utilizes the dipole moment, yet there is coherent signal present in the residuals of many inversions. Part of the residual is due to multipole moments such as the quadrupole and octupole. We approximate the response of a compact source body using both dipole and quadrupole moments. Synthetic models resembling UXO and non-UXO targets, with induced magnetization distributions obtained from integral equation solutions, are used to examine the nature of the multi-pole responses. We decompose the full response of the models into separate dipole, quadrupole, and higher-order moment responses. Analyses indicate that the field due to quadrupole and other higher-ordered moment responses can be as large as 10% of the dipole response. Based on this understanding, we developed a new approach to UXO discrimination by using a weighted ratio of residual over dipole field. The ratio yields a measure of the strength of the higher-order moments relative to the dipole and indicates the possibility of an irregular shaped non-UXO item. Extensive simulation and application to field data sets show that this new discrimination criterion is viable with currently available magnetic data from UXO clearance.