Modeling the EMI Decay Response of Medium and Large UXO with Conventional and B-field Sensors

We compare the response of a conventional dB/dt EMI system (Geonics EM63) with a new system which utilizes a 3-component fluxgate B-field sensor. The B-field system has the advantage of providing decay curve information to times a factor of 5 to 10 greater than is achieved by the conventional dB/dt system. The transition from intermediate to late-time decay behavior is predicted theoretically to occur about a factor of two earlier in time for the B-field system, and this is confirmed by plotted observed data. Longer pulse times, lower transmitter frequencies, and longer decay curves are possible with the B-field system due to its wider bandwidth and lower noise at the low-frequency end of the spectrum. The decay constant for the medium and large munitions items considered here ranges from 12 to 25 msec and all decays when excited and measured with the EM63 EMI metal detector are incorrectly estimated as too small, due to the limited pulse-length and off-time intervals in the EM63 system. We propose that the improved ability of the B-field system to estimate the fundamental decay constant of a conductive permeable object will assist in UXO target discrimination, especially for situations where decay curves at earlier sample times are perturbed by ancillary scrap, or the response of magnetic earth.