<title>Detection and identification of mines from natural magnetic and electromagnetic resonances</title>

Abstract

The detection ofland mines has two fundamental goals: The first is a high detection rate (low probability of missing a mine) and the second is a low false alarm rate. Detection of mines and mine-like objects is generally not difficult; the problem is the high false-alarm rate caused by detection of innocuous objects such as shrapnel or metal junk, or even rocks or voids in the soil. The problem is one of discrimination, not one of detection. In order to maximize the success of achieving this goal, a mine detector needs to incorporate many complementary sensor technologies and to utilize the concept ofsensor data fusion. Two subsystems employ new signal processing techniques which extract certain features from the data that are unique identifiers ofthe mines. These features are the natural magnetic and electromagnetic resonances, which form the impulse response function, or equivalently, the natural frequencies represented by poles in the complex frequency plane. For different objects these are sufficiently distinct that pattern recognition processes can be used to arrive at a probability ofa match to a particular mine.

On the low-frequency natural response of conducting and permeable targets

“…For example, we can use it for concealed-weapon identification [31] as well as for identification of buried metal mines (in which case discrimination mitigates the need to dig up each piece of anthropic metal clutter). With regard to the latter example, the effects of the lossy soil become important as the size of the metal target diminishes (this is especially relevant for plastic mines that possess very small metal content).…”

“…The resonant frequencies are therefore given by (31) where represents the th zero of the derivative of the Bessel function For nonpermeable targets, the corresponding TE -modes have lower resonant frequencies because the magnetic field components and inside the target have finite values at the boundary. For increasing permeability, however, the resonant frequencies of the TE -modes converge to those of the TM -modes given in (31) (pairs of resonances in Fig.…”

“…We discuss an approximate solution for modes with later, but let us first compare numerical results with measured decay times for a solid cylinder of radius cm, height cm, and different conductivities. The decay times derived from the measured step responses in [31]- [32] and converted into imaginary resonant frequencies are compared to the calculated ones (Table III). The slight discrepancies could be due to the fact that the true conductivities of the targets are unknown.…”

“…In [31]- [32], an impulse detector was used to measure the step response of certain canonical objects, in which the impulse response of the detector itself has been deconvolved from the measured data. Using a Helmholtz coil, the generated incident magnetic field was nearly uniform, as assumed in Section II-A.…”

“…While such analyses have been applied to the case of scattering from perfectly conducting and lowloss dielectric BOR's [21]- [24], [25]- [30], its application to calculation of natural modes for highly (but not perfectly) conducting, permeable targets is new. To validate the accuracy of the numerical data, comparisons are made with measured data [31]- [32] for several variations in material properties. Additionally, we present numerical results for the decay coefficients and natural-mode currents of other conducting and permeable targets.…”