This paper discusses the ability of time and frequency domain electromagnetic induction systems to discriminate unexploded ordnance from clutter. Toward this end, time and frequency domain electromagnetic induction systems were built and the responses of a wide variety of targets including loops, spheres, cylinders and inert UXOs were measured. Also, time and frequency responses of test targets are numerically modeled using finite element methods to validate the experimental work. Target information is more distinct in the frequency domain than time domain. Moreover, discrimination performance of the frequency domain electromagnetic induction system was enhanced by almost a factor of two when the usual the low frequency spectrum (30 Hz to 24 kHz) was extended down to extremely low frequencies (1 Hz to 30 Hz). However, data acquisition at extremely low frequencies is a time consuming process especially if data averaging is required to achieve acceptable SNR. Therefore, in practice, it would be better to have two operating modes when using a frequency domain electromagnetic induction system; one with very few operating frequencies and the other operating in the entire band (1 Hz to 24 kHz). Once a target location is marked using the first mode, the system can be used as a “cued” sensor in the second mode, thus improving the discrimination.
Extremely low frequency measurements, below 30 Hz, of solid, thin-, and, thick-walled steel (permeable) cylinders with length-to-diameter ratios of approximately 4 are described and compared with the predicted response computed using a frequency domain finite element method (FDFEM). Measurements were made using a conventional EMI test setup consisting of a Hewlett Packard 89410 vector signal analyzer, rectangular transmitting and a figure-eight (bucked) receiving coil, along with appropriate transmitter and receiver coil amplifiers. All cylinders were measured with the predominant component of the excitatory magnetic field both aligned with and orthogonal to (two distinct measurements) the cylinder's axis. Measurements were made with and without a centered copper ring on the cylinders. The ring simulates the so-called rotating bands on actual UXO. Not surprisingly, we observed that the quadrature peak of the response shifts down in frequency much more when the axis of the ringed cylinder is aligned with the excitatory magnetic field than when perpendicular to it. Our measurements indicated that the real part of the response of the smallest cylinders measured asymptotically approaches its DC value around 1 Hz while the largest of the cylinders measured does not asymptote until well below 1 Hz. It appears that target information that may be crucial for discrimination purposes, especially for larger targets, exists at frequencies well below 30 Hz. Extremely low frequency measurements, especially with data averaging (stacking), can be a rather time consuming process, and therefore it is not likely that such measurements can be made from a moving platform. However, once an object of interest has been detected, the target can be reacquired and the measurement taken with the sensor stationary with respect to the target (sometimes referred to as a qued approach). As our measurements and simulations indicate, the qued method may be necessary if large solid UXO are to be distinguished from large thin-walled clutter objects.