Extremely low-frequency response (below 30 Hz) of UXO-like objects

Abstract

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 filed 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.