Spatial response characteristics of an electromagnetic induction sensor as it passes over a metallic object are investigated using a simple analytical technique. In this low-frequency technique, one replaces a metallic object with equivalent electric and magnetic dipoles and then applies the principles of reciprocity to obtain the induced EMF in a sensor coil. Analysis is carried out for a sensor employing rectangular coils, and the object set is confined to a sphere and a prolate spheroid. The simple approach, which is illustrated with both numerical and experimental data, is found to he adequate to understand the effect on the response characteristics of parameters such as object depth, orientation, aspect ratio, and material properties. The results clearly bring out the limitations of simple detection and location strategies and reinforce the need for more sophisticated processing to provide accurate buried-object location.
Some soils can adversely affect the operation of sensitive metal detectors widely used to detect buried landmines. Although there has been some related work in geophysics, researchers in metal detection techniques, until very recently, seem to have largely ignored the issue of problem soil. As a result, rigorous scientific investigations of how soil electromagnetic properties may affect the operation of metal detectors are lacking. Thus, there is a need for theoretical and experimental studies to clarify which electromagnetic properties are important and to what extent they affect the performance of metal detectors of various designs. The paper presents a systematic analytical framework, based on existing work in geophysics and non-destructive testing, for studying the effects of soil electromagnetic properties on the functioning of metal detectors. For this initial study the burial medium is modelled as a half-space. While soil electrical conductivity has been assumed to be real and independent of frequency, soil magnetic susceptibility has been modelled as complex and frequency dependent. Simplified versions of the analysis techniques have been applied to three selected cases of practical importance, namely, non-conducting soil with constant susceptibility, non-conducting soil with frequency-dependent susceptibility and non-magnetic soil with constant conductivity. Results from a preliminary analysis of even these simple cases explain a number of previous experimental observations, for example, the greater influence of magnetic properties than of electrical conductivity on the performance of metal detectors.
A solution is presented to the problem of magnetostatic location and identification of compact ferrous objects of arbitrary shape. It is shown that, in practice, the inversion of the magnetostatic dipole field or field gradient is a necessary first step toward determining object location and identity. Several iterative and noniterative methods of determining the dipole moment and location from field or gradient measurements are described and compared. It is shown that given the dipole-moment estimates, it is possible to determine the identity of the dipole source in practical situations by pattern recognition. A unique prototype total field magnetometer is described, which is capable of explicitly and accurately locating and identifying axially symmetric compact ferrous objects. It has performed well in preliminary tests using spheres and spheroids.
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