Electromagnetic induction spectroscopy for identifying hidden objects

Abstract

A spectrogram of secondary magnetic field strength as a function of frequency and spatial relationship is used to identify hidden objects. A time-varying multi-frequency primary electromagnetic field is generated, preferably over the range 100 Hz to 300 kHz, which induces a time-varying multi-frequency secondary electromagnetic field about the hidden object. The strength of the secondary field, typically inphase and quadrature, is plotted as a spectrogram over a low frequency broadband spectrum as a function of frequency and spatial relationship between the hidden object and the secondary field strength detector. From this spectrogram, indications may be had of the hidden object's characteristics such as location, size and shape, and material composition. Preferably, the measured spectrogram is compared against a library of reference spectrograms by a computer to identify the hidden object.

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