Active broadband electromagnetic detection and classification of buried naval mines

We present a new active broadband electromagnetic (EM) sensor to detect and classify buried naval mines. The sensor operates at multiple programmable frequencies suitable to a given littoral environment. Once the sensor detects a potential target, it measures the target's spectral responses over the entire operating bandwidth. The sensor then compares the measured spectrum with a library of spectra stored in mines that are known or presumed to occur in the area. Target classification based on the spectral fingerprints is called electromagnetic induction spectroscopy or EMIS. It is known that an EMIS spectrum depends on the object's shape and metal compositions in terms of electrical conductivity and magnetic permeability. Apart from detecting explosives, EMIS is the only rigorous, physics-based phenomenology for identifying buried mines. A major drawback of an active EMI sensor is its relatively short detection range. We have recently found, however, that we may able to significantly increase the range for a metallic mine embedded in a conductive medium (e.g. mines in saturated sediments), in comparison with the range for the same mine placed in the ground. We describe this potential range extension in terms of "current channeling" that occurs in the vicinity of a mine in a conductive medium. The current channeling response can be quite strong and, under some conditions, can exceed the eddy current response by nearly an order of magnitude, which can significantly increase the range for underwater mine hunting.