Three-Dimensional Steerable Magnetic Field (3DSMF) Sensor System for Classification of Buried Metal Targets

Most EMI sensors do not take advantage of all the available information that is inherent in the metal target response. The sensors tend to measure only a single dimension of a target response or, in the case of a spatially scanned target, try to infer a multi dimensional response. Experimental EMI sensors that do attempt to generate a 3D magnetic field and measure a 3D target response do so with magnetic field antennas that have fixed and complex spatial magnetic field distributions With a conventional pulsed EMI metal detector, a current loop transmitter is placed in the vicinity of the buried metal target, and steady current flows in the transmitter for a sufficiently long time to allow the turn-on transients in the soil to dissipate. The transmitter loop current is then turned off. According to Faraday's Law, the collapsing magnetic field induces an electromotive force (emf) in nearby conductors, such as the metal target. This emf causes eddy currents to flow in the conductor. Because there is no energy to sustain the eddy currents, they begin to decrease with a characteristic decay time that depends on the size, shape, and electrical and magnetic properties of the conductor. The decay currents generate a secondary magnetic field that is detected by a magnetic field receiver located above the ground and coupled to the transmitter via a data acquisition and control system. The direction of the magnetic field and the field strength generated by a conventional loop EMI metal detector are a complex function of the distance of the antenna to the target. As the antenna is moved over the target, data are collected from different antenna-target aspect angles. An automatic target recognition (ATR) algorithm attempts to combine the measured time or frequency signature from the target with the spatial data to perform target classification.
The original document contains color images. Rept. No., NSTD-05-693, SERDP Project MM-1314.