Bomb Detection in Magnetic Soils: AC versus DC Methods

Abstract

We compare the performance of eddy current metal detectors for finding small bombs and other items of unexploded ordnance (UXO) with the potencial of DC gradiometers of scalar and resonant type. Extensive database of results was collected during laboratory measurements and field trials performed by JRC. There are two main obstacles for bomb detection: false alarms caused by metal clutter and effect of magnetic soil and rocks. A number of soil samples were collected at mine fields and UXO affected areas. Magnetic properties of these soils were examined both in time and frequency domain. The results were compared with the detection depths measured both in soil and in air for various targets. In general, the most difficult soils show frequency dependent susceptibility, which creates signal similar to metal object. We show how to compensate such soils. DC methods give signals between 1 nT and 100 nT. Gradiometer should be used, because often even the natural variations of the Earth's field mask the signature from metal object. Scalar gradiometers need no calibration, but give less information than multi axis vectorial systems. We show Achilles'heel of scalar gradiometers which we believe has never been reported in the literature.JRC.G.6-Security technology assessmen