DC Gradiometers for Bomb Detection: Scalar Versus Vectorial

Location of UXO (unexploded ordnance) using its DC magnetic signature requires precise gradiometers: resonant scalar and vectorial fluxgate sensors are used. The needed resolution is 1 nT in 50 000 nTEar th’s field. Scalar gradiometers are very stable, but in some positions the scalar gradient is very small and the target may be overlooked. The disadvantage of the double-sensor fluxgate vectorial gradiometer is its high sensitivity to angular mismatch. This may be partly suppressed by using a 3-axial gradiometer arrangement with 6 sensors, which also gives more information from one reading. We developed field calibration procedures for such an instrument which allow the correction of misalignments down to 0.05 deg. DC gradiometers are typically used for deep and large objects. We demonstrate that they are also able to find small cluster bomb submunitions and that they are less affected by metal scrap than existing eddy-current UXO detectors. The field shape from a small bomb strongly depends on local field inclination; the magnetically hard component is not negligible, so that the signature depends also on the magnetic history and orientation of the target.

RIPKA Pavel;  LEWIS Adam;  KASPAR Petr;  VCELAK Jan; 

2007-01-29

American Scientific Publishers

JRC35457