Dual-mode, fluxgate-induction sensor for UXO detection and discrimination
We describe an innovative, dual-mode, fluxgate-induction sensor (FIS) that combines a DC magnetometer and an electromagnetic (EM) sensor. This sensor will enable one-pass unexploded ordnance (UXO) detection and discrimination using the joint measurements. The FIS is based on a 5-inch long, high-permeability magnetic core and a sensing coil that are shared by both EM and magnetic (MAG) modes. This integration makes the sensor very compact and removes the potential crosstalk problem of the core material of one sensor dominating the response of the other. A prototype receiver was developed and true serial, dual-mode operation demonstrated. The FIS achieved sensitivities of 1 nT rms in MAG mode and 0.2 pT/Hz at 1 kHz in EM mode, which compare favorably to existing commercial instruments. Triaxial dipole modeling confirms that three-component EM data are better for shape characterization than one (vertical) component. Assuming axisymmetry, inversions of three-component EM measurements of 22 cylindrical and disk-shaped targets yielded 100% correct classification of UXO-like objects (cylinders) and 38% misclassification of disks as cylinders. Degraded classification performance with increasing vertical distance may be caused by the small cross-sectional area of the FIS; an array will provide better field sampling. Triaxial dipole modeling of the MAG data confirmed its utility for detecting deeper targets using only the vertical component. Based on these results, the FIS is ready for field demonstration and validation.