Future advanced unexploded ordnance (UXO) detection and discrimination systems may require a combination of alternating current (AC) electromagnetic (EM) and direct current (DC) magnetic (Mag) measurements and 3-axis vector sensors to provide characterization of target shape and reduce the false alarm rate. The need for independent EM and Mag measurements increases system size, weight, and cost. Furthermore, a rigorously co-located measurement of the EM and Mag target response is difficult to achieve in practice. Present EM sensors used for UXO detection are predominantly air-core induction coils of order 0.5-m diameter (e.g., the EM61 coil has a size of 0.5 m x 1m, and the EM63 coil has a size of 1m x 1m), a fact that makes them bulky and difficult to integrate into a compact 3-axis configuration. To measure the DC field, a second sensor, usually an optically pumped magnetometer (OPM), is required. An OPM is a total field sensor that does not contain vector information. A fluxgate magnetometer has sufficient sensitivity for DC measurement of UXO. It is much cheaper than an OPM. However, using a separate induction sensor and fluxgate in close proximity is problematic because the signal detected will be dominated by the high permeability core and metal components of the other sensor, rather than by the target.
The objective of this project was to develop and demonstrate a single compact receiver that operates both as a fluxgate magnetometer to sense DC magnetic fields and as an electromagnetic induction sensor for AC magnetic fields, using the same high-permeability material for both sensors.
Researchers developed and demonstrated an innovative dual mode, fluxgate-induction sensor (FIS) that combined a fluxgate magnetometer and an electromagnetic induction sensor to sense DC Mag field and AC EM field respectively. The FIS is based on a 5-inch long, high-permeability magnetic core and a sensing coil that are shared by both EM and Mag modes.
The FIS has a sensitivity of 1 nT for the fluxgate and 0.2 pT/rtHz at 1 kHz for the induction sensor. It compared favorably to the Geonics EM63 system in induction mode and to a commercial fluxgate in fluxgate mode. Triaxial dipole modeling confirmed that three-component EM data are better for shape characterization than one (vertical) component.
Assuming axis symmetry, 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. The discrimination performance of the FIS was comparable to that achieved using the EM61-3D sensor at the Blossom Point test grid (probability of detection = 91%, probability of false alarm = 32%). In fluxgate mode, the FIS yielded high quality fits of the data and relatively accurate target locations and depths. Triaxial dipole modeling of the Mag data confirmed the FIS’s utility for detecting deeper targets using only the vertical component.
The dual-mode, 3-axis measurement results demonstrated the feasibility of using the FIS for UXO surveys and showed potential for one-pass surveys and reduction of false alarm rates. The compact design of the sensor coil makes it feasible to integrate an array of 3-axis sensors into a next-generation receiver, which could increase the receiver signal-to-noise ratio and scanning speed.