Compact, Low-Noise Magnetic Sensor with Fluxgate (DC) and Induction (AC) Modes of Operation

Abstract

Abstract : In this SERDP project, QUASAR Federal Systems (QFS) developed and demonstrated an innovative dual-mode, fluxgate-induction sensor (FIS) that combines a fluxgate magnetometer and an electromagnetic (EM) induction sensor to sense DC magnetic (Mag) field and EM field respectively. The FIS is based on a 5 long, high-permeability magnetic core and a sensing coil that are shared by both EM and 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 has 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 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. The discrimination performance of the FIS was comparable to that achieved using the EM61-3D at the Blossom Point test grid (PD = 91%, PFA = 32%). In fluxgate mode, the FIS yields very 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 demonstrate the feasibility of using the FIS for UXO surveys, and show great 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 SNR and scanning speed.