The detection of small munitions is particularly difficult because of the fine spatial sampling required to detect them. The geophysical systems typically used for ordnance detection generally are not deployed in an array adequate for detecting small targets or do not have the necessary resolution. Giant magnetoresitive (GMR) sensors are small (order of 1 centimeter) and could be combined in a compact, lightweight array. The objective of this project was to characterize the fundamental capabilities of GMR sensors, particularly the noise floor, and determine the expected performance in terms of signal-to-noise at specified distances for a variety of munitions types.
GMR sensors manufactured by Honeywell and NVE Corporation were evaluated to identify differences in performance characteristics (sensitivity, bandwidth, saturation, hysteresis, etc.) and features. The sensors evaluated were the Honeywell HMC100x series and the NVE AA00x series. These GMR sensors have similar operating range and sensitivity specifications that are suitable for geophysical applications. Regardless of sensor type, there is a tradeoff between linear operating range (or dynamic range) and sensitivity; increased sensitivity is associated with decreased dynamic range. The sensitivity of a GMR sensor operating in passive mode can be enhanced by use of a flux concentrator (a strip of high permeability material that aids in channeling magnetic flux into the sensor).
GMR sensors were evaluated for use as both static (magnetometer) and pulsed magnetic field measurements. As passive magnetometers, researchers demonstrated that both the Honeywell and NVE sensors performed similarly and, performance-wise, there is no preference for one over the other. However, the Honeywell GMR sensor does offer two features that make it desirable, Set/Reset strap and OFFSET strap. These features aid with, among other things, improving linearity and sensor biasing. Also, Honeywell offers the HMR2300 Smart Digital Magnetometer, a three-component sensor, which incorporates the HMC100x GMR sensors. The HMR2300 package is compact and has an onboard analog-to-digital converter, serial interface, and selectable sample rate. The Honeywell sensors are recommended as the preferred GMR sensor for magnetic measurements because of these additional features. Because of the signal attenuation feature offered by the Honeywell HMR2300, the HMR2300 can only be used for passive measurements and not for active GMR measurements.
Researchers showed that a GMR sensor can be used in active mode (pulsed or continuous wave), i.e., as a receiver sensor in combination with a coil transmitter, to measure the eddy current response of a target. The late-time response of a single-turn copper wire, multi-turn wire, and solid copper sphere were measured. Three distinct decay rates were observed. The sensitivity of a GMR sensor operating in active mode can be biased by using a static (permanent magnet) or time varying magnetic field, or combination of the two. It is important to note that measuring the eddy current response of ferrous targets with a GMR sensor is more difficult than measuring the eddy current response of non-ferrous targets because ferrous targets, in general, have a non-zero direct current (DC) response. A ferrous target near the GMR sensor will upset the bias, making stable eddy current measurements more difficult. The project team did not explore ways to mitigate this problem; however a high-pass filter with a suitably low 3 dB cutoff frequency is a possible method for resolving this issue.
The performance of GMR sensors is comparable to magnetometers typically used for geophysical applications. The GMR sensor studied has a resolution < 7 nT, which is generally suitable for near-surface geophysical purposes. A unique feature of a GMR sensor is its ability to detect magnetic fields over a wide frequency range, from DC to several megahertz. Thus, a single GMR sensor is capable of measuring responses presently requiring both a magnetometer and a frequency-domain or time-domain electromagnetic induction sensor. Further research is required to evaluate the combined static and dynamic aspects of a GMR sensor and to evaluate its use in arrays for unexploded ordnance discrimination and identification.