Ideal sensors for electromagnetic induction (EMI) surveys conducted for unexploded ordnance (UXO) detection and classification have high sensitivity over an ultra-wideband (DC to hundreds of kHz), measure all polarization combinations, and are inexpensive and easy to manufacture. Ideal sensors are also small enough to facilitate pinpoint measurements rather than averaging over a large area (as with large coils) and lend themselves to deployment in large arrays. Although magnetoresistive (MR) sensors possess most of these ideal attributes, a decade ago they did not have sufficient sensitivity for most geophysical applications.
The objective of this SERDP Exploratory Development (SEED) project is to show that magneto-resistive sensors are suitable for use in EMI and magnetometer instruments for UXO detection and discrimination.
Noise and sensitivity measurements were conducted in the frequency domain using a Helmholz coil. Magnetic biasing, electrical bridge biasing, chopper techniques, and open and closed loop configurations were investigated. The optimal configuration produced a noise density of 6 pT/√Hz for frequencies above 200 Hz. At 1 Hz, the noise density is approximately 30 pT/√Hz. These results were obtained in both open loop and closed loop operation.
A laboratory prototype instrument was built using the magneto-resistive sensors to test the magnetometer and EM induction response to actual UXO targets and compare it with readings from similar instruments. When operating in magnetometer mode, the prototype detected an 81 mm target at a distance of over two meters. When operating in time-domain EM induction mode, measurements could be made as early as 20 µs after the transmitter turn-off. Instruments using coils as receivers typically can only receive signals after 100 µs. The induced currents at early-time provide more information about the shape of the target than the late-time currents. Furthermore, the small size of the magneto-resistive sensor does not spatially average the received signal like a large coil does. These sensor attributes provide more information for discrimination than conventional EM induction instruments.
The results of this investigation show that a man-portable instrument similar in physical configuration to the metal detectors commonly used by the public on beaches can be built for UXO detection and discrimination. The instrument can be operated as both a magnetometer to detect targets and in EM induction mode for discrimination. The small size and low-cost enables the production of portable array systems with many receivers that are similar to the ‘advanced’ discrimination systems available such as the MetalMapper.
The primary recommendation for further development is to build a man-portable instrument for UXO detection and ‘advanced’ discrimination. The unique functionality of anisotropic magneto-resistance (AMR) sensors facilitate ‘advanced’ (i.e. multi-sensor, multi-polarization) EMI measurements in a portable instrument that can access most terrain types and navigate heavy vegetation. The envisioned man-portable instrument contains the EMI instrument, magnetometer, navigation instruments, and an acquisition personal computer. It is also recommended that future research examine methods for reducing magnetic noise. Finally, it is possible to increase AMR sensitivity with reverse magnetic biasing of the permalloy film. Although this is not practical with the Honeywell sensors, it is recommended for future research.