The environmental problem addressed with this technology concerns the task of detecting buried unexploded ordnance (UXO) in a cluttered site, with discrimination between them. A technology that can accomplish UXO detection with clutter discrimination in an efficient manner (high production rates, low cost of operation) would provide the Department of Defense with a tool that could have great impact on the cost of UXO cleanup.
The coaxial coil configuration electromagnetic induction (EMI) sensor is motivated by the potential advantages of the common mode rejection of electromagnetic noise from external sources. The balanced differential receiver (gradient) measurements reject voltages induced by noise fields that are uniform over distances on the scale of the receiver coil separation, including natural sources such as ferics (distant lightning induced), geomagnetic storms (sun spot induced), platform motion in the geomagnetic field, and man-made sources such as power line fields. The platform motion induced noise has been shown to be particularly problematic for the vehicular towed concentric-coil system (GEM-3) in the operational frequencies below 100 Hz. One penalty paid with the coaxial geometry is an increased height of the transmitter coil, reducing the excitation field strength over the target. There is a trade-off between increasing the coil separation to increase the difference signal and reducing the separation to reduce the transmitter-target distance, and the design must provide a good compromise for the anticipated target depth envelope. Also, small separations pose an engineering challenge at achieving adequate bucking (receiver coil balance).
The objectives of this demonstration were to show that data were not significantly degraded by motion on a wheeled-cart platform from normal motion during surveying and that there was greater immunity to ambient noise than more conventional EMI sensors. Standard performance goals for EMI sensors in terms of probability of detection (Pd) and false alarm rate (FAR) also apply.
This project demonstrated the validity of the hypotheses that the coaxial configuration was more immune to both motion-induced and ambient noise by showing little increase in noise between static and dynamic (rolling cart) conditions and showing less noise from an adjacent transmitter than observed with a GEM-3. Depending on target depth, the static signal-to-noise ratio (SNR) is sometimes better for the GEM-3 owing to the transmitter coil distance. Pd, probability of false alarm (Pf), and other metrics for static (stopping over grid points) and dynamic (continuous rolling) conditions were calculated.
Decision-making issues associated with the applicability of the coaxial-coil EMI technology for end users’ specific UXO detection and discrimination mission needs were addressed. Operational performance under static and dynamic conditions was assessed and comparisons made with existing concentric-coil systems (GEM-3).