State-of-the-art towed sensor systems for detecting unexploded ordnance (UXO) include arrays of electromagnetic and/or magnetometric sensors mounted on either hand-towed or vehicle-towed carts. The survey speed of these carts is limited by the need for high spatial resolution of target signatures and the need for high sensor signal-to-noise ratios (SNR). Spatial resolution is a function of the sensor dwell time and sampling rate, and SNR is a function of motion-induced sensor noise.
The objective of this project was to develop a prototype, laboratory proof-of-concept survey sensor system called the moving belt metal detector (MBMD), which operates by increasing sensor speed over the ground while maintaining adequate sensor dwell time over the target for good SNR and reducing motion-induced sensor noise.
The MBMD uses an array of metal detection sensors mounted on a flexible belt similar to a tank track. The belt motion is synchronized with the forward survey speed so individual sensor elements remain stationary relative to the ground. In the prototype MBMD, a single pulsed transmitter coil was configured to provide a uniform magnetic field along the length of the receivers in ground contact. Individual time-domain electromagnetic induction (EMI) receivers were designed to sense a single time-gate measurement of the total metal content. Each sensor module consisted of a receiver coil, amplifier, digitizing electronics, and a low-power ultra-high frequency wireless transmitter.
Researchers determined that the MBMD speed is limited by the time-response of the metal detector and the length of the track/belt that is on the ground. Compared to a conventional vehicle-towed system, the MBMD has the potential to reduce the time required to survey a site by a factor of 5 to 10. Because the concept places the sensor in a fixed earth reference frame during the sensor’s measurement time, improved sensor performance may be realized through reduced motion-induced noise and increased signal processing averaging time. Conventional ensemble averaging, or adaptive signal processing, could be applied to sensor data. In addition, extra target dwell time would allow computer processors to do computationally intensive real-time detection and target classification.
Improving survey speeds with high-resolution spatial signatures and high SNR can lead to improved ordnance detection and characterization, as well as lower site remediation costs. The MBMD concept is potentially useful for different metal detection technologies while still achieving speed improvements. One example would be to implement an array of total field or vector magnetometers into the belt array. Because the belt array allows extra time over a target, the metal detector and magnetometer could use a time-multiplex data collection scheme, thus gathering simultaneous data over a target.