The Department of Defense (DoD) Base Realignment and Closure (BRAC) process requires environmental restoration and mitigation of millions of acres of former military bases to render them safe before they are returned to public use. Remediation of land contaminated with unexploded ordnance (UXO) entails the detection of buried metallic anomalies. A commonly used sensor for UXO detection is the electromagnetic induction (EMI) metal detector. Conventional EMI metal detectors, using either frequency-domain (FD) or time-domain (TD) eddy current methods, can detect large metal targets such as metal-cased high-metal content mines and UXOs at both shallow and deep depths under a wide range of environmental and soil conditions. However, other metal objects (clutter) commonly found in the environment pose a major problem. Clutter comprises a large fraction of detected anomalies and represents a major cost contribution to UXO clean-up efforts because, in the absence of anomaly classification as UXO or as clutter object, the anomaly must be remediated or dug up. Time-efficient and cost-effective remediation will be realized only when the detected metal targets can be accurately classified, preferably in real- or near real-time.
The project objectives were to develop the Three-Dimensional Steerable Magnetic Field (3DSMF) Sensor System and demonstrate that it directly measures the three components of the magnetic polarizability tensor (MPT) of a metal object for target classification. The 3DSMF Sensor System employs wide bandwidth TD EMI sensor technology originally developed for a landmine detection project for the U.S. Army using custom designed and constructed electronics optimized for metal detection sensitivity and data collection speed.
The project began with sensor concept modeling and simulation, bread-boarding, testing of the system components, and development of the control software. The prototype antenna and control electronics were then fabricated and integrated. A number of technical challenges were revealed during the integration test phase which resulted in hardware and software modifications to resolve the problems. Data analysis of laboratory testing with targets was conducted after configurations of the transmitters and receivers were finalized in 2005.
The 3DSMF Sensor System fully functioned in the laboratory with three steerable magnetic field transmitters, a ten-turn loop Z-receiver antenna, and ten-turn quadrapole X- and Y-receiver antennas. High quality laboratory data was collected with calibration objects (wire loops, spheres, rings, and plates) as well as from some axially symmetric targets (pipes). Direct measurement of the magnetic fields in addition to data analysis from receiver measurements of simple two-dimensional targets confirmed the validity of the 3DSMF concept. The project team then collected additional laboratory data for algorithm development efforts instead of undergoing field testing due to concerns about the “non-robust” nature of the prototype design.
Results show promise of confirming the 3DSMF identification concept when using libraries of target time decay constants. The spatial orientation of targets was determined at different depths using library constants which had been derived from calibration measurements made with that target at a single depth. There is also evidence that the orientation of a target can be determined even if the target is off-center of the antenna. Further data collection and analysis would characterize the effectiveness of the 3DSMF Sensor System in discriminating between different targets.
The improved target classification capability of the 3DSMF sensor system has the potential to reduce the false alarm rates associated with UXO site cleanups, leading to reductions in the cost of site remediation.