Cost-effective wide-area reconnaissance methods are needed to identify bombing targets and other unexploded ordnance (UXO)-laden areas that occur within larger uncharacterized areas. Airborne geophysical surveys can accomplish this goal at some sites. However, success cannot be achieved at all sites where UXO characterization is required because the stand-off distance is too great between the sensors and the ordnance as a result of vegetation or terrain; small ordnance items are below the detection threshold of current cesium vapor magnetometers; or other detrimental effects such as background geology that raise the noise floor or reduce target signal strength.
The increasing availability of robust Superconducting QUantum Interference Device (SQUID) technologies offers an opportunity to develop airborne sensors that have greater resolution and vector analysis capabilities. Such capabilities would open up more areas to wide-area assessment at lower cost. The objective of this project was to develop and demonstrate an airborne full-tensor magnetic gradiometer for detection and precision mapping of UXO.
The system design is based on a liquid nitrogen-cooled high-temperature SQUID developed in a project funded by the U.S. Department of Energy and the U.S. Navy’s Explosive Ordnance Disposal Technology Division. Under this SERDP project, researchers designed and tested the SQUID sensor and characterized the noise signatures in flight. Processing and analysis tools for the tensor data were developed to reduce or remove the noise signatures and to maximize the detection thresholds at large sensor-target offsets.
The high-temperature SQUID functioned well in a stationary mode. Noise levels in both shielded and unshielded environments were low, and response linearity was good. Sensor orthogonality was not as good, but even the worst combination of elements appeared to be within 0.5°. Sample UXO targets were detected at considerable distances with good signal-to-noise ratios. Vibration tests of the helicopter booms and rotor noise studies were all within reasonable limits. Data processing and inversion algorithms were researched and tested with synthetic data and shown to be successful.
The transition to a moving platform, however, was ultimately unsuccessful. The mechanisms designed to allow it to function within a larger range of field values were sound and have been successfully deployed in other systems. With this system, the mechanisms introduced unusual artifacts into the data, which negated the possibility of completing basic calibration procedures. Even when avoiding these artifacts, continuing problems with random data shifts introduced so much noise that results were generally poorer than the supplementary fluxgate magnetometer. The lack of an absolute reference made quantitative analysis impossible for all but the strongest anomalies. The final result is that the operational noise level is so high that it outweighs the additional benefit of interpreting full tensor data for UXO detection.
Ongoing research in the field indicates that a low-temperature (liquid helium) intrinsic gradiometer system would be better suited to a moving platform despite the added complexity of a pressurized cryogenic vessel. Such a system is operated in a towed-bird helicopter and stinger-mounted fixed-wing configuration. Its use as an electromagnetic receiver may also produce some benefit by virtue of operating in the system’s lowest noise bandwidth.