Current airborne unexploded ordnance (UXO) detection is dominated by Cesium (Cs)-vapor, direct current (DC) total field magnetometers. These DC magnetometers are expensive, consume high power, and provide limited information about the shape or orientation of the target. Researchers previously developed a compact induction sensor (IS) magnetometer with equivalent sensitivity to current state-of-the-art ISs. The IS is an alternating current (AC) sensor, which is insensitive to DC and near-DC noise and can provide vector magnetic information for targets. The objective of this SERDP Exploratory Development (SEED) project was to conduct a proof-of-concept study for airborne UXO detection using AC IS magnetometers.
The 3-axis IS is 6” long, 0.440” in diameter, 1.8 oz in weight for each axis, and includes a “main” tri-axial sensor, and a single axis reference sensor. The magnetic IS has the sensitivity and dynamic range of current technology, but is the same linear dimension and one-tenth the weight. These advances have made possible the first 3-axis orthogonal IS module. The sensitivity of the sensor module (10 - 20 pT/√Hz at 1 Hz) was verified during a field test in October 2003 at the Nevada Test Site. By using three orthogonal sensors, one can detect signatures from a buried UXO target in all three directions and therefore acquire information on the distance and direction of magnetic sources.
In recent years, a Helicopter-based Magnetometer Mapping System (Helimag) was developed by the Naval Research Laboratory and further modified by Sky Research. This system provides efficient digital mapping at survey rates of approximately 300-500 acres per day, with detection capabilities approaching that of ground-based methods. The Helimag system consists of a Kevlar boom mounted on a Bell 206L or MD 530F helicopter. The boom has an array of seven Cs-vapor, total-field magnetometers mounted along an axis perpendicular to the centerline of the helicopter.
Researchers mounted the 3-axis IS module and a reference sensor on Sky Research’s Helimag sensor platform to detect magnetic field distortion generated by a UXO object buried in the ground. The moving platform converts the DC magnetic signature into an AC signature with frequency components determined by the target size and orientation, as well as the speed and the flight height of the moving platform.
The sensors were then deployed on flight tests. The main sensor was mounted in the center of the boom, and the reference sensor was mounted in the far right side of the boom. The measured noise floor during these tests was 500 times higher than the sensor noise floor, approximately 10 nT/√Hz at 1 Hz. The post processed data gave an in-flight noise floor of approximately 3 nT/√Hz at 1 Hz and was able to achieve an approximate signal-to-noise ratio of 13 for the largest target (100 lb bomb). During the flight tests, the reference sensor was not rigidly mounted to the main sensor. Thus, the reference sensor was largely incoherent with the main sensor and did not provide effective cancellation. However, some cancellation of motion-induced noise was possible with data from the off-axis sensor data in the 3-axis assembly, which were coherent with the main sensor.
Following the flight tests, it became clear that the reference sensor should be mounted directly above the main sensor in a gradiometer configuration to maximize noise mitigation. Post-flight experiments were carried out to compare a lash-up IS gradiometer to the original IS magnetometer. The gradiometer, aligned to the Earth’s field, showed a factor of 10 times lower noise than the IS. The team concluded that a noise floor of 1 nT/√Hz at 1 Hz is possible to achieve during flight.
The project concluded that the IS gradiometer holds promise for airborne UXO detection. A 3-axis IS gradiometer is an alternative to the Cs-vapor magnetometer because it can be made more sensitive, allowing detection from greater heights and of smaller targets. The system is a vector sensor, enabling target detection and discrimination by providing information about the target shape and orientation. In addition, the IS gradiometer can improve flight safety and efficiency because it can be made lighter and smaller.