In 2000, the ESTCP-funded project "Electromagnetic Induction and Magnetic Sensor Fusion for Enhanced UXO Target Classification (MR-199812)" was completed. This project was based on the premise that classification based on shape is central to the problem of discriminating between unexploded ordnance (UXO) and clutter. Most UXO fit a specific profile in that they are long and slender with typical length-to-diameter aspect ratios of four or five. On the other hand, many clutter items do not fit this profile. Using pulsed-induction sensor data, the project team developed a model-based estimation procedure to determine whether or not a target is likely to be a UXO. The model relies on exploiting the dependence of the induced field on target size, shape, and orientation. The final demonstration of the project was conducted on a research range that turned out to have only a limited distribution of clutter. To better measure the utility of these methods at a variety of sites, the team undertook this demonstration on a former gunnery range.
The Multi-Sensor Towed Array Detection System (MTADS) field hardware consists of a low-magnetic-signature that tows an overlapping horizontal array of modifed EM61 time-domain induction sensors. For this demonstration, both the original MTADS EM61 (one sampling gate) sensors and EM61 MkII (four sampling gates) sensors were used. The model that was applied has been described in the literature. It relies on the fact that the EM61 signal can be described as originating from an induced dipole moment at the target location. The target response can be characterized by three orthogonal response functions. For an axisymmetric object, two of these response functions are equal and there are only two unique response functions corresponding to the longitudinal and transverse responses.
MTADS magnetometer, EM61, and EM61 MkII surveys were conducted on 10 acres at the Badlands Bombing Range Impact Area on the Pine Ridge Reservation in South Dakota. Each survey was analyzed independently, and the results were compared. The early model EM61 MkII used in this survey exhibited elevated noise levels that rendered the extra data available from the additional sampling gates unusable. The manufacturer has since corrected this design problem. Using the MTADS EM61 survey data, 70 anomalies were selected that had signal levels at least twice the relatively high site background caused by heavy coverage of small metal fragments littering the range. Several discrimination methodologies were developed and applied to these 70 anomalies. Each result was compared to the results of the magnetometer analysis.
There are two lessons to be learned from this demonstration. At a live site with heavy coverage of small fragments and clutter, time-domain electromagnetic induction methods are limited by the high background at early times. The obvious solution is to record the signal later in the decay although this introduces a host of problems being addressed in several SERDP and ESTCP projects. Second, measurement or control of sensor platform motion is vital if one hopes to achieve good classification performance. (Project Completed - 2003)