Cost-effective reconnaissance methods are needed to identify bombing targets, impact areas, and other unexploded ordnance (UXO)-contaminated areas that occur within larger uncharacterized Department of Defense (DoD) sites. Airborne magnetic surveys can accomplish this goal at some sites, as established by the demonstrations of ESTCP project MR-200037 at Badlands Bombing Range (BBR) in South Dakota. The success at BBR could not be achieved at all sites where UXO characterization is required because of geologic noise (e.g., in basaltic areas), non-magnetic metallic ordnance, or other detrimental effects. The objective of this project was to evaluate an airborne high-resolution time-domain electromagnetic system for the detection and mapping of probable UXO-related contamination.
The Oak Ridge Airborne Geophysical System Time-domain Electromagnetic (ORAGS-TEM) system is based on a single transmitter coil and two receiver coils mounted on a rigid 12 m x 3 m rectangular boom structure rigidly fixed to the airframe of a commercial helicopter. The receivers are located at the ends of the boom structure. Ancillary equipment included a laser altimeter and a real-time differentially corrected global positioning system (GPS) for navigation and data positioning. This configuration enabled operation at a nominal flight altitude of 1 to 3 meters above ground level. The survey methodology consisted of parallel flight lines, interleaved to achieve full coverage. The survey process concluded with data processing, analysis, interpretation, and mapping using commercial software to generate digital images depicting locations and magnitudes of electromagnetic anomalies indicative of metal concentrations that may represent UXO.
To validate the detection capabilities of the system, several controlled test sites developed under previous ESTCP-funded projects were surveyed in addition to surveys conducted on actual UXO-contaminated sites at BBR. Seeded items included engineering items, inert ordnance, and simulants that were selected to bracket the expected detection parameters of the system. Actual ordnance items at the survey sites represented a limited range of ordnance—M38 practice bombs, 105-mm projectiles, 81-mm mortar rounds, and 2.75-inch rockets. Under favorable field conditions, the ORAGS-TEM was able to detect ordnance items as small as were detectable with the more mature helicopter total magnetic field system (i.e., 60-mm rounds). In field operation, it is anticipated that a smaller percentage of 60-mm rounds would be detected and that this sensitivity is more dependent on altitude than for magnetometer systems. Several configurations of the ORAGS-TEM were tested with two different coil sizes. At survey altitudes below 1.5 m, the small multiple turn receiver coils in the vertical gradient configuration produced the highest signal-to-noise over most ordnance, but the advantage of the gradient configuration was lost at higher survey heights because gradient fields decay more rapidly than single coil responses. Furthermore, the large 3 m x 3 m receiver loop produced equivalent or higher signal-to-noise than the small coils at 3-m survey altitude. Responses from ordnance measured at a 3-m survey altitude dropped below background for most mid- and small-sized ordnance, irrespective of transmitter-receiver configuration.
Large ranges, such as those located in the arid regions of the Western United States, will benefit most from the application of airborne electromagnetic characterization. The sparse vegetation at arid and semi-arid sites permits the airborne systems to be flown closer to the ground, enabling improved target detection. Airborne electromagnetic systems for UXO detection will augment magnetic systems at some ranges to reduce the number of false positives and will provide an alternative to magnetic systems when impacted by geologic conditions or other effects. (Project Completed - 2007)