As a result of past military training and weapons testing activities, an estimated 6 million hectares (ha) (approximately 15 million acres) of U.S. land is potentially contaminated with unexploded ordnance (UXO) and/or weapons testing- and training-related artifacts. These contaminated areas include sites designated for base realignment and closure (BRAC) and Formerly Used Defense Sites (FUDS). Using current technologies, costs associated with the detection, identification, and mapping of this contamination have been estimated to be in the tens of billions of dollars. Current surface-based technologies are generally labor intensive, slow, and expensive. Significant cost savings could be achieved if it is demonstrated that advanced airborne methods can provide a substitute for a portion of the surface-based applications. Typically, airborne magnetometers have not been used for UXO detection due to limitations in the physics and an inability to position the magnetic sensors in close proximity to the targets at or beneath the earth’s surface. Recent demonstrations and advances in airborne magnetic systems have led to significantly improved performance over prior generation airborne systems. Although airborne systems do not match the resolution and sensitivity of ground-based surveys, an airborne approach provides the option for personnel to conduct surveys without contacting potentially explosive devices and offers a relatively nonintrusive approach by reducing the disturbance of indigenous plant and animal habitat that often accompanies ground geophysical activities (i.e., brush cutting).
The fourth-generation airborne system developed and utilized in these demonstrations was based on eight airborne-quality cesium vapor magnetometers mounted in three rigid 6 m booms (one forward, two lateral) that are mounted to the airframe of a commercial helicopter. 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 m above ground level (AGL). The survey methodology consisted of parallel lines traversing the areas of interest with the survey lines adjacent to one another (as opposed to being interleaved as with the second-generation system) so that eight traces of total magnetic field data were collected for each flight line, providing a nominal data spacing of 1.75 m with a flight line spacing of 12 m. The survey process concludes with data processing, analysis, interpretation, and mapping using commercial software to generate digital images depicting locations and magnitudes of anomalies that may represent UXO.
The objective of this project was to evaluate an improved airborne high-resolution magnetic system for the detection and mapping of probable UXO-related contamination by validating the detection and characterization of ordnance and ordnance-related debris at large previously unsurveyed areas and at controlled test sites. These demonstration surveys produced results confirming that this improved airborne magnetometer technology is both practical and cost-effective for the detection and mapping of certain categories of UXO as well as wide-area surveillance associated with footprint reduction activities.
To validate the detection capabilities of the system, several controlled test sites (calibration sites) developed under previous ESTCP-funded projects or other DoD-funded projects were surveyed in addition to surveys conducted on actual UXO-contaminated sites (e.g., Aberdeen Proving Ground, Badlands Bombing Range, Sierra Army Deport, Nomans Land Island). 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 included all manner of ordnance ranging from 60-mm mortar rounds up to 1,000-lb general purpose air-deployed bombs. Detection rates varied with the size of the targets and site conditions. Results show that the system typically achieves detection rates of better than 70% (and sometimes 100%) for larger ordnance, while rates of 30-70% are more typical for 60-mm, 81-mm, and smaller items. The rate of coverage for the surveys ranged between about 40 and 140 acres/hr (16-57 ha/hr), and the average survey speed was about 20 m/s, except where the survey area was too small for efficient operation. The average distance between the actual locations of the excavated items and the predicted locations from helicopter anomalies was consistently less than 1 m. Noise levels were typically 1-3 nanotesla (nT) in the raw data and less than 0.1 nT in the filtered data.
Issues related to these demonstrations center on the appropriate use of the technology. Clearly, the improved airborne system is unable to detect all UXO items of potential interest. The technology continues to be constrained by the presence of tall vegetation and rough terrain that increases the distance between the system and the UXO items of interest, thereby limiting detection ability. It remains apparent that application of the technology to small survey areas will not be cost-effective due to the large cost associated with mobilization/demobilization and considerable helicopter costs. Users should consider both the intended UXO targets and survey area (size, terrain, and vegetation) before considering the use of airborne systems for UXO detection, mapping, and/or footprint reduction.