Interpretation of high-resolution low-altitude helicopter magnetometer surveys over sites contaminated with unexploded ordnance

Throughout the world, millions of acres of potentially productive land are contaminated with unexploded ordnance due to either past conflicts or to military training activities. Low-level helicopter magnetometry (HeliMag) is currently being used to rapidly survey large areas and identify regions that are potentially clear of hazardous munitions. One configuration currently in use comprises seven cesium vapor magnetometers, horizontally spaced 1.5 m apart and mounted on a boom several meters in front of a Bell 206L helicopter. Magnetometer data are collected at 400 Hz at altitudes as low as 1.5 m above the ground along transects spaced 7 meters apart. From this dense, high-resolution data, potential metallic targets as small as a 60 mm mortar are identified using manual and/or automatic target picking methods. The target picks are then used to estimate densities of potential contamination. 100% detection is generally not feasible, so that HeliMag is usually applied in a characterization rather than in a clearance mode. We describe a HeliMag survey collected over a UXO contaminated site at Yekau Lake, near Edmonton, Canada. The objective was to identify the location and extent of an 11.5 pound bomb target area at a former training range. The target density estimates derived from manual picks were strongly influenced by geology and clutter and did not reflect the underlying density of ordnance and ordnance related clutter. By fitting a dipole model to each target pick, and comparing it to the expected response of the target item, we could estimate the density of objects with similar size/shape to an 11.5 pound bomb. This analysis clearly identified an area of elevated contamination in the same region where 11.5 pound bombs were found during ground reconnaissance. In summary, the new methodology significantly improves the interpretability of HeliMag data when used for UXO site assessment.

Research Highlights► Helicopter magnetics used for rapid assessment of unexploded ordnance contamination. ► Flying height of 1.5 m or less required for high detection probability. ► A dipole model is fit to each anomaly and used to assess the UXO likelihood. ► Distribution of UXO like models more accurately reflects the actual density of UXO.