Magnetic Response of Clustered UXO Targets

Abstract

The objective of many recent UXO surveys has been described as “wide-area assessment” with the purpose of obtaining better definition of a known problem area. The targets of interest are clusters of ordnance, fragments and debris which are all indicators of greater contamination, higher risk of UXO hazard and higher remediation or construction costs. This is a different problem from the detection and discrimination of individual anomalies. This paper provides a definition of a “cluster” based on the amount of overlap between individual dipole signatures. In total field surveys, magnetic anomalies overlap significantly and show an increased amplitude response once the individual sources are spaced closer than 0.5 times the sensor height. When this condition is extended over a large area, such as the center of a target site, the result can be comparable to a horizontal sheet of dipoles. The equations to simulate a horizontal sheet are derived, and from these the relative density of targets may be calculated from the measured data by assuming a nominal target moment. Two field tests support both the qualitative and quantitative predictions. Extending this concept to field practice, we examine some of the implications for standard operational procedures. For example, if we accept that QA/QC metrics should represent the targets of interest, then we should require wide-area assessment surveys to create impractically large grids of surface frag. Likewise, for detection of clusters the concepts of detection probability and search radius based on single items are irrelevant. Discrimination techniques that rely on dipole fitting will be extremely inaccurate. Instead, QA parameters and models suitable for horizontal sheets will have to be derived.