UXO Clusters and Sheet‐Like Anomalies in Wide‐Area Assessment Surveys

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, frag 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. With spacing of individual sources closer than 0.5 times the sensor height, their magnetic anomalies overlap significantly and increase the response amplitude. The result is 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 or vise versa. A field test supports both the qualitative and quantitative theory. Finally, in practice QA/QC metrics should represent the targets of interest, which means creating dense grids of surface frag. For detection of clusters, the concepts of detection probability and search radius based on single items are irrelevant. Likewise, 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.