MEC Target Classification using EM61 Interrogation during Reacquisition

The value of digital geophysical mapping (DGM) for the remediation of sites containing Munitions and Explosives of Concern (MEC) is well documented. In recent years, the state of the science has shifted to the application of classification techniques which limit intrusive operations to those DGM anomalies most likely to represent subsurface MEC. While new target classification techniques show significant promise, they are still impractical at many of the sites with more challenging terrain and vegetation conditions. This paper will describe the use of target interrogation during reacquisition using existing, proven technology which provides a real-time field alternative to anomaly classification applicable to most sites and with the potential to significantly reduce the number of DGM targets that require intrusive investigation. NAEVA utilized a differential reacquisition threshold and innovative methods of evaluating terrain response to eliminate subsurface targets that are unlikely to be subsurface MEC. During processing of DGM data, target selections must be made conservatively knowing that data are recorded at a single orientation (parallel to line direction) and that the instrument often will not pass over a subsurface object in a way that creates the maximum response amplitude. During target reacquisition, the original mapping instrument is maneuvered over an anomaly at multiple orientations until that maximum response is found. The first implication of this technique is that a less conservative criterion could be applied to the selection of targets than would be reasonable during data processing, thus reducing the number of targets that require intrusive investigation. In addition, the ability to evaluate anomaly characteristics from multiple orientations in real-time allows the skilled operator to make sound judgments about the nature of a subsurface anomaly in the presence of non-metallic terrain response. The size of the McClellan project offered NAEVA the opportunity to evaluate Geonics EM61- MK2 targeting and reacquisition peak response data with respect to the dig results for over 195,000 targeted DGM anomalies in terrain which is generally unfavorable for anomaly classification using existing (large) advanced sensors. An analysis of this data showed that the anomaly peak response measured during reacquisition could be utilized to eliminate a significant (10-20%) number of digs without significantly impeding the ability to detect and remove MEC. The classification of targets in areas exhibiting elevated terrain response were evaluated under two different scenarios and offered reductions in intrusive effort of more than 50%.