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Target Prioritization in TEM Surveys for Sub-surface UXO Investigations Using Response Amplitude, Decay Curve Slope, Signal to Noise Ratio, and Spatial Match Filtering

Publisher —
Zonge.

Authors —
Darrell B. Hall, Earth Tech, Inc., Colton, CA;
Scott C. MacInnes, Zonge Engineering and Research Organization, Soldotna, AK;
John Dickerson, Earth Tech, Inc., Colton, CA;
Jennifer Hare, Zonge Engineering and Research Organization, Tucson, AZ.

Paper — [pdf] UXO_TEM_Target

Abstract
To reduce the number of false-positives in identifying UXOs and related debris for ongoing site investigation, a target prioritization routine was developed based on the likelihood of a metallic source.  Numerous parameters were evaluated statistically using over 2000 existing intrusive investigation results.  Those showing the best ability to discriminate between metallic and non-metallic sources were incorporated into the scheme, including peak amplitude response, power-law decay slope, signal to noise ratio, and spatial match filter response.  Parameters were assigned increasing numeric values based on likelihood of a metallic source, and summed to produce a target rank.
Subsequent application to over 4000 new intrusive investigations revealed that the highest ranked 25 percent of targets were nearly five times more likely to yield a metallic source when investigated than the lowest ranked 25 percent.  These results show that target selection routines in UXO investigations would benefit from using more parameters than just magnitude response, allowing more aggressive target identification and reduced costs by lowering the number of false positives that are investigated.  Proposed refinements may further increase predictive capabilities.
Introduction
Time-domain electromagnetic (TEM) methods are widely used in surveys for unexploded ordnance (UXO).  Since typically most anomalous TEM responses are not associated with UXOs, significant effort in recent years has been directed towards discriminating the source of anomalies.  The question of which responses should be targeted for intrusive investigation remains open. Often surveys will simply target all responses that exceed a specified amplitude for a given time-gate after the pulse. This results in the selection of numerous responses that physically do not correspond to a subsurface metallic source. 
Further, some responses that are due to a subsurface metallic source can be missed because the peak amplitude does not exceed the specified threshold.  An ongoing Site Investigation (SI) for UXO provided an opportunity to develop a target prioritization scheme to discriminate between metallic and non-metallic and thus, help to reduce the number of ‘false positives’ (i.e., no metallic source could be identified after intrusive investigation of a given target).