Optimizing the Metalmapper for Static "Cued ID" Measurements over UXO

The MetalMapper is a commercially available advanced EM system that includes an array of 3 transmitter loops and 7 tri-axial receiver cubes that is used for detecting and classifying UXO. In the current instrument, the number of receivers was determined by hardware limitations (24 analog channels max), and the receivers were positioned to provide uniform coverage when deployed for mapping surveys. With support from SERDP, we have undertaken a model study aimed at answering important questions about the optimality of the MetalMapper antenna configuration when deployed for static cued ID measurements. These questions include: Is there a significant advantage to tri-axial receiver loops as compared to a single loop receiver?; Are 7 tri-axial receivers necessary or might we obtain similar performance from an array of 5 tri-axial receivers that are better positioned? Our study extends earlier studies by Grimm and Sprott (Grimm 2002) at Blackhawk Geoservices (now Zapata/Blackhawk) and Smith et al (Smith 2005) at Lawrence Berkeley Labs. Those studies have guided us during the early development of the MetalMapper and its forbearer, the AOL system. Using synthetic data sets for a conductive and permeable sphere placed at random locations beneath the MetalMapper, we show that the current MetalMapper receiver array performs better than either an array of 7 single-component (dBz/dt) receivers or an array of 5 tri-axial receivers having several different dimensions. We obtained similar performance when we tested these arrays with a second synthetic data set that better simulates a UXO. Our study also confirms empirical observations from past demonstrations of the MetalMapper that the quality of shape-based target parameters is significantly degraded when the target has a horizontal offset from the center of the MetalMapper array of more than 40cm.