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Fast TEM for UXO mapping at Gambell, Saint Lawrence Island, Alaska

Publisher –
Zonge, 2001. Presented at UXO -Countermine Forum, Session 30, Location and Mapping, New Orleans, Louisiana, April 9-12, 2001

Authors –
Scott MacInnes *, Skip Snyder, Ken Zonge, Zonge Engineering and Research Organization, Tucson, Arizona;
Jon Durham, US Army Corps of Engineers, Huntsville, Alabama;
John Dickerson, Earth Tech, Colton, California

Paper – [pdf] FastTEM_UXO_Gambell

Abstract
A fast-sampling, transient electromagnetic (TEM) system was assembled for mapping probable UXO submerged in a shallow fresh-water lake near Gambell, Saint Lawrence Island, Alaska. The survey was designed to generate optimal data given the survey area, UXO characteristics, and the required depth of exploration. Since the search area was a fresh-water lake in an arctic climate, the survey was conducted in early May. The lake ice was still sufficiently thick to provide safe access, but most of the snow cover had melted, making it easy to move equipment across the search area. A continuous-sampling TEM system that records averaged data every three seconds was mounted on a sled and towed at a slow walk by the equipment operator.
A real-time differential GPS unit recorded the sled’s location every five seconds. Using this system, a total of 426,000 line-feet of profile with an average sample interval of 7.5 ft were surveyed over a period of 14 days. At each sample point, vertical-component transient data were recorded at 26 delay times, ranging from 7 to 570 microseconds. In this paper, we describe in detail the design of the antenna system and the survey. Area maps of the TEM response at several gates are presented. Using the maps and transient profile plots, target locations were picked. Analysis of the transient decay curves observed over the targets provides additional information about target size and conductivity.