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The Very Early Time Electromagnetic (VETEM) system, the High-Frequency Sounder (HFS), and the Tensor Magnetic Gradiometer System (TMGS) were developed by the U.S. Geological Survey (USGS) for applications other than unexploded ordnance (UXO). However, features of these systems may make them adaptable to UXO applications. VETEM is a time-domain electromagnetic (EM) induction system that utilizes full waveform recording. The HFS is a flexible frequency-domain EM system. The TMGS uses four sensitive 3-axis fluxgate magnetometers so that gradients of field components are recorded. Tests were conducted in air using a selection of inert UXO objects ranging from 20 mm to 105 mm in diameter. A nulled overlapped antenna arrangement was used for both VETEM and the HFS. It was found that this antenna configuration resulted in distinctly different responses to horizontal UXO objects aligned along the direction of traverse versus horizontal and perpendicular to the traverse. Vertical orientation of UXO objects resulted in smaller amplitude signals and an interesting slightly asymmetric “crater” shape response in plan view. The HFS response was a strong function of frequency, which is good for discrimination. The TMGS data are the most sensitive to ferromagnetic objects and the multi-component data hold the promise of accurate location, including depth, to UXO objects using a dipole mapping algorithm. Plans include increased sensitivity and multicomponent capability for the EM systems, advanced algorithms to enhance UXO discrimination for EM and TMGS systems and field evaluations at standard UXO test sites of the Strategic Environmental Research and Development Program (SERDP). Introduction UXO is one of the most pressing problems facing the Department of Defense and other government agencies that have lands that were once used for military training and are now closed or closing and being returned to civilian use. Cleanup of all UXO-contaminated lands using existing methods would be prohibitively expensive, so a great deal of effort is being directed to finding better ways to detect, locate and identify buried UXO. It is not sufficient to merely detect buried metal objects, because many of these objects are not UXO and pose no hazards. In many cases of range cleanup 70% or more of the cost consists of locating and removing harmless metal including soda cans, broken plow blades and fragments of ordnance that exploded as designed. Among the primary geophysical methods for detecting and classification are various time and frequency domain EM systems and magnetometer systems. Since the USGS developed some prototype EM and magnetic systems for applications other than UXO, and because some of these prototype systems have some unusual characteristics, we proposed to test, evaluate, and modify three of these systems, the VETEM system, the HFS, and the TMGS. We are now in the process of carrying out this work and this paper describes some of the results from laboratory tests. |
