Alltem, A Triangle Wave On-Time Time-Domain System for Uxo Applications

Abstract

Data from tests of two electromagnetic prototype systems, the High Frequency Sounder (HFS), and the Very Early Time Electromagnetic (VETEM) system at the Unexploded Ordnance (UXO) Calibration Test Lanes at the Yuma Proving Ground, Arizona, showed good sensitivity to UXO objects, but also considerable response to the electrical conductivity of the ground at frequencies above 20 kHz or at the very early times provided by VETEM. To overcome these problems, we use a triangle wave excitation similar to the one used by the UTEM system developed at the University of Toronto. One of the favorable characteristics of such a system is that ferrous and non-ferrous metals behave quite differently so that an operator can tell at a glance whether a response involves ferrous metals or not. With such a system we can measure the transients to much later times than were possible with VETEM. In addition, the ability to generate and measure three field components aids target discrimination and identification. Results from laboratory test prototypes show good sensitivity to surrogate and actual inert UXO items. Introduction UXO remains one of the most difficult and costly problems facing the Department of Defense. UXO seriously impacts the Base Realignment and Closure (BRAC) program, for example. One of the greatest costs is digging up pieces of metal that are not UXO because it is difficult to distinguish UXO from shrapnel and other harmless metal objects. Last year we reported on tests of VETEM, the High Frequency Sounder, and the Tensor Magnetic Gradiometer System (TMGS) at the UXO Standard Calibration Grid at Yuma Proving Ground (YPG), Arizona (Wright and others, 2004; Smith and Bracken, 2004). From those tests we concluded that the TMGS should be reconfigured and that a new electromagnetic induction (EMI) system should be designed, built and deployed. We decided to follow the practice of using a triangle wave current excitation, used in the UTEM system developed at the University of Toronto, and named the new system ALLTEM because it is a time domain electromagnetic (TEM) system whose current excitation is on all the time. In the present paper we consider some of the benefits and design issues for such a system and show results of modeling and laboratory experimental data obtained with prototype sensors. Concept and Benefits The VETEM system is unusual in that it records data both while the transmitter is on and after turn-off (Wright and Chew, 2000). Examination of the VETEM data from YPG demonstrated a higher signal-to-noise ratio while the transmitter was in its ramp-up rather than after turn-off. However, the VETEM pulse length, typically 2 μs to a maximum of 5 μs, was too short to avoid strong response from the earth, nor could we measure to late enough time after transmitter turn-off to avoid earth response because the VETEM receiver measures the current in a shorted-turn loop using a pulse transformer that