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Aspects Of System Design For Airborne Electromagnetic Detection Of Unexploded OrdnanceNormal access

Authors: L.P. Beard, W.E. Doll, T.J. Gamey, J.S. Holladay and J.L.C. Lee
Event name: 16th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
Session: Unexploded Ordnance
Publication date: 06 April 2003
Organisations: EEGS
Language: English
Info: Extended abstract, PDF ( 481.66Kb )

In recent field tests at a former bombing range, we examined factors affecting signal-tonoise
in a low-flying, helicopter-borne time-domain electromagnetic system specifically
designed for detection of unexploded ordnance. We isolated the most useful base frequencies
specific to our system, and found that under good survey conditions we were able to reliably
detect ordnance as small as 60 mm rounds. We tested different receiver geometries and sizes
and found that small receiver coils yielded higher amplitude anomalies than did large loop
receivers when directly over ordnance items, though the large loops more reliably detected items
offset from the receiver center. We examined the design of transmitter coils, and found
advantage in a lobed transmitter design over a rectangular loop. Vertical gradient measurements
rarely revealed any ordnance that was not also detected by the lower of the two receivers in the
gradient pair, but could be useful in determining whether an object was deeply buried or near the
ground surface. The most significant factor in the ability to detect small ordnance is sensor
height. Controlled source electromagnetic fields decay rapidly, both from the transmitter to the
ordnance item and upon return from ordnance to receiver, so that above a sensor height of 3
meters only large ordnance items could be reliably detected.

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