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


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