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Airborne electromagnetic (EM) systems have evolved into two basic platforms since their introduction in the 1950s. The helicopter-borne frequency-domain systems (HEM) use narrow-band, low-moment transmitters and closely spaced receivers with a rigid geometry between the transmitter and receiver coils. A wide range of conductance discrimination, excellent spatial resolution, and moderate depth penetration characterizes these systems. The fixed-wing time-domain systems (AEM) use wide-band, high-moment transmitters and separated receiver coils in a geometry that is not rigid. These systems have a moderate range of conductance discrimination, moderate spatial resolution, and much greater depth penetration than HEM systems. Since 1995 there have been a number of attempts at adapting the advantages of the fixed-wing time-domain systems to the helicopter platform. The AeroTEM system, the result of one such effort, is based on a rigid, concentric-loop geometry with the receiver coils placed in the center of the transmitter loop (Figure 1). The advantages of this configuration include: maximum coupling to all target geometries regardless of the depth below the surface; sharper anomalies with simpler shapes compared to fixed-wing systems; anomaly shapes independent of the flight-line direction; and coincident transmitter-receiver coils have lower sensitivity to conductive overburden than separated transmitter-receiver systems.