oa Case histories illustrating the characteristics of the Heligeotem System

The HeliGEOTEM system was introduced in 2005 to provide greater operational flexibility and improved lateral resolution compared with a fixed-wing system (Fountain et al., 2005). The system, described in more detail by Fountain et al. (2005), is a vertical-axis dipole transmitter towed below and behind a helicopter. The receiver, also attached to the tow cable is about 15 m in front and 35 m above the transmitter. The system measures the response in the time domain when a half-sine current pulse excites the ground. The dB/dt and B-field responses are measured in the x, y and z orientations. The geometry of the system and the coordinate system are shown on Figure 1. Compared with fixed-wing systems, the helicopter systems have their transmitter / receiver closer to the ground surface, which is why the spatial resolution is greater (the response is much sharper); also, the response of shallow bodies is much larger. However, when the bodies are deeper, the responses are more comparable.