Case histories illustrating the characteristics of the HeliGEOTEM system

Abstract

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. 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. Since its introduction, the HeliGEOTEM airborne electromagnetic (AEM) system has undergone a number of improvements. The introduction of broadband coils results in greater sensitivity to poor conductors. Increases in the dipole moment (signal level) and reductions in the noise level have improved the signal to noise ratio, allowing the response of deeper conductors to be detected. These characteristics are illustrated with case histories from the Sudbury and Timmins areas of Ontario, Canada and the Mattagami area of Quebec, Canada. A comparison with data from the DighemV AEM system flown at Maimon, Dominican Republic, shows that the HeliGEOTEM is able to see deeper than DighemV, but does not have quite as good capability to resolve features close to the surface. Another comparison in an area of northern Alberta shows that the HeliGEOTEM has poorer resolution compared to the frequency-domain RESOLVE system, but better near-surface resolution that the fixed-wing GEOTEM system. This example is also used to illustrate the depth of penetration of the three systems in a conductive environment. Also, interpretation and display tools developed for fixed-wing systems can also be applied to the HeliGEOTEM data.