The MEGATEM fixed-wing transient EM system applied to mineral exploration: a discovery case history

Abstract

Richard Smith and David Fountain of Fugro Airborne Surveys (Ottawa, Canada) and Michel Allard of Canadian mining company Noranda (Laval, Quebec) describe the development of a new airborne electromagnetic survey system for mineral exploration. In the mid 1980s, Geoterrex (now part of Fugro Airborne Surveys) introduced the GEOTEM system, a fully digital airborne electromagnetic (AEM) receiver, using the same transmitter that had previously been used, very successfully, on the INPUT system (Annan, 1990). The GEOTEM system is mounted on a twin engine CASA 212 aircraft, and the transmitter loop is wound around the nose, wing tips and the tail of the aircraft (Figure 1). This transmitter excites eddy currents in the subsurface with periodic pulses of the ‘primary’ magnetic field. The decay of these currents is measured with a receiver towed behind the aircraft in a ‘bird’. When the eddy currents decay slowly, this is generally indicative of material in the subsurface that is conductive. Figure 2 shows a schematic diagram of the system showing the towed receiver bird. Airborne electromagnetic systems typically also measure the intensity of the Earth’s geomagnetic field. For the GEOTEM system the magnetometer is in a second bird. also towed behind and below the aircraft. Having magnetic data is useful for mineral exploration, as it can help to distinguish between conductive suphides (which are often associated with magnetic sulphides – for example pyrrhotite) and other conductive features like clay, graphites and shear zones (which are not magnetic). The magnetometer towed-bird location puts the magnetometer sensor close to the ground and provides high-resolution data. One measure of the strength of an AEM system is the transmitter dipole moment, the product of the current flowing in the transmitter loop, the area of the loop and the number of turns of wire in the transmitter. For the GEOTEM system, the peak dipole moment has been 0.69 million Am2 since the early 1990s. The system has been used in many different types of terrain and for many applications (Smith and Annan, 1997). It was recently attributed with discovering the Storliden deposit in Sweden (Posey, 1999). In the mid 1990s, the GEOTEM system on a CASA was deployed by BHP (now BHP Billiton) for exploration at relatively high altitudes (less than 2400 m) in the Altiplano regions of the Andes Mountains of South America. By 1996, BHP was interested in exploring in areas above 2400 m altitude.