A square-wave current inverter for aircraft-mounted electromagnetic surveying systems

An inverter topology that has been developed for geophysical surveying applications makes it possible to achieve very fast current reversal in magnetic field coils. The inverter also makes it possible to generate waveforms that contain a variable high-frequency spectrum superimposed on a high-amplitude low-frequency fundamental. A feature, which is attractive from an implementation point of view, is that a low-voltage source such as a battery can be combined with high-voltage transistors to meet high dynamic requirements. Two variations of the topology are discussed, namely, a clamped and unclamped version. The proposed inverter was successfully implemented in two aircraft-mounted geophysical surveying systems achieving substantial performance improvement over the older systems. A 40-kW system was constructed for a fixed-wing aircraft and a smaller, battery-operated system was implemented on a helicopter.