DESMEX: A novel system development for semi-airborne EM exploration
in: Geophysics (2020)
Semi-airborne electromagnetics combines the benefits of powerful transmitters deployed on the ground with efficient helicopter-borne mapping of the magnetic field response in the air. The penetration depth can exceed those of classical airborne EM systems, since low frequencies and large transmitter-receiver offsets can be realized in practice. Albeit introduced already in the 1970-ies, the use of semi-airborne electromagnetics for exploration purposes has been rare in the past. However, with the demand to increase detection depths in the context of raw material exploration programs, the semi-airborne concept may become more widely applicable. In this paper, we present the technical details of a novel system that combines a high-moment horizontal electric dipole transmitter on the ground with low-noise three-axis induction coil magnetometers, a three axis fluxgate magnetometer and a laser gyro inertial measurement unit integrated within a helicopter-towed airborne platform. The emphasis of this work is to describe the airborne system and to illustrate the processing of the data and the data quality which can be achieved with our setup. As opposed to existing semi-airborne systems, we analyze the data in the frequency domain and estimate transfer functions between the received magnetic field components and the synchronously recorded injection current. This approach is similar to the procedure employed in controlled source electromagnetics. We find that for typical source dipole moments of 20-40kAm and for rectangular current waveforms with a fundamental frequency of about 10 Hz, we can estimate reliable transfer functions in the frequency range from 10?8000Hz and over a measurement area of 4x5km2 for each source installation.