b'Minerals geophysics There are challenges in getting aGeosystems Inc (USA) based on naturalare the tipper components as the transfer measurable response in relativelysource AMT/MT was listed under thefunction of a vertical magnetic field; resistive terrain (commonly higher thantesting/R&D status. The Dicon system, in itsTipper measurements (Hz/Hx or Hz/1000 ohm-m) and subtle resistivitytest configuration, measured orthogonalHy) are dimensionless, cannot resolve contrasts. components of the E and H fields. layered geology, and tipper responses There are parasitic IP and SPM effectsare considerably diminished for compact on measured induction under specificHigh-Sense Geophysics (Canada) further3D bodies compared to 2D geological near-surface conditions. developed an AFMAG system in 1998strikes. Since the tilt of the flying coil with Petr Kuzmin. The first field test, inis unstable during a survey, ZTEM uses Methods that exploit natural1999, was successful and promising.attitude sensors to correct the source of electromagnetic fields (magneto telluricThe development was terminated afterthe error. Still, the error correction quality and magneto variational, AFMAG) canconsolidation with Fugro. is affected by unknown differences in overcome the limitations of airbornethe horizontal components between the systems with controlled primary fieldOther airborne AFMAG prototypes in 2001 and 2002 developed by Geotechaudio-magnetic field magnitudes at the sources. A comparative estimation ofreference and in-flight positions.the depth of investigation of differentLtd. played the role of transitional and airborne electromagnetic principles isnon-commercial designs. The AirMtAll the systems from the AFMAG family presented in Figure 1. system with three orthogonal inductivesuffer from bias effects of the recorded receiver coils was announced as being atdata, arising from weak natural signals. The first period of theoreticalan R&D stage and was not commerciallyThese distortions cannot be corrected by development and practical usageavailable. The AirMt system measured thesignal filtering, and, as a result, significant of AFMAG (audio-frequencyrotational invariant part of the in-phasesystematic errors and false anomalies magnetic technique) as an inductiveand quadrature transfer functions in theoccur. This technical problem has been electromagnetic method exploited audio- frequency domain for three magneticsolved with the development of the frequency natural magnetic fields. Onegeometrical components from theMobileMT technology.of the main reasons for the developmentairborne receiver and three magnetic was the potential to provide significantgeometrical components at a stationaryThe MobileMT system, introduced depth information without exploitingreference base station. in 2018, was developed by Expert technically limited primary field,Geophysics Limited to continue controlled sources. McPhar GeophysicsThe tipper-type, magnetovariationalthe evolution of the airborne Limited commercially used the airborneairborne platform ZTEM became the nextelectromagnetic natural fields AFMAG system in the 1960s and earlygeneration of AFMAG technology andtechnology 12 years after the 1970s. The company then shifted theirthe first commercial airborne AFMAGintroduction of ZTEM and 60 years after commercial focus to radiometry, and thussystem more than 40 years after thethe first commercial AFMAG system.the development of electromagneticoriginal AFMAG. The ZTEM system is methods was terminated. based on measuring the induced signals vertical magnetic component, primarily The Dicon/Q-Trac airborne EM systemcaused by lateral resistivity variations in introduced in 1997 by Barringerthe subsurface geology. ZTEM outputs MobileMT in Western Australia.MobileMT employs an airborne receiver which comprises three orthogonal induction coils to take measurements of alternating magnetic fields, and a ground electric base station, which measures reference and signal electric fields in two perpendicular directions with four pairs of electrodes. The E-field base station includes the reference orthogonal pair of grounded lines, utilised to eliminate local noise, and correct for data bias distortions. In the MobileMT technology, the E-field data is used to reference the primary natural electromagnetic field variations to facilitate the separation of the time-variance from the space-variance of the Figure 1.Approximate depth of investigation estimation of different airborne electromagnetic principles. measured fields.AUGUST 2023 PREVIEW 34'