Measuring AEM waveforms with a ground loop Aaron Davis 1 and James Macnae 1 1 RMIT University Applied Physics GPO Box 2476V Melbourne, Victoria Australia 3001 (February 21, 2008) Running head: AEM waveforms with a ground loop ABSTRACT Measurement of a transmitter current waveform provides critical data not otherwise avail- able for some AEM systems; yet which is needed for quantitative modelling of AEM data. We thus derive a method of measuring an airborne transmitter waveform by means of monitoring the current induced in a closed, multi-turn, insulated ground loop of known inductance L and resistance R. The transmitter waveforms of several different time-domain systems are deconvolved from the measured ground loop response when excited by the pri- mary electromagnetic field of the airborne electromagnetic (AEM) system itself. In general, our measurements agree well with contractor-described transmitter current waveforms; al- though crucial differences exist between our deconvolved waveforms and those described in the literature. Using the pulse-per-second feature of a GPS antenna the induced ground loop can monitor the frequency drift of a frequency-domain system. The ground loop be- haves like a lossy electric field antenna when the resistance closing the ground loop is made 1