Measuring AEM waveforms with a ground loop

Abstract

Measuring a transmitter-current waveform provides critical data unavailable for some airborne electromagnetic (AEM) systems yet needed to model AEM data quantitatively. We developed a novel experimental method of measuring an airborne transmitter waveform by monitoring the current induced in a closed, multiturn, insulated ground loop of known inductance [Formula: see text] and resistance [Formula: see text]. The transmitter waveform of five different time-domain systems is deconvolved from the measured ground-loop response when excited by the primary electromagnetic field of the AEM system. In general, our measurements agree well with contractor-described transmitter current waveforms, although crucial differences exist between our deconvolved waveforms and those described in the literature. Using the pulse-per-second feature of a GPS antenna, the ground loop can monitor the frequency drift of a frequency-domain system. The ground loop behaves like a lossy electric-field antenna when the resistance closing the ground loop is too large. This leads to negatives in the response of coincident-loop systems without including induced polarization effects. After observing exponentially decaying, oscillating-current responses in high-resistance ground loops, we model the observed current with an LRC circuit whose resistance and capacitance represent generalized effective antenna and free-space values. Our model predicts responses that closely match the damped oscillations seen in the airborne response during flyover; however, it does not work well on conductive ground.