Measuring AEM waveforms with a ground loop

Davis, A and MacNae, J 2008, 'Measuring AEM waveforms with a ground loop', Geophysics, vol. 73, no. 6, pp. F213-F222.


Document type: Journal Article
Collection: Journal Articles

Title Measuring AEM waveforms with a ground loop
Author(s) Davis, A
MacNae, J
Year 2008
Journal name Geophysics
Volume number 73
Issue number 6
Start page F213
End page F222
Total pages 10
Publisher Soc Exploration Geophysicists
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 L and resistance R. 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.
Subject Electrical and Electromagnetic Methods in Geophysics
Keyword(s) antennas
electric fields
electromagnetic fields
electromagnetic waves
method of moments
time domain analysis
transmitters
DOI - identifier 10.1190/1.2976791
Copyright notice © 2008 Society of Exploration Geophysicists. All rights reserved.
ISSN 0016-8033
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