Quantifying AEM system characteristics using a ground loop

Abstract

Quantitative interpretation of time-domain airborne electromagnetic ͑AEM͒ data is hampered by uncertainty in altimetry, system geometry, transmitter waveform, data averaging, and timing. We present a simple calibration method that serves to define these issues by the use of a closed multiturn loop of known electrical and physical properties that is insulated from the ground beneath it. By predicting the secondary response of the AEM receiver and comparing it with the measured data, we have identified and quantified systematic errors mentioned above in several systems. In addition, we identify an alternative subprocess that uniquely calculates altimeter and geometry errors by measuring the current induced in a ground loop of known properties and comparing it with predictions. The ground-loop method is used best over resistive cover to minimize limitations caused by nonuniform conductive ground and is a calibration tool that makes AEM data consistent with quantitative models. Fluctuating geometric errors caused by bird swing limit the accuracy of applying the geometry corrections from one flyover to an entire survey.

An investigation of seawater and sediment depth using a prototype airborne electromagnetic instrumentation system – a case study in Broken Bay, Australia

“…Airborne electromagnetic instrumentation stability, calibration (Brodie and Sambridge 2006; Davis and Macnae 2008; Vrbancich and Fullagar 2007b) and the ability to accurately track the orientation and altitude of the airborne electromagnetic sensor system over seawater during the survey (Davis, Macnae and Robb 2006; Kratzer and Vrbancich 2007) prevent the full potential of the airborne electromagnetic method from being realized in applications that require accurate mapping of shallow conductive features such as ground salinity and coastal seawater depth. In order to overcome the above limiting factors, a time‐domain helicopter airborne electromagnetic system is currently being developed for the Defence Science and Technology Organisation in several stages that will be optimized for shallow water bathymetric mapping.…”

Metalliferous mining geophysics — State of the art after a decade in the new millennium

“…A procedure involving a ground loop was devised by Davis and Macnae (2008a, b) to improve understanding of the time-domain waveform shape and of timing and altimeter geometry errors; that procedure also results in derivation of better fidelity information from the data. One advantage of this work is that the precise nature of most of the time-domain system waveforms becomes apparent (Davis and Macnae, 2008a). All contractors now are trying harder to make high-quality waveform information available.…”

Measuring AEM waveforms with a ground loop

“…The envelope starts and ends with small shoulders, crossing to a main lobe in the center where the mutual inductance coupling the transmitter to the loop is greatest. The clear current nulls at 3 and 12 s indicate that the ground beneath the loop is very resistive; the secondary fields generated in the earth produced negligible currents in the ground loop ͑Davis and Macnae, 2008͒. To complete the deconvolution of the VTEM waveform, we took several complete ground-current waveforms stacked together to increase the ratio of signal to ͑random͒ noise. For our stacking process, we first calculated N, the number of DAQ samples per transmitter waveform, by counting the number of samples over 100 repetitions of the recorded ground signal.…”