Late-time effects in airborne EM: induced polarisation, superparamagnetism and rotation
Creators: name: Kratzer, Terence
Publisher: name: RMIT University
Year: 2024

description: The aim of this thesis is to characterise and correct for three major late-time effects that are becoming increasingly important in airborne EM (electromagnetic) systems: induced polarisation, superparamagnetism, and rotation noise. The primary importance in characterising these effects is to improve the performance of AEM systems at late delay times. This aim is becoming increasingly important as near surface deposits are exhausted and the need for successful exploration to greater depths and in difficult environments mounts. <br>I identify the three late delay time effects from literature review. The approach to characterisation of each of these effects is similar: model the underlying phenomenon; fitting of the models to the data, thus obtaining deconvolved signals for each of the fitted models; subtraction of the unwanted signal(s) to leave pure AEM responses; and investigation of the coherence of the predicted SPM and AIP (Airborne IP) source parameters. <br>I model Inductive IP (IIP) using a combination of Warburg and exponential decay models as a basis for fitting electromagnetic data from ground TEM and airborne VTEM surveys. Observed decays are deconvolved into EM and IP constituents by constrained least squares fitting of basis functions, modified to account for transmitter waveforms. The method has been confirmed through synthetic modelling of 2D and 3D structures, and when applied to ground TEM or airborne TEM data, obtains an estimate of apparent chargeability at each station or fiducial. <br>I use a 1/t decay, a time-dependance associated with magnetic viscosity, to model and fit SPM effects in AEM data. I identify the presence of SPM effects, as distinct from the decay of good conductors, by using this model as additional basis functions in constrained least-squares fitting. <br>I use the output of tri-axial rotation-rate sensors as a basis of a model for rotation noise, to predict and subtract the rotation noise from rigidly-coupled ARMIT magnetic field sensors. <br>In order to test out the usefulness of both the IP characterisation and rotation correction, I then use data from a low base frequency airborne IP and EM survey, which employed a low-noise B-field sensor coupled with rotation rate sensors. I then calculate chargeability over a synthetic IP source. <br>In the course of characterising these effects for better low-frequency performance, I make useful progress towards airborne induced polarisation. This includes characterisation of IP effects in an existing dB/dt airborne EM system. In the case of a VTEM survey in Africa, the apparent chargeabilities mapped graphitic sediments and provided spatially consistent indications of clay concentrations. <br>I distinguish SPM effects in airborne electromagnetic survey data from the response of good conductors. Application of the method to airborne TEM (time-domain electromagnetic) surveys shows that the method allows correction of SPM and hence aids significantly in conductive target identification. <br>The approach towards rotation noise removal is successful in reducing rotation noise by one to two orders of magnitude at low frequencies. The survey over a synthetic IP source is easily able to determine location thanks to rotation correction, and the use of a B-field sensor is significantly more useful than a dB/dt system, due to the low frequency response of such a sensor. <br>I also find that as these three effects (IP, SPM, and rotation noise) are all late-time, they often impact on each other. Specifically, I find that correction for rotation noise is important for accurate characterisation of induced polarisation, and is likely to be important for improved characterisation of superparamagnetism. Additionally, further investigation indicated that in some cases SPM and IP could be equal and opposite, either reducing the amplitude of the larger effect, or even resulting in a negligible net effect. This cancellation effect is also likely to be reduced with the improved late-time sensitivity that effective rotation correction brings.