Modelling the superparamagnetic response of AEM data

Several lines of airborne EM data flown at different system elevations across a known sulphide and surface cover with elevated superparamagnetic (SPM) properties were analysed with MAXWELL, layered-earth inversions and LEROIAIR. The SPM material was modelled with frequency-dependent magnetic susceptibilities at shallow depth. The SPM responses can be confused with responses of deep conductors and vice versa. Depending on the parameter weighting used, 1D inversions model all late-time responses as deep conductive material or as surficial SPM material. However, the joint 1D inversion of data acquired at different system elevations manages to recover a deep conductor from the sulphide anomaly and elevated SPM values at the location of the SPM response. For the modelled parameters, the VTEM data require a vertical system separation of at least 5-10 m to allow for the discrimination between the SPM and sulphide responses. In the absence of vertically separated AEM profiles, the x-component data, if available, might offer some model discrimination. For laterally extensive surficial SPM material, synthetic x-data computed with a modified version of LEROIAIR show a negligible SPM response. Following the determination of SPM parameters from VTEM survey data, these values were used to predict the SPM response for other AEM systems, including MEGATEM, AEROTEM, HELITEM and RESOLVE. Whereas the transient helicopter EM systems VTEM, AEROTEM and HELITEM data can be strongly affected by SPM effects, fixed-wing MEGATEM data are unaffected, due to the high elevation and large transmitter – receiver separation of the EM system. SPM effects on frequency-domain systems such as RESOLVE data are also small.