Detection and correction of SPM effects in airborne EM surveys

Abstract

Recent noise reductions in airborne electromagnetic (AEM) systems have allowed detection of conductors at great depths, but systems now have also become sensitive to superparamagnetic (SPM) effects. We distinguish SPM effects in airborne electromagnetic survey data from the response of good conductors. In electromagnetic data processing, off-time data can be accurately represented as amplitudes of a set of basis functions that are comprised of decays that decrease exponentially as a function of time. The SPM impulse response can be approximated by a decay that is proportional to time to the inverse power, a time dependence associated with magnetic viscosity. We identify the presence of SPM effects, as distinct from the decay of good conductors, by using inverse power-law decays as additional basis functions in constrained least-squares fitting. Application of the method to airborne time-domain electromagnetic (TEM) surveys shows that the method allows correction of SPM and hence aids significantly in conductive target identification.

Key words::

• airborne EM
• magnetic viscosity
• SPM
• superparamagnetism

Acknowledgements

Thanks to IMX Resources for giving us access to the Mwese and Kapalagulu datasets. Thanks to James Hanneson (Adelaide Mining Geophysics Pty Ltd) for preparing the AMGplate model responses for us. Computing resources for this project came from RMIT University, with support for Terence Kratzer from an Australian Postgraduate Award. James Macnae’s contribution was funded through RMIT University. We thank associate editor Aaron Davis, reviewers Richard Smith and an anonymous reviewer for their constructive comments on the manuscript.