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.