Superparamagnetism in ground and airborne electromagnetics: Geometrical and physical controls

Superparamagnetic (SPM) effects are not uncommon in ground and airborne electromagnetic (AEM) geophysical data, with sources in the regolith typically being maghemite grains. Using laboratory parameters for maghemite and the Neel equation, a typical geophysical electromagnetic (EM) system is only sensitive to maghemites with dimensions in the 9-12 nm range. The power-law decay observed in a dB/dt system depends on the magnetic nanoparticle volume distribution: Log normally distributed volumes centered on the detection window produce approximately 1/t decays after the electromagnetic responses have vanished. If most of the nanoparticles are smaller than the center of the system sensitivity, the power-law decays faster than the 1/t result, if most nanoparticles are larger, a response slower than 1/t results. Geologic origins of regolith maghemite favor a thin-layer geometry in the near surface. Shape-demagnetization effects imply that SPM responses observed will only arise from magnetization in the horizontal plane. For a ground transmitter, this is the very small area directly under the transmitter loop wire, causing only locally detectable effects. For an airborne transmitter, an extensive radial ring in which the magnetic field is subhorizontal is not demagnetized, and this results in detectable SPM airborne anomalies. For SPM sources in hard rock, a magnetization model consisting of spherical particles enclosed in a finite volume is adapted for SPM. This model shows that the falloff in SPM amplitude with airborne system altitude is rapid for surficial and finite-bedrock sources. As a result, fixed-wing surveys with a transmitter at a 90 m or higher altitude is much less likely to show SPM effects in the EM data. Because the secondary magnetic field of spherical particles is parallel to the transmitter dipole source direction at a colocated receiver, the model explains the empirical observation that concentric loop AEM systems do not detect any x-component of an SPM response.