SPM and airborne EM

Abstract

Superparamagnetic (SPM) artefacts in EM survey data cause difficulties in the interpretation of time-domain data. Developments in modern, commercial, high power, low noise, airborne systems now mean that SPM artefacts in survey data may be a problem that all interpreters should be aware of. SPM sources are very small surficial ferromagnetic particles however they produce anomalies in time domain data that are similar to the response from deep basement conductors. When present in Heli-EM data SPM anomalies manifest as lengthy decays and may comprise most, if not all of the received signal during ?late-time? readings. SPM anomalies frequently have similar wavelengths to those caused by base metal ore bodies making spatial discrimination difficult. To complicate matters further, large amplitude, discrete SPM anomalies may also be superimposed on the response of economic base metal ore bodies. Methods to discriminate SPM anomalies in Heli-EM data from those caused by inductive effects have focused on decay analysis. In commercial Heli-EM data however the time base is usually not sufficiently long and the noise levels are too high to provide a definitive discrimination tool. The most definitive technique found to date to identify SPM anomalies has been to take ground measurements using custom mini-coil sensors. Offset receiver configurations have been found to provide a reasonably cost-effective technique to avoid the worst SPM artefacts during follow-up ground surveying. Trials have shown that one way to reduce and identify SPM effects is for the aircraft to fly higher. This appears to significantly diminish the amplitude of surficial SPM while genuine basement conductors are still energised.