Superparamagnetic effects in EM surveys for Mineral Exploration

Abstract

Superparamagnetic effects in soil and rock cause responses in a variety of EM survey types that are routinely used in mineral exploration surveys. The identification of these effects can be difficult and the response may be very similar to that from targeted base metal orebodies. Identification of these effects in TDEM datasets is critical for prioritising ground follow-up of targets and failing to correctly identify SPM anomalies frequently leads to inaccurate inversions, misdirected field work, and unnecessary, deep drill holes. As signal-to-noise ratios in acquisition systems increase, SPM effects will become an increasingly common issue in both airborne and ground EM surveys. This paper presents examples of SPM responses in data collected with the latest airborne, ground, and drill hole EM systems used in minerals exploration. Ground follow-up results confirm the SPM responses. Techniques that have been found to be effective to firstly avoid, and secondly to identify SPM anomalies in survey data are presented. These techniques are based around minimising the primary energising magnetic field at the source of the SPM, and by distancing the sensor so that it is separated from the source. The main technique for recognising SPM effects directly in data remains the fairly predictable power law decay rate. Secondary techniques such as association of anomalies with alluvial sediments, topographical features (e.g. drainage, hills), and palaeochannel patterns also provide indications of SPM. Associations of anomalies with low transmitter loop height in airborne systems is also indicative of SPM, however systems that can better discriminate SPM need to be developed.