The calculation of magnetic components and moments from TMI: a case study from the Tuckers igneous complex, Queensland

Abstract

This paper re-examines some largely forgotten methods that are eminently adaptable to modern computing environments. The methods were originally formulated before the advent of electronic computers and do not appear to have been fully developed, or given the attention they deserve. It is apparent that if an anomaly is small with respect to the geomagnetic field, which is usual, components of the anomaly can be derived from total magnetic intensity observations, or indeed, from any other component irrespective of its direction. Furthermore, for an isolated anomaly, integrals of first order moments of the components yield information such as the direction and magnitude of the total anomalous magnetic moment of the source. Improvements to the calculation of components are proposed that, i) compensate for the finite nature of survey areas, and ii) correct for the departure from a true potential field of total magnetic intensity anomalies. Examples of the application of the methods are derived from a survey flown by Aerodata over the Tuckers Range area of north Queensland. This survey was chosen because the magnetic properties of the lithologies giving rise to the magnetic anomalies have been thoroughly studied. The selected areas include the north margin and an outlier, which contain many anomalies showing effects of magnetic remanence of reverse polarity. The results of the methods used herein agree well with those expected from the measured magnetic properties and magnetic modelling. The calculation of the total moment of the north margin yielded a declination of 35.8° and an inclination of 39.0°. A comparison with the measured properties shows that the reverse remanence of the overprint zone appears to account for about 70% of the total anomaly over the north margin. Similarly, the outlier yielded a declination of 13.7° and an inclination of 4.5°. Over the outlier the overprinted zone appears to account for about 50% of the magnetic anomaly. It seems that magnetic interpretation could be significantly enhanced by developing these methods to their full potential and incorporating them in commercial packages.