Abstract
The remote determination of magnetic remanence in rocks is a method that has largely been ignored because of the ambiguity associated with the estimation of both the Koenigsberger ratio and remanent magnetisation direction. Our research shows that the resultant magnetisation direction can be derived directly through inversion of magnetic data for an isolated magnetic anomaly from a compact magnetic source. The resultant magnetisation direction is a property of the target magnetic rocks and a robust inversion parameter. The departure angle of the resultant magnetisation vector from that of the inducing magnetic field is an important indicator of the existence of remanent magnetisation and the inversion process can detect departures that are not easily detected by visual inspection. This departure angle is called the apparent resultant rotation angle or ARRA.
The induced field vector, remanent magnetisation vector and resultant magnetisation vector lie on the plane of a great circle. We find the intersection of the transformed polar wander vector trace with the great circle plane to obtain one or more possible solutions for the remanent magnetisation vector. Geological deduction will normally allow us to reduce the ambiguity for multiple solutions to obtain the most likely remanent magnetisation direction. Once the remanent magnetisation direction is established, it is then possible to determine the Koenigsberger ratio and magnetic susceptibility for the target.
We illustrate the methodology using survey data over the Black Hill Norite which also has extensive palaeomagnetic data available for comparison with the inversion results. We then apply the remote remanence estimation (RRE) method to a systematic study of a large number of intrusive pipes in the Thomson Orogen, New South Wales. The corrected magnetic susceptibility and remanence properties, spatial distribution and underlying uncertainties are evaluated for their potential use by diamond explorers. The additional information assists with differentiating kimberlites from other intrusive pipes based on age and remanence properties.
The resultant magnetisation vector of a compact magnetic source can be obtained through simultaneous inversion for geometry and magnetisation. The resultant magnetisation vector and induced field vector lie on a plane that intersects the transformed apparent polar wander path to provide estimates for magnetic remanence, susceptibility and age.
Acknowledgements
ModelVision from Tensor Research was used to perform the magnetic inversions for both the research and field applications. Discover PA from pbEncom was used for the generation of the image displays. The authors would like to thank Phil Schmidt (MagneticEarth), Clive Foss (CSIRO) and Robert Musgrave (NSW Geological Survey) for their helpful input on magnetic polar wander data and the magnetic moment methods.