The remote determination of magnetic remanence

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 magnetization direction. Our research shows that the resultant magnetization direction can be derived directly through inversion of magnetic data for an isolated magnetic anomaly. The resultant magnetization direction is a property of the target magnetic rocks and a robust inversion parameter. The departure angle of the resultant magnetization vector from that of the inducing magnetic field is an important indicator of the existence of remanent magnetization 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 magnetization vector and resultant magnetization vector lie on a great circle. We find the intersection of the polar wander vector trace with the great circle to obtain one or more possible solutions for the remanent magnetization direction. Geological deduction will normally allow us to reduce the ambiguity for multiple solutions to obtain the most likely remanent magnetization direction. Once the remanent magnetization direction is established, it is then possible to determine the Koenigsberger ratio and magnetic susceptibility for the target. We illustrate the methodology with some synthetic models and targets from Australian magnetic surveys. Magnetic remanence is a physical property of the rock that is distinct from susceptibility and this methodology provides a new tool to help with the categorization and prioritization of exploration targets.