Recovery of magnetization direction from magnetic field interpretation of a ring dyke in northern Queensland

Simultaneous recovery of complex geometry and unknown magnetization direction from inversion of magnetic field data poses considerable challenges. Never theless a range of methods address such problems and solutions are feasible. We illustrate application of several methods to the magnetic anomaly over a reversely magnetized ring dyke in the Permo-Carboniferous Newcastle Range Volcanics of northern Queensland. Palaeomagnetic study of these rocks isolated two steep reverse magnetization directions of Permian and mid-Carboniferous ages and it is clear from the magnetic field anomaly that one of these dominates the total magnetization. To recover the magnetization direction we first applied magnetic moment analysis (MMA). MMA is best applied to compact sources and must be located over the centre of magnetization. We had hoped that the circular symmetry of the ring dyke might enable this analysis to succeed, but it was disrupted by the eccentric distribution of magnetization in an annulus and produced consistently low inclination values. We then applied several methods which scan a range of trial magnetization directions, applying reduction to pole or equatortransforms from which the optimum value of a chosen statistic is taken to specify the best magnetization direction. These methods are less sensitive to distribution of magnetization and provided superior recovery of inclination, but with inconsistent declination values. Finally we applied inversions which recovered inclinations close to the palaeomagnetic measurements, but again with uncer tain declination values. Further forward modelling with test magnetization directions confirmed the higher sensitivity in recovering inclination rather than declination of magnetization from this anomaly.