3-D magnetic model inversion for structure mapping

Abstract

A series of three-dimensional magnetic models has been designed to represent a wide range of geological structures. The models have been incorporated into automatic inversion routines, so that their parameters may be derived quickly from magnetic anomalies. The objective in developing this modelling scheme was to devise tools for applying to aeromagnetic data that would yield quantitative structural information, complementing the qualitative interpretations usually carried out. The models are based on simple slab components, and include single and double slabs, horizontal and plunging fold structures. The number of critical parameters for each model has been minimised to facilitate stable and rapid inversion. The inversion modelling scheme has been tested in the area of the Kurrawang Syncline near Kalgoorlie, WA. Here a magnetic conglomerate marker horizon has been folded and subsequently faulted. The results from the inversions of aeromagnetic data show excellent agreement with the known geology. It has been found that orientations tend to be determined more reliably than do thicknesses and depths. Other structures that have been modelled are the Mount Edwards folded ultramafic belt, near Widgiemooltha, WA, which is complicated by cross-cutting dykes; and a series of banded ironstones that host copper-gold mineralisation at Osborne, Queensland. The modelling technique is useful for testing an initial qualitative interpretation by trying the appropriate model and finding out if it can be made to fit. If a model fits well, then valuable structural information (strikes, dips, plunge etc) can be obtained. Although the modelling procedures were developed for use in reconnaissance mapping with airborne data, they are equally applicable to prospect exploration and ground survey measurements.