Sharp-Focus Magnetic-Source Modelling to Map Depth to Prospective Albany-Fraser Basement beneath the Gunbarrel Basin

Abstract:

Mapping basement beneath cover is a substantial challenge for mineral and petroleum exploration within Australia. We have excellent magnetic field coverage over extensive areas of prospective geology buried beneath cover, but at present a wide range of sub-optimal, sparsely documented methods are used in analysis of that data. A sensitivity analysis of magnetic source depth estimation reveals that meaningful depth values are only feasible from suitable, carefully selected samples of the magnetic field data, isolated and subjected to intensive individual study. We term such analysis ‘sharp-focus’. CSIRO is developing a semi-automated sharp-focus methodology to recover magnetic source depth values with associated sensitivity estimates from magnetic field data. A key component of the method is inclusion of the Automag process, developed from the Naudy depth estimation technique. The major advantage of Automag is that solutions can be converted to parametric models, which can then be passed to inversion for further improvement and sensitivity analysis. Associated magnetisation, thickness and dip values assist in assigning geological interpretation to the source magnetisations. The sensitivity analysis substitutes for provision of uncertainties on the depth values (which are undefined by any depth estimation method because of fundamental non-uniqueness of this inverse problem). Currently we are investigating individual components in the workflow of this methodology, before assembling those components into a semi-automated process which will allow us to analyse the vast volumes of magnetic field data across Australia with a uniform but flexible optimised process. We illustrate some aspects of our methodology in a case study which forms part of an investigation of the Albany-Fraser zone, which is highly prospective for nickel and copper, but is partially covered by the Gunbarrel Basin. Mapping depth to the top of this sequence, together with its major structures and the plunges of the variably magnetised units is crucial for well targeted mineral exploration. Detailed elevation of the basement surface is required to understand the distribution of mineralisation indicators at and above that surface, and those details can only be resolved if individual magnetic depth estimates are reliable