An iterative approach to optimising depth to magnetic source using the spectral method

Abstract

Knowledge of the depth of cover is poor across large areas of Australia. The spectral method is an efficient method of producing reliable depth to magnetic basement estimates across large regions of the continent. A semi-automated work-flow has been created that enables the generation of depth to magnetic source estimates from windowed magnetic data using the Spector and Grant method. The work-flow allows for the correction of the power spectra, prior to the picking of straight-line segments, to account for the fractal distribution of magnetic sources. The fractal parameter (ß) varies with depth and was determined by picking multiple depth estimates in regions of outcropping magnetic basement which have been upward continued to different levels in order to simulate different amounts of burial beneath non-magnetic sediments. A power law function best approximates the decay of ß with depth. An iterative schema used to determine the optimum ß where the depths of magnetic sources are unknown, has been incorporated into the workflow. Preliminary testing in a region of known magnetic basement depth has produced encouraging results, although further testing is required. The decrease of ß with increasing depth suggests that the fractal distribution of magnetisation becomes less correlated, or fractal, over larger volumes of observation.