Construction of a regional-scale 3D geological model from geologically constrained potential field inversion

Abstract

Our approach to the generation of a regional 3D geological model is through geologically constrained potential field inversion of crosssections. Wavelet edges form a scaffold on which the major physical property discontinuities can be linked to yield a 3D model. We have recently completed such a model for the Koonenberry Belt in far northwest NSW. The potential field modelling in this process constitutes a form of lithologic inversion, involving iterative refinement of parametric bodies. Detailed magnetic and gravity modelling of structurally complex geology required us to employ multiple polygonal bodies and to iteratively modify these in a way that closely honoured structural controls on the geology. Models evolved very substantially during the process. This procedure contrasts with the statistical approach to lithologic inversion, which relies on a well-defined geological a priori model as a seed for random single-voxel iterations. Geological complexity, limited outcrop, and initial uncertainty between competing tectonic interpretations precluded construction of such a starting model for the Koonenberry Belt. After completion of the 2D inversions, wavelet edges corresponding to the boundaries of major bodies were identified. Sets of edges define contours of surfaces, with depth approximated as half the upward continuation. These surfaces were linked between corresponding bodies on adjacent sections, and also served to extend the model beyond the limits of the 2D inversion. The completed model for the Koonenberry Belt reveals a complete assemblage of the elements of a Cambrian arc. The model predicts features seen on recently reprocessed seismic sections beyond the area covered by the original cross-sections.