Second horizontal derivatives of ground magnetic data applied to gold exploration in the Yilgarn Craton of Western Australia

Abstract

Ground magnetic surveys for gold exploration in the Archaean Yilgarn Craton of Western Australia are rapidly being replaced by low-level, high-resolution, fixed-wing and helicopter aeromagnetic surveys for detailed geological mapping of large areas. However, for exploration of areas that are either small, or where the buried magnetic sources are located near the surface, or where the magnetic response of the regolith is of interest, ground magnetic surveys are required in order to measure the high-frequency magnetic responses associated with these environments. Surface and subsurface geological features are often of interest when exploring for near-surface gold mineralisation. Earlier work, by S. Mudge, has shown that bipole plots of second horizontal derivatives of aeromagnetic data are effective in resolving detail in aeromagnetic data. These have been applied to ungridded ground magnetic data and are effective in resolving high-frequency detail in the magnetic responses of subsurface rocks and the regolith. The large high-frequency component of ground magnetic data acquired from maghemite-rich regolith areas presents different and often difficult data processing and presentation problems compared with data acquired from higher-level aeromagnetic surveys. Despite this, second horizontal derivatives of ground magnetic data resolve important detail of magnetic subsurface rocks and the regolith that would otherwise be lost in images and contours of the gridded data. For an area in the Kalgoorlie district of Western Australia, data from a ground magnetic survey and a low-level aeromagnetic survey were transformed with the second horizontal derivative filter to reveal different degrees of resolution of magnetic sources located in the regolith and the subsurface. Line profiles, and images and contours of the gridded data, from both surveys resolved subsurface sources which have assisted with the identification of drill targets. Importantly, however, magnetic features in the regolith were only resolved in the enhanced ground survey data which has assisted with geological mapping of the regolith.