Interpretation of airborne magnetic data over selected areas of Witbank coalfield, South Africa : an aid to mine planning

Abstract

The main objective of this work is to aid coal mine planning by determining geological structures that affect mining such as faults, dykes and sills. Faults can displace the coal seams by breaking and throwing the coal seam on one side of the fault deeper; thereby constituting safety risks or even making exploitation uneconomical. Dykes are associated with devolatilisation of the coal around them. Both dykes and faults are also important in terms of groundwater management. The sills have the same devolatilisation effect as dykes but on a larger scale as they cover more area than the dykes. They also make mining difficult if one has to mine through them, especially if they are not weathered. The study was based on high resolution airborne magnetic data which was flown on selected areas in the Witbank coalfield. The combined areas cover a total of ~171001 hectares. Total field magnetic data were collected. The data were processed and gridded. The resulting total field magnetic grids were enhanced for better delineation of structures by: reducing the data to the pole, calculation of the first vertical derivatives and computation of the analytic signal. Using the enhanced grids together with other datasets; topographic maps, borehole log information and infrastructure information from the surrounding mines, the geological structures were interpreted. Intense geological activity is observed on the eastern areas of the coalfield (Belfast and Arnot). This is the region where numerous geological activities of different ages were recorded, e.g., the intrusion of Bushveld Complex, the Olifants River Dyke Swarm (ORDS) and the Karoo Basalts. Associated with the intense geological activity is the presence of dykes, faults and sills, which are observed throughout the areas. In the central region, Vandyksdrif area 1 reveals the pre-Karoo basement rocks (felsites and diabases of the Bushveld Igneous Complex) and Vandyksdrif area 2 reveals the Karoo dolerite in the form of a sill. The sill corresponds to the high elevation area on the digital terrain model data suggesting that the sill might be unweathered. Unfortunately, the borehole logs do not mention the state of the dolerite. The western areas reveal mainly the dolerite sill. Like in Vandryksdrif area 2, there is no mention of the state of the dolerite and the suspicion is that the dolerite is also unweathered there as well. Copyright