To minimise risk, a more effective system of mine planning and operation is required which calls for advance knowledge of structural and geological conditions ahead of the coal working face. This research project attempts to identify, by means of a literature survey, different exploration and geophysical methods that could be used to detect dykes ahead of underground coal mine workings. In this study a range of geophysical and exploration methods have been identified which are generally used in the mining industry. Each different method exploits a different physical property of the earth’s crust, e.g. seismic and acoustic methods measure the velocities of sound waves, while magnetic methods distinguish the variance in magnetic susceptibility of different rock types. Gravitational survey methods detect and measure lateral variations in the earth’s gravitational pull that are associated with near surface changes in density.
Dykes generally display a high level of magnetic susceptibility and magnetic survey methods are therefore commonly used by most coal mining companies to identify the presence of this geological feature. The sophistication of aeromagnetic survey techniques has increased significantly over the past decade and is now routinely used by local and overseas coal mining companies to explore relatively large target areas fairly successfully. Horizontal, in-seam drilling was first tested in the United States during the late 1950’s. This technique was initially developed to degasify the coal seam ahead of mining operations. Similar trials were conducted in Australia during 1980 and by mid-1981 the Australlian Coal Industries Research Laboratories Limited (ACIRL) reported that “600 m longholes can be achieved on a regular basis”. In-seam drilling is now practiced on a number of South African collieries and hole lengths of up to 1200 m are standard. During the early 1990’s Amcoal introduced incline-directional drilling from surface as a means to detect dykes and other geological features ahead of mining. Average borehole lengths achieved with this method are now in the order of 1200 metres, however, development work is in progress to increase the effective reach to 2000 metres. Similar work is currently in progress at Sasol Coal.
The use of ground penetrating radar as a means to detect dykes has been tested on various South African gold mines. Initial reports indicate that the presence of dykes and faults could be detected successfully up to a distance of 40 metres ahead of the mining face. The same level of success could, however, not be achieved on coal mines.
At present most South African coal operations use a combination of vertical drilling, aeromagnetic surveying and in-seam drilling techniques to detect and identify the presence of dykes ahead of the mining face with a relative high degree of accuracy