Drilling is expensive, so this next phase of exploration only begins when all the data have been gathered from pre-existing geological and topographic maps, aerial/satellite photographs, geological mapping and from seismic surveying.
The thickness and quality of a coal seam in an area are first determined by drilling boreholes a few kilometres apart using a grid pattern. Mobile drilling rigs (Figure 11) use a powerful motor to rotate a drill bit attached to a series of steel rods within the hole. The bit, made of tungsten carbide or studded with diamonds, grinds away the rock, cutting a cylindrical hole through the rock sequence as pressure is applied to it. Specialized drilling fluids are used to lubricate the bit. The same fluids bring small fragments of rock, or cuttings, to the surface, where they can be examined by the geologist.
If substantial samples are required when a coal seam is penetrated, a cutting barrel can be used in place of the solid drill bit to drill out a cylinder of rock, called a core (Figure 12). Coring sequences of strata is slow and expensive, and is only undertaken when details from cores are essential.
Given that drilling a borehole currently costs around £190 per metre, and that coring is two and a half times more expensive than this, calculate the cost of drilling a borehole 800 m deep, which includes 130 m of core.
If drilling a borehole costs £190 per metre, coring must cost £190 per metre × 2.5 = £475 per metre. The cost of drilling the borehole is therefore:
The cost of coring the remainder is therefore:
The total cost is therefore £61 750 + £127 300= £189 050
Although expensive, cores do make it possible to constrain the thickness and depth of the coal seam. Furthermore, detailed analysis of other sediments in the cores can reveal the environment in which the rocks were originally deposited. Recalling Figure 5, this may permit geologists to predict parts of the rock sequence where coal is either more or less likely to be found. Coal samples are usually recovered from the core for chemical analysis, to measure its carbon (i.e. rank), sulphur and ash content.