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2.7.1 Recognizing geological problems

The most profitable coal mines are those that possess unbroken, horizontal seams of constant thickness and quality. In mines where this is not so, profit levels will depend on the ability of the mine geologist to predict changes in the seam before they are encountered at the face.

Geological problems fall into two categories — gradual changes and sudden changes. Where a change is gradual, such as a seam thinning or splitting, data from boreholes in advance of the workings, supplemente
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2.7 Geological problems in coal mines

A modern coalface is a very complex operation that represents a large investment in terms of capital, labour and planning. Cutting machines and lengthy conveyors are inflexible and require uniform geological conditions to maximize output. What then are the effects of geological variations on such a mining system?

Geological factors control the selection of working areas. The two principal geological conditions that affect mining operations are, first, the nature of the coal-bearing rock
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2.6 Underground mining

Coal extraction is of course less straightforward using underground mining techniques. The associated costs are higher, and these begin with the sinking of two shafts, an ‘upcast’ and a ‘downcast’ shaft for ventilation (Figure 22). Sinking these to a depth of a kilometre may take a few years and during thi
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2.4 Modern mine planning

Once the geological data gathered during the exploration phase has been evaluated, geologists will estimate the quality and quantity of coal present. Coal reserves (in tonnes) are calculated from volume × density (Section 5). The volume of coal is controlled by seam area and seam thickness.

Hence:

tonnage = se
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2.3.4 Geophysical methods — borehole logging

If a core is not recovered from a borehole, another way to assess the types of rock that it penetrates is to measure their physical properties. Mounting a string of electronic instruments behind the drill bit most conveniently does this: it allows the properties of the rock to be monitored as the borehole is drilled. An alternative is to lower instruments down the completed borehole by cable; hence the name wireline logging.

Such logging measures several physical properties of th
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2.3.3 Drilling

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 (Author(s): The Open University

2.3.2 Geophysical methods — seismic surveying

Geophysical survey methods use measurements made at or near the Earth's surface to investigate the subsurface geology. The most widely used geophysical method is seismic reflection surveying; a rapid and highly cost-effective way of gathering data.

A seismic source (produced either by the explosive release of compressed air in a shallow borehole, or a heavy pad vibrated hydraulically at the surface) generates seismic waves that travel through the ground (
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2.3.1 Geological mapping of coalfields

Coalfields can be divided into two categories: exposed coalfields, where the coal-bearing strata outcrop at the surface, and concealed coalfields, where they are hidden beneath younger rocks. Exposed coalfields can be defined with considerable precision by surface geological investigations; indeed geologists recording field data still represent the cheapest exploration ‘tool’ available to the coal industry.

In populated regions, the locations of coal outcrops are well
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2.3 Exploring for coal

Early miners would have found it easy to trace the distinctive black colour of coal along an outcrop (for example, a coastline or river valley), and surface trenches were used to locate less obvious outcrops. However, tracing an outcrop underground was problematical as the only means of exploration was by digging costly trial shafts. The development of exploratory steam-powered drilling in the early 19th century improved matters, but it was not until the mid- to late- 20th century that more a
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2.2 Winning coal in former times

Coal was probably first used as a fuel by early Chinese civilizations, and there is evidence for coal working in the UK since Roman times. However, early approaches to mining were limited by the available technology, and left much of the coal behind.

At first, coal was dug from seams exposed at the surface in shallow excavations into valley sides that followed the coal seam. The amount of coal that could be extracted from these trenches and from adits (short horizontal tunnels) w
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2.1 Finding and extracting coal

Coal is often regarded as the principal fossil fuel, and with good reason. There is almost three times more energy available from the global proven coal reserves as there is from proven oil and gas reserves taken together. Therefore, it is unsurprising that even today much time and effort is spent locating it.

This section considers the techniques used in coal exploration and how coal is produced from surface and underground mines. But first, a brief look at a few of the historical aspe
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1.7.1 Carboniferous mires

During the late Carboniferous, mires developed over vast areas of the UK. Much of today's land area was an extensive, low-lying plain bordering a sea to the south (a sea that was soon to be the site of a mountain-building episode). Any mountains that existed lay hundreds of kilometres to the north. Large river systems meandered southwards across these plains.

At that time, the UK lay in tropical latitudes, almost on the Equator (see Author(s): The Open University

1.7 How old is coal?

Not surprisingly, the distribution of coal deposits through time corresponds closely to the origin and distribution of land plants. (This is discussed further in Section 4.) Coals are commonly found in rocks from Carboniferous times onwards, Devonian coals are rare, and pre-Silurian true coals are never found. This coincides with evidence for the evolution of land plants, which first appeared in Silurian times about 400 Ma (million years) ago, colonized the land surface rapidly through the De
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1.6 Impurities in coal

Coal rank reflects the maturity of a coal, but another variable is the ratio of combustible organic matter to inorganic impurities found within the coal. As discussed earlier, impurities result mainly from clay minerals washed into the mire prior to its eventual burial. In addition, some impurities are formed from the plant material itself during coalification.

These inorganic impurities are non-combustible and therefore leave an inert residue or ash after coal combustion. High-a
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1.5 The physics and chemistry of coal formation

Coal is a type of sediment made up mainly of lithified plant remains. But how does spongy, rotting plant debris become a hard seam of coal? As discussed earlier, plant material growing in mires dies, and then rots under anoxic conditions to form peat (by the process of humification). With time, the mire becomes covered with layers of sediment, the weight of which squeezes water and gas out of the pore spaces and compacts the vegetation. As subsidence allows deposition of further mire
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1.4 Coal-forming environments in the geological record

Figure 5 simplifies a typical vertical succession of sedimentary rocks found in many coalfields. The sequence from the base of the section upwards reveals the following:

  1. When a mire starts to form, the first plants take root in underlying clays or sands that form the soil. Their r
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1.3.2 Peat formation in raised mires

Mires can also form inland within low-lying depressions, provided the rate of precipitation exceeds the rate of evaporation (Figure 4a). Peat is impermeable and so its accumulation progressively impedes drainage. This attribute gives mires the ability to maintain a water table independent of the area surrounding them.
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1.3.1 Peat formation in deltas and coastal barrier systems

Since mires require poor drainage, low-lying land close to coastal areas might provide the right conditions for peat to form. Most extensive areas of modern peat formation are indeed situated not far above sea-level, and as Figure 2 shows, they are commonly associated with river deltas and coastal barriers. Such enviro
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1.3 Coal-forming environments today

Coal formation begins with preservation of waterlogged plant remains to produce peat and then slow compression as the peat is buried. About 10 m of peat will compress down to form about 1 m of coal; clearly large amounts of plant debris must be available for preservation. Even so, for a significant thickness of peat to accumulate there must be a balance between the growth of plants and the decay of underlying dead material to form peat (a process known as humification).

Su
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1.2 The origins of coal

If you examine a piece of coal, at first sight it appears black and rather homogenous. However, closer inspection generally shows a series of parallel bands up to a few millimetres thick. Most obvious are shiny bands that break into angular pieces if struck. Between them are layers of dull, relatively hard coal and thin weak layers of charcoal-like carbon. Coal splits easily along these weak layers, which crumble to give coal its characteristic dusty black coating.

Microscopic examinati
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