Energy resources: Coal
Energy resources: Coal

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Energy resources: Coal

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 the rocks surrounding the borehole. These include the velocity at which sound travels through each rock type, their density and their emission of natural gamma radiation (from unstable isotopes of uranium, thorium and potassium that vary a great deal between different rock types). Because a single physical property cannot define a rock type, the logging geologist must compare a range of properties at each depth to interpret the rock type present.

As the location, quality and quantity of coal in a coalfield will affect the profitability of a mine, mining will not commence until the mining company and its financial partners are satisfied that these crucial parameters have been sufficiently constrained. Even after mining has commenced, further exploration (including the drilling of more boreholes) periodically allows initial estimates to be refined.

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