Energy resources: Coal
Energy resources: Coal

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Energy resources: Coal

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 (Figure 9). These are reflected at buried geological boundaries and return to the surface where their amplitude and time of arrival are recorded by an array of detectors (Figure 9). Data produced by moving the positions of both source and detectors along a surface traverse is processed by computer to produce a seismic section through the Earth along the line of the survey, which take the form of a vertical cross-section. Further details of seismic surveying follow in Section 3.3.3, where its principal use in petroleum exploration is discussed.

Figure 9
Figure 9 The layout of a seismic survey, showing the reflection of seismic waves by two reflecting layers.

Figure 10 shows an example of a seismic section. A series of light and dark lines (or reflectors) approximately show the orientation of the sedimentary strata at depth. Coal has very different seismic characteristics from those of most common rocks. This results in large variations across coal-sediment boundaries that produce strong reflections. Seismic sections can also indicate geological problems that might be encountered in a coalfield. In particular, the displacement of a number of reflectors might indicate the presence of a fault, of which there are several in Figure 10.

Figure 10
Figure 10 An example of a seismic section. The (vertical) arrival time axis in milliseconds (ms) is roughly equivalent to increasing depth. Towards the top of the section a pair of dark lines indicate major coal seams. They are displaced by a fault near the centre of the traverse (marked by the dashed red line). Many other features show up, including greater complexity in the deeper part of the section, and towards the left of the section deep, more steeply dipping reflectors are truncated by the simpler ones at shallower depths: this is an unconformity (solid blue line).

The data in a seismic section enables the subsurface geological structure to be visualized. However, eventually, the interpretation has to be related to reality by drilling vertical boreholes to sample inferred rocks directly.

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