Surface trenches were used to located what?
what are the broad tools available to geologist
Coal is the one that brought Industrialisation of United Kingdom. In the 1800 hundreds there was a Coal boom in England.
What is Coal boom?
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.
Early miners would have found it easy to trace the distinctive black colour of coal along an outcrop, 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 advanced techniques made it possible to significantly reduce the uncertainties associated with estimating the size of a coalfield.
Modern exploration techniques are aimed at accurately assessing the location, quality and quantity of coal in a coalfield. In order to achieve this there are three broad categories of tools available to geologists:
• geological mapping,
• geophysical methods (seismic surveying) and
• drilling methods.
They are considered here in the order in which they are likely to be employed.
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 known and mapped. However, in remote areas new finds are still possible. In such areas data acquired from satellites or aircraft are assessed before geologists start exploring on the ground.
The Global Positioning System (GPS), now increasingly used for navigation, uses signals from satellites to pinpoint locations precisely, so that geologists can more easily create accurate maps in the field. Field data are increasingly processed using spatial analysis software to create digital maps of coal outcrops and to model the likely extension of the coal beneath the surface.
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. 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.
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.
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 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.
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