As a mining expert with extensive knowledge in the field of geology and mining engineering, I am well-versed in the various techniques and depths associated with coal mining operations. The depth of coal mines can vary significantly based on a multitude of factors including the geology of the area, the thickness of the coal seam, and the mining technology available.

Deep mining is typically employed when coal seams are located at depths of 180 to 300 feet (55 to 90 meters). This method involves the use of tunnels and shafts to access the coal seams below the surface. Deep mining requires significant infrastructure and is often more expensive than surface mining due to the costs associated with excavation, ventilation, and worker safety.

However, there are exceptions to this general rule. In some cases, even coal that is found at depths exceeding 200 feet (60 meters) can be mined using surface mining techniques. This is particularly true in the western United States, where the coal seams are often quite thick, ranging from 60 to 90 feet (20 to 25 meters). The decision to use surface mining in these instances is influenced by the economic feasibility and the environmental impact of the mining operation.

Surface mining, which includes methods such as open pit mining, is more suitable when the coal seams are close to the surface and are thick enough to justify the removal of the overburden (the layer of earth and rock above the coal seam). This method is less costly and can be more efficient in terms of extraction rates compared to deep mining.

It's important to note that the choice between deep mining and surface mining is not solely based on depth. Other factors such as the quality of the coal, the environmental regulations in place, and the market demand for coal also play a significant role in determining the most appropriate mining method.

Moreover, safety is a paramount concern in deep mining operations. The deeper the mine, the greater the challenges in terms of safety, including the risk of rock falls, flooding, and the need for effective ventilation systems to manage harmful gases.

In conclusion, while deep coal mines are generally found at depths of 180 to 300 feet, the actual depth can vary, and in some cases, surface mining techniques can be adapted to extract coal from greater depths. The decision to employ a particular mining method is a complex one, taking into account geological, economic, and safety considerations.

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Coal that occurs at depths of 180 to 300 ft (55 to 90 m) are usually deep mined, but in some cases surface mining techniques can be used. For example, some western U.S. coal that occur at depths in excess of 200 ft (60 m) are mined by the open pit methods, due to thickness of the seam 60-C90 feet (20-C25 metres).read more >>