Complex sentence: The choice of mining method depends primarily on depth, density, overburden and thickness of the coal seam; seams relatively close to the surface, at depths less than approximately 180 ft (50 m), are usually surface mined.
Split into simpler sentences: The choice of mining method depends primarily on depth of burial, density of the overburden and thickness of the coal seam. Seams relatively close to the surface, at depths less than approximately 180 ft (50 m), are usually surface mined.