短语词组短语thin coal seam shearer 薄煤层采煤机extremely thin coal seam 极薄煤层thin coal seam group 薄煤层群thin coal seam mines 薄煤层矿井thin coal seam face 薄煤层工作面thin -seam 薄煤层extremely thin protective seam 极薄保护层thin coal seam mining 薄煤层开采 返回 thin seam