The proportion of underground coal production as a percentage of overall Australian production has fallen in the last 10 years, with underground production remaining more or less static at about 53 Mt (58 million st). However, underground coal mining remains a viable industry in Australia, due to use of modern longwall mining systems. In 1993-1994, 26 longwalls were in operation, with total production exceeding 39 Mt/a (43 million stpy). Some of these longwalls achieve productivity levels approaching efficient opencut operations, and in some cases, they exceed them. These high-production longwall mines are not without problems when depth and distances from outcrop increase and the gas content of the seams approaches or exceeds critical limits. When gas content of a coal seam exceeds 6 m~3A (233 cu ft/st), gas flows into the mine may be sufficient to interrupt production from time to time. If other seams surround the mined seam, the quantity of gas emitted from all sources is often sufficientto overcome the ventilation system. This problem occured early in some Australian mines, especially in the New South Wales Southern District, and ventilation solutions were put in place. These solutions have, in many cases, subsequently required augmentation by gas drainage networks - both in-seam and downhole - to capture and direct gas away from the longwall ventilating district.
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机译：在过去的十年中，地下煤炭产量在澳大利亚总产量中所占的比例下降了，地下产量或多或少保持不变，大约为53 Mt（5800万标准吨）。但是，由于使用了现代长壁开采系统，地下煤炭开采在澳大利亚仍然是一个可行的行业。在1993年至1994年期间，有26座长壁管投入运营，总产量超过39吨/年（4300万标准杆）。这些长壁墙中的一些达到了接近有效的明挖操作的生产率水平，在某些情况下，它们超过了它们。当露头的深度和距离增加并且煤层中的瓦斯含量接近或超过临界极限时，这些高产长壁矿井并非没有问题。当煤层的瓦斯含量超过6 m〜3A（233 cu ft / st）时，流入矿井的瓦斯可能足以不时中断生产。如果其他煤层围绕开采的煤层，则从所有气源发出的气体量通常足以克服通风系统。这个问题在澳大利亚的一些矿山中很早就出现了，特别是在新南威尔士州南部地区，并且采取了通风措施。在许多情况下，这些解决方案随后都需要通过煤层内和井下的瓦斯抽采网络进行扩充，以捕获和引导长壁通风区的天然气。
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