变质岩矿区，由于经过了多次构造运动和变质作用，岩层倾角往往变得很陡。同时，岩层节理极为发育，岩性软硬不均，软硬互层递变频繁。在这种矿区钻进，如设计钻孔角度，不按地层特点选择确定，就会造成钻孔施工时，发生恶劣的镜曲现象。这种现象，尤其是在采用钢砂作磨料时，更为显著而且常见。 Metamorphic rock mining area, due to many tectonic movements and metamorphism, strata tend to become very steep. At the same time, the joints of rock formations are extremely developed, the lithology is hard and soft, and the layers of soft and hard layers are changing frequently. In this mining area drilling, such as the design of drilling angle, not to choose according to the characteristics of the formation, it will result in drilling construction, the occurrence of severe mirror phenomenon. This phenomenon, especially when using grit as an abrasive, is more pronounced and common.