大中型水电站，大多座落于河流中上游河谷地段，其河床覆盖层特点是松散，少胶结，渗透性大。钻进中护壁困难，冲洗液赔失严重，还有局部架空层；取心取样难。由于松散性和软硬不均，常规岩心钻探只能采取部分大粒径漂砾石，薄砂层和细颗粒往往被冲洗液冲跑。因而，全面查明覆 Large and medium-sized hydropower stations, mostly located in the upper reaches of the river valley, the riverbed cover is characterized by loose, less cemented, permeability. Difficult to drill into the retaining wall, flushing fluid lost serious damage, as well as partial overhead layer; coring sampling difficult. Due to looseness and unevenness of hardness, conventional core drilling can only take part of the large size of the drift gravel, sand and fine particles often run flushing liquid. Therefore, a comprehensive identification of coverage