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Reverse circulation drilling is also called central sample collection or double-wall drilling. This type of drilling usually uses a double-wall drill pipe. Drilling force is high wind pressure gas, which enters into the hammer through the gap between the outer pipe and the inner pipe. The hammer hit the drill bit to break rock, and the broken rock is collected into the pores on the drill bit surface, and then the rock samples are collected through the sample collection pipe and the drill pipe.

Advantages

1. No pollution, the reverse-circulation DTH hammer can quickly pick up samples on the drill bit surface at the moment of rock fragmentation. The sample does not need to pass through the hammer length, thus avoiding sample contamination and loss.

2. High productivity. In broken and fractured rock, the penetration speed is superior to the ordinary hammer.

3. Dry samples can be collected even in water condition, because drilling cuttings are collected as soon as they produced on the drill bit surface.

4. High sample recovery. Samples are collected from the drill bit surface, so samples loss can be avoided in broken and fractured rock formations. Since the bit diameter is almost equal to the external diameter of the wear sleeve, the sample collection recovery rate can reach 98%.