When you need to drill high quality holes with diameters between 100-254 mm, DTH (Down the Hole) drilling is by far the best and most effective method. DTH drilling can be performed in most types of rock, hard or soft. It's a commonly used method in the mining industry (blast hole drilling), water well drilling, construction, and in the oil and gas industry.
In DTH drilling, the drill string rotates while the drilling hammer continuously strikes down into the rock. Inside the hammer, a piston powered by compressed air gives the drill bit its striking power. This combined with the rotational movement means the rock is crushed in a very efficient manner.