When drilling a well, the standard procedure is to set casing to whatever depth is required by law or that conditions dictate. Then, you drill below the casing to find the aquifer. This is when it gets interesting. The driller has to not only find and identify the producing formations, he also has to minimize damage to the formation.
Mud drilling requires a mud weight slightly higher than the formation pressure to keep the hole stable. But this also damages the formation by intrusion of fines and cuttings into the producing aquifer. It is not as bad with air drilling, because the pressure in the wellbore is always less than the formation pressure. Still, the force of the air and the hammer does push some cuttings into vugs, crevices and other parts of the formation. The least damaging method of drilling is air-assist reverse circulation. With that method, the formation pressure is always higher than the pressure at the face of the bit.