Cable drilling rigs are operated by repeatedly lifting and dropping a heavy string of tools into a borehole. These tools break consolidated rock into small fragments and loosens materials from the side of the borehole. Water is then added by the driller and mixed with the crushed particles, creating a slurry in the bottom of the hole. An experienced driller can tell when there is too much slurry at the bottom of the borehole, and it is reducing the drilling to an unacceptably slow level. When this happens, a portion of the slurry is removed, and the drilling will continue. This process will continue until the borehole is finished.
Rotary drilling rigs use a sharp, rotating drill bit to break through the layers of rock. Similar to a handheld drill, the spinning motion of the drill bit will allow for penetration of even the hardest rock. Rotary drilling cuts by rotating at the bottom of the hole. In addition to rotation, the drill must exert downward pressure as the drill bit cuts its way through the rock. Part of the art of rotary drilling is to match the bit type and pull down pressure with the rock we are attempting to drill through. The use of drilling fluids to maintain circulation and to keep the hole lubricated and free of cuttings is essential.