Drilling Methods

Drilling is used to obtain very detailed information about rock types, mineral content, rock fabric and the relationships between rock layers close to the surface and at depth. Drilling is only used in areas that have been selected as "targets" from geological, geophysical and/or geochemical methods.

Drilling platform

Four drilling methods are widely used, each depending on the type of information required and/or the rock types being drilled.

Air-rotary drilling-this is a quick and economical method of producing a sample. Compressed air is forced down the drill pipe and, as it returns to the surface, it carries with it rock chips made by the rotating bit. This method is used in soft rock materials to about 25m deep, or to drill through the top layer of decomposed rock and soil to get a fresh rock sample from the bedrock below.

Air-percussion drilling-this method is used for penetrating hard rock types to a depth of around 300m. Rather than a rotating bit, a hammer bit is forced through the rock by compressed air. As in air-rotary drilling, rock chips are brought to the surface by the returning air.

Examining the samples

Mud-rotary drilling-this method is used for drilling through soft rocks, sand and clay layers especially in the search for coal, oil or gas. A special mix of clay and water is forced down the drill hole turning a rotary bit, with rock chips returning in the "mud" slurry. This method is used for holes up to 3km deep and can be done from ships or offshore platforms especially in the search for oil and/or gas. A person who collects the chips and identifies the rock type and other information is known as a "mud-logger".

Mud-rotary drill bit

Diamond core drilling-this method involves a pipe encrusted in industrial diamonds being used to drill through rock layers, with a "core" of rock being left in the centre of the pipe. This core is recovered and gives information not only about the rock types, but also about the relationships between the rock layers in detail.

Diamond core drill bit

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