The suitable drilling technique is selected depending upon the depth and size of the borehole, mineral to be investigated, sampling requirements, and nature of formation. Conventional core drilling is used for shallow-to-medium depths in difficult clayey and fractured formation. Wire-line drilling is used for deep holes with consolidated formation.
When representative soil samples are required, Split Spoon Sampler device is generally used, preferably with self-tripping assembly. Selection of drilling method is also determined by the availability of proper equipment and accessories. Selection of drilling fluid depends on nature of formation. Selection of methods of drilling is also affected by the availability of equipment and relevant standardized series accessories.
The selection of drill depends on following factors:
Borehole depth: The proposed depth and size of the borehole are the guiding factor for the selection of the drill. The capacity of a diamond core drill is based on its capacity to lift the drill string. The power consumed by the auxiliaries is around 20% and so the required capacity of the drill is to be increased by 20%.
Character of rock: The character of rock determines the speed and feed requirement. The drillability depends mainly upon formation hardness and toughness. Hard formations require hydraulic feed rigs.
Terrain condition: In difficult and hilly terrain lightweight rig and pumps are preferred. The Geologist-in-Charge determines the borehole location, drilling procedures (coring/ noncoring), total depth, and sample intervals and documents the requirements in the Drilling plan and Sampling plan prior to the start of field activities. The Drilling plan and Sampling plan may be prepared for each borehole. The Geologist-in-Charge will observe all handling of the core by the drill crew, will prepare a lithology log, and will collect core samples at specific intervals for purposes of chemical analyses and physical testing.
