Which of the following tubewell drilling methods are correctly matched to their hydrogeological operating conditions?
Options:
(A) Cable tool percussion drilling: Suitable for boulder, gravel, and coarse alluvial formations where a heavy chisel bit crushes the formation
(B) Direct mud rotary drilling: Suitable for deep unconsolidated alluvial clay and sand formations using bentonite drilling mud for wall stabilization
(C) Reverse circulation rotary drilling: Ideal for large diameter (gt; 60\text{ cm}$) gravel-packed wells in coarse alluvium, minimizing formation clogging
(D) Down-the-Hole (DTH) hammer drilling: Operates via a pneumatic reciprocating hammer, highly efficient in hard crystalline igneous and metamorphic rocks
Answer Key:
A, B, C, D
Step-by-Step Solution & Explanation
All four drilling methods are correctly matched to standard geological formations in water well construction:
- Cable tool: High versatility in boulder/cobble alluvium.
- Direct rotary: Fast drilling in deep sedimentary basins using mud to hold hole walls.
- Reverse rotary: Water flows down annulus and rises inside drill string at high velocity, keeping hole clean without dense bentonite cake.
- DTH: Air percussion breaks hard basalt/granite rock quickly.