In mineral exploration, a drilling structure describes the complete process from initial hole opening to achieving target depth.

This includes variations in borehole diameter and depth across different segments. Generally, greater diameter variation leads to increased structural complexity.

Key operational challenges arise in specific geological conditions: 

- Sandy clay/gravel layers: High risk of wall collapse 

- Water-sensitive formations: Potential borehole diameter shrinkage 

- Quicksand layers: Significant hole formation difficulties 

When standard mud systems or specialized wall-protection agents prove inadequate, casing installation becomes critical to isolate unstable zones. Multi-layer casing (up to three tiers) might be required even at moderate depths, resulting in a multi-stage stepped drilling profile. This necessitates an initial opening diameter substantially larger than the final required hole size.