Mine Development & Surveying

Structural Geology

Dip, strike, folds and faults — and the apparent-dip and true-thickness trigonometry that turns a geological section into mineable numbers.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Structural geology describes how rock layers are oriented and deformed. A planar bed is fixed by its strike (the horizontal line on the plane) and its true dip

δ

(the steepest inclination, measured perpendicular to strike). Any section cut oblique to strike shows a gentler apparent dip

α

. Deformation produces folds (anticlines/synclines), faults (normal — extension, hanging wall down; reverse/thrust — compression, hanging wall up; strike-slip — lateral), and joints (fractures with no displacement). For mine planning the two quantities that matter most are the apparent dip seen in a given drive direction and the true thickness of a seam from its outcrop/borehole width.

The Core Formula Matrix

Apparent dip:

tan α = tan δ sin β

(

β

= angle between section and strike)

True thickness from horizontal width

w

(across-strike, beds dipping

δ

t = w sin δ

True thickness from a vertical borehole depth

d

through the seam:

t = d cos δ

Limits: along strike

β = 0 \Rightarrow α = 0

; perpendicular

β = 9 0^{\circ} \Rightarrow α = δ

The ‘IIT Traps’

⚠
**Apparent dip $\leq$ true dip, always.** If a calculation gives $α > δ$ , the section angle $β$ was mis-measured.
⚠
**Horizontal width uses $sin δ$ ; vertical depth uses $cos δ$ .** Picking the wrong projection swaps the answer.
⚠
Normal vs reverse fault. Normal = hanging wall down (extension); reverse/thrust = hanging wall up (compression).
⚠
** $β$ is measured from the strike**, not from the dip direction; using the complement flips $sin$ and $cos$ .

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

In a normal fault, the hanging wall moves:

Q2MEDIUM2 Marks · NAT

A coal seam has a true dip of

3 0^{\circ}

. A roadway is driven at

4 0^{\circ}

to the strike. The apparent dip along the roadway is ______ degrees. (Round off to two decimal places.)

deg

Q3HARD2 Marks · NAT

A seam dipping at

3 5^{\circ}

shows a horizontal outcrop width of

12 m

measured across the strike. Its true (perpendicular) thickness is ______ m. (Round off to two decimal places.)