---
title: "Coal seam attitude: square 300 m"
subject: "Engineering Geology II"
chapter: 6
key: "semester_4/engineering_geology_ii:bh-coal-seam-square-300"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/bh-coal-seam-square-300
site: BCT Engineering
---

# Coal seam attitude: square 300 m

Engineering Geology II · Chapter 6: Measurement, analysis and interpretation of structural geological data

- Asked in: 2078 Chaitra, 5 marks
- Video: [Structural Geology - How to do 3-point problems](https://www.youtube.com/watch?v=vnsB7LePrlk) — ESS234 online field course
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/bh-coal-seam-square-300
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-6

## Question

Three bore holes are sunk at SW, SE and NW corners of a square level ground. The side of the square is 300 m long. The bore holes are A, B, C respectively. The bore holes meet the coal seam at 15 m in A, 45 m in B and 60 m in C. Determine the attitude of the coal seam.

## Short answer

Depth gradients are 0.10 m/m east and 0.15 m/m north. Coal seam attitude is strike N56.3°W, dip 10.2° toward N33.7°E (NNE).

## Answer

**Method:** the coal seam is a plane. Take A (SW corner) as origin, x east, y north. B (SE) = (300, 0), C (NW) = (0, 300).

#### Data
| Borehole | Position (x, y) m | Depth to seam (m) |
|---|---|---|
| A (SW) | (0, 0) | 15 |
| B (SE) | (300, 0) | 45 |
| C (NW) | (0, 300) | 60 |

#### Depth gradients
$$
\begin{aligned}
a &= \frac{45 - 15}{300} = 0.10\ \text{(depth increases eastward)} \\
b &= \frac{60 - 15}{300} = 0.15\ \text{(depth increases northward)}
\end{aligned}
$$
Depth $d = 15 + 0.10\,x + 0.15\,y$.

#### Dip and dip direction
$$
\tan\delta = \sqrt{0.10^2 + 0.15^2} = 0.1803 \Rightarrow \delta = 10.22^\circ
$$
$$
\text{dip direction} = \arctan\left(\frac{0.10}{0.15}\right) = 33.69^\circ\ \text{from north (NNE)}
$$
The strike is perpendicular: $33.69^\circ - 90^\circ = -56.31^\circ$, i.e. N56.3°W (= 303.69°).

**Answer: attitude of the coal seam = strike N56.3°W, dip 10.2° towards N33.7°E (NNE).**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How are the depth gradients found?
   A. a = 45/300
   B. a = 60/300 and b = 15/300
   C. a = 15/300
   D. a = (45 − 15)/300 east and b = (60 − 15)/300 north
   Answer: D. Gradient is depth difference over distance along each side from A. (Key point: Gradients)

2. What is the dip?
   A. 10.2° from tan = sqrt(0.10² + 0.15²)
   B. 15°
   C. 33.7°
   D. 0.18°
   Answer: A. tan(dip) = 0.1803. (Key point: Dip)

3. How is the dip direction found?
   A. N56.3°W
   B. N33.7°E, arctan(0.10/0.15) from north
   C. due east
   D. due north
   Answer: B. Depth increases 0.10 eastward and 0.15 northward. (Key point: Dip direction)

4. What is the strike?
   A. N33.7°E
   B. due east-west
   C. N56.3°W, perpendicular to the dip direction
   D. N45°W
   Answer: C. Strike is 90° from the dip direction. (Key point: Strike)

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