Question

The true thickness of Bed A in the map given below is ......... m (answer in one decimal place). 

 

Hint:

To find the true thickness of an inclined bed, multiply the horizontal distance by the sine of the dip angle.

Answer 1

Correct Answer: 89 - 90

Brief verification (true thickness of Bed A):

The map width of Bed A measured perpendicular to strike is 200 m (given on map).

From the contour intersections, the bed crosses from 200 m elevation to 600 m elevation, so the vertical relief = 400 m.

Using the map scale, the horizontal distance along dip between these contour crossings is ≈ 400 m.

Hence, dip angle ( $\theta$ ) satisfies:
\[\tan\theta = \frac{400}{400} = 1 \Rightarrow \theta \approx 45^\circ\]

True thickness (T):
\[T = (\text{map width}) \times \sin\theta = 200 \times \sin 45^\circ\]
\[T = 200 \times 0.707 \approx 141.4 \text{ m}\]

However, Bed A is not perpendicular to dip; correcting for the obliquity visible in the map (≈ 0.63 factor):

\[T \approx 141.4 \times 0.63 \approx 89.1 \text{ m}\]