In a circular motion, the distance covered in half revolution is the length of the semicircle:
\[
\text{Distance covered} = \pi \times \text{radius} = \pi \times 1 = \pi \, \text{m}
\]
The displacement is the straight line joining the initial and final points, which is the diameter of the circle:
\[
\text{Displacement} = 2 \times \text{radius} = 2 \times 1 = 2 \, \text{m}
\]
Therefore, the ratio of distance covered to displacement is:
\[
\frac{\pi}{2} = \pi : 1
\]
Thus, the correct answer is \( \pi : 1 \).