# A geologist is investigating layers of rock beneath the ground using seismic waves produced by a small controlled explosion at the surface. The waves travel downward, partially reflect at each boundary between different rock layers, and return to detectors at the surface. Explain how this method allows the geologist to determine both the depth of a rock layer boundary and the nature of the boundary.

> OCR A-Level Physics B: Advancing Physics (H557) — 5.1 Thermal physics · Explain · 5 marks

> Seismic waves travel at a known, constant speed through a given rock type. When they reach a boundary between two different rock types, some of the wave energy is reflected back to the surface while the remainder continues downward into the next layer.

## Mark scheme (5 marks)

1. When seismic waves reach a boundary between two different media/rock types, they are partially reflected back
2. The time between emission and detection of the reflected wave is measured
3. Because the wave velocity/speed is known and constant, the depth of the boundary can be calculated from the travel time
4. A stronger/larger amplitude reflected wave indicates a greater difference in the properties of the two rock types at the boundary
5. Multiple reflections arriving at different times correspond to different boundaries, allowing the geologist to map several rock layers

## Key terms

- [seismic waves](https://www.gradenine.co.uk/glossary/seismic-waves)
- [boundary](https://www.gradenine.co.uk/glossary/boundary)

## Related

- [Revision notes for OCR A-Level Physics B: Advancing Physics (H557)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-geologist-is-investigating-layers-of-829687ed) · Published by Druglandscape Ltd.