# A geologist is investigating the structure of rock layers beneath the Earth's surface. She sends seismic waves downward from the surface and detects the waves that return. Explain how the geologist can use the returning seismic waves to determine both the depth at which a rock boundary lies and whether more than one boundary is present underground.

> OCR A-Level Physics A (H556) — 5.3 Astrophysics and cosmology · Explain · 5 marks

> Seismic waves are produced by earthquakes or by controlled explosions at the surface. When they reach a boundary between two different rock layers, they are partially reflected back to the surface, where detectors record them. The speed of seismic waves through rock is known.

## Mark scheme (5 marks)

1. When seismic waves reach a boundary between two media / rock layers, they are partially reflected back
2. The time between emitting the wave and detecting the reflected wave is measured
3. Since the speed of the waves is known (constant through a given medium), the depth of the boundary can be calculated using distance = speed × time (divided by two because the wave travels to the boundary and back)
4. If more than one boundary is present, multiple reflected waves will return at different times
5. The remainder of the waves that are not reflected continue through the boundary (are transmitted), allowing them to reach and reflect from deeper boundaries

## 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 A (H556)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-geologist-is-investigating-the-structure-84457149) · Published by Druglandscape Ltd.