# A geologist uses ultrasound pulses to investigate the depth of a rock layer buried beneath the ground. The ultrasound is emitted from the surface, travels downward, and some of the waves are reflected back when they reach the boundary between two different rock types. Explain how the geologist can use the reflected ultrasound waves to determine the depth of the rock layer boundary, and explain why ultrasound is better suited to this task than visible light.

> OCR A-Level Physics B: Advancing Physics (H557) — 5.2 Circular motion and gravitational fields · Explain · 5 marks

> Ultrasound waves have a frequency above 20 kHz, which is higher than the upper limit of human hearing. When ultrasound reaches a boundary between two media, it is partially reflected back. The speed of sound through rock is approximately 3000 m/s.

## Mark scheme (5 marks)

1. When ultrasound reaches the boundary between the two rock types, it is partially reflected back to the surface
2. A receiver at the surface detects the reflected ultrasound and the time between emission and detection is measured
3. Because the speed of the ultrasound through the rock is known (constant), the depth can be calculated using distance = speed × time, with the time halved because the wave travels to the boundary and back
4. Visible light cannot penetrate through opaque rock, whereas ultrasound can travel through solid media
5. Ultrasound is a non-invasive method that can image beneath surfaces without damaging the rock or requiring excavation

## Key terms

- [ultrasound waves](https://www.gradenine.co.uk/glossary/ultrasound-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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-geologist-uses-ultrasound-pulses-to-81981c36) · Published by Druglandscape Ltd.