# A geologist investigating the seafloor uses sonar pulses from a research ship to map the depth of sediment layers beneath the seabed. The sonar system emits ultrasound pulses downward. Some pulses reflect from the top of the sediment layer and some reflect from the solid rock beneath it. Explain how the sonar system is able to detect two distinct reflected pulses and how the geologist can use these reflections to determine the thickness of the sediment layer.

> OCR A-Level Physics A (H556) — 5.2 Circular motion and gravitational fields · Explain · 5 marks

> Ultrasound pulses are emitted downward from a research ship. The pulses travel through seawater, then through a layer of soft sediment, before reaching solid rock below. A receiver on the ship detects the reflected pulses.

## Mark scheme (5 marks)

1. Ultrasound pulses are partially reflected at each boundary between two different media
2. The first reflected pulse returns from the boundary between the seawater and the top of the sediment layer
3. The second reflected pulse returns from the boundary between the sediment and the solid rock below
4. The speed of ultrasound through the sediment is known/constant, so the time difference between the two received pulses can be used to find the distance travelled through the sediment
5. The thickness of the sediment is half the distance calculated, because the pulse travels down through the sediment and back up again

## 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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