# Sonar is used on a research vessel to map the seabed. The vessel emits ultrasound pulses that travel down to the seabed, reflect, and are detected on their return. Explain how the sonar system determines the depth of the seabed, and why ultrasound waves rather than sound waves audible to humans are used for this purpose.

> WJEC A-Level Physics (Wales) — 5.2 Gravitational fields (A-Level only) · Explain · 5 marks

> A research vessel uses sonar to create detailed maps of the ocean floor. Ultrasound pulses are emitted from the hull of the vessel and detected after reflection from the seabed.

## Mark scheme (5 marks)

1. Ultrasound waves are emitted from the vessel and travel down through the water to the seabed
2. The ultrasound waves reflect off the seabed (boundary) and return to the vessel
3. The time between emission and detection of the reflected pulse is measured
4. The depth is calculated using the wave velocity and the measured time (depth = velocity × time / 2)
5. Ultrasound has a frequency above 20 kHz, which is above the upper limit of human hearing, so it cannot be heard and does not cause interference / disturbance to marine life or crew; OR ultrasound has a shorter wavelength giving better resolution / more detailed images of the seabed

## Key terms

- [sonar](https://www.gradenine.co.uk/glossary/sonar)
- [ultrasound waves](https://www.gradenine.co.uk/glossary/ultrasound-waves)

## Related

- [Revision notes for WJEC A-Level Physics (Wales)](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/sonar-is-used-on-a-research-e9f819cb) · Published by Druglandscape Ltd.