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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
Ultrasound pulses are emitted from the vessel and travel through the water until they reach the seabed. The pulse is reflected at the seabed–water boundary and returns to the vessel. The time interval between emission and reception of the reflected pulse is measured. The depth is calculated from the measured time and the known speed of sound in water using the formula depth = (velocity × time)/2, because the pulse travels the distance twice. Ultrasound is used instead of audible sound because its frequency is above 20 kHz, beyond human hearing, so it does not disturb the crew or marine life, and its shorter wavelength gives higher resolution of the seabed.
Examiner tips
- Show the full sequence: emission → travel → reflection → return
- Include the depth formula with the division by 2
- Explain the frequency advantage (above 20 kHz) and resolution benefit
Common mistakes
- Forgetting the division by 2 in the depth calculation
- Confusing audible sound with ultrasound, or omitting the frequency reason
- Not mentioning the short wavelength advantage
Mark scheme (5 marks)
- Ultrasound waves are emitted from the vessel and travel down through the water to the seabed
- The ultrasound waves reflect off the seabed (boundary) and return to the vessel
- The time between emission and detection of the reflected pulse is measured
- The depth is calculated using the wave velocity and the measured time (depth = velocity × time / 2)
- 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 in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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