A ship uses sonar to map the ocean floor. The sonar system emits ultrasound pulses downward and a receiver detects the reflected pulses returning from the seabed. Explain how the sonar system produces information about the depth of the ocean floor, and explain why ultrasound is more suitable for this purpose than audible sound.

OCR A-Level Physics B: Advancing Physics (H557) — 5.1 Thermal physics · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Sonar systems on research vessels emit pulses of ultrasound waves vertically downward. The time between emission and detection of each reflected pulse is recorded. Ultrasound has a frequency above 20,000 Hz, which is above the upper limit of human hearing.

Model answer (5 marks)

Ultrasound pulses are emitted downward and reach the water–seabed interface. A portion of the wave is reflected back to the receiver. The speed of sound in water (≈1500 m s⁻¹) is known, so the distance to the seabed can be calculated from the travel time: (d= frac{1}{2}v,t). A short time therefore means a shallow depth, a long time a deeper depth.

Ultrasound is preferred to audible sound because its frequency is above 20 kHz, outside the range of human hearing and most marine life, so it causes no audible disturbance. In addition, the shorter wavelength of ultrasound gives higher resolution, allowing smaller seabed features to be detected.

Examiner tips

  • Use the formula (d= frac{1}{2}v,t) to show how depth is calculated; include the speed of sound in water. Mention both the lack of audible disturbance and the higher resolution due to shorter wavelength. Keep the answer concise and use the exact terminology from the mark scheme.
  • common_mistakes
  • :
  • Forgetting the factor ½ when converting time to distance. Using audible sound instead of ultrasound. Not linking the high frequency to both noise avoidance and better resolution.

Mark scheme (5 marks)

  1. Ultrasound waves reach the boundary between the water and the seabed and are partially reflected back towards the receiver.
  2. The speed of the ultrasound waves through water is constant/known, so the time taken between emission and detection can be used to determine the distance to the seabed.
  3. A shorter time between emission and detection indicates a shallower depth; a longer time indicates greater depth.
  4. Ultrasound has a frequency above the upper limit of human hearing (above 20,000 Hz / 20 kHz), so it does not cause noise disturbance to crew or marine life in the audible range.
  5. Ultrasound has a shorter wavelength than audible sound of the same medium, giving better resolution/ability to detect smaller features of the seabed.

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