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 · View as Markdown

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

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.

Model answer (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.

Examiner tips

  • Use the command word ‘Explain’ – give a clear, step‑by‑step description. Show the time‑difference calculation and state that distance = speed × time. Remember to divide the total distance by two to get the layer thickness.

Common mistakes

  • Confusing the speed of sound in seawater with that in sediment. Failing to divide the total distance by two, giving the round‑trip distance as the thickness. Not recognising that two separate reflections are produced by the two interfaces.

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

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