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Physics · Waves

Ultrasound and seismic waves

Explain how ultrasound and seismic waves are used for detection and exploration, including echo sounding and evidence for the structure of the Earth (Physics only, Higher tier).

  • Higher
  • 6 key terms
  • All boards
Download the full pack · 3 files

Warm-up

Answer each one, then check.

  1. 1

    What is an echo?

    Show answerHide answer

    A reflected sound

  2. 2

    What is the speed equation?

    Show answerHide answer

    \(v = s \div t\)

  3. 3

    What is ultrasound?

    Show answerHide answer

    Sound with frequency above 20 kHz

  4. 4

    What causes an earthquake?

    Show answerHide answer

    Movement of the Earth's tectonic plates

  5. 5

    Is a P-wave longitudinal or transverse?

    Show answerHide answer

    Longitudinal

Learning Objectives

  1. 1Explain how ultrasound is partially reflected at boundaries and used for imaging.
  2. 2Use the time taken for an echo to calculate a distance.
  3. 3Describe P-waves and S-waves and how they provide evidence about the Earth's structure.
  4. 4Describe echo sounding.

DETECTION AND EXPLORATION

Ultrasound waves are partially reflected when they meet a boundary between two media. The time taken for the echo to return can be used to work out how far away the boundary is.

Seismic waves from earthquakes (P-waves and S-waves) give evidence of the structure of the Earth.

P-waves and S-waves

P-waves

  • Longitudinal seismic waves.
  • Travel through solids and liquids.
  • Travel faster than S-waves.
  • Speed changes between solids and liquids.

S-waves

  • Transverse seismic waves.
  • Cannot travel through liquids.
  • Travel more slowly.
  • Stopped by the liquid outer core.

Echo Sounding

A ship sends an ultrasound pulse down and receives the echo 0.40 s later. The speed of sound in sea water is 1500 m/s. Calculate the depth.

Show the solutionHide the solution
  1. 1 Distance there and back \(1500 \times 0.40 = 600\) m
  2. 2 Depth is half of this \(600 \div 2 = 300\) m

Answer300 m

Medical Ultrasound

An ultrasound pulse is reflected from a boundary inside the body after 0.000060 s. The speed in tissue is 1500 m/s. Calculate the depth of the boundary.

Show the solutionHide the solution
  1. 1 Total distance \(1500 \times 0.000060 = 0.090\) m
  2. 2 Depth \(0.090 \div 2 = 0.045\) m = 4.5 cm

Answer4.5 cm

Evidence for the Core

Explain how S-waves give evidence that part of the Earth's core is liquid.

Show the solutionHide the solution
  1. 1 S-waves Cannot travel through liquids
  2. 2 Observation They are not detected on the far side of the Earth
  3. 3 Conclusion Part of the core (the outer core) must be liquid

AnswerS-waves are blocked by the outer core, so it must be liquid.

Uses

Other examples.

  • Medical imaging

    Ultrasound scans of foetuses and organs are safe (non-ionising).

  • Industrial imaging

    Ultrasound detects cracks in metal.

  • Echo sounding

    Used for water depth and to detect fish or shipwrecks.

  • Seismology

    New evidence from seismic waves led to discoveries about parts of the Earth that cannot be observed directly.

Depth Finder

A boat sends out ultrasound and hears the echo after 0.20 s (speed 1500 m/s). Calculate the depth of the water. Explain why the time must be halved.

1. Use s = vt.

2. Halve the distance.

A good answer shows: Distance there and back = 1500 × 0.20 = 300 m; depth = 150 m. The pulse travels to the bottom and back.

Can I...?

  1. 1Define ultrasound.
  2. 2Explain echo timing.
  3. 3Calculate depth by echo.
  4. 4Describe P-waves.
  5. 5Describe S-waves.
  6. 6Explain the liquid core.
  7. 7Give uses of ultrasound.
  8. 8Explain why the time is halved.

Summary & Exam Focus

  • Ultrasound is partially reflected at boundaries.
  • Depth = speed × time ÷ 2.
  • P-waves: longitudinal; S-waves: transverse, not through liquids.
  • Seismic evidence shows a liquid outer core.

Exam focus

A ship sends out ultrasound and receives the echo 0.40 s later. The speed is 1500 m/s. Calculate the depth of the sea. (3 marks) (3 marks)

Distance there and back, then halve.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

Ultrasound
Sound with frequency above 20 kHz.
Echo
A reflection of a sound wave.
Seismic wave
A wave produced by an earthquake.
P-wave
A longitudinal seismic wave.
S-wave
A transverse seismic wave that cannot travel through liquids.
Echo sounding
Using reflected sound to find depth.

Practice questions

Have a go at each one before you open its answer.

  1. Question 1 Calculate 3 marks

    A ship sends a pulse of ultrasound towards the sea bed and detects the echo 0.40 s later. The speed of sound in sea water is 1500 m/s. Calculate the depth of the sea at this point.

    Show answerHide answer

    Model answer

    Distance there and back = 1500 × 0.40 = 600 m; depth = 600 ÷ 2 = 300 m.

    Mark scheme

    • Correct substitution — 1 mark
    • 600 m — 1 mark
    • 300 m (halved) — 1 mark
  2. Question 2 Explain 3 marks

    Explain how ultrasound waves can be used to produce an image of an organ in the body.

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    Model answer

    Ultrasound is partially reflected at boundaries between different tissues. The time for the echoes to return to a detector is used to work out the distance to each boundary, and the signals are used to build up an image.

    Mark scheme

    • Partially reflected at boundaries — 1 mark
    • Time taken for echoes measured — 1 mark
    • Distance calculated and used to form an image — 1 mark
  3. Question 3 Explain 4 marks

    S-waves cannot travel through the Earth's outer core. Explain what this tells scientists about the outer core, and how the properties of P-waves give further evidence about the Earth's structure.

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    Model answer

    S-waves cannot travel through liquids, so the outer core must be liquid. P-waves travel through solids and liquids at different speeds, so their changes in speed and direction show the boundaries between layers of different materials.

    Mark scheme

    • S-waves cannot travel through liquids — 1 mark
    • So the outer core is liquid — 1 mark
    • P-waves travel through solids and liquids at different speeds — 1 mark
    • Changes show boundaries between layers — 1 mark
  4. Question 4 State 2 marks

    State two differences between P-waves and S-waves.

    Show answerHide answer

    Model answer

    P-waves are longitudinal and S-waves are transverse. P-waves travel through solids and liquids but S-waves travel only through solids (or P-waves are faster).

    Mark scheme

    • Longitudinal and transverse — 1 mark
    • A second difference — 1 mark
  5. Question 5 Explain 3 marks

    Suggest why ultrasound is used rather than X-rays to scan an unborn baby.

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    Model answer

    Ultrasound is non-ionising so it does not damage cells, whereas X-rays are ionising and could cause mutations.

    Mark scheme

    • Ultrasound non-ionising — 1 mark
    • X-rays ionising — 1 mark
    • Could damage cells of the baby — 1 mark

Quick check

  1. Ultrasound has a frequency...

    1. Abelow 20 Hz
    2. Bof 50 Hz
    3. Cabove 20 kHz
    4. Dof 1 Hz
    Show answerHide answer

    C: above 20 kHz

    Above human hearing.

  2. In echo sounding the time is...

    1. Aone way only
    2. Bzero
    3. Chalf the real time
    4. Dthere and back
    Show answerHide answer

    D: there and back

    So halve it.

  3. S-waves cannot travel through...

    1. Aliquids
    2. Bsolids
    3. Crock
    4. Dmetal
    Show answerHide answer

    A: liquids

    They are transverse.

  4. P-waves are...

    1. Atransverse
    2. Blongitudinal
    3. Celectromagnetic
    4. Dstatic
    Show answerHide answer

    B: longitudinal

    Compressions and rarefactions.

  5. Seismic evidence shows that the Earth's outer core is...

    1. Asolid
    2. Bgas
    3. Cliquid
    4. Dempty
    Show answerHide answer

    C: liquid

    S-waves do not pass through.

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