Waves for detection and exploration
We use waves — especially ultrasound and seismic waves — to "see" inside objects and the Earth.
Ultrasound and echo sounding
- Ultrasound transmitter sends a pulse.
- When the pulse meets a boundary between materials of different density, some is reflected.
- Receiver picks up the echo; computer records the time delay.
- Distance to boundary: $d = v \Delta t / 2$ (factor of 2 because pulse goes there and back).
Uses:
- Medical ultrasound — imaging fetuses, tumours; safer than X-rays.
- Sonar — depth measurement, fish-finding, submarine detection.
- Industrial flaw detection — find cracks in metals.
Seismic waves
Earthquakes produce two types of seismic waves:
- P-waves (primary, longitudinal) — fast, travel through both solids and liquids.
- S-waves (secondary, transverse) — slower, only travel through solids (because liquids can't sustain shear waves).
By recording arrival times of P and S waves at seismic stations, we map the inside of the Earth.
How we know about Earth's interior
The S-wave shadow zone: at certain angles from an earthquake, no S-waves are detected. This shows the Earth has a liquid outer core (S can't pass through it).
P-waves bend (refract) at the core-mantle boundary, leaving a smaller P-wave shadow zone.
The Earth's structure (from outside in):
- Crust (thin, solid).
- Mantle (mostly solid, can flow over millions of years).
- Liquid outer core (~2900 km below surface).
- Solid inner core (centre).
✦Worked example— Worked example — sonar
A ship sends a sonar pulse and receives an echo 0.50 s later. Speed of sound in water ≈ 1500 m/s. Find depth.
- Distance there and back: $1500 \times 0.50 = 750$ m.
- Depth = 750/2 = 375 m.
⚠Common mistakes
- Forgetting the factor of 2 in echo distance.
- Saying ultrasound is dangerous — it's much safer than X-rays.
- Confusing P and S waves.
- Saying the inner core is liquid — it's solid (the outer core is liquid).
AI-generated · claude-opus-4-7 · v3-deep-physics