Using waves to explore hidden structures

1. Many structures cannot be observed directly, such as the inside of the Earth or the seabed beneath several kilometres of ocean.
2. Scientists explore them by sending waves into them and detecting the waves that come back or pass through.
3. When a wave meets the boundary between two materials, some of it may be reflected, some transmitted into the new material, and some absorbed.
4. The wave's velocity usually changes as it enters a new material, and if it crosses the boundary at an angle it changes direction, which is refraction.
5. How much of a wave is reflected or absorbed, and how fast it travels, depends on both the type of wave and the material it travels through.
6. Measuring when waves arrive, where they arrive and how strong they are therefore gives evidence about the materials and boundaries they have passed through.

Seismic waves

1. An earthquake releases energy that spreads out through the Earth as seismic waves.
2. Seismometers at monitoring stations all over the world record the vibrations when the waves arrive.
3. Comparing the records from many stations shows which waves reached each place, when they arrived and how strong they were.
4. The two types of seismic wave that travel through the inside of the Earth are P-waves and S-waves.

P-waves and S-waves

1. P-waves, or primary waves, are longitudinal, so the particles vibrate parallel to the direction in which the wave travels.
2. P-waves can travel through both solids and liquids, because both can be squashed and then spring back.
3. S-waves, or secondary waves, are transverse, so the particles vibrate perpendicular to the direction in which the wave travels.
4. S-waves can travel through solids but not through liquids, because the particles of a liquid slide past each other and cannot pass on a sideways vibration.
5. P-waves travel faster than S-waves through the same rock, so P-waves always reach a seismometer first.
6. The time gap between the arrival of the P-waves and the arrival of the S-waves gets longer the further a station is from the earthquake.
7. Combining the distances worked out by three or more stations pinpoints where the earthquake happened, which is how earthquakes are detected and located.
8. The speed of seismic waves increases gradually with depth in the mantle, so the waves refract gradually and travel along curved paths.

Evidence for a liquid outer core

1. After a large earthquake, S-waves are detected across the side of the Earth nearest the earthquake.
2. No S-waves are detected anywhere beyond about $105^{\circ }$ from the earthquake, measured around the Earth's centre, which leaves a huge S-wave shadow zone on the far side.
3. S-waves cannot pass through liquids, so the S-wave shadow zone shows that the Earth contains a liquid layer that blocks them.
4. The size of the shadow zone shows how big this liquid layer is, which reveals a liquid outer core with a radius of a little over half the Earth's radius.
5. P-waves can pass through the liquid outer core, but they slow down suddenly as they enter it and refract sharply at its boundary.
6. This sharp bending leaves a ring-shaped P-wave shadow zone, between about $105^{\circ }$ and $140^{\circ }$ from the earthquake, where almost no P-waves arrive.
7. The sharp refraction shows that there is a clear boundary where the solid mantle meets the liquid outer core.

Evidence for a solid inner core

1. Faint P-waves are detected inside the P-wave shadow zone, where none would arrive if the core were liquid all the way through.
2. These P-waves have been refracted or reflected at another boundary deep inside the core, where they speed up again.
3. This is evidence for a solid inner core at the centre of the Earth, which was first proposed by the seismologist Inge Lehmann in 1936.
4. Together, the seismic evidence gives a layered model of the Earth, with a solid crust, a solid mantle, a liquid outer core and a solid inner core.

Exploring deep water with sonar

1. Light cannot travel far through seawater because it is quickly absorbed, so the deep ocean floor cannot be seen.
2. Sound and ultrasound travel well through water, at about $1500m/s$, so they are used to explore deep water instead.
3. A ship's sonar transmitter sends a short pulse of ultrasound down into the water.
4. The pulse is reflected when it reaches the seabed or any object in the water, and a detector on the ship picks up the echo.
5. The time between sending the pulse and receiving the echo is the time taken for the sound to travel down and back.
6. Using $distance=speed\times time$ and then halving the result gives the depth of the seabed or the distance to the object.
7. Sonar is used to map the shape of the seabed, find shipwrecks, detect submarines and locate shoals of fish.
8. Hard rock reflects a strong echo, while soft mud absorbs more of the sound and reflects a weaker echo, so the strength of the echo shows what the seabed is made of.
9. Some of the sound passes into the seabed and is reflected from the boundaries between layers of rock and sediment beneath it, which helps geologists search for oil and gas.