Investigation inside the Earth
The surest way to see what the Earth is like is to penetrate and see it yourself. However, there is a limit to penetration because the temperature and pressure rise as you enter the Earth.
Therefore, scientists use indirect methods such as seismic wave studies, mineral synthesis experiments, and meteorite studies.
Types and characteristics of seismic waves
There are several types of seismic waves.
The primary wave (P wave)is the fastest among seismic waves. It can pass through solids, liquids, and gases.
The secondary wave (S wave) is slower than the P wave. S wave is observed after the P wave is observed. Also, only solid substances can pass through. Therefore, it was used to infer the Earth's interior structure because it cannot pass through the liquid portion (mainly the nucleus).