What me this is used till study the composition of the earths core?

Seismic waves are measured.

Explanation:

During an earthquake, there is a build-up of stress. When it is released it sends different shock waves or seismic waves. While we feel this travelling to the surface as earthquakes, the waves also travel towards the core. As it passes through the mantle, the outer core and the inner core the waves refract around the different mediums and are measured on other areas of the surface.