How do scientists study earth's layers?
By studying how seismic waves travel through the Earth
Seismic waves, which are generated when an earthquake occurs, travel throughout the entire Earth. Since the different layers of the earth can allow seismic waves to pass through them differently, scientists use this to know about the earths layers.
And so, by studying the rate at which these waves travel through the layers of earth, the Scientists study about them.
~Hope this helps.