The inner Earth is inaccessible. Unlike the surface, where we can collect rocks and study them directly, further than a few kilometres in we can only deduce the geology through physical reasoning and experiment. This part of the web site gives a guide to how we can establish the geological nature of the inner Earth. We can start by establishing the shape and size of the Earth (it is not a perfect sphere). We can apply Newton's Law of Gravitation to estimate the mass and density of the Earth. To establish how this mass is distributed - and thus get a first idea of the inner structure - we must establish the Moment of Inertia. All this allows us to deduce that there is a crust, mantle and core.
To get more information we must use the science of seismology - studying how seismic waves (especially from earthquakes) are transmitted. Different waves are transmitted with different velocities. Measuring the travel time of seismic waves from source (earthquake focus) to receivers distributed around the Earth can be used to establish the classical layered structure - confirming the models of Earth structure deduced from gravity and orbit.