Inferences about the interior of the Earth, so far from being all inferior to those in the ‘exact’ sciences, range from those which are indeed flimsily based to inferences that are now as well established as commonly accepted results in standard physics.K. E. Bullen, The Earth's Density (1975)
All the information we have about the inaccessible interior of the Earth is embodied in Earth models which, if they are well constrained by observations and physical laws, are, at least in some respects, open to as little doubt as accepted tenets of, for instance, astronomy.
The previous chapters were devoted to laying the groundwork of the physics and thermodynamics that apply to the materials constituting the deep Earth, emphasizing the contribution of laboratory experimentation. We are now in a position to summarily present the recent view of the inner Earth that results from the conjunction of these physical constraints with a corpus of ever-improving geophysical observations.
We will follow the traditional, and convenient, habit of separately considering seismological, thermal and compositional (mineralogical) Earth models. It must, of course, be kept in mind that they strongly interact (Fig. 7.1).
The seismological models are based on velocity–depth profiles determined from the travel-time–distance curves for seismic waves and on periods of free oscillations (see Bullen and Bolt, 1985 and, for a clear elementary presentation, Bolt, 1982). Due to the development of worldwide networks of three-component broad-band seismographs, there are more and more data, of better and better quality.