Observations and methods

In Chapters 6–8 we have seen how travel time curves depend on the characteristics of the media through which the seismic waves propagate and that the velocity's distribution with depth can be deduced from them. In this chapter we will apply these results to observations regarding the Earth and discuss the results concerning its internal structure obtained. For short distances (less than 1000 km) we can use the flat-Earth approximation and plane geometry. Seismic waves for that range of distances give information on depths of about 100 km, that is, on the crust and part of the upper mantle. For this range of distances we can apply the theory derived in Chapters 6 and 7. For greater distances the spherical shape of the Earth must be considered, so the results of Chapter 8 must be applied. The effects due to the deviations of the form of the Earth from a sphere, that is, mainly its flatness, can be taken into account by using corrections to the spherical model. In seismology these effects are not very important.

The first seismic waves used for the study of the Earth's structure were those produced by earthquakes. Even today this is the main source of information, especially for the deep interior. Among the first tables and curves of travel times of seismic waves were those of Oldham, who in 1906 deduced the existence of the Earth's core. These tables were completed by Zöppritz and Turner and later, in 1914, by Gutenberg.