We thus described the waveform inversion technique and its challenges in global seismology, continuing during the fifth lecture session. This methodology requires sophisticated tools for calculating the wave field in the heterogeneous medium of the Earth's mantle. Introduced in the 1980s, and developed in the 1990's using the Earth's eigenmode theory and its1st order approximations to calculate the wave field, it has benefited in the last fifteen years from high-performance numerical tools (the " des éléments spectraux " method), which have replaced approximate calculations based on eigenmodes. Combined with increasingly comprehensive databases of broadband seismic recordings, the full waveform inversion method has already uncovered a set of " slow " columns (indicating temperatures several hundred degrees higher than the surrounding mantle), anchored at the base of the mantle in the LLSVPs, oriented vertically and extending across the lower mantle in geographical proximity to hot spot volcanoes. These structures, larger than those expected for purely thermal plumes, must also include a compositional element. Some of them contain, at their base, " ULVZ " (ultra low velocity zones), low-lying structures a few hundred kilometers in diameter, with extreme elastic property anomalies, which are detected by direct modeling of the diffracted waveform on the solid mantle/liquid core boundary.
After illustrating these recent results, and thus presenting the current state of deep mantle imaging, we devoted the end of the fifth session to imaging techniques of the Earth's crust, allowing us to present, very briefly, a technique introduced only fifteen years ago, that ofambient noise tomography. This is based on the principles of interferometry, generally applied to pairs of recording stations, and exploits the presence of seismic background noise generated continuously not by earthquakes, but by the interaction of wind with the ocean surface. This latter interaction in turn generates waves that interact with the seabed, producing seismic energy sources. This technique overcomes the constraints imposed by the very non-uniform distribution of earthquakes, and is currently used extensively to obtain high-resolution models of continental crust in regions with dense seismic networks, by exploiting the signal corresponding to surface waves. It is currently making rapid progress, extending to volume waves and reaching structures at greater depths.
