We introduced another type of seismic wave, surface waves, which complete the illumination of the upper mantle beneath the oceans. In the fourth session, we illustrated some of the discoveries made with imaging from surface waves, which propagate along the Earth's surface, sampling greater and greater depths depending on their period, manifested by frequency-dependent propagation velocities. A first notable observation is the dependence of the structure on the age of the oceanic plate, and another is the variation of the anisotropic structure, which maps the thickness of the lithosphere, both in the oceans and on the continents, as well as the present and past direction of plate movement.
In this fourth session, we show how, by combining data on surface wave dispersion (fundamental mode and harmonics) and volume wave travel times, we have been able to build increasingly accurate large-scale models of variations in the global structure of the mantle, focusing on LLSVPs whose enigmatic origins never cease to intrigue us : they have steep edges, which rules out an interpretation in terms of purely thermal variations, and exhibit less anisotropy than the cooler ring structure surrounding them. Debates about their nature and role in mantle convection rage on at : are they a reservoir of primordial matter (untouched by convection since the time of Earth's formation), or are they made up of material " recycled " from subduction plates ? Are they denser or less dense than neighboring regions ? How high do they reach above the core-mantle boundary ? Are they compact structures, or do they represent clusters of mantle plumes ?
To further refine tomographic images and settle these debates, new tools are needed to compensate for the very imperfect illumination provided by the small parts of the seismic recordings used " classically " in global imaging (travel times of P and S waves, and dispersion of surface waves). This has led to the gradual development of a methodology that attempts to exploit all the information contained in the recordings, with the aim of interpreting the entire " waveform ", which contains not only the waves that propagate " directly " between source and station, but also numerous waves that reflect and refract at the surface and at the core-mantle boundary, as well as at internal structural discontinuities in the mantle, thus enabling structures to be illuminated at higher spatial resolution.
