This lecture was devoted to the various seismic methods used to detect vertical structural discontinuities and thus study the fine structure of the lithosphere and upper mantle. This mainly involves the observation of converted or reflected waves on these discontinuities. We review recent results obtained in the oceanic domain, thanks in particular to data from the few underwater stations installed in the western Pacific by Japanese teams. We described the difficulties involved in instrumenting the ocean floor, and then described recent results from electrical conductivity measurements near the East Pacific ridge, and off the Nazca plate subduction zone (South America). Taken as a whole, recent results seem to lean towards an interpretation in terms of domains of differing hydration levels, without ruling out the possible presence of "pockets" of partial melting at the base of the oceanic lithosphere.
The final part of this lecture focused on the role of viscosity stratification on mantle convection, and in particular that of a low-viscosity zone in the upper mantle, in conferring on convection characteristics close to those observed in the Earth by global seismic imaging, in particular the presence of long-wavelength structures.
