Scattering of S waves diffracted at the core-mantle boundary:: forward modelling

The lowermost 200-300 km of the Earth's mantle, known as the D " layer, is an extremely complex and heterogeneous region where transfer processes between the core and the mantle take place. Diffracted S waves propagate over large distances and are very sensitive to the velocity structure of this region. Strong variations of amplitudes and waveforms are observed on recordings from networks df broad-band seismic stations. We perform forward modelling of diffracted S waves in laterally heterogeneous structures in order to analyse whether dr not these observations can be related to lateral inhomogeneities in D ". We combine the diffraction due to the core and the scattering due to small-scale volumetric heterogeneities (10-100 km) by coupling single scattering (Born approximation) with the Langer approximation, which describes S-diff wave propagation. The influence on the direct as well as on the scattered wavefields of the CMB as well as of possible tunnelling in the core or in D " is fully accounted for. The SH and the SV components of the diffracted waves are analysed, as well as their coupling. The modelling is applied in heterogeneous models with different geometries: isolated heterogeneities, vertical cylinders, horizontal inhomogeneities and random media. Amplitudes of scattered waves are weak and only velocity perturbations of the order of 10 per cent over a volume of 240 x 240 x 300 km(3) produce visible effects on seismograms. The two polarizations of S-diff have different radial sensitivities, the SH components being more sensitive to heterogeneities closer to the CMB, However, we do not observe significant time-shifts between the two components similar to those produced by anisotropy. The long-period S-diff have a poor lateral resolution and average the velocity perturbations in their Fresnel zone. Random small-scale heterogeneities with +/- 10 per cent velocity contrast in the layer therefore have little effect on S-diff, in contrast to their effect on PKIKP.