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 betw een the core and the mantle take place. Diffracted S waves propagate over l arge distances and are very sensitive to the velocity structure of this reg ion. Strong variations of amplitudes and waveforms are observed on recordin gs from networks df broad-band seismic stations. We perform forward modelli ng of diffracted S waves in laterally heterogeneous structures in order to analyse whether dr not these observations can be related to lateral inhomog eneities 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 w avefields 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 wa ves are analysed, as well as their coupling. The modelling is applied in heterogeneous models with different geometries: isolated heterogeneities, vertical cylinders, horizontal inhomogeneities a nd random media. Amplitudes of scattered waves are weak and only velocity p erturbations of the order of 10 per cent over a volume of 240 x 240 x 300 k m(3) produce visible effects on seismograms. The two polarizations of S-dif f have different radial sensitivities, the SH components being more sensiti ve to heterogeneities closer to the CMB, However, we do not observe signifi cant time-shifts between the two components similar to those produced by an isotropy. The long-period S-diff have a poor lateral resolution and average the velocity perturbations in their Fresnel zone. Random small-scale heter ogeneities with +/- 10 per cent velocity contrast in the layer therefore ha ve little effect on S-diff, in contrast to their effect on PKIKP.