Impact of grain shape on the seismic properties of heterogeneous materials, and applications to the uppermost inner core.

Abstract:

The strong attenuation of PKIKP waves which bottom in the inner core and the strong coda of reflected P-waves at the inner core boundary suggest that the uppermost inner core is highly heterogeneous. Moreover, the eastern hemisphere of the inner core is characterized by isotropic P-wave velocity and strong attenuation whereas the western hemisphere exibits lower P-wave velocity, less overall attenuation and anisotropy both in velocity and attenuation. While the hemispherical pattern of velocity and attenuation can be explained by variable grain size, anisotropy calls for the existence of preferential orientations in the material.

We have investigated the role of grain elongation and flattening on the propagation of elastic waves in two-phase materials and single-mineral-cubic aggregates with overall transverse isotropic symmetry. Based on the Dyson equation, explicit expressions of attenuation and phase velocity for q-P, q-SV and q-SH waves have been derived. Attenuation and velocity strongly vary with the frequency, the direction of propagation and the grain shape. We find that the fast/slow direction of propagation is not systematically aligned with the direction of low/high attenuation. The hemispherical pattern of seismic properties in the uppermost inner core can be explained by lateral variations in texture with radially elongated grains in the Western hemisphere and equi-axed grains in the Eastern hemisphere. This texture remains compatible with an eastward translation of the inner core.