We described methods for estimating the radius of the ICB(inner core boundary ) and the thickness of the liquid-solid transition, important for understanding the solidification process and core dynamics, as well as the topography of the seed. Next, we describe the successive work on the seismological estimation of the density jump at the ICB, which provides constraints on the composition of the seed: since the latter is composed of purer iron than the outer core, the aim is to determine which light element (or elements) is (are) expelled into the liquid core as the seed solidifies, thereby helping to maintain compositional convection in the liquid core. In the 1990s, estimates obtained from the analysis of earth vibrations and volume waves reflected on the seed surface (PKiKP) gave divergent results, from less than 0.3 -103 kg/m3, to more than 1 -103 kg/m3, but the most recent work is in better agreement, giving a value of around 0.6-0.8 - 10-3 kg/m3.
We then turned to the question of the seed's solidity, indirectly proven by the analysis of the eigenfrequencies of core-sensitive modes. If the seed is solid, we predict the existence of a PKJKP volume wave propagating as a shear wave inside the seed (and a compressional wave outside it). The PKJKP wave is very difficult to observe, as the conversion coefficient at the surface of the seed is very low. A first detection in 1972 was later invalidated. Since then, attempts have been made to observe it using methods based on trace summations(stacking) on regional arrays. So far, only four other papers have proposed the detection of this phase, three at relatively low frequency (~ 10 s) and one at higher frequency (~ 0.5 s). This question remains open, to say the least, and some argue that it is not possible to observe it with current seismic arrays (Shearer, 2010). Only one estimate of the quality factor (inverse of attenuation) in shear in the seed has been published (Cao and Romanowicz, 2009), proposing a Qβ value greater than 300.
