In the preceding section, it has been shown that simple models with homogeneous layers usually under-estimate the posterior uncertainty. A solution to this issue is proposed in this section by the introduction of vertically heterogeneous layers. A linear and a power law increase of the velocity with depth are considered here. The dispersion curve computation is designed for layers with homogeneous properties. Consequently, in both cases, the variation is discretized by several sub-layers for which properties are managed by the characteristics of the main heterogeneous layer.