Heterogeneity of the lowermost mantle

Strong heterogeneity at a variety of scale lengths has been imaged in the lowermost mantle using different forward and inverse methods. Coherent patterns in differential travel times of waves that sample the base of the mantle-such as diffracted shear waves (Sdiff) and compressional waves (Pdiff)-are readily apparent, and are compared with results from tomographic studies. Travel time and waveform modeling studies have demonstrated the presence of intense lateral variations in a variety of mapped features, such as a regionally detected high velocity D″ layer, ultra-low velocity zones, D' anisotropy, strong scattering and heterogeneity. Such shortwavelength variations currently preclude confident mapping of D″ structure at smaller scales. Issues of seismic resolution and uncertainties are emphasized here, as well as the limitations of one-dimensional modeling/averaging in highly heterogeneous environments.