Evidence for small scale structure of the upper mantle

Seismological research has provided a major contribution to our present knowledge of the structure of the Earth. In the 1930's, the radially symmetric seismic velocity distribution was established, showing regions of different seismic character (crust, upper mantle, transition zone, lower mantle, inner and outer core). The interpretation of the seismological findings in terms of the compositional, thermal, and geodynamical constitution of the Earth has, since then, been an important subject in solid Earth geophysics. Presently, there are still major outstanding questions in this field that require high resolution seismological data for their solution. The research of this thesis must be seen in this context: it addresses two aspects that concern the detailed structure of the upper mantle. One is related to the fine structure of the sharp seismic velocity increases ('discontinuities') at depths of about 400 and 670 km, marking the upper and lower boundary of the transition zone. The other concerns the degree of lateral (seismic velocity) heterogeneity in the upper mantle. In the following, a brief outline is presented of the discussion about the nature of the upper mantle discontinuities, and of several aspects associated with upper mantle lateral heterogeneity.