Global seismic tomography of the mantle

At the end of the previous decade, a general consensus has developed that it is feasible to map on the global scale some of the largest scale 3‐D features in the Earth's deep interior and that such mapping may have fundamental significance for our understanding of the geodynamic processes, such as the flow in the mantle. General reviews of the state‐of‐the‐art for that period can be found in Dziewonski and Woodhouse [1987], Woodhouse and Dziewonski [1989] and Romanowicz [1991]. In the first four years of the 1990's, significant progress has been achieved on several fronts. One of these is introduction of the new class of data: travel times measured from the long period components of digital seismograms. These data are much more accurate than, for example, those reported by the ISC, and geophysically important patterns could be detected without inversion.