9706663 Ritsema This research involves the use of the expanding data base of high- quality, broad-band digital seismic networks, both permanent and temporary to invert for earth structure in regions previously undersampled, especially the lower mantle. Several source- receiver path geometries provide unique sampling of the lowermost mantle beneath the Indian, South Pacific and Atlantic oceans that cannot be obtained with comparable detail with past seismic station coverage. The construction of seismic models for unmapped regions and the analysis of the full spectrum of length scales over which seismic velocity structure varies in the lower mantle are primary goals. These aspects remain crucial to advancing our understanding of the relation of seismic structure to the thermal and chemical composition of the region, and their influence on dynamical processes near the core-mantle boundary. ***
