HR: 17:30h
AN: S34C-07    [Abstracts]
TI: Investigating the lowermost mantle using migrations of long-period S-ScS data
AU: * Chambers, K
EM: Kit.Chambers@earth.ox.ac.uk
AF: University of Oxford, Dept. of Earth Sciences Parks rd, Oxford, OX1 3PR United Kingdom
AU: Woodhouse, J H
EM: John.Woodhouse@earth.ox.ac.uk
AF: University of Oxford, Dept. of Earth Sciences Parks rd, Oxford, OX1 3PR United Kingdom
AB: Knowledge of structure in the lowermost mantle is of central importance to our understanding of mantle processes such as the nature of the thermo-chemical boundary layer, mantle mixing and the distribution of compositional heterogeneity, plume formation, and core-mantle coupling. Despite this, the lower mantle is poorly understood. Recent seismological investigations have either concentrated on the use of travel time and amplitude data to retrieve detailed structure over small regions or used global travel time datasets to constrain very long wavelength structure. This study attempts to bridge the gap between these techniques by applying migration processing methods to a global dataset of seismograms. We use two migration techniques: a simple backprojection and a more sophisticated migration with weights based on the Generalised Radon Transform (GRT). For the GRT migration, a forward model for the scattered wavefield from a distribution of point heterogeneities was developed using the Born approximation and ray theoretical Green's functions. The influence of biases in the source-receiver distribution on the migration results was tested by computing synthetic datasets using various distributions of point scatterers and the forward model for the scattered wavefield. Our results suggest the presence of a variety of structures in the lowermost mantle, including complicated structure near the CMB and a D'' reflector. In some regions our results can be simulated using fairly simple distributions of point scatterers, elsewhere the results require more complicated structures. The results suggest that the D'' reflector is not a globally present feature, nor a continuous surface thus requiring that the D'' discontinuity is caused by either localised structures, such as thermal or chemical heterogeneity, or a global boundary with a variable impedance contrast.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Seismology [S]
MN: Fall Meeting 2005