HR: 1340h
AN: S33A-1078    [Abstracts]
TI: Imaging Lower Mantle Structure by Waveform Migration in 3D Tomographic Models
AU: * Hutko, A R
EM: ahutko@es.ucsc.edu
AF: Universtiy of California Santa Cruz, Earth Sciences 1156 High St, Santa Cruz, CA 95064 United States
AU: Lay, T
EM: tlay@es.ucsc.edu
AF: Universtiy of California Santa Cruz, Earth Sciences 1156 High St, Santa Cruz, CA 95064 United States
AU: Garnero, E
EM: garnero@asu.edu
AF: Arizona State University, Dept. of Geological Sciences Box 871404, Tempe, AZ 85287-1404 United States
AU: Revenaugh, J
EM: justinr@umn.edu
AF: University of Minnesota Twin Cities, Department of Geology and Geophysics, Minneapolis, MN 55455-0219 United States
AB: Lower mantle migration applications have generally used a 1D reference model for computing travel times. Resulting images of reflectors or scatterers are intrinsically biased by inaccuracies of the background model. Given that differential time variations for shear wave phases can range over 5 s over 1000 km dimension regions, failure to account for volumetric velocity heterogeneity can produce substantial apparent reflector topography or mislocation of structures by hundreds of kilometers. Alignment of observations on a reference phase or application of travel time corrections for aspherical structure affecting a reference phase have been used as a first-order approach to correction for volumetric heterogeneity, but these approaches cannot account for significant heterogeneity within the migration volume. We present the first lower mantle migration in a 3D heterogeneous reference structure applied to shear wave paths traversing the deep mantle below the Cocos Plate. We calculate travel time perturbations throughout the scattering medium relative to ScS, our aligned reference phase, which we know must reflect from the core-mantle boundary. Tomographic mantle shear velocity models from Grand (2002) and Hung et al. (2004) are used to compute travel time anomalies on all path segments, computed as perturbations to ray paths for a 1D reference structure (full 3D ray tracing is not attempted in this application because of the mammoth number of travel time calculations involved). Models obtained by this 3D migration are compared with corresponding regional structures obtained by 1D migration (Thomas et al., 2004), stochastic Kirchhoff migration (Hutko and Revenaugh, 2004), and forward modeling of local seismogram stacks (Lay et al., 2004). Given the small differential travel times typically involved in deep mantle, wide-angle migration applications, our results demonstrate that accounting for volumetric velocity heterogeneity is imperative to avoid biased images. Iterative tomography/migration approaches will be required to attain model accuracy, as is the case for exploration seismology.
DE: 7260 Theory and modeling
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting