HR: 10:20h
AN: S32A-01    [Abstracts]
TI: Surface wave tomography: where does ray theory break down on a global scale?
AU: * peter, d
EM: dpeter@erdw.ethz.ch
AF: ETH Zurich, Institute of Geophysics, Schafmattstr. 30, ETH-Honggerberg HPP, Zurich, 8093, Switzerland
AU: Boschi, L
EM: boschi@tomo.ig.erdw.ethz.ch
AF: ETH Zurich, Institute of Geophysics, Schafmattstr. 30, ETH-Honggerberg HPP, Zurich, 8093, Switzerland
AU: Woodhouse, J H
EM: john.woodhouse@earth.ox.ac.uk
AF: University of Oxford, Department of Earth Sciences, Parks Road, 3PR, Oxford, OX1, United Kingdom
AB: In global seismic tomography, almost all inversions thus far have relied on ray theory. Ray theory is valid at high frequencies where wavelengths are short compared to the size of the heterogeneous structure of the Earth. Particularly for surface waves at longer periods, we need to go beyond this approximation. Born theory is expected to perform well even for heterogeneities of approximately the same size as the wavelength. Today, it has become feasible to apply Born sensitivity kernels in global scale inversions, but it is still unknown how well ray theory and Born theory compare with one another especially for surface wave tomography; a systematic investigation on global-size scales with a realistic source/receiver distribution is lacking. A common way to investigate the resolution of the ill-posed inverse problem consists of conducting a set of different checkerboard tests with the same inverse algorithm. Thus, regions of high and poor resolution can be distinguished. In this study, we focus on phase-velocity inversions of intermediate to long-period surface waves. We use the source/station distribution of a real phase-anomaly measurement database to investigate the resolution limit of ray-theoretical and Born-theoretical inversions. Our synthetic datum is calculated by cross-correlating waveforms obtained numerically from a membrane model which accounts implicitly for all nonlinear propagation effects; it therefore surpasses known problems where synthetic data are computed in a linear model. For this 2-D case, we are also able to iterate the inverse scheme with Born sensitivity kernels calculated in a heterogeneous background Earth via the adjoint wavefield method. Our results show a clear improvement for high harmonic degree when Born theory is used to invert phase-anomaly measurements of long-period surface waves.
DE: 1734 Seismology
DE: 7255 Surface waves and free oscillations
DE: 7270 Tomography (6982, 8180)
DE: 7290 Computational seismology
SC: Seismology [S]
MN: 2007 Fall Meeting