Date of Award

2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Specialization

Marine Geology and Geophysics

Department

Oceanography

First Advisor

Yang Shen

Abstract

At seismic discontinuities in the crust and mantle, part of the compressional-wave energy converts to shear wave, and vice versa. These converted waves have been widely used in receiver function (RF) studies to image discontinuity structures in the Earth. While generally successful, the conventional RF method has its limitations and is suited mostly to flat or gently dipping structures. Among the efforts to overcome the limitations of the conventional RF method is the development of the wave-­‐theory-­‐based, passive-­‐source reverse-­‐time migration (PS-­RTM) for imaging complex seismic discontinuities and scatters. To date, PS-­RTM has been implemented only in 2D in the Cartesian coordinate for local problems and thus has limited applicability. In Chapter 1, we introduce a 3D PS-­RTM approach in the spherical coordinate, which is better suited for regional-­and global-­scale seismic imaging. New computational procedures are developed to reduce artifacts and enhance migrated images, including back-­‐propagating the main arrival and the coda containing the converted waves separately, using a modified Helmholtz decomposition operator to separate the P and S modes in the back­‐propagated wavefields, and applying an imaging condition that maintains a consistent polarity for a given velocity contrast. The new approach allows us to use migration velocity models with realistic velocity discontinuities, improving accuracy of the migrated images. We present several synthetic experiments to demonstrate the method, using regional and teleseismic sources. The results show that both regional and teleseismic sources can illuminate complex structures and this method is well suited for imaging dipping interfaces and sharp lateral changes in discontinuity structures. In Chapter 2, we test the 3D PS-­RTM method with sparse and unevenly distributed seismic arrays using synthetic data and discuss the station spacing limitation in its application. A cubic spline interpolation method is used to interpolate the sparse and unevenly recorded seismic signal onto numerical grids to reconstruct the full wavefield. We found that as long as the station spacing is smaller than half apparent wavelength of the converted wave, and the noise level is comparable or even slightly larger than the converted wave, the cubic spline method is sufficient to interpolate the wavefield for the PS-­RTM method. In Chapter 3, we apply the 3D PS-­RTM method to the Yellowstone hotspot to image the mantle transition zone discontinuities. We develop a data regularization procedure that includes iterative deconvolution, principle component analysis and interpolation. The RTM volumes from different earthquakes are weighted and stacked based on the distribution of stations that record each earthquake. We discuss how the earthquake distribution, reference velocity model, dominant frequency and principle component analysis affect the image results. The resulting RTM model shows a depressed 410‐km discontinuity, an uplifted 660‐km discontinuity and a thinner­‐than‐normal mantle transition zone thickness beneath Yellowstone, which may suggest a deep mantle plume rising from the lower mantle.

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