Imaging the Earth's small-scale structure using full-waveform inversion: Theory and application to the imaging of mantle plumes

• Main Author: Rickers, F.
• Other Authors: ISES: Hunting for plumes in the mantle, Seismology, Trampert, Jeannot, Fichtner, Andreas
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: UU Department of Earth Sciences 2013
• Subjects: North Atlantic
• Online Access: https://dspace.library.uu.nl/handle/1874/267499

Within the last decade, increasing computational power and theoretical advances have initiated the transition from approximation-based tomography to iterative full-waveform tomography in regional and global seismology. Such full-waveform methods allow for the exploitation of complete seismograms to constrain the 3-D structure of the Earth, which potentially leads to much higher resolution compared to approximation-based methods. The method used in this thesis relies on numerical wavefield simulations with the spectral-element method, which honours the full complexity of seismic wave propagation in realistic 3-D media. Gradients are constructed with the adjoint method. Full-waveform models are affected by a number of subjective inversion choices, such as the type of misfit and the regularisation of the gradient. The complexity of seismic wave propagation together with the non-linearity of the problem make it a challenging task to determine optimum inversion strategies. For this thesis, we first performed synthetic experiments to investigate and optimise the capability of full-waveform tomography to constrain small-scale structures in the Earth's mantle. These experiments were focused on the tomographic detection of possible mantle plumes, which are a challenging target for seismic tomography due to their small lateral extent and their extension deep into the lower mantle. Using a misfit based on the time-continuous and amplitude-independent instantaneous phase difference, we succeeded to fully recover an idealised narrow mantle plume. The accurate measurement and inclusion of diffracted waves arriving in the body-wave coda was identified as essential for the recovery in the lower mantle. We further confirmed that simple cross-correlation traveltime misfits do not permit the recovery of plumes in the lower mantle, wavefront healing largely conceals the acquired time delays. We applied these results to the construction of a high-resolution 3-D model of the S-wave speed beneath the North Atlantic region, extending ...