Master Thesis Defense by Luisa Elisabeth Hirche

Title: Evolution of Olivine Crystal Fabrics in Earth’s Upper Mantle

Abstract:
Olivine, the most abundant mineral in the Earth’s upper mantle, influences seismic wave propagation through its crystallographic preferred orientation (CPO), which develops during deformation. As a result, seismic anisotropy serves as a key tool for constraining large-scale geodynamic models by linking seismic observations to mantle flow processes via the orientation of olivine crystals.

The main objective of this thesis is to develop and evaluate an inversion method capable of inferring olivine fabric distributions from seismic data. As a foundation, I first examine fabric evolution models commonly used in the geoscience community, including a discrete model and a second-order structure tensor approach. I then focus on a spectral representation that describes the orientation distribution function (ODF) of olivine using spherical harmonics. This spectral model is applied to simulate the evolution of A-type olivine fabric within a simplified single-cell thermal convection model of the Earth’s upper mantle. The resulting fabrics are then used to compute synthetic seismic velocities, which serve as input for the inversion.

I adapt an inversion method originally developed for polycrystalline ice to recover olivine fabrics from seismic observations. The approach combines (i) a harmonic expansion of the unknown ODF, (ii) a fourth-order tensor approximation to reduce dimensionality, and (iii) Voigt averaging to homogenize the elastic response of orthotropic crystals.

The inversion results show that the success of recovering olivine fabric distributions strongly depends on the truncation level of the spherical harmonics expansion and the quality of prior information about the fabric.

Supervisors: Nicholas Mossor Rathmann and Klaus Mosegaard
Censor: Jens Olaf Pepke Pedersen from DTU