Numerical and experimental investigations of elastic wave anisotropy in monomineral and polymineral rocks

Seismic anisotropy is a key property to understand the structure of the crust and mantle. In this contribution, we investigate the influence of shape (morphological) preferred orientation (SPO), crystallographic preferred orientation (CPO) and the spatial distribution of grains on seismic anisotropy in rocks (Bazargan et al., 2018). A numerical toolset has been developed with COMSOL to investigate these effects numerically, which has been benchmarked analytically and against other numerical models. Numerical samples modelled in 2D and 3D can determine anisotropy, by measurements along different sample axes, using different geometrical setups and mineral compositions. This numerical tool can include a variety of mineral arrangements and propagate P and S waves from different directions to calculate anisotropy. Current numerical results confirm directly the relations between the structural framework of the rocks (foliation, lineation) and velocity anisotropy, shear wave splitting and shear wave polarisation. This has been proven numerically with the effects of layering, which represents foliation and lineation in 2D. One of the aims of this work is to apply the fundamental results and effects of effective medium to improve our finite element method (FEM) toolbox to provide a numerical modelling tool for seismic data that have been collected in the field. Since the numerical and laboratory measurements are worked on together to verify the numerical results, to compare the models and explain the laboratory measurements have been conducted.

Here we also present laboratory measurements of directional dependence of elastic waves velocity and shear wave splitting to the internal rock structure. In the experimental part of this study, we illustrate the contribution of microstructural parameters (grain sizes, SPO and microcracks) to the elastic anisotropy of relatively similar quartzites and granites. An objective with the laboratory measurements is to investigate the effect of grain size and its possible influence on elastic wave speed and potential scattering effects due to wavelength effects. Granites are the one we use to investigate anisotropy related to SPO and CPO. Our experimental data consist of the measurements of elastic wave velocities (Vp, Vs₁ and Vs₂) at confining pressures up to 600 MPa (Bazargan et al., 2019).  numerical modelling together with laboratory measurements are used to obtain a better understanding of the role of microstructures in elastic wave propagation and its anisotropy

 

Bazargan, M. Almqvist, B. Hieronymus, Ch. Piazolo, S., Employing Finite Element Method using COMSOL multiphysics to predict seismic velocity and anisotropy: Application to lower crust and upper mantle rocks. EGU 2018.

Bazargan, M. Motra, H. B. Almqvist, B. G. Hieronymus, Ch. Piazolo, S., Elastic wave anisotropy in amphibolites and paragneisses from the Swedish Caledonides measured at high pressures (600 MPa) and temperatures (600 ^oC). EGU 2019.