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- Bazargan, Mohsen
(författare)
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Numerical and laboratory studies of seismic properties and elements of rock fabric from the microscale to the macroscale
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Physical properties of rocks studied in the laboratory are useful to provide constraints on the dynamics of Earth’s interior. This may include direct constraints on in-situ seismic properties, such as elastic wave velocity measurements that can be compared to seismological data, or petrofabric indicators such as anisotropy of magnetic susceptibility (AMS). Another approach that provides predictive insight into the physical properties of Earth’s interior are computer models. Numerical modelling, in particular, can be used to investigate the dynamic propagation of elastic waves or the flow of a material to generate a fabric or texture (i.e., petrofabric in rocks). This thesis focuses on an integrative approach, utilizing both laboratory measurements and numerical modelling, to understand physical properties and petrofabric development in rocks originating in Earth’s crust. The physical properties of rocks are affected by both intrinsic sources (e.g., inherent to crystals) and extrinsic sources (e.g., layering, microcracks, shape preferred orientation of crystals, grain size, presence of geological fluids). A versatile numerical elastic wave propagation model is constructed with COMSOL Multiphysicsâ and benchmarked against a stainless-steel standard used for laboratory elastic wave measurements. The numerical model is flexible and enables setup of composite materials with different sample geometries, which is of importance when modeling the physical properties of rocks in realistic geological scenarios. Using the elastic wave propagation model, this thesis explores different scenarios and their influence on seismic properties, including the effect of grain size on bulk elastic wave speed and compositional layering on seismic anisotropy. The first application focuses on a joint laboratory and numerical study of similar composition gabbro samples, with distinctly different grain size. The numerical model is used to evaluate the relationship between wavelength and grain size. The second application utilizes laboratory measurements as input data for the models, to determine seismic properties of compositionally layered materials. It is shown that the seismic properties, and in particular anisotropy, of a layered material depends on 1) the combination of the inherent rock properties and layering and 2) the wavelength (l) to layer thickness ratio (d). Importantly, independent of scale, the physical properties are wavelength dependent, showing a decrease (apparent dispersion) in velocity, when transitioning from a ray medium (l/d < 1) to an effective medium (l/d > 10). In the second part of this thesis, another COMSOL Multiphysics modelling approach is used to investigate how crystals rotate in a magmatic flow, and how a petrofabric in different magmatic rocks may develop. A set of different magmatic flow scenarios are explored, with direct application to natural examples of dykes and magma chambers. These numerical models may serve as useful predictors of petrofabric in igneous rocks where determination of flow direction of magma is of interest.
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