Fracture in rock is a major factor that affects the rock's physical properties and it also provides the route for the passage of fluids that can transport potentially hazardous substances and hydrothermal fluids. Assessment of the degree of fracture in rocks is important as they play an essential role in many geomechanical issues (stability of boreholes, stimulation of oil and geothermal reservoirs, the design of civil structures, tunnels and hazardous waste disposals), and in understanding a number of processes in the Earth's crust such as magmatic intrusions, plate tectonics, fault mechanics and sedimentary basins. The fundamental understanding of how seismic waves are altered when they pass through fractured rock are currently poorly understood, hence a comprehensive study is timely. An improved understanding of how fractures affect the physical properties (such as seismic velocity and attenuation) would significantly enhance our ability to predict the fracture state of rock at depth remotely. The main focus of this thesis is to characterize P and S wave velocity, their ratio, shear wave splitting and attenuation and their dependence on the fracture density of the rock. Laboratory experiments were carried out in uniaxial compressive condition to increase microfracture density and hydrostatic confining condition to close microfractures. Experiments were performed on a single rock type (Westerly granite) to keep the mineralogy, chemical composition, and grain size constant. The condition of the microfractures was dry to remove the complexity of saturation and fluid type. Through transmission technique was used to measure P and S wave velocities and spectral ratio technique was used to measure attenuation. P and S wave velocities were measured at 1.5MHz. Attenuation measurements were made in the frequency range of O.8MHz to 1.7MHz. Elastic properties can be measured statically where strain data are recorded and related to stress during slow loading of a specimen, or dynamically, where the elasticity can be calculated from the velocity of P and S waves. In order to understand the elastic properties of the crust at depth using seismology, the relationship between the static and dynamic properties must be known. Increasing-amplitude, uniaxial cyclic loading experiments were carried out to investigate and quantify the effect of microcracking on the elastic properties, and to establish a relationship between static and dynamic measurements. There is a linear relationship between static and dynamic Young's moduli, and a significant discrepancy between the static and dynamic Poisson's ratio. We attribute the differences in the static and dynamic elastic properties to the size distribution of the crack population relative to the amplitude and frequency of the applied stress, frictional sliding on closed cracks during loading/unloading, and the assumption of isotropic elasticity in the sample. Strong stress-dependency exists in the uniaxial compressive and hydrostatic confining conditions due to closure of microcracks. This resulted in: an increase in the P and S wave velocities, their ratio, static and dynamic Young's modulus, and static and dynamic Poisson's ratio; and a decrease in the P and S wave attenuation. The increase of fracture density caused: a decrease in the P and S wave velocities and static and dynamic Young's modulus; a small increase in the dynamic Poisson's ratio and VpNs; and a large increase in the static Poisson's ratio, and P and S wave attenuation. Seismic wave attenuation is more sensitive than seismic wave velocity to closure of microcracks and Increase of microfracture density. The effect of varying crack density on the P and S wave velocities and elastic properties under confining pressure (depth) were quantified. The elastic wave velocities and Young's modulus of samples that have a greater amount of microcrack damage required higher confining pressure to be equal to those of samples with no induced microcrack damage. We found that fractures are completely closed at ~5km (~130MPa) in crystalline rocks. At shallow depth (less than 5km), fracture density affects seismic wave velocities. We observed an overall 6% and 4% reduction in P and S wave velocities respectively due to an increase in the fracture density. The overall reduction in the P and S wave decreased to 2% and 1 % at ~2km. Consequently, assessing the degree of fracture between 2km and 5km using seismic wave velocities may be difficult
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:569902 |
| Date | January 2011 |
| Creators | Blake, Oshaine Omar |
| Publisher | University of Liverpool |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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