Compressional elastic wave velocities of serpentinized pyroxenite at high pressures and high temperatures and its geological significance
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Elastic wave velocity measurement in rocks at high pressures and high temperatures plays a key role in researching the state, properties and movement of the earth interior materials.Cite
We review the methods of measuring the velocities of elastic-waves in rocks and summarize the temperature-dependence of elastic-wave velocities under high-temperature and high-pressure conditions. The elastic-wave velocities in rocks are strongly affected by several phenomena such as thermal cracking, phase transition of minerals, partial melting of rocks, and dehydration of hydrous minerals. These phenomena are strongly affected by pressure-temperature conditions and chemical compositions of rocks and minerals. Thus, it is very difficult to predict the elastic wave velocities of rocks and minerals under high-pressure and high-temperature conditions theoretically. Laboratory measurements of the velocities of elastic-waves in rocks under high-pressure and high-temperature conditions have provided useful data for estimating physical and geological properties in the crust and upper mantle. We also mention the next issues to be studied in relation to the velocity of elastic waves in rocks. It is important to measure elastic-wave velocities in rocks under high-temperature and high-pressure conditions in the presence of pore-fluids.
Elasticity
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Overburden pressure
Shear waves
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Compressional wave velocities have been measured in granite, granulite, amphibolite, and peridotite specimens under temperatures up to 700 °C and confining pressures up to 6 kbars. In general, velocity increases with pressure and decreases with temperature. Quartz-bearing rocks show an anomalous behavior of their compressional wave velocities. The velocity-temperature relations exhibit a velocity-minimum due to the high-low inversion of the constituent quartz crystals. The intrinsic effect of temperature on velocities is hard to determine, due to thermal expansion and the consequent loosening of the structure. The opening of new cracks and the widening of old cracks causes a large decrease in compressional wave velocities. The effects of grain boundary characteristics on elastic wave propagation are progressively eliminated as the pressure is raised and the loosening of structure in the rocks caused by the effect of temperature is balanced by the confining pressure. The minimum pressure to prevent damage at a given temperature should, therefore, be about 1 kbar/100 °C. The values obtained under these conditions are considered to be as correct as the intrinsic properties of the compact aggregates. Velocity anisotropies at high confining pressures and high temperatures correlate with preferred lattice orientation of the constituent minerals. The effect of dimensional orientation and microcracks on seismic anisotropy seems to be of minor importance in dry rocks. The higher the confining pressure the less important it becomes. Seismic anisotropy induced by preferred lattice orientation of the constituent minerals, however, remained unaffected even at pressures of 6 kbars and temperatures of more than 700 °C.
Overburden pressure
Peridotite
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Ultramafic rock
Elasticity
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Carbonate reservoirs with varied depositional environments and diagenetic processes have assorted mineral composition and diverse pore structures, making it difficult to investigate the correlation between pressure and elastic velocity. The pressure-velocity relationship is influenced by the variation of pore structures and can be studied as a function of mineralogy, porosity, grain size, and pore shape. An elastic parameter called frame flexibility factor or pore structure parameter (γ) from a rock-physics model is proposed as an indicator to study the effect of various pore structures on elastic wave velocity with differing pressures. Wave velocities measured on room-dry core samples from a Lower Cretaceous carbonate reservoir under varying differential pressures up to 30 MPa are used to analyze the impact of grain/crystal size and pore types on velocity variation with pressure. The pore structure parameter γ is related to a grain size index (D), in addition to aspect ratio. The gradient of velocity change with pressure is controlled by the initial aspect ratio of the rock. As pressure increases, aspect ratio increases and γ decreases so that wave velocity increases. For different rocks of similar porosity at a given pressure, γ also decreases as D increases. For the studied samples, a power-law relationship occurs between the grain size index (D) and the grain diameter estimated from thin sections. Grain size correlates very well with porosity and permeability. These results can be helpful to estimate grain size from wave velocities, using well logs and seismic data.
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A description is presented of an experimental assembly which has been developed to conduct concurrent measurements of compressional and shear wave velocities in rocks at high temperatures and confining pressures and with independent control of the pore pressure. The apparatus was used in studies of the joint effects of temperature, external confining pressure, and internal pore water on sonic velocities in Westerly granite. It was found that at a given temperature, confining pressure has a larger accelerating effect on compressional waves in dry rock, whereas at a given confining pressure, temperature has a larger retarding effect on shear waves.
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Velocities of compressional waves have been measured at pressures of 100 to 10,000 bars for various metamorphic rocks which are believed to be important constituents of the earth's crust. Detailed petrographic and petrofabric analyses are reported from, thin sections cut from the specimens. The principal factors contributing to the velocities of compressional waves in igneous and metamorphic rocks are discussed. Particular attention is given to the dependence of velocity on porosity, mineral orientation, and mineral composition. The principal conclusions are: (1) the initial changes in velocity with pressure are related to the arrangement and shape of pore spaces in a rock, (2) variation of velocity with propagation direction in metamorphic rocks is a consequence of preferred mineral orientation, and (3) velocities calculated from single-crystal data agree with most of the measured velocities.
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Introduction. The propagation of elastic waves in the earth's crust is governed by the elastic properties of the constituent minerals within crustal rocks. The ubiquity of the feldspars, together with their abundance in crustal rocks, indicates that seismic velocities in the crust are largely determined by the elastic properties of feldspar. Measurements of the velocity of compressional and shear waves in single crystals of potassium-sodium and plagioclase feldspars have been reported by Alexandrov and Ryzhova [1962], Ryzhova [1964], and Ryzhova and Alexandrov [1965].
Alkali feldspar
Shear waves
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Cataclastic rock
Overburden pressure
Brittleness
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