Measurement of the adiabats of quartz,forsterite, and magnesium oxide at high pressures and high temperatures and adiabatic gradient in the mantle

The geophysically-important adiabat (?T/?P)_s has been measured at pressures up to 50 kbar and temperatures up to 1000°C. A simple power law describes the relationship between (?T/?P)_s and the compression of the material. The power is independent of the material and of the temperature within the uncertainty. This consistency in the power allows the extrapolation of the adiabat to pressure and temperature conditions of the mantle of the earth. The adiabatic gradient is shown to be significantly smaller than the melting gradient.     

Mechanical hysteresis in rocks at low strain amplitudes and seismic frequencies

A sensitive capacitance displacement transducer has been used to record hysteresis loops in the stress-strain diagrams of laboratory samples of granite, basalt, sandstone and concrete subjected to cyclic axial strains with amplitudes of order 10^?5 and periods of 10–300 sec. The ends of the loops are always cusped, whether the load cycle is sinusoidal or not, and at low strain amplitudes the loop shape becomes symmetrical and appears to be independent of amplitude. Thermal relaxation influences the observed loop shapes, so that the strain cycles represent a compromise between adiabatic and isothermal compressions. However, this does not affect the conclusion that stress-strain loops are always cusped. This observation does not appear to be consistent with linear theories of damping of acoustic and seismic waves, which indicate elliptical loops.     

Evaluation of active and passive seismic earth pressures considering internal friction and cohesion

The Log-Spiral-Rankine (LSR) model, which is a generalized formulation for assessing the active and passive seismic earth pressures considering the internal friction and cohesion of backfill soil, is reviewed and improved in this study. System inconsistencies in the LSR model are identified, which result from an inaccurate assumption on the vertical normal stress field (σ_z=γz) in a general c–ϕ soil medium, and from omitting the effect of soil cohesion when solving for the stress states along the failure surface. The remedies to the said inconsistencies are presented, and local and global iteration schemes are introduced to solve the resulting highly coupled multivariate nonlinear system of equations. The modified LSR model provides a more complete and accurate solution for earth retaining systems, including the geometry of the mobilized soil body, the stress state along the failure surface, as well as the magnitude and the point of application of the resultant earth thrust.     

Absorption of strain waves in porous media at seismic frequencies

An understanding of strain wave propagation in fluid containing porous rocks is important in reservoir geophysics and in the monitoring in underground water in the vicinity of nuclear and toxic waste sites, earthquake prediction, etc. Both experimental and theoretical research are far from providing a complete explanation of dissipation mechanisms, especially the observation of an unexpectedly strong dependence of attenuationQ ^–1 on the chemistry of the solid and liquid phase involved. Traditional theories of proelasticity do not take these effects into account. In this paper the bulk of existing experimental data and theoretical models is reviewed briefly in order to elecidate the effect of environmental factors on the attenuation of seismic waves. Low fluid concentrations are emphasized. Thermodynamical analysis shows that changes in surface energy caused by weak mechanical disturbances can explain observed values of attenuation in real rocks. Experimental dissipation isotherms are interpreted in terms of monolayered surface adsorption of liquid films as described by Langmuir's equation.In order to describe surface dissipation in consolidated rocks, a surface tension term is added to the pore pressure term in the O'Connell-Budiansky proelastic equation for effective moduli of porous and fractured rocks. Theoretical calculations by this modified model, using reasonable values for elastic parameters, surface energy, crack density and their geometry, lead to results which qualitatively agree with experimental data obtained at low fluid contents.     

Phase relations in the system diopside-jadeite at high pressures and high temperatures

Phase behaviour in the system diopside-jadeite (CaMgSi₂O₆NaAlSi₂O₆) have been investigated in the pressure region 100–300 kbar at about 1000°C in a diamond-anvil press coupled with laser heating. The omphacite solid solution extends from 30 to at least 200 kbar for the entire system. Omphacites, ranging in composition from pure diopside to more than 40 mole % jadeite, transform to diopside (II) at pressures greater than 230 kbar. Diopside (II), which probably possesses a perovskite-type structure, cannot be preserved when experiments are quenched to ambient conditions. Jadeite-rich omphacites were found to decompose into an assemblage of NaAlSiO₄(CaFe₂O₄-type structure) + stishovite + diopside (II) (?) at pressures greater than about 260 kbar. These results suggest that an eclogitic model mantle would not display the 400-km seismic discontinuity. Moreover, sodium in the transition zone and lower mantle would most likely be accommodated in phases of omphacite and diopside (II).     

Effect of interface friction on tunnel liner internal forces due to seismic S- and P-wave propagation

The effects on the hoop force and bending moment developed in the lining of a circular tunnel of the contact properties of the soil-lining interface are investigated numerically for both cases of S- and P-seismic wave propagation. Development of a robust model for the dynamic simulation of this problem includes: (i) the implementation of a hysteretic model of the non-linear soil response under cyclic loads in the finite element code ABAQUS; and (ii) validation of the analyses results against centrifuge tests from the literature and closed-from elasticity solutions. Accordingly, a parametric study is conducted to quantify the effect of adopting different values of the friction coefficient of the tunnel liner interface, while assuming that the relaxation load is transferred only to the temporary support shell of the tunnel; a hypothesis applicable mainly to tunnels constructed with the NATM method where an unreinforced concrete final lining is usually installed. Practical findings of this study suggest that the full-slip assumption should be used in conjunction with closed-form expressions for preliminary estimates of the tunnel response. On the contrary, for tunnels where the lining is designed to bear the soil loads, numerical tools should be used for the rational assessment of their seismic response. In the latter case, more experimental studies are needed to evaluate the friction properties at the interface, since common expressions correlating the friction coefficient with the friction angle of the surrounding soil do not appear compatible with the centrifuge test results examined herein.     

Influence of subsurface injection on time‐lapse seismic: laboratory studies at seismic and ultrasonic frequencies

Seismic monitoring of reservoir and overburden performance during subsurface CO₂ storage plays a key role in ensuring efficiency and safety. Proper interpretation of monitoring data requires knowledge about the rock physical phenomena occurring in the subsurface formations. This work focuses on rock stiffness and elastic velocity changes of a shale overburden formation caused by both reservoir inflation induced stress changes and leakage of CO₂ into the overburden. In laboratory experiments, Pierre shale I core plugs were loaded along the stress path representative for the in situ stress changes experienced by caprock during reservoir inflation. Tests were carried out in a triaxial compaction cell combining three measurement techniques and permitting for determination of (i) ultrasonic velocities, (ii) quasistatic rock deformations, and (iii) dynamic elastic stiffness at seismic frequencies within a single test, which allowed to quantify effects of seismic dispersion. In addition, fluid substitution effects connected with possible CO₂ leakage into the caprock formation were modelled by the modified anisotropic Gassmann model. Results of this work indicate that (i) stress sensitivity of Pierre shale I is frequency dependent; (ii) reservoir inflation leads to the increase of the overburden Young's modulus and Poisson's ratio; (iii) in situ stress changes mostly affect the P‐wave velocities; (iv) small leakage of the CO₂ into the overburden may lead to the velocity changes, which are comparable with one associated with geomechanical influence; (v) non‐elastic effects increase stress sensitivity of an acoustic waves; (iv) and both geomechanical and fluid substitution effects would create significant time shifts, which should be detectable by time‐lapse seismic.     

Orthorhombic perovskite phases observed in olivine,pyroxene and garnet at high pressures and temperatures

Ferromagnesian silicate olivines, pyroxenes and garnets with Mg/(Mg + Fe)?0.3 (molar) have been found to transform to high-pressure phases characterized by the orthorhombic perovskite structure when compressed to pressures above 250 kbar in a diamond-anvil press and heated to temperatures above 1,000°C with a YAG laser. The zero-pressure density of the perovskite phase of (Mg,Fe)SiO₃ is about 3–4% greater than that of the close-packed oxides, rocksalt plus stishovite. For (Mg,Fe)₂SiO₄ compounds, the perovskite plus rocksalt phase assemblage is 2–3% denser than the mixed oxides. The experimental synthesis of such high-density perovskite phases in olivine, pyroxene and garnet compounds suggests that (Mg,Fe)SiO₃-perovskite is the dominant mineral phase in the earth's lower mantle.     

The system enstatite-wollastonite at high pressures and temperatures,with emphasis on diopside

High-pressure phase transformations for three intermediate compositions (including diopside) in the system enstatite (MgSiO₃)-wollastonite (CaSiO₃) were investigated in the pressure range 100–300 kbar at about 1000°C in a diamond-anvil press coupled with laser heating. The phase behaviour of the two end components (enstatite and wollastonite) at high pressure has been reported earlier. The results of this study reveal that there is very limited solid solution of diopside (CaMgSi₂O₆) in the various high-pressure phase assemblages of enstatite. At pressures greater than about 200 kbar, diopside and a composition between diopside and wollastonite were found to transform into non-quenchable phases, as does wollastonite. It is thought probable that diopside and wollastonite form solid solutions with the perovskite structure at high pressure, but that on release of pressure it is not possible to preserve the high-pressure modification.     

各向异性介质中的弹性阻抗及其反演

地震反演已成为油藏描述中的重要组成部分.绝大多数的常规地震反演是叠后地震数据体上进行,很少考虑各向异性存在的情况.随着勘探开发的发展,地震各向异性和叠前地震波阻抗反演引起了人们极大关注.本文在各向同性介质中弹性阻抗研究基础上,推导出了各向异性介质中的弹性阻抗方程,提出了地震各向异性介质中用弹性阻抗进行储层参数描述的技术路线和框架,并对反演过程中存在的问题进行了有益探讨.     

A rotary-shear low to high-velocity friction apparatus in Beijing to study rock friction at plate to seismic slip rates

This paper reviews 19 apparatuses having highvelocity capabilities,describes a rotary-shear low to highvelocity friction apparatus installed at Institute of Geology,China Earthquake Administration,and reports results from velocity-jump tests on Pingxi fault gouge to illustrate technical problems in conducting velocity-stepping tests at high velocities.The apparatus is capable of producing plate to seismic velocities（44 mm/a to 2.1 m/s for specimens of 40 mm in diameter）,using a 22 kW servomotor with a gear/belt system having three velocity ranges.A speed range can be changed by 103 or 106by using five electromagnetic clutches without stopping the motor.Two cam clutches allow fivefold velocity steps,and the motor speed can be increased from zero to 1,500 rpm in 0.1-0.2 s by changing the controlling voltage.A unique feature of the apparatus is a large specimen chamber where different specimen assemblies can be installed easily.In addition to a standard specimen assembly for friction experiments,two pressure vessels were made for pore pressures to 70 MPa;one at room temperature and the other at temperatures to 500 °C.Velocity step tests are needed to see if the framework of rate-and-state friction is applicable or not at high velocities.We report results from velocity jump tests from 1.4 mm/s to 1.4 m/s on yellowish gouge from a Pingxi fault zone,located at the northeastern part of the Longmenshan fault system that caused the 2008 Wenchuan earthquake.An instantaneous increase in friction followed by dramatic slip weakening was observed for the yellowish gouge with smooth sliding surfaces of host rock,but no instantaneous response was recognized for the same gouge with roughened sliding surfaces.Instantaneous and transient frictional properties upon velocity steps cannot be separated easily at high velocities,and technical improvements for velocity step tests are suggested.     

Gas retention in fine-grained pyroclastic flow materials at high temperatures

The ability of a dense pyroclastic flow to maintain high gas pore pressure, and hence low friction, during runout is determined by (1) the strengths and longevities of gas sources, and (2) the ability of the material to retain residual gas once those sources become ineffective. The latter is termed the gas retention capacity. Gas retention capacity in a defluidizing granular material is governed by three timescales: one for the evacuation of bubbles (t _be ; brief and not considered in this paper), one for hindered settling from the expanded state (t _sett), and one for diffusive release of residual pore pressure from the non-expanded state (t _diff). The relative magnitides of t _sett and t _diff depend on bed thickness, t _sett dominating in thin systems and t _diff in thick ones. Three pyroclastic flow materials, two ignimbrites and a block-and-ash flow sample, were studied experimentally to investigate expansion behaviour under gas flow and to determine gas retention times. Effects of particle size were evaluated by using two size cuts (<4 mm and <250 μm) from each sample. Careful drying of the materials was necessary to avoid effects of humidity-related cohesion. Two sets of experiments were carried out: (1) expansion in the non-bubbling regime at 50–200°C, (2) bed collapse tests from the initially bubbling state at 50–550°C. Provided that gas channelling was avoided by gentle stirring, all the samples exhibited a regime of uniform expansion prior to the onset of bubbling. Fine particle size (in particular high fines content), low particle density and high temperature all favoured smoother fluidization by increasing the maximum expansion possible in the non-bubbling state. An empirical equation describing the uniform expansion of the materials was determined. High temperature also favoured greater gas partitioning into the dense phase of the bubbling bed, as well (in finer-grained samples) as higher voidage in the settled bed. Large values of t _sett and t _diff were favoured by fine particle size. Temperature had less influence, suggesting that experimental results at low temperatures (50–200°C) can be extrapolated to higher temperatures. Gas retention times provide insight into the ability of pyroclastic flows in expanded (t _sett) or non-expanded (t _diff) flow states to retain gas once air ingestion or gas production have become ineffective. Finer-grained pyroclastic flows are expected to retain gas longer, and hence to have higher apparent ‘mobilities’, than coarser-grained ones of comparable volume, as has been observed on Montserrat.     

Flow and fracture maps for basaltic rock deformation at high temperatures

Understanding how the strength of basaltic rock varies with the extrinsic conditions of stress state, pressure and temperature, and the intrinsic rock physical properties is fundamental to understanding the dynamics of volcanic systems. In particular it is essential to understand how rock strength at high temperatures is limited by fracture. We have collated and analysed laboratory data for basaltic rocks from over 500 rock deformation experiments and plotted these on principal stress failure maps. We have fitted an empirical flow law (Norton’s law) and a theoretical fracture criterion to these data. The principal stress failure map is a graphical representation of ductile and brittle experimental data together with flow and fracture envelopes under varying strain rate, temperature and pressure. We have used these maps to re-interpret the ductile–brittle transition in basaltic rocks at high temperatures and show, conceptually, how these failure maps can be applied to volcanic systems, using lava flows as an example.     

饱和碳酸盐岩地震频带弹性参数响应的实验机理分析研究: 压力与孔隙流体影响

碳酸盐岩储层中蕴藏着全球近一半油气储量,也是我国深层、超深层的重点勘探领域.然而,深层碳酸盐岩复杂孔隙结构导致其储层条件下岩石物理响应机理不明,这对有效地震储层表征与流体预测带来了巨大挑战.目前,国内尚缺乏系统开展针对深层碳酸盐岩储层的地震频带岩石物理实验测试研究.厘清深层油藏开发中因流体流动而引起的储层压力场与孔隙流体性质变化对地震频带弹性参数影响机制,在一系列地球物理应用中具有十分重要的意义.通过开展跨频带（地震低频+超声高频）弹性参数实验测试,本文系统研究了压力与孔隙流体对深层碳酸盐岩弹性模量及其衰减的影响机制.针对中低孔渗碳酸盐岩储层岩芯,分别测量了其在干燥、饱和盐水与甘油下的弹性参数,分析了弹性模量对压力与频率敏感性.干燥岩芯杨氏模量频变特性显著低于饱和流体岩芯实测数据,且饱和甘油岩芯模量及其衰减均高于饱和盐水测量结果.部分饱和盐水岩芯的地震频带实测结果表明,杨氏模量与泊松比均随含水饱和度升高而增大,其频散程度却略有降低.此外,基于干燥岩芯弹性参数不依赖频率的前提,受迫振动法低频实验所得岩石弹性模量和衰减数据与三孔隙类型喷射流模型预测结果相一致,证实喷射流是引起复杂孔隙微观结构储层岩芯频变黏弹性响应的主控因素.

     

Composition of the core,I. Solubility of oxygen in molten iron at high temperatures

Experiments on the solubility of FeO in molten iron have been carried out at temperatures between 2100 and 2550°C. The results show that liquid FeO is extensively soluble in molten iron at 2500°C and indicate that they probably become completely miscible above 2800°C. Liquid iron in equilibrium with crystalline magnesiowüstite (Mg_0.8Fe_0.2)O which is believed to be an important mineral in the lower mantle, would dissolve about 14 mol.% of FeO at 2500°C and 40 mol.% of FeO at 2800°C. The geochemical implications of these results are discussed. It is concluded that the outer core probably contains a large amount of dissolved FeO and that oxygen is probably the principal light element in the outer core.     

Experimental studies of electrical conductivities and P-wave velocities of gabbro at high pressures and high temperatures

The studies on the physical properties of minerals and rocks in combination with the work in petrology, mineralogy and geochemistry are not only a useful mean to look into the composition and structure of the earth抯 interior, but also can provide extreme…     

Improvements to Griggs-type apparatus for mechanical testing at high pressures and temperatures

New and improved techniques and apparatus for testing the mechanical properties of materials at high presures and temperatures are described. These include an improved Griggs-type deformation apparatus designed to operate to 5 GPa and associated servo-controlled hydraulic drive and electronics, the design of hydrostatic (molten alkali halide mixtures) pressure assemblies to measure flow stresses as low as a few MPa, the characterization of temperature gradients and friction in such assemblies, measurement of the melting curve of an alkali halide mixture used as a confining pressure medium, and the measurement of acoustic emissions.     

Compressional elastic wave velocities of serpentinized pyroxenite at high pressures and high temperatures and its geological significance

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. Nowadays dehydration is believed to be as one of the most important reasons responsible for the abnormality of seismic velocity (Kern, 1982; Ito, 1990; Christensen, 1989; Popp, Kern, 1993; SONG, et al, 1996; ZHOU, 1998; ZHAO, et al, 1996). Geophysical, geochemical and mineralogical data have revealed that on…     

Hematite grains: Size and coercive force from AF demagnetization at high temperatures

The coercivity spectrum of low-field high-temperature partial thermoremanent magnetization (PTRM) of a synthetic hematite powder, extremely high at room temperature, decreases very slowly with increasing temperature up to 500°C then decreases rapidly, especially above 600°C. From the AF demagnetization curves at 600 and 650°C it is calculated, following the Néel's theory of single-domain particles that the grains carrying the PTRM have a mean coercive force of 23 ± 5 kOe and a mean grain size of 0.40 ± 0.15 μm, which is not significantly different from the mean grain size of 0.48 ± 0.03 μm from electron micrograph observations.     

Effect of viscous,viscoplastic and friction damping on the response of seismic isolated structures

In this paper the efficiency of various dissipative mechanisms to protect structures from pulse‐type and near‐source ground motions is examined. Physically realizable cycloidal pulses are introduced, and their resemblance to recorded near‐source ground motions is illustrated. The study uncovers the coherent component of some near‐source acceleration records, and the shaking potential of these records is examined. It is found that the response of structures with relatively low isolation periods is substantially affected by the high‐frequency fluctuations that override the long duration pulse. Therefore, the concept of seismic isolation is beneficial even for motions that contain a long duration pulse which generates most of the unusually large recorded displacements and velocities. Dissipation forces of the plastic (friction) type are very efficient in reducing displacement demands although occasionally they are responsible for substantial permanent displacements. It is found that the benefits by hysteretic dissipation are nearly indifferent to the level of the yield displacement of the hysteretic mechanism and that they depend primarily on the level of the plastic (friction) force. The study concludes that a combination of relatively low friction and viscous forces is attractive since base displacements are substantially reduced without appreciably increasing base shears and superstructure accelerations. Copyright © 2000 John Wiley & Sons, Ltd.