Simulating the effects of induced anisotropy on liquefaction potential using a new constitutive model

The effects of induced anisotropy on the undrained behaviour of very loose and saturated sands have been a subject of intensive investigation, both experimentally and theoretically, by several authors in the past few years. This paper proposes an original constitutive model well‐adapted to simulate the behaviour of sands subject to complex stress histories, in particular, the preloading cycle along the classical drained stress path in compression. The developed model belongs to the family of critical state models. Its construction is based on a few theoretical concepts taken from the theory of ‘Bounding Surface Plasticity’ developed among others by Y. Dafalias and Popov (1975), the ‘Clay And Sand Model’ (CASM) of H. Yu (2006), the CJS model (B. Cambou and K. Jafari (1988)) and the hyperelastic isotropic model of P. Lade (1987). To accurately simulate volume changes, which represent a key element in soil behaviour, a state‐dependent dilatancy rule is proposed, which can account for the influences of stress and void ratio. The current void ratio depends implicitly on the irreversible strains already accumulated hence the strain history. A kinematic hardening is combined with an isotropic hardening, involving rotation and distortion of the bounding surface, in order to capture correctly the experimental observations. Comparisons of experimental results to numerical simulations show that the model is able to simulate with a good precision the major trends of undrained responses of loose and presheared sands. It predicts correctly rapid static liquefaction at small or null drained preloading, as well as the progressive transition to a completely stable behaviour typical of dense sands, while the sample is loose in reality. At intermediate to large amplitudes of preloadings, the model also predicts correctly the temporary stage of instability when the deviatoric stress decreases slightly before rising up again. Copyright © 2013 John Wiley & Sons, Ltd.     

横向各向同性介质和斜方介质各向异性参数的约束条件

Thomsen提出的横向各向同性(TI)介质各向异性参数(α0、β0、ε、δ、γ)是各向异性理论研究和实际资料处理中的常用参数,Thomsen参数的取值必须符合物理学定律和实际地学情况,随意的取值可能导致无意义乃至错误的结果.本文根据热力学定律和弹性常数的物理意义,结合大量的实测数据,提出常见TI介质的Thomsen参...     

用剪切波分裂研究台湾北部地壳各向异性

研究主要使用台湾北部13个地震台站记录到的1991年7月～2002年12月的波形数据,采用剪切波分裂SAM分析方法,对台湾北部地区的剪切波分裂特征进行了研究.发现位于宜兰盆地内的台站的快剪切波优势偏振方向为近E-W方向,而位于山脉(西部麓山带、雪山山脉和中央山脉)的台站的快剪切波优势偏振方向为NNE向或NE方向.位于海...     

地壳介质非均匀性对华北地区强震活动的影响

华北地区是我国地震灾害最频繁的地区之一,该区域的强震空间分布可能与地壳介质的不均匀性有关.本文建立了华北地区岩石圈三维Maxwell模型,并尝试采用剪切波分裂参数为新的约束条件,对区域地壳运动场和应力应变场进行了模拟,探讨地壳介质不均匀性与区域地震活动的关系.研究结果表明:外部动力源控制了华北地区整体的地壳运动方式,地...     

Seismic anisotropy in a hydrocarbon field estimated from microseismic data

The study of seismic anisotropy in exploration seismology is gaining interest as it provides valuable information about reservoir properties and stress directions. In this study we estimate anisotropy in a petroleum field in Oman using observations of shear‐wave splitting from microseismic data. The data set was recorded by arrays of borehole geophones deployed in five wells. We analyse nearly 3400 microearthquakes, yielding around 8500 shear‐wave splitting measurements. Stringent quality control reduces the number of reliable measurements to 325. Shear‐wave splitting modelling in a range of rock models is then used to guide the interpretation. The difference between the fast and slow shear‐wave velocities along the raypath in the field ranges between 0–10% and it is controlled both by lithology and proximity to the NE‐SW trending graben fault system that cuts the field formations. The anisotropy is interpreted in terms of aligned fractures or cracks superimposed on an intrinsic vertical transversely isotropic (VTI) rock fabric. The highest magnitudes of anisotropy are within the highly fractured uppermost unit of the Natih carbonate reservoir. Anisotropy decreases with depth, with the lowest magnitudes found in the deep part of the Natih carbonate formation. Moderate amounts of anisotropy are found in the shale cap rock. Anisotropy also varies laterally with the highest anisotropy occurring either side of the south‐eastern graben fault. The predominant fracture strikes, inferred from the fast shear‐wave polarizations, are consistent with the trends of the main faults (NE‐SW and NW‐SE). The majority of observations indicate subvertical fracture dip (>70° ). Cumulatively, these observations show how studies of shear‐wave splitting using microseismic data can be used to characterize fractures, important information for the exploitation of many reservoirs.     

裂隙参数对衰减各向异性影响的数值模拟

理论、观测和实验均证实,在地壳和上地幔存在对应力变化敏感的直立裂隙,在整体上呈现方位各向异性.在Hudson裂隙理论基础上,系统全面地数值模拟了裂隙介质几何、物性和弹性波频率等参数对各向异性衰减的影响.结果显示,裂隙密度、裂隙纵横比、泊松比、裂缝填充物、弹性波频率和未破裂岩石母体纵横波速度等对各向异性衰减有着显著影响,...     

Characterization of the D beneath the Galapagos Islands using SKKS and SKS waveforms

SKS and SKKS waveforms from 16 events occurring between 2003 and 2005 in the Tonga Trench that were recorded by the BOLIVAR array are analyzed to determine the structure of the D layer beneath the Galapagos Islands.248 differential travel-time residuals of SKKS-SKS are measured and reveal a region of positive residuals of differential travel times in the northeast portion of the sampled region.Analyzing correlation statistics between the measured SKKS-SKS residuals and the observed absolute travel time dela...     

各向异性介质中频率相关横波分裂参数的提取算法(英文)

基于曾新吾和MacBeth提出的时域内双源累积旋转方法,建立了频率相关介质中分析多分量VSP数据的横波分裂参数提取算法(DCTF)。该算法可以在频域中针对单个频率提取横波各向异性参数(快横波的极化方向以及快、慢横波间的时间差),从而避免了目前常用方法中使用窄带通滤波可能带来的误差。通过对地震合成记录的数值分析,确定了该算法的可行性和正确性,并与目前常用方法的应用结果进行了比较。结果显示,频率相关横波分裂参数可以利用DCTF从地震四分量数据中直接提取。在地震频率范围内,含较大尺度裂缝时各向异性参数将表现出频率相关性,这意味着在地震频率范围会出现频散。随着频率的增加,各向异性有降低的趋势。     

A simulation study of microstructure evolution inside the shear band in biaxial compression test

We study the development of microstructure inside the shear band in granular media consisting of elliptical‐shaped particles. Plane strain biaxial compression test was simulated using two‐dimensional distinct element method. The generation of large voids and concentration of excessive particle rotation inside a shear band are found in a quite similar manner to those observed in natural soils. Evolution of the microstructure inside and outside the shear band is studied. The magnitude and direction of particle rotation inside the shear band is influenced by orientation of long axes of elliptical particles. Because of such particle rotations inside the shear band, the preferred alignment of particles becomes horizontal in the residual state, which results in a more anisotropic contact normal distribution oriented along the major principal stress axis. Copyright © 2010 John Wiley & Sons, Ltd.     

苏北盆地沙埝油田阜三段储层微观特征及其与驱油效率的关系

在对沙埝油田阜三段储层微观特征进行研究的基础上,利用真实砂岩微观模型水驱油实验得到驱油效率,探讨了物性和孔隙结构特征对驱油效率的影响,解释实验中出现渗透率相近的样品驱油效率却相差较大的特殊现象,并对退汞效率作为驱油效率的准确性进行分析。结果表明,储层岩石碎屑颗粒粒度细、分选较差、填隙物含量高,且经历了较强的压实作用,是造成孔隙喉道细小、非均质性较强的主要原因;驱油效率随物性变好、喉道变大而提高,但其与喉道分选性的关系较为复杂,大喉道一方面加剧了喉道间的非均质性,另一方面则提供了孔隙间良好的连通通道,分选较好的小喉道则保证了注入水的均匀推进;驱油效率主要受物性与最大连通喉道控制;真实砂岩微观模型水驱油实验所得的驱油效率与压汞实验所得的退汞效率相关性较好,但在低值区存在一定差异。