基于二维三分量伪谱法模拟数据的EDA介质中横波分裂研究

为了检测定向裂隙介质中横波分裂的方位属性特征,分析地震属性随裂隙密度和方位变化,采用人工吸收边界和反周期扩展边界,用伪谱法获得不同裂隙密度和不同方位地质模型三分量地面记录;应用时频分析和剪切波偏振分析研究由于裂隙方位和密度引起的横波分裂.结果显示,裂隙密度和方位决定着横波分裂的时差和偏振.快慢横波的延迟时间随裂隙密度增大而增加,不同方位相同裂隙密度的横波分裂时差有微小的变化.在45°方位检测时间延迟时间最大.通过时频分析,可以看到不同方位的瞬时主频有显著的变化,在横波分裂处瞬时主频有明显变化.因此,瞬时主频和快横波的偏振以及延迟时间可以作为裂隙方位和密度的指示.     

SKS波对地壳裂隙各向异性的响应——理论地震图研究

穿透含裂隙、裂缝地壳8s视周期的SV波的理论地震图研究表明,当地壳平均裂隙密度高于0.01即横波各向异性高于1%时,非对称面内不同方位的SKS波均发生分裂;地震图中直接的记录显示是切向T分量上出现SKS波的振动,其振幅随地壳平均裂隙密度的增大而增强,甚至能与径向R分量上的振幅相当.局限于上地壳的强裂缝各向异性同样能引起SKS分裂.长周期SKS波分裂对地壳内裂隙、裂缝的分布缺乏分辨率.直立平行排列裂隙、裂缝使得SKS分裂T分量记录特征具有方位对称性,这来自于HTI介质中快、慢波偏振和到时差随方位变化的对称性;而倾斜裂隙、裂缝使得该方位对称性丧失.对实际观测SKS分裂的偏振解释需要考虑地壳裂隙各向异性,特别是断裂附近的强裂缝各向异性.     

玉树地震震源区Pg波方位各向异性及其意义

地壳介质中,特别是上地壳广泛存在着微裂隙,如果这些微裂隙定向排列,在宏观上就形成了各向异性介质,当地震波通过时就会产生特殊的现象,对剪切波而言就会发生横波分裂,对P波而言则会看到随方位的变化.利用玉树地震的余震数据,通过层析成像的方法研究震源区附近P波速度的横向变化和方位各向异性变化,探讨微裂隙的分布及意义.     

2008年汶川地震震源区横波分裂的变化特征

对2008年汶川地震序列的横波分裂的时空变化现象进行了研究．通过在横波窗内S波质点运动图的分析,从地震余震区附近台站的近场地震记录中提取了S波分裂的快波偏振方向和快慢波之间的时间延迟．龙门山断裂附近7个台站的S波分裂的结果显示了快波偏振方向的空间分布特征,断裂带东南侧（四川盆地一侧）台站的快波偏振方向总体为北东方向,而在断裂西北侧高原内部的PWU（平武台）则为近东西方向．观测到了快慢波之间到时差的趋势性变化,靠近余震带南端的L5501台的到时差在主震后持续减小,北端PWU台的到时差比主震前明显增大．横波分裂现象主要由台站下方岩层中随应力场方向排列的微裂隙控制,结果显示了主震和余震过程中区域应力场的变化,汶川地震余震活动带南部的应力在主震和余震中得到了释放,地壳应力场在余震带北部集中,导致了PWU台在主震后介质各向异性强度的增强．     

S波偏振方法在确定汶川8.0级地震震源深度中的应用

当横波进入由微裂隙和微孔这些比波长小的非均匀物质组成的岩石中,分裂成两个以不同方向偏振的组成部分.这两列波的偏振与裂隙面成平行和垂直方向,并以不同的速度传播.由于速度的不同,使这两列偏震波在裂隙介质的传播中产生“分裂”,横波分裂,使得横波波形产生一种新特征,即它离开裂隙区后仍然保持其偏振方向不变.地下几千米处,裂隙有垂直的也有不垂直的.     

裂缝介质横波分裂时差影响因素定量研究

裂缝介质数值模拟是研究裂缝型油藏各向异性及裂缝检测的关键.横波分裂现象是波在各向异性介质中传播的一个显著特征,横波分裂时差能够反映地下裂隙密度和介质物性等信息.本文给出二维均匀TTI介质在不同极化角度下的横波群速度公式,进而得到偏移距表征的横波分裂时差表达式;同时采用二维三分量交错网格高阶有限差分方法模拟不同极化角度下的三分量炮记录,与理论值对比分析,定量研究分裂时差与极化角、偏移距、各向异性参数的关系,得到如下主要结论:1)在不同各向异性强度下,横波分裂时差与极化角呈现不同的变化规律,而快慢横波振幅比与极化角的关系不受各向异性强度的影响;2)从能量角度,在45°～55°极化角内,横波传播的能量对比最强,是观测横波分裂的最佳角度范围;3)表征各向异性强度的三个参数中γ对横波分裂时差的影响最大,其从0变化到0.3,分裂时差增幅达432.4%.     

卢龙地区S波偏振与上地壳裂隙各向异性

由三分量数字地震仪组成的小孔径流动台网记录了1982年10月19日河北卢龙M_s=6.1级地震的部分余震.用质点运动图的方法对横波的偏振进行了分析。研究结果表明,在横波窗内的各观测点都存在横波的分裂现象.不同离源角和方位角快波偏振的水平投影都具有近NE40°方向的优势取向,与根据卢龙地震两组断层错动在各向同性介质中所辐射的横波的偏振方向不一致.这可以由传播介质中应力所导致裂隙的定向排列来解释.这一观测结果提供了卢龙地区脆性上地壳大范围膨胀各向异性（EDA）的证据,并表明这一地区直立平行排列裂隙取向和水平主压应力的方向为NE40°.     

对大同地震横波分裂的研究

由三分向数字地震仪构成的小孔径流动台网记录了1989年10月山西大同二个强震之后15天内的余震。纵波和横波的尾波在直达波的后续波列中表现得非常明显,估计是由近地表不规则沉积层对地震波的聚焦效应所形成的。由于近地表不规则沉积层的作用,在较宽的离源角的范围内都可以辨认出横波分裂现象。用大范围膨胀各向异性(EDA)裂隙对横波的影响解释了快波偏振的排列方向与区域应力场最大主压应力方向的一致性。对一天中包括一个较大地震以及它前后较小地震形成的系列,用理论到时差数值模拟与实际记录对比的方法,确认了横波分裂到时差在一个较大地震发生前后的上升和下降,用应力导致壳内裂隙随时间的变化对横波传播的作用解释了这种变化。到时差在短时间内发生的迅速变化说明含有EDA裂隙的上地壳对区域应力变化具有快速的弹性响应。     

伊朗中东部Ghaen-Birjand地区上地壳各向异性研究c

通过对1997年5月10日Zirkuh ( Ghaen-Birjand )破坏性地震若干余震横波分裂现象的观测，研究了该地区上地壳各向异性特征. 采用质点运动图和纵横比两种不同手段获取了横波分裂参数. 所选余震记录显示了明显的横波分裂，说明该地区介质具有强烈的各向异性. 通过质点运动的方法得出:快横波的偏振方向为22N19E；且快、慢横波的延时之差为6516 ms. 用纵横比的方法得到的结果为:快横波的偏振方向35N18E, 且快、慢横波的延时之差为(4910)ms. 对于厚约5 km各向异性均匀分布的水平层， 我们采用了一个沿应力分布的裂纹模型，并采用垂直于N22E排列, 且密度为0.01的裂纹情况来解释本文结果. 由于研究范围是上地壳，因此假设裂纹被水填充. 最后通过Hudson模型，对裂纹密度为0.005, 0.01和0.03的模型，给出横波速度随传播方向与裂纹对称轴夹角的变化曲线. 结果表明，当横波的偏振平行于裂纹表面时波速不随角度变化；而偏振垂直于裂纹表面时，速度曲线则有明显变化.      

地震波在粘弹性各向异性地壳介质中的传播及其应用研究

实际地球介质中普遍存在着多尺度、多取向、填充各种流体的孔隙、裂隙和裂缝,这种介质常常被称为双相介质或多相介质。裂隙及其填充流体的存在一方面使得介质的弹性模量(体模量和剪切模量)减小,改变流体输运性质,另一方面也导致地震波传播的各向异性、衰减和频散。介质的双相和各向异性特征是现代地球物理学的两个难点。一般各向异性弹性介质的弹性矩阵有21个独立参量,满足Christoffel方程;在高频近似条件下,满足射线追踪方程。然而直接求解波动方程往往十分困难,常常不能得到解析解。考虑到实际地球介质大多是弱各向异性,因此可以利用扰动理论近似描述,将相速度、偏振矢量线性化并表示为弱各向异性参数(Weakly Anisotropy,WA)的形式。我们给出了TTI介质中qP波的相速度和偏振矢量的弱各向异性参数线性化反演公式。20世纪70年代以来,科学家们通过地震波速度的方位各向异性、横波分裂、接收函数法、面波偏振等方法研究地球介质的各向异性。其中横波分裂方法是各向异性最直观的表现,具有相对简单、横向分辨率高、对裂隙密度及纵横比较敏感等优点,是研究介质各向异性性质的一种主要方法,也是研究大构造(如岩石圈等)的重要手段。本文以EDA(Extensive Dilatancy Anisotropy),APE(Anisotropic Poro-Elasticity),Chapman多尺度模型为例介绍了双相介质中的等效介质模型。EDA、APE理论的研究对象是微观尺度的单组微裂隙,其裂隙密度定义为张性扩容各向异性。EDA被认为是地壳各向异性的主要因素,当横波通过这种介质会分裂成快慢横波,且快横波的优势偏振方向往往与区域最大水平应力相同。基于SOC(Self Organized Criticality)理论双相介质的破裂特性,可以对由于应力作用处于临界状态下的微裂隙与流体的动态特征进行描述的APE模型表明,横波分裂对地壳介质的微裂隙的几何特征(如裂隙的纵横比)十分敏感,而微裂隙的几何特征变化对应力场的变化十分敏感。因此将横波分裂结果在地震前后的变化解释为横波对地应力场变化的响应,并提出可以利用这一特征进行应力检测。在不同地质构造时期,由于区域应力场的作用,可能导致实际地球介质中的微裂隙是多尺度的,并且有多个优选方向。本文介绍的Chapman多尺度模型将中等尺度断裂加入分析中,并考虑两种尺度的微裂隙和裂缝之间的相互作用,获得了频率相关的等效弹性矩阵。通过引入复数弹性张量,可考虑各向异性粘弹性介质的波动理论。由于弹性矩阵的虚数部分的引入,且是频率相关的,因此解析解的计算比一般各向异性弹性介质更加复杂。在高频近似时,可将一般各向异性弹性介质的射线追踪方程以及扰动理论应用到弱各向异性弱衰减(Weakly Anisotropy-Attenuation,WAA)介质中,此时其固体骨架由各向同性变为各向异性介质,且弹性矩阵的扰动由实数变为复数。本文给出了P波在一般WAA介质中随方位衰减的线性化反演公式。通过2个、3个、6个等方位间距的剖面组成的walkawayVSP模拟实验,使用TTI模型和一般各向异性模型对模拟数据进行了反演,验证了反演公式的正确性和可靠性。然后,对一个来自JavaSea的由三条剖面组成的walkaway VSP实际观测数据进行了反演计算,获得了钻井中不同深度检波器处的WA参数。中国地震台网中心测定,北京时间2008年5月12日14:28:04,在青藏高原东缘龙门山推覆构造带上发生了逆冲为主带有少量右旋、挤压型M S8.0大地震(31.01°N,103.42°E)。为提高时频分辨率,分析时频局部化信息,引入Meyer-Yamada正交小波基MYW。在粘弹性各向异性线性化理论的基础上,运用小波分析方法对以汶川地震为中心,200km为半径的范围内的地震进行了分析,获得了汶川震中区9个固定台站的P波小波波谱的平均衰减结果和方位各向异性衰减结果。该方法能够有效利用小震数据研究震源区介质各向异性随时空的变化规律,具有广阔的应用前景。     

Observation and analysis of frequency-dependent anisotropy from a multicomponent VSP at Bluebell-Altamont field, Utah

In this paper, we present results from the analysis of a multicomponent VSP from a fractured gas reservoir in the Bluebell-Altamont Field, Utah. Our analysis is focused on frequency-dependent anisotropy. The four-component shear-wave data are first band-pass filtered into different frequency bands and then rotated to the natural coordinates so that the fast and slow shear-waves are effectively separated. We find that the polarisations of the fast shear-waves are almost constant over the whole depth interval, and show no apparent variation with frequency. In contrast, the time delays between the split shear-waves decrease as the frequency increases. A linear regression is then applied to fit the time-delay variations in the target and we find that the gradients of linear fits to time delays show a decrease as frequency increases. Finally, we apply a time-frequency analysis method based on the wavelet transform with a Morlet wavelet to the data. The variation of shear-wave time delays with frequency is highlighted in the time-delay and frequency spectra. We also discuss two mechanisms giving rise to dispersion and frequency-dependent anisotropy, which are likely to explain the observation. These are scattering of seismic waves by preferentially aligned inhomogeneneities, such as fractures or fine layers, and fluid flow in porous rocks with micro-cracks and macro-fractures.     

Fluid‐dependent shear‐wave splitting in fractured media

Azimuthal anisotropy in rocks can result from the presence of one or more sets of partially aligned fractures with orientations determined by the stress history of the rock. A shear wave propagating in an azimuthally anisotropic medium splits into two components with different polarizations if the source polarization is not aligned with the principal axes of the medium. For vertical propagation of shear waves in a horizontally layered medium containing vertical fractures, the shear‐wave splitting depends on the shear compliance of the fractures, but is independent of their normal compliance. If the fractures are not perfectly vertical, the shear‐wave splitting also depends on the normal compliance of the fractures. The normal compliance of a fluid‐filled fracture decreases with increasing fluid bulk modulus. For dipping fractures, this results in a decrease in shear‐wave splitting and an increase in shear‐wave velocity with increasing fluid bulk modulus. The sensitivity of the shear‐wave splitting to fluid bulk modulus depends on the interconnectivity of the fracture network, the permeability of the background medium and on whether the fracture is fully or partially saturated.     

Numerical analysis of seismic wave propagation in fluid-saturated porous multifractured media

Elastic wave propagation and attenuation in porous rock layers with oriented sets of fractures, especially in carbonate reservoirs, are anisotropic owing to fracture sealing, fracture size, fracture density, filling fluid, and fracture strike orientation. To address this problem, we adopt the Chapman effective medium model and carry out numerical experiments to assess the variation in P-wave velocity and attenuation, and the shear-wave splitting anisotropy with the frequency and azimuth of the incident wave. The results suggest that velocity, attenuation, and anisotropy vary as function of azimuth and frequency. The azimuths of the minimum attenuation and maximum P-wave velocity are nearly coincident with the average strike of the two sets of open fractures. P-wave velocity is greater in sealed fractures than open fractures, whereas the attenuation of energy and anisotropy is stronger in open fractures than sealed fractures. For fractures of different sizes, the maximum velocity together with the minimum attenuation correspond to the average orientation of the fracture sets. Small fractures affect the wave propagation less. Azimuth-dependent anisotropy is low and varies more than the other attributes. Fracture density strongly affects the P-wave velocity, attenuation, and shear-wave anisotropy. The attenuation is more sensitive to the variation of fracture size than that of velocity and anisotropy. In the seismic frequency band, the effect of oil and gas saturation on attenuation is very different from that for brine saturation and varies weakly over azimuth. It is demonstrated that for two sets of fractures with the same density, the fast shear-wave polarization angle is almost linearly related with the orientation of one of the fracture sets.     

Shear Wave Splitting Analysis to Estimate Fracture Orientation and Frequency Dependent Anisotropy

Shear wave splitting is a well-known method for indication of orientation, radius, and length of fractures in subsurface layers. In this paper, a three component near offset VSP data acquired from a fractured sandstone reservoir in southern part of Iran was used to analyse shear wave splitting and frequency-dependent anisotropy assessment. Polarization angle obtained by performing rotation on radial and transverse components of VSP data was used to determine the direction of polarization of fast shear wave which corresponds to direction of fractures. It was shown that correct implementation of shear wave splitting analysis can be used for determination of fracture direction. During frequencydependent anisotropy analysis, it was found that the time delays in shearwaves decrease as the frequency increases. It was clearly demonstrated throughout this study that anisotropy may have an inverse relationship with frequency. The analysis presented in this paper complements the studied conducted by other researchers in this field of research.     

含有倾斜薄裂缝孔隙地层中的井孔声场

应用三维交错网格应力-速度有限差分方法,数值模拟了含有倾斜裂缝孔隙介质地层中点声源所激发的井孔声场问题.为满足薄裂缝计算需求,开发了不均匀网格有限差分算法,提高了计算精度及计算速度.利用将孔隙介质方程参数取为流体极限的办法来处理裂缝中的流体,实现了流体-孔隙介质界面处的差分方程统一,使界面处的计算更加灵活方便.在验证了方法正确性的基础上,分别考察了单裂缝宽度、裂缝带宽度、裂缝倾斜角度以及孔隙介质渗透率等参数的变化对井轴上阵列波形的影响并进行了分析.结果表明,声波经过裂缝时可能产生反射横波及斯通利波,后者随裂缝宽度的减小而减小,而前者随裂缝宽度的改变,变化不大,在裂缝很小(20μm)时依然存在;裂缝带的宽度、密度越大,反射斯通利波越强;当裂缝(裂缝带)倾斜时,反射横波消失,但反射斯通利波受裂缝倾斜角度的影响较小;渗透率的改变对斯通利波的衰减影响较为明显.     

Pore fluid viscosity effects on P‐ and S‐wave anisotropy in synthetic silica‐cemented sandstone with aligned fractures

Ultrasonic (500 kHz) P‐ and S‐wave velocity and attenuation anisotropy were measured in the laboratory on synthetic, octagonal‐shaped, silica‐cemented sandstone samples with aligned penny‐shaped voids as a function of pore fluid viscosity. One control (blank) sample was manufactured without fractures, another sample with a known fracture density (measured from X‐ray CT images). Velocity and attenuation were measured in four directions relative to the bedding fabric (introduced during packing of successive layers of sand grains during sample construction) and the coincident penny‐shaped voids (fractures). Both samples were measured when saturated with air, water (viscosity 1 cP) and glycerin (100 cP) to reveal poro‐visco‐elastic effects on velocity and attenuation, and their anisotropy. The blank sample was used to estimate the background anisotropy of the host rock in the fractured sample; the bedding fabric was found to show transverse isotropy with shear wave splitting (SWS) of 1.45 ± 1.18% (i.e. for S‐wave propagation along the bedding planes). In the fractured rock, maximum velocity and minimum attenuation of P‐waves was seen at 90° to the fracture normal. After correction for the background anisotropy, the fractured sample velocity anisotropy was expressed in terms of Thomsen's weak anisotropy parameters ε, γ & δ. A theory of frequency‐dependent seismic anisotropy in porous, fractured, media was able to predict the observed effect of viscosity and bulk modulus on ε and δ in water‐ and glycerin‐saturated samples, and the higher ε and δ values in air‐saturated samples. Theoretical predictions of fluid independent γ are also in agreement with the laboratory observations. We also observed the predicted polarisation cross‐over in shear‐wave splitting for wave propagation at 45° to the fracture normal as fluid viscosity and bulk modulus increases.     

Investigation of upper crust anisotropy in Ghaen-Birjand region, east-central Iran

IntroductionWhenpropagatingthroughananisotropicmedium,ashearwavesplitsintotwo(quasi)shearwaveswithdifferentpropagationspeedsandpolarizedorthogonally.Owingtotherecentdevel-opmentofseismicobservationsystem,detectionofshearwavessplittingwithverysmalldelaytimesbetweenfasterandslowershearwavesbecameavailableandprovidedpowerfulapproachfordetectionofcrustalanisotropy.Crampin(1978)emphasizedtheroleofalignedmicrocracksasacauseofcrustalanisotropyandpointedoutthatforverticallyalignedmicrocracksthedirecti…     

涪陵地区井下页岩岩芯裂缝体密度与声学性质关系实验研究

本研究对涪陵地区井下产气的龙马溪组页岩岩芯,采用三轴压机压裂制造裂缝,用工业CT扫描压裂前后岩芯,应用图像识别技术统计出裂缝体密度,又用超声脉冲透射法测定样品破裂前后裂缝方向上的纵波、单偏振方向与裂缝不同夹角的横波,来研究裂缝对页岩岩芯声学性质的影响.实验结果表明:压裂后样品的纵波速度略微降低,只有含较多内部裂隙的150#样品纵波速度减小幅度明显.压裂前后样品的纵波波形差别不大,纵波主频随裂缝体密度呈下降趋势,即压裂后纵波频谱主频向低频端移动.压裂前横波速度随自身与裂隙方位角变化而变化,与0°和180°相比,在45°和135°时略微减小,在90°时速度降低幅度最为明显并且发生相位反转.典型样品的横波主频随偏振方向与裂缝夹角的增大而逐渐向低频移动;压裂后,横波频谱杂乱,出现多处局部峰值,速度和主频较压裂前更低,平均横波波速随裂缝体密度呈明显减小趋势.平均纵横波速比随裂缝密度呈近线性增加,表明其与裂缝体密度有较强相关性.     

Frequency‐dependent anisotropy of porous rocks with aligned fractures

Naturally fractured reservoirs are becoming increasingly important for oil and gas exploration in many areas of the world. Because fractures may control the permeability of a reservoir, it is important to be able to find and characterize fractured zones. In fractured reservoirs, the wave‐induced fluid flow between pores and fractures can cause significant dispersion and attenuation of seismic waves. For waves propagating normal to the fractures, this effect has been quantified in earlier studies. Here we extend normal incidence results to oblique incidence using known expressions for the stiffness tensors in the low‐ and high‐frequency limits. This allows us to quantify frequency‐dependent anisotropy due to the wave‐induced flow between pores and fractures and gives a simple recipe for computing phase velocities and attenuation factors of quasi‐P and SV waves as functions of frequency and angle. These frequency and angle dependencies are concisely expressed through dimensionless velocity anisotropy and attenuation anisotropy parameters. It is found that, although at low frequencies, the medium is close to elliptical (which is to be expected as a dry medium containing a distribution of penny‐shaped cracks is known to be close to elliptical); at high frequencies, the coupling between P‐wave and SV‐wave results in anisotropy due to the non‐vanishing excess tangential compliance.     

Effect of fracture fill on frequency‐dependent anisotropy of fractured porous rocks

In fractured reservoirs, seismic wave velocity and amplitude depend on frequency and incidence angle. Frequency dependence is believed to be principally caused by the wave‐induced flow of pore fluid at the mesoscopic scale. In recent years, two particular phenomena, i.e., patchy saturation and flow between fractures and pores, have been identified as significant mechanisms of wave‐induced flow. However, these two phenomena are studied separately. Recently, a unified model has been proposed for a porous rock with a set of aligned fractures, with pores and fractures filled with two different fluids. Existing models treat waves propagating perpendicular to the fractures. In this paper, we extend the model to all propagation angles by assuming that the flow direction is perpendicular to the layering plane and is independent of the loading direction. We first consider the limiting cases through poroelastic Backus averaging, and then we obtain the five complex and frequency‐dependent stiffness values of the equivalent transversely isotropic medium as a function of the frequency. The numerical results show that, when the bulk modulus of the fracture‐filling fluid is relatively large, the dispersion and attenuation of P‐waves are mainly caused by fractures, and the values decrease as angles increase, almost vanishing when the incidence angle is 90° (propagation parallel to the fracture plane). While the bulk modulus of fluid in fractures is much smaller than that of matrix pores, the attenuation due to the “partial saturation” mechanism makes the fluid flow from pores into fractures, which is almost independent of the incidence angle.