ulti-azimuth three-component surface seismic modeling for viscoelastic cracked monoclinic media

针对粘弹性裂隙型单斜介质,本文应用高阶交错网格差分法,对其传播快照以及多方位地面三分量记录进行模拟,结果证明了裂隙填充物的性质引起的各向异性与粘弹性性质对单斜介质波场有明显的影响,总结了不同观测方位地震记录呈现出的规律性,这为进一步了解波场在介质中的传播情况,提出更合理的理论模型,提供一种尝试.     

粘弹性裂隙型单斜介质多方位地面三分量记录模拟

针对粘弹性裂隙型单斜介质,本文应用高阶交错网格差分法,对其传播快照以及多方位地面三分量记录进行模拟,结果证明了裂隙填充物的性质引起的各向异性与粘弹性性质对单斜介质波场有明显的影响,总结了不同观测方位地震记录呈现出的规律性,这为进一步了解波场在介质中的传播情况,提出更合理的理论模型,提供一种尝试。     

裂隙各向异性介质中的NMO速度

推导了各向异性介质中由弹性系数表示的方位动校NMO速度的具体表达式,表明各向异性介质中方位NMO速度程椭圆形状,并分别对具水平对称轴的横向各向同性介质(HTI)、正交介质和单斜各向异性介质及在不同的裂隙填充物的性质下方位NMO速度进行了计算,结果表明裂隙的存在对NMO速度的影响不仅与裂隙密度有关,还取决于裂隙填充物的性质.同时,研究表明对于裂隙型单斜各向异性介质,其方位NMO速度椭圆轴向并不象HTI介质和正交介质中的那样与自然坐标系的坐标轴一致,而是发生了一定角度的偏离,其大小与裂隙填充物的性质、两组裂隙密度的比值及裂隙间的夹角等因素有关,研究结果为进一步区分裂隙介质的类型及裂隙填充物的性质提供依据.     

单斜介质中方位NMO速度Thomsen参数反演方法研究

采用启发式共轭梯度法，即随机爬山法 + 共轭梯度法，利用单斜介质中P波方位NMO速度椭圆轴向偏转角度接近于零这一特性，简化P波方位NMO速度公式，并利用多方位P波NMO速度，反演出某一初始CMP观测线与自然坐标系之间的夹角，作为进一步进行Thomsen各向异性参数反演的基础. 根据各向异性介质中方位NMO速度与Thomsen参数之间的关系，建立了利用三种波的多方位NMO速度及垂直传播速度反演单层单斜各向异性介质Thomsen各向异性参数的目标函数. 对计算的理论值添加具有一定标准差的正态分布的随机噪声，用以模拟实际观测存在的误差，通过对加噪后的数据进行多次反演的误差分析，表明了所建立的目标函数及选用的反演方法是有效可行的，而且相对稳定.     

三维单斜裂隙介质地震正演模拟

多组任意夹角直立裂隙的排列方式比单组平行排列直立裂隙更具一般性,因此本文基于线性滑动理论和Hudson理论,将上述多组任意夹角直立裂隙介质等效为单斜各向异性介质,给定物性参数计算刚度矩阵,运用交错网格高阶有限差分法求解三维单斜介质弹性波一阶速度-应力方程来模拟弹性波在单斜介质中的传播过程.通过分析不同物性下的波场和共炮点道集,不同偏移距下的方位角道集,说明裂隙物性与波场特征之间的关系,以及其方位特征信息.     

HTI煤层方位AVO响应与裂隙识别

煤层在顶板压实作用下水平裂隙近乎闭合,多垂向裂隙发育,因此将该类煤层等效为HTI介质,研究裂隙密度和填充物类型对方位AVO的影响,可以为预测裂隙提供理论依据,对煤田勘探有重要意义.本文基于Thomsen各向异性理论和模型等效理论建立上层为各向同性介质而下层为垂向裂隙发育的HTI煤层的正演模型,推导出适用于所建模型的更高精度反射系数近似公式对不同裂隙密度、不同填充物类型的模型进行方位AVO数值模拟,并利用反射系数进行地震记录合成;从反射系数空间曲面和地震记录两方面分析不同模型随着入射角、方位角、裂隙密度、裂隙填充物的改变而产生的不同地震响应.结果表明:裂隙密度的增大导致反射系数空间曲面波幅增大,地震记录波振幅增大;裂隙密度相同时,饱和水裂隙模型和干裂隙模型的反射系数空间曲面和合成地震记录存在显著差异;方位角为0°时,饱和水裂隙模型和干裂隙模型的反射系数差值达到0.2,分辨性较强;在方位角为0.时,入射角大于45.时地震记录差异最显著.测试数据中,含饱和水裂隙煤层的反射系数绝对值低于干裂隙煤层,地震记录的振幅也低于干裂隙煤层.     

裂缝型HTI介质的纵波弹性阻抗

裂缝发育导致的地震波场各向异性,是裂缝型油气藏的一个重要特征.然而针对该类油气藏的叠前弹性阻抗技术,一直没有通过各向异性弹性阻抗公式建立与裂缝参数的直接关系.本文在回顾封闭平行硬币状裂缝模型和线性滑动模型的基础上,将裂缝填充物性质、分布密度与围岩的横纵波速度比,引入裂缝型HTI介质弹性阻抗公式,并对相应的归一化弹性阻抗响应特征进行模拟分析.分析表明,弹性阻抗受以上三参数的影响规律存在明显差异,其中含气裂隙介质随裂缝密度的变化要明显高于对应的流体裂隙介质,流体填充裂隙介质随横、纵波速度比的变化幅度要高于对应的含气裂隙介.该结论初步为裂缝型油气藏流体识别工作提供了依据.     

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,...     

基于随机各向同性背景的粘弹性单斜介质二维三分量正演模拟

将Von Karman型的随机各向同性背景引入粘弹性单斜各向异性波动方程,并应用交错网格技术进行模拟.结果表明在这种非均匀复合介质中波场特性既体现了规律性较强的粘弹性单斜各向异性性质,又体现了介质的随机性特点.具体表现为:胀缩、剪切品质因子造成了振幅的显著衰减;相同方差条件下,尺度越小随机介质干扰强度越大.为进一步研究裂隙型油气藏基本特征提供了有益的参考.     

一种三维正交方位各向异性介质岩石 物理建模及弹性波正演模拟方法

通过线性滑动理论和岩石物理等效理论, 将两组正交直立裂隙介质等效为一种正交方位各向异性介质进行三维岩石物理建模, 通过高阶交错网格有限差分求解弹性波动方程模拟地震波在该种介质中的传播过程. 在建模过程中改变物性参数, 分析不同裂隙密度条件下的炮集和波场特征, 以及正交各向异性的方位特征. 研究结果表明, 各向异性强度随裂隙密度等物性增大而增强, 而且这些特征在共炮点道集和波场中均有所体现.      

Finite-difference modelling of S-wave splitting in anisotropic media

We have implemented a 3D finite‐difference scheme to simulate wave propagation in arbitrary anisotropic media. The anisotropic media up to orthorhombic symmetry were modelled using a standard staggered grid scheme and beyond (monoclinic and triclinic) using a rotated staggered grid scheme. The rationale of not using rotated staggered grid for all types of anisotropic media is that the rotated staggered grid schemes are more expensive than standard staggered grid schemes. For a 1D azimuthally anistropic medium, we show a comparison between the seismic data generated by our finite‐difference code and by the reflectivity algorithm; they are in excellent agreement. We conducted a study on zero‐offset shear‐wave splitting using the finite‐difference modelling algorithm using the rotated staggered grid scheme. Our S‐wave splitting study is mainly focused on fractured media. On the scale of seismic wavelenghts, small aligned fractures behave as an equivalent anisotropic medium. We computed the equivalent elastic properties of the fractures and the background in which the fractures were embedded, using low‐frequency equivalent media theories. Wave propagation was simulated for both rotationally invariant and corrugated fractures embedded in an isotropic background for one, or more than one, set of fluid‐filled and dry fractures. S‐wave splitting was studied for dipping fractures, two vertical non‐orthogonal fractures and corrugated fractures. Our modelling results confirm that S‐wave splitting can reveal the fracture infill in the case of dipping fractures. S‐wave splitting has the potential to reveal the angle between the two vertical fractures. We also notice that in the case of vertical corrugated fractures, S‐wave splitting is sensitive to the fracture infill.     

垂向裂隙各向异性煤层地震波响应

煤层中存在的裂隙会导致介质表现为各向异性,本文以HTI型煤层为例,结合各向异性介质弹性矩阵和各向异性裂隙理论,推导出不同充填物的垂直裂隙中各向异性参数表达式,将其应用于地震波响应分析;通过改进的交错网格差分法和各向异性Christoffel方程波场分解法,得到地震波合成记录和分解后的P波和SV波记录;将Thomsen群速度与相速度公式,经过坐标轴旋转变换,得到HTI型煤层中不同各向异性参数的地震波速度响应表达式;建立不同类型煤层地质模型,分析了裂隙密度、裂隙充填物以及煤层厚度等参数变化时的地震波响应特征.研究结果为分析垂向裂隙各向异性薄煤层地震波传播规律提供工具,为选用相应地震数据进行地震波各向异性参数反演提供依据.     

正交各向异性介质中qP波入射的二阶近似反射系数与透射系数

地震波在各向异性介质中以一个准P波(qP)和两个准S波(qS1和qS2)的形式传播.研究三种波的相速度、群速度以及偏振方向等传播性质能够为各向异性介质中的正反演问题提供有效支撑.具有比横向各向同性(TI)介质更一般对称性的正交各向异性介质通常需要9个独立参数对其进行描述,这使得对传播特征的计算更为复杂.当两个准S波速度相近时具有耦合性,从而令慢度的计算产生奇异性.因此,奇异点(慢度面的鞍点和交叉点)附近的反射与透射(R/T)系数的求解不稳定,会导致波场振幅不准确.本文首次通过结合耦合S波射线理论和基于迭代的各向异性相速度与偏振矢量的高阶近似解,得到了适用于正交各向异性介质以qP波入射所产生的二阶R/T系数的计算方法.与基于一阶近似的结果相比,基于二阶近似的方法提高了qP波R/T系数的精度,能得到一阶耦合近似无法表达的准确的qP-qS转换波的R/T系数解,且方法适用于较强的各向异性介质.     

Seismic wavefield modeling based on time-domain symplectic and Fourier finite-difference method

Seismic wavefield modeling is important for improving seismic data processing and interpretation. Calculations of wavefield propagation are sometimes not stable when forward modeling of seismic wave uses large time steps for long times. Based on the Hamiltonian expression of the acoustic wave equation, we propose a structure-preserving method for seismic wavefield modeling by applying the symplectic finite-difference method on time grids and the Fourier finite-difference method on space grids to solve the acoustic wave equation. The proposed method is called the symplectic Fourier finite-difference (symplectic FFD) method, and offers high computational accuracy and improves the computational stability. Using acoustic approximation, we extend the method to anisotropic media. We discuss the calculations in the symplectic FFD method for seismic wavefield modeling of isotropic and anisotropic media, and use the BP salt model and BP TTI model to test the proposed method. The numerical examples suggest that the proposed method can be used in seismic modeling of strongly variable velocities, offering high computational accuracy and low numerical dispersion. The symplectic FFD method overcomes the residual qSV wave of seismic modeling in anisotropic media and maintains the stability of the wavefield propagation for large time steps.     

Aligned vertical fractures, HTI reservoir symmetry and Thomsen seismic anisotropy parameters for polar media

Certain crack-influence parameters of Sayers and Kachanov are shown to be directly related to Thomsen's weak-anisotropy seismic parameters for fractured reservoirs when the crack/fracture density is small enough. These results are then applied to the problem of seismic wave propagation in polar reservoirs, i.e., those anisotropic reservoirs having two axes that are equivalent but distinct from the third axis), especially for horizontal transversely isotropic seismic wave symmetry due to the presence of aligned vertical fractures and resulting in azimuthal seismic wave symmetry at the Earth's surface. The approach presented suggests one method of inverting for fracture density from wave speed data. A significant fraction of the technical effort in the paper is devoted to showing how to predict the angular location of the true peak (or trough) of the quasi-SV-wave for polar media and especially how this peak is related to another angle that is very easy to compute. The axis of symmetry is always treated here as the x ₃-axis for either vertical transversely isotropic symmetry (due, for example, to horizontal cracks), or horizontal transversely isotropic symmetry (due to aligned vertical cracks). Then, the meaning of the stiffnesses is derived from the fracture analysis in the same way for vertical transversely isotropic and horizontal transversely isotropic media, but for horizontal transverse isotropy the wave speeds relative to the Earth's surface are shifted by  90^o  in the plane perpendicular to the aligned vertical fractures. Skempton's poroelastic coefficient B is used as a general means of quantifying the effects of fluids inside the fractures. Explicit Biot-Gassmann-consistent formulas for Thomsen's parameters are also obtained for either drained or undrained fractures resulting in either vertical transversely isotropic or horizontal transversely isotropic symmetry of the reservoir.