Seismic dispersion and attenuation in layered porous rocks with fractures of varying orientations

Wave‐induced fluid flow plays an important role in affecting the seismic dispersion and attenuation of fractured porous rocks. While numerous theoretical models have been proposed for the seismic dispersion and attenuation in fractured porous rocks, most of them neglect the wave‐induced fluid flow resulting from the background anisotropy (e.g. the interlayer fluid flow between different layers) that can be normal in real reservoirs. Here, according to the theories of poroelasticity, we present an approach to study the frequency‐dependent seismic properties of more realistic and complicated rocks, i.e. horizontally and periodically layered porous rock with horizontal and randomly orienting fractures, respectively, distributed in one of the two periodical layers. The approach accounts for the dual effects of the wave‐induced fluid flow between the fractures and the background pores and between different layers (the interlayer fluid flow). Because C₃₃ (i.e., the modulus of the normally incident P‐wave) is directly related to the P‐wave velocity widely measured in the seismic exploration, and its comprehensive dispersion and attenuation are found to be most significant, we study mainly the effects of fracture properties and the stiffness contrast between the different layers on the seismic dispersion and attenuation of C₃₃. The results show that the increasing stiffness contrast enhances the interlayer fluid flow of the layered porous rocks with both horizontal and randomly orienting fractures and weakens the wave‐induced fluid flow between the fractures and the background pores, especially for the layered porous rock with horizontal fractures. The modelling results also demonstrate that for the considered rock construction, the increasing fracture density reduces the interlayer fluid flow while improves the dispersion and attenuation in the fracture‐relevant frequency band. Increasing fracture aspect ratio is found to reduce the dispersion and attenuation in the fracture‐relevant frequency band only, especially for the layered porous rock with horizontal fractures.     

周期成层Patchy模型中纵波的频散和衰减研究

本文用孔隙介质力学的方法来研究纵波在介观周期成层Patchy模型的传播问题.相对于White方法,该方法少了不同介质分界面处的流体压力不等的假设,推导过程更为严密;和Dutta的双相方程解耦方法比较,文中方法直接求解双相方程,形式上更为直观.当纵波通过周期成层孔隙模型时,在低频段用孔隙弹性力学得到纵波频散和衰减结果与用White公式得到的结果符合得很好;周期成层的Patchy孔隙模型由于其空间排列的周期性使其成为孔隙弹性声子晶体,在其频散和衰减曲线上的高频段会出现多个禁带和通带,这使得速度不会随频率单调递增,衰减峰也不只一个.对于地震频段,随着渗透率降低模型的衰减峰向低频移动,这和实际观测结果一致,和传统Biot理论预测结果正相反;随着含气量上升纵波衰减峰值先上升后下降,其最大值出现在含气量0.1左右,这和相关实验结果一致.研究结果表明该模型可以定性解释观测到的地震衰减结果.     

Seismic wave attenuation in fluid-saturated porous media

Contrary to the traditional view, seismic attenuation in Biot's theory of fluid-saturated porous media is due to viscous damping of local (not global) pore-fluid motion. Since substantial inhomogeneities in fluid permeability of porous geological materials are to be expected, the regions of highest local permeability contribute most to the wave energy dissipation while those of lowest permeability dominate the fluid flow rate if they are uniformly distributed. This dichotomy can explain some of the observed discrepancies between computed and measured attenuation of compressional and shear waves in porous earth. One unfortunate consequence of this result is the fact that measured seismic wave attenuation in fluid-filled geological materials cannot be used directly as a diagnostic of the global fluid-flow permeability.     

含流体复杂孔隙介质地震波衰减与频散

含流体孔隙介质中地震波的速度频散和衰减在指导复杂储层含油气性识别领域具有重要意义.本文构建了包含微观挤喷流与介观层间流影响的跨尺度模型, 并使用求解介质等效模量的方式得到了模型中地震波的频散速度与衰减因子, 克服了前人在高频段计算结果出现异常值以及不同尺度衰减峰无法分离的缺陷.在该双尺度模型的基础上, 本文又综合考虑宏观尺度Biot流, 将三种地震波衰减理论耦合, 建立了相对统一的三尺度地震波衰减岩石物理模型.在Biot理论框架下, 分析了非均匀性流体、孔裂隙结构、微观挤喷流以及介观层间流对流体压力与弹性模量的影响, 得到了该三尺度模型中地震波的波动方程, 并求解得到了相应的地震波衰减与频散曲线, 分析了不同介质参数对衰减与频散曲线的影响.我们推导了在该模型上覆均匀各向同性介质情况下, 分界面处各类极性波的反射、透射系数特征方程, 并得到了随频率以及入射角变化的反射、透射系数三维曲面.

     

基于Hessian矩阵的地震随机噪声压制方法

随机噪声的压制在提高地震资料信噪比方面发挥重要作用.考虑到传统去噪方法在构造复杂地区难以取得理想的去噪结果,本文提出基于Hessian矩阵特征值对应的线性目标关系在多个尺度上对随机噪声进行压制.该方法将地震信号看作不同尺度的曲线,从而利用Hessian矩阵在曲线检测方面表现出的良好性能实现信噪分离.该方法与传统方法相比不受地层倾角的限制,因此能够处理构造较为复杂地区的地震数据.利用模型及实际资料对该方法进行了验证并与传统方法F-X反褶积的去噪结果做对比,结果表明基于Hessian矩阵的随机噪声压制方法在构造复杂地区能够保持有效信号的完整性.

     

White球状Patchy模型中纵波传播研究

在球坐标系下用直接求解孔隙弹性方程的方法计算了介观尺度下空间周期排列的White球状Patchy模型中纵波传播问题.首先对纵波的衰减和频散进行了计算,并引入了物理学上声子晶体原理来解释高频时纵波在White球状模型中传播的异常现象.在含水饱和度和速度关系的研究中发现,在低频段用等效流体理论和Gassmann理论估计流体Patchy饱和岩石中的纵波速度完全能够满足当前地震勘探的要求.随后的具有相同含气饱和度但有不同周期的Patchy模型研究结果表明,随着空间周期变大,低频的纵波频散变得明显,纵波衰减峰频率向低频移动,但峰值几乎不变.最后,对单元外层含水中心含油的White球状Patchy模型和中心含气White球状Patchy模型进行研究、对比,发现孔隙流体流动对孔隙介质中的纵波频散、衰减影响显著.另外,在具体数值求解过程中用缩减方程组规模的方法解决了线性方程组严重病态得不到正确结果的问题.     

低信噪比地震资料FDOC-seislet变换阈值消噪方法研究

地震数据本质上是时变的,不仅有效同相轴表现出确定性信号的时变特征,而且复杂地表和构造条件以及深部探测环境总是引入时变的非平稳随机噪声.标准的频率-空间域预测滤波只适合压制平面波信号假设下的平稳随机噪声,而处理非平稳地震随机噪声时,需要将数据体分割为小窗口进行分析,但效果不够理想,而传统非预测类随机噪声压制方法往往适应性不高,因此开发能够保护地震信号时变特征的随机噪声压制方法具有重要的工业价值.压缩感知是近年出现的一个新的采样理论,通过开发信号的稀疏特性,已经在地震数据处理中的数据插值以及噪声压制中得到了应用.本文系统地分析了压缩感知理论框架下的地震随机噪声压制问题,建立了阈值消噪的数学反演目标函数;针对时变有效信息具有的可压缩性,利用有限差分算法求解炮检距连续方程,构建有限差分炮检距连续预测算子（FDOC）,在seislet变换框架下,提出一种新的快速稀疏变换域——FDOC-seislet变换,实现地震数据的高度稀疏表征;结合非平稳随机噪声不可压缩的特征,提出了一种整形迭代消噪方法,该方法是一种广义的迭代收缩阈值（IST）算法,在无法计算稀疏变换伴随算子的条件下,仍然能够对强噪声环境中的时变有效信息进行有效恢复.通过对模型数据和实际数据的处理,验证了FDOC-seislet稀疏变换域随机噪声迭代压制方法能够在保护复杂构造地震波信息的前提下,有效地衰减原始数据中的强振幅随机噪声干扰.

     

基于经验模态分解的分数维地震随机噪声衰减方法

经验模态分解算法（EMD）是一种基于有效波和噪声尺度差异进行波场分离的随机噪声压制方法,但由于实际地震数据波场复杂,导致模态混叠较严重,仅凭该方法进行去噪很难达到理想效果.本文基于EMD算法对信号多尺度的分解特性,结合Hausdorff维数约束条件,提出一种用于地震随机噪声衰减的新方法.首先对地震数据进行EMD自适应分解,得到一系列具有不同尺度的、分形自相似性的固有模态分量（IMF）;在此基础上,基于有效信号和随机噪声的Hausdorff维数差异,识别混有随机噪声的IMF分量,对该分量进行相关的阈值滤波处理,从而实现有效信号和随机噪声的有效分离.文中从仿真信号试验出发,到模型地震数据和实际地震数据的测试处理,同时与传统的EMD处理结果相对比.结果表明,本文方法对地震随机噪声的衰减有更佳的压制效果.

     

Seismic isolation foundations with effective attenuation zones

In this paper, a new configuration of seismic isolation foundation containing several concrete layers and some rubber blocks is proposed. The concrete layers and the rubber blocks are placed periodically to form a periodic foundation. To study the isolation ability of this new configuration of periodic foundation, an equivalent analytical model is established. For practical applications, two very useful formulas are obtained. Using these formulas, the low bound frequency and the width of the first attenuation zone can be directly approximated without the calculation of dispersion structure. This new configuration of seismic isolation foundation enjoys the first attenuation zone between 2.15 Hz and 15.01 Hz, which means that the components of seismic waves with frequencies from 2.15 Hz to 15.01 Hz cannot propagate upward in the foundation. To illustrate the efficiency of this seismic isolation foundation, the seismic responses of a 6-story frame with three different foundations are simulated. Numerical simulations show that the seismic responses of the structure with the periodic foundation are greatly attenuated as compared with those of the structure with no isolation base or with traditional rubber bearings.     

Seismic attenuation and velocity dispersion in fractured rocks: The role played by fracture contact areas

The presence of fractures in fluid‐saturated porous rocks is usually associated with strong seismic P‐wave attenuation and velocity dispersion. This energy dissipation can be caused by oscillatory wave‐induced fluid pressure diffusion between the fractures and the host rock, an intrinsic attenuation mechanism generally referred to as wave‐induced fluid flow. Geological observations suggest that fracture surfaces are highly irregular at the millimetre and sub‐millimetre scale, which finds its expression in geometrical and mechanical complexities of the contact area between the fracture faces. It is well known that contact areas strongly affect the overall mechanical fracture properties. However, existing models for seismic attenuation and velocity dispersion in fractured rocks neglect this complexity. In this work, we explore the effects of fracture contact areas on seismic P‐wave attenuation and velocity dispersion using oscillatory relaxation simulations based on quasi‐static poroelastic equations. We verify that the geometrical and mechanical details of fracture contact areas have a strong impact on seismic signatures. In addition, our numerical approach allows us to quantify the vertical solid displacement jump across fractures, the key quantity in the linear slip theory. We find that the displacement jump is strongly affected by the geometrical details of the fracture contact area and, due to the oscillatory fluid pressure diffusion process, is complex‐valued and frequency‐dependent. By using laboratory measurements of stress‐induced changes in the fracture contact area, we relate seismic attenuation and dispersion to the effective stress. The corresponding results do indeed indicate that seismic attenuation and phase velocity may constitute useful attributes to constrain the effective stress. Alternatively, knowledge of the effective stress may help to identify the regions in which wave induced fluid flow is expected to be the dominant attenuation mechanism.     

Effective bulk modulus of the pore fluid at patchy saturation

We explore a package of parallel porous layers, each filled with a different fluid. Assume that this package is sampled by an elastic wave with the wavelength much larger than the thickness of an individual layer. Also assume that the layers are hydraulically isolated from each other, meaning that the diffusion length is smaller than that of the individual layer. This assumption is relevant to a patchy saturation scenario. Suppose that we wish to conduct the fluid substitution operation on this package treated as a single porous elastic body. What is the effective bulk modulus of the pore fluid to be used in this operation that will result in the same elastic modulus as computed by Backus averaging the individual moduli of the layers? We address this question analytically by assuming that the porosity, dry frame, and the mineral matrix properties of the individual layers are the same for all layers. The only difference between the layers is the pore fluid. We find that the resulting effective bulk modulus of the fluid thus derived falls between the arithmetic and harmonic averages of the fluid bulk moduli in the layers. It can be approximated by a linear combination of these two bounds where the weights are 0.50 and 0.50 or 0.75 for the arithmetic average and 0.25 for the harmonic average, depending on the elastic moduli of the dry frame, the mineral, and the pore fluids. This solution also provides a relation between the effective bulk modulus of the pore fluid in the system under examination and water saturation to be used in the fluid substitution operation at a coarse spatial scale.     

Seismic attenuation in Faroe Islands basalts

We analysed vertical seismic profiling (VSP) data from two boreholes at Glyvursnes and Vestmanna on the island of Streymoy, Faroe Islands, to determine the magnitude and causes of seismic attenuation in sequences of basalt flows. The work is part of SeiFaBa, a major project integrating data from vertical and offset VSP, surface seismic surveys, core samples and wireline log data from the two boreholes. Values of effective seismic quality factor (Q) obtained at Glyvursnes and Vestmanna are sufficiently low to significantly degrade the quality of a surface reflection seismic image. This observation is consistent with results from other VSP experiments in the North Atlantic region. We demonstrate that the most likely cause of the low values of effective Q at Glyvursnes and Vestmanna is a combination of 1D scattering and intrinsic attenuation due to seismic wave‐induced fluid flow within pores and micro‐cracks. Tests involving 3D elastic wave numerical modelling with a hypothetical basalt model based on field observations, indicate that little scattering attenuation is caused by lateral variations in basalt structure.     

基于f-x域流式预测滤波器的地震随机噪声衰减方法

随机噪声的影响在地震勘探中是不可避免的,常规的随机噪声压制方法在处理中往往会破坏具有时空变化特征的非平稳有效地震信号,影响地震数据的准确成像.当前油气勘探的目标已经转变为“两宽一高”,随着数据量的增大,对去噪方法的处理效率也提出了更高的要求.因此,开发高效的非平稳地震数据随机噪声压制方法具有重要意义.预测滤波技术广泛用于地震随机噪声的衰减,本文基于流式处理框架提出一种新的f-x域流式预测滤波方法,通过在频率域建立预测自回归方程,运用直接复数矩阵逆运算代替迭代算法求解非平稳滤波器系数,实现时空变地震同相轴预测,提高自适应预测滤波的计算效率.通过与工业标准的FXDECON方法和f-x域正则化非平稳自回归(RNA)方法进行对比,理论模型和实际数据的测试结果表明,提出的f-x域流式预测滤波方法能更好地平衡时空变有效信号保护、随机噪声压制和高效计算三者之间的关系,获得合理的处理效果.     

陕西地区地震动衰减关系研究

通过分析陕西省丰富的地震活动资料,包括鲜明的地震活动与震害分布的地域性特点以及地震地质环境特点,将陕西省分为三个分区,来进行地震动衰减关系的研究.研究内容主要包括地震烈度衰减关系和基岩地震动衰减关系.选择美国西部作为参考区,三个分区分别作为研究区,采用可逆法进行映射转换计算,得到了三个分区的基岩地震动衰减关系,在转换过...     

地震波衰减规律及其恢复方法

针对地面地震分辨率低,不能有效识别薄层储层、薄层地层等问题,本文通过地震波衰减规律的分析,提出了一种恢复地震波高频衰减获得宽频带地震剖面的方法,从而大幅度地提高了地震分辨率.应用双井微地震测井资料对松辽盆地地震波动力学特征研究表明,地震波衰减规律是在近地表低速层和近地表低速层的近震源区地震波高频衰减巨大,而在高速层地震波高频衰减很小.药量大近震源区地震波高频衰减大,药量小近震源区地震波高频衰减小.那么,近震源区和近地表低速层对地震波的衰减是地面地震资料频带窄、分辨率低的主要原因.据此提出了如下确定性反褶积方法,用双井微地震测井资料求取近震源区、近地表低速层和虚反射等滤波因子,用其对地面地震资料作确定性反褶积处理,从而恢复近震源区、近地表低速层等几种因素的地震波衰减,将大药量激发地表接收的地面地震延拓成小药量激发高速层接收的宽频带地震.应用该方法对松辽盆地优势频带宽5~90 Hz,视主频50 Hz的地面地震资料处理后,地震剖面优势频带宽达5~360 Hz,视主频达180 Hz,使常规地震剖面分辨率提高2倍.具体的说松辽盆地中部含油组合的地面地震分辨能力由9~15 m提高到3~5 m.宽频带地震剖面与160 Hz的人工合成地震记录对比符合的很好,表明其处理结果是正确.该成果在油气勘探开发中的油气储层预测、构造学研究、沉积学研究等方面有重要的意义.     

Elastic properties of saturated porous rocks with aligned fractures

Elastic properties of fluid saturated porous media with aligned fractures can be studied using the model of fractures as linear-slip interfaces in an isotropic porous background. Such a medium represents a particular case of a transversely isotropic (TI) porous medium, and as such can be analyzed with equations of anisotropic poroelasticity. This analysis allows the derivation of explicit analytical expressions for the low-frequency elastic constants and anisotropy parameters of the fractured porous medium saturated with a given fluid. The five elastic constants of the resultant TI medium are derived as a function of the properties of the dry (isotropic) background porous matrix, fracture properties (normal and shear excess compliances), and fluid bulk modulus. For the particular case of penny-shaped cracks, the expression for anisotropy parameter ε has the form similar to that of Thomsen [Geophys. Prospect. 43 (1995) 805]. However, contrary to the existing view, the compliance matrix of a fluid-saturated porous-fractured medium is not equivalent to the compliance matrix of any equivalent solid medium with a single set of parallel fractures. This unexpected result is caused by the wave-induced flow of fluids between pores and fractures.     

基于非稳态多项式拟合的地震噪声衰减方法研究(英文)

基于非稳态多项式拟合理论,针对地震数据中同相轴振幅变化这一特征,我们提出了一种地震噪声衰减的新方法。非稳态多项式拟合系数是时变的,通过整形正则化约束多项式拟和系数的光滑性,自适应的估计地震数据的相干分量。基于动校正后的共中心点道集(CMP)中地震信号的相干性,利用非稳态多项式拟合估计有效信号,从而衰减随机噪声。对于线性相干噪声,如地滚波,首先利用径向道变换(RadialTraceTransform,RTT)将地震数据变换到时间一视速度域,在时间—视速度域利用非稳态多项式拟合估计出相干噪声,然后减去相干噪声。该方法可以有效的估计振幅变化的相干分量,不需要相干分量振幅为常量的假设。模拟和实际资料处理结果表明,与传统的稳态多项式拟合和低切滤波相比,该方法可以更为有效的衰减地震噪声,同时保真了地震有效信号。     

Seismic ground-roll noise attenuation using deep learning

We propose to adopt a deep learning based framework using generative adversarial networks for ground-roll attenuation in land seismic data. Accounting for the non-stationary properties of seismic data and the associated ground-roll noise, we create training labels using local time–frequency transform and regularized non-stationary regression. The basic idea is to train the network using a few shot gathers such that the network can learn the weights associated with noise attenuation for the training shot gathers. We then apply the learned weights to test ground-roll attenuation on shot gathers, that are not a part of training input to obtain the desired signal. This approach gives results similar to local time–frequency transform and regularized non-stationary regression but at a significantly reduced computational cost. The proposed approach automates the ground-roll attenuation process without requiring any manual input in picking the parameters for each shot gather other than in the training data. Tests on field-data examples verify the effectiveness of the proposed approach.