PROCESSING OF PS-REFLECTION DATA APPLYING A COMMON CONVERSION-POINT STACKING TECHNIQUE1

Converted waves require special data processing as the wave paths are asymmetrical. The CMP concept is not applicable for converted PS waves, instead a sorting algorithm for a common conversion point (CCP) has to be applied. The coordinates of the conversion points in a single homogeneous layer can be calculated as a function of the offset, the reflector depth and the velocity ratio v_P/ v_s. For multilayered media, an approximation for the coordinates of the conversion points can be made. Numerical tests show that the traveltime of PS reflections can be approximated with sufficient accuracy for a certain offset range by a two-term series truncation. Therefore NMO corrections can be calculated by standard routines which use the hyperbolic approximation of the reflection traveltime curves. The CCP-stacking technique has been applied to field data which have been generated by three vertical vibrators. The in-line horizontal components have been recorded. The static corrections have been estimated from additional P- and SH-wave measurements for the source and geophone locations, respectively. The data quality has been improved by processes such as spectral balancing. A comparison with the stacked results of the corresponding P- and SH-wavefield surveys shows a good coherency of structural features in P-, SH- and PS-time sections.     

Elastic interferometry for ocean bottom cable data: Theory and examples‡

Elastic imaging from ocean bottom cable (OBC) data can be challenging because it requires the prior estimation of both compressional‐wave (P‐wave) and shear‐wave (S‐wave) velocity fields. Seismic interferometry is an attractive technique for processing OBC data because it performs model‐independent redatuming; retrieving ‘pseudo‐sources’ at positions of the receivers. The purpose of this study is to investigate multicomponent applications of interferometry for processing OBC data. This translates into using interferometry to retrieve pseudo‐source data on the sea‐bed not only for multiple suppression but for obtaining P‐, converted P to S‐wave (PS‐wave) and possibly pure mode S‐waves. We discuss scattering‐based, elastic interferometry with synthetic and field OBC datasets. Conventional and scattering‐based interferometry integrands computed from a synthetic are compared to show that the latter yields little anti‐causal response. A four‐component (4C) pseudo‐source response retrieves pure‐mode S‐reflections as well at P‐ and PS‐reflections. Pseudo‐source responses observed in OBC data are related to P‐wave conversions at the seabed rather than to true horizontal or vertical point forces. From a Gulf of Mexico OBC data set, diagonal components from a nine‐component pseudo‐source response demonstrate that the P‐wave to S‐wave velocity ratio (V_P/V_S) at the sea‐bed is an important factor in the conversion of P to S for obtaining the pure‐mode S‐wave reflections.     

P-to-S conversion for a thin anisotropic zone produced by vertical fracturing

Point-source synthetic seismic responses for a thin, fractured bed are generated, interpreted and processed. The synthesis is carried out for a compressional source and multicomponent surface receivers. The anisotropy considered has hexagonal symmetry, with a horizontal symmetry axis, and represents oil- and gas-filled, aligned vertical fractures for a broad range of fracture densities and aspect ratios. P-to-S reflected conversions recorded on the horizontal geophones show both kinematic and dynamic anomalies that increase with increasing fracture density and are only weakly dependent on aspect ratio. In contrast, the vertical component P-wave reflections provide a much poorer diagnostic of fracturing. Analytic expressions for the eigenvalues and eigenvectors of a vertically fractured system are presented, that have the same simplicity as those for transverse isotropy. New linearized expressions for mode-converted amplitudes are developed for small angles of incidence and are used to interpret the synthetic response.     

天然地震转换波偏移成像方法

当地震波在层状介质中传播,P波通过速度界面时,会产生PP型折射波和PS型折射转换波．本文利用PS转换波与初至P波的到时差,提出一种不需要拾取P波和转换波震相的转换界面偏移成像方法．文中对三种不同的理论模型,在正演计算的基础上,采用了上述方法进行数值计算,计算结果表明,这种方法能够确定转换界面的位置和形态．这一方法的实现将有利于利用天然地震资料研究地球深部结构。     

Fracture characterization using converted waves

We present a method for inversion of fracture compliance matrix components from wide‐azimuth noisy synthetic PS reflection data and quantitatively show that reflection amplitude variations with offset and azimuth for converted PS‐waves are more informative than P‐waves for fracture characterization. We consider monoclinic symmetry for fractured reservoir (parameters chosen from Woodford Shale), which can be formed by two or more sets of vertical fractures embedded in a vertically transverse isotropic background. Components of effective fracture compliance matrices for a medium with monoclinic symmetry are related to the characteristics of the fractured medium. Monte Carlo simulation results show that inversion of PS reflection data is more robust than that of PP reflection data to uncertainties in our a priori knowledge (vertically transverse isotropic parameters of unfractured rock) than PP reflection data. We also show that, while inversion of PP reflections is sensitive to contrasts in elastic properties of upper and lower media, inversion of PS reflections is robust with respect to such contrasts.     

Application of 3D vertical seismic profile multi‐component data to tight gas sands‡

Due to the complicated geophysical character of tight gas sands in the Sulige gasfield of China, conventional surface seismic has faced great challenges in reservoir delineation. In order to improve this situation, a large‐scale 3D‐3C vertical seismic profiling (VSP) survey (more than 15 000 shots) was conducted simultaneously with 3D‐3C surface seismic data acquisition in this area in 2005. This paper presents a case study on the delineation of tight gas sands by use of multi‐component 3D VSP technology. Two imaging volumes (PP compressional wave; PSv converted wave) were generated with 3D‐3C VSP data processing. By comparison, the dominant frequencies of the 3D VSP images were 10–15 Hz higher than that of surface seismic images. Delineation of the tight gas sands is achieved by using the multi‐component information in the VSP data leading to reduce uncertainties in data interpretation. We performed a routine data interpretation on these images and developed a new attribute titled ‘Centroid Frequency Ratio of PSv and PP Waves’ for indication of the tight gas sands. The results demonstrated that the new attribute was sensitive to this type of reservoir. By combining geologic, drilling and log data, a comprehensive evaluation based on the 3D VSP data was conducted and a new well location for drilling was proposed. The major results in this paper tell us that successful application of 3D‐3C VSP technologies are only accomplished through a synthesis of many disciplines. We need detailed analysis to evaluate each step in planning, acquisition, processing and interpretation to achieve our objectives. High resolution, successful processing of multi‐component information, combination of PP and PSv volumes to extract useful attributes, receiver depth information and offset/ azimuth‐dependent anisotropy in the 3D VSP data are the major accomplishments derived from our attention to detail in the above steps.     

Observation of shear‐wave splitting in the multicomponent node data from Atlantis field,Gulf of Mexico

In 2005, a multicomponent ocean bottom node data set was collected by BP and BHP Billiton in the Atlantis field in the Gulf of Mexico. Our results are based on data from a few sparse nodes with millions of shots that were analysed as common receiver azimuthal gathers. A first‐order look at P‐wave arrivals on a common receiver gather at a constant offset reveals variation of P‐wave arrival time as a function of azimuth indicating the presence of azimuthal anisotropy at the top few layers. This prompted us to investigate shear arrivals on the horizontal component data. After preliminary processing, including a static correction, the data were optimally rotated to radial (R) and transverse (T) components. The R component shows azimuthal variation of traveltime indicating variation of velocity with azimuth; the corresponding T component shows azimuthal variation of amplitude and phase (polarity reversal). The observed shear‐wave (S‐wave) splitting, previously observed azimuthal P‐wave velocity variation and azimuthal P‐wave amplitude variation, all indicate the occurrence of anisotropy in the shallow (just below the seafloor) subsea sediment in the area. From the radial component azimuthal gather, we analysed the PP‐ and PS‐wave amplitude variation for the first few layers and determined corresponding anisotropy parameter and V_P/V_S values. Since fracture at this depth is not likely to occur, we attribute the observed azimuthal anisotropy to the presence of microcracks and grain boundary orientation due to stress. The evidence of anisotropy is ubiquitous in this data set and thus it argues strongly in favour of considering anisotropy in depth imaging for obtaining realistic subsurface images, at the least.     

2D PP/PS-stereotomography: Application to a real 2D-OBC dataset

It has been shown on an ‘ideal’ synthetic dataset that PP/PS‐stereotomography can estimate an accurate velocity model without any pairing of PP‐ and PS‐events. The P‐wave velocity model is first estimated using PP data and then, fixing this velocity field, the S‐wave velocity is estimated using the PS data. This method needed to be evaluated further and we present here the first application of PP/PS‐stereotomography to a real dataset: the 2D East‐West Mahogany OBC line (Gulf of Mexico). We are here confronted with data which do not fit our working assumptions: coherent noise (due to an approximate separation of PP‐ and PS‐events and some remaining multiples), probably some anisotropy and 3D effects. With a careful selection of the stereotomographic picks, which allows one to decrease the effect of the picked coherent noise by the automatic picker, our application can demonstrate the relevance of our approach in the upper part of the profile, where anisotropy and 3D effects might be low. We can thus estimate, without any pairing of PP‐ and PS‐events, a velocity field which provides not only flat common image gathers, but also PP‐ and PS‐depth migrated images located at the same positions. For the deeper part of the profile, a significant shift in depth appears. In addition to possible anisotropy, 3D effects and a more complex velocity field (‘salt body’), this is due to the quality of the PZ‐ and X‐components profiles: The PZ‐component profile where the PP‐stereotomographic picking is performed, is polluted by conflicting converted or multiple events and the X‐component profile, where the PS‐stereotomographic picking is performed, is highly noisy. This study emphasizes the need to develop accurate selection criteria for the stereotomographic picks.     

Characterization of dipping fractures in a transverselyisotropic background

Although it is believed that natural fracture sets predominantly have near‐vertical orientation, oblique stresses and some other mechanisms may tilt fractures away from the vertical. Here, we examine an effective medium produced by a single system of obliquely dipping rotationally invariant fractures embedded in a transversely isotropic with a vertical symmetry axis (VTI) background rock. This model is monoclinic with a vertical symmetry plane that coincides with the dip plane of the fractures. Multicomponent seismic data acquired over such a medium possess several distinct features that make it possible to estimate the fracture orientation. For example, the vertically propagating fast shear wave (and the fast converted PS‐wave) is typically polarized in the direction of the fracture strike. The normal‐moveout (NMO) ellipses of horizontal reflection events are co‐orientated with the dip and strike directions of the fractures, which provides an independent estimate of the fracture azimuth. However, the polarization vector of the slow shear wave at vertical incidence does not lie in the horizontal plane – an unusual phenomenon that can be used to evaluate fracture dip. Also, for oblique fractures the shear‐wave splitting coefficient at vertical incidence becomes dependent on fracture infill (saturation). A complete medium‐characterization procedure includes estimating the fracture compliances and orientation (dip and azimuth), as well as the Thomsen parameters of the VTI background. We demonstrate that both the fracture and background parameters can be obtained from multicomponent wide‐azimuth data using the vertical velocities and NMO ellipses of PP‐waves and two split SS‐waves (or the traveltimes of PS‐waves) reflected from horizontal interfaces. Numerical tests corroborate the accuracy and stability of the inversion algorithm based on the exact expressions for the vertical and NMO velocities.     

利用近震转换波探测地壳深部构造的研究

本文对近震PS折射转换波的运动学和动力学特征进行了初步的研究,并且提出了识别近震PS转换波的主要参考标志。利用这些标志分析了邢台地区某些测点的近震转换波资料,所测出的转换界面深度与爆炸地震法得出的主要结果基本一致。因此,近震PS转换波法是探测区域地壳深部构造的一种有效方法。      

3D description and inversion of reflection moveout of PS‐waves in anisotropic media

Common‐midpoint moveout of converted waves is generally asymmetric with respect to zero offset and cannot be described by the traveltime series t²(x²) conventionally used for pure modes. Here, we present concise parametric expressions for both common‐midpoint (CMP) and common‐conversion‐point (CCP) gathers of PS‐waves for arbitrary anisotropic, horizontally layered media above a plane dipping reflector. This analytic representation can be used to model 3D (multi‐azimuth) CMP gathers without time‐consuming two‐point ray tracing and to compute attributes of PS moveout such as the slope of the traveltime surface at zero offset and the coordinates of the moveout minimum. In addition to providing an efficient tool for forward modelling, our formalism helps to carry out joint inversion of P and PS data for transverse isotropy with a vertical symmetry axis (VTI media). If the medium above the reflector is laterally homogeneous, P‐wave reflection moveout cannot constrain the depth scale of the model needed for depth migration. Extending our previous results for a single VTI layer, we show that the interval vertical velocities of the P‐ and S‐waves (V_P0 and V_S0) and the Thomsen parameters ε and δ can be found from surface data alone by combining P‐wave moveout with the traveltimes of the converted PS(PSV)‐wave. If the data are acquired only on the dip line (i.e. in 2D), stable parameter estimation requires including the moveout of P‐ and PS‐waves from both a horizontal and a dipping interface. At the first stage of the velocity‐analysis procedure, we build an initial anisotropic model by applying a layer‐stripping algorithm to CMP moveout of P‐ and PS‐waves. To overcome the distorting influence of conversion‐point dispersal on CMP gathers, the interval VTI parameters are refined by collecting the PS data into CCP gathers and repeating the inversion. For 3D surveys with a sufficiently wide range of source–receiver azimuths, it is possible to estimate all four relevant parameters (V_P0, V_S0, ε and δ) using reflections from a single mildly dipping interface. In this case, the P‐wave NMO ellipse determined by 3D (azimuthal) velocity analysis is combined with azimuthally dependent traveltimes of the PS‐wave. On the whole, the joint inversion of P and PS data yields a VTI model suitable for depth migration of P‐waves, as well as processing (e.g. transformation to zero offset) of converted waves.     

Triplications for the converted wave in transversely isotropic media with a tilted symmetry axis

In seismic data processing, serious problems could be caused by the existence of triplication and need to be treated properly for tomography and other inversion methods. The triplication in transversely isotropic medium with a vertical symmetry axis has been well studied and concluded that the triplicated traveltime only occurs for S wave and there is no triplication for P and converted PS waves since the P wave convexity slowness always compensates the S wave slowness concavity. Compared with the vertical symmetry axis model, the research of the triplication in transversely isotropic medium with a tilted symmetry axis is still keeping blank. In order to analyse the triplication for the converted wave in the tilted symmetry axis model, we examine the traveltime of the triplication from the curvature of averaged P and S wave slowness. Three models are defined and tested in the numerical examples to illustrate the behaviour of the tilted symmetry axis model for the triplicated traveltime with the change of the rotation angle. Since the orientation of an interface is related to the orientation of the symmetry axis, the triplicated traveltime is encountered for the converted wave in the tilted symmetry axis model assuming interfaces to be planar and horizontal. The triplicated region is influenced by the place and level of the concave curvature of the P and S wave slowness.     

中海油多分量地震处理技术的发展与应用实例

20世纪末中海油在南海油气勘探中成功试验了二维多分量地震技术.其后十多年,中海油进一步在不同近海油气盆地采集了三维多分量OBC地震资料.但由于针对三维转换波的处理未能形成有效的关键技术,除双检叠加利用了水压分量和陆检垂直分量外,大量水平分量数据未被使用.鉴于此,中海油在十二五期间开展三维多分量地震数据处理的关键技术攻关：针对弹性波场的矢量特征、转换波射线路径不对称以及双程旅行时多时间尺度等问题,成功研发了矢量化的信号处理技术、弹性波速度建模以及叠前时间偏移成像等技术;集成了相对完善的三维弹性波成像EWI软件系统;完成了多个试验区的三维四分量OBC地震转换波的处理,取得了好于以往技术的效果;建立了海上多分量OBC地震资料处理流程.但相对于成熟的纵波处理技术,海洋多分量地震处理技术仍需要不断完善与发展.     

Common-Reflection-Surface Stack for Converted Waves

The finite-offset (FO) common-reflection-surface (CRS) stack has been shown to be able to handle not only P-P or S-S but also arbitrarily converted reflections. It can provide different stack sections such as common-offset (CO), common-midpoint (CMP) and common-shot (CS) sections with significantly increased signal-to-noise ratio from the multi-coverage pre-stack seismic data in a data-driven way. It is our purpose in this paper to demonstrate the performance of the FO CRS stack on data involving converted waves in inhomogeneous layered media. In order to do this we apply the FO CRS stack for common-offset to a synthetic seismic data set involving P-P as well as P-S converted primary reflections. We show that the FO CRS stack yields convincing improvement of the image quality in the presence of noisy data and successfully extracts kinematic wavefield attributes useful for further analyses. The extracted emergence angle information is used to achieve a complete separation of the wavefield into its P-P and P-S wave components, given the FO CRS stacked horizontal and vertical component sections.     

基于Russell近似的纵横波联合反演方法研究

PP波和PS波联合反演方法作为有效的地震技术,比单纯纵波反演精度要高,能够提高地震储层识别的精度.以Russell近似理论为基础,推导了新的转换波AVO近似公式,双层模型界面的反射特征数值模拟显示,新公式具有较高的近似精度,且具备直接反演流体因子f、剪切模量μ和密度ρ等参数的优势,有效避免间接反演带来的误差.结合纵横波联合反演理论,提出了基于贝叶斯理论的新型联合反演算法.在实际应用中,对纵波和转换波角道集进行同相轴匹配处理,综合利用纵波和转换波资料携带的信息,实现基于Russell近似的多波联合反演.模型数据和实际资料测试结果表明,反演结果与真实值或测井结果匹配度较高,证实该方法真实有效.     

三分量数字检波器在ZY油田应用效果分析

在水平分量的炮集记录中,数字三分量检波器直达波的椭圆率明显呈现出线性偏振的形态,并具有较高的分辨率,而模拟三分量检波器直达波的线性偏振不清晰,抗干扰能力差.在频谱分析和能量曲线分析的对比中,数字检波器的性能也高于模拟检波器.在PP波弱波阻抗界面的沙河组沙三12段,PS波表现出强反射界面,而且构造幅度比PP波剖面明显.在PS波剖面上,剖面深处两边的构造联系清晰可见,而在PP波剖面上很难解释剖面两边构造之间的关系,表明PS波资料对深层界面的成像能力有时不比PP波能力差,多分量资料在构造勘探上具有互补性.在沙四层底钻遇气层的马62井和马11井的测线CDP位置上,PP波和PS波的振幅比剖面上表现出低振幅比异常区,与钻井结果吻合.用多分量衰减系数或衰减比剖面,能够识别砂岩含气/水产生所产生的高频衰减异常.多分量地震属性及其比值剖面是储层含气异常指示的有效参数.     

Joint PP and PS AVO inversion based on Zoeppritz equations

Considering Zoeppritz equations, reflections of PP and PS are only the function of ratios of density and velocity. So the inversion results will be the same if the ratios are the same but values of density, velocities of P-wave and S-wave are different without strict constraint. This paper makes efforts to explore nonlinear simultaneous PP and PS inversion with expectation to reduce the ambiguity of AVO analysis by utilizing the redundancy of multi-component AVO measurements. Accurate estimation of ratio parameters depends on independence of input data. There are only two independent AVO attributes for PP reflectivity (i.e. intercept and gradient) and two for PS reflectivity (i.e. pseudo-intercept and pseudo-gradient or extreme amplitude), respectively. For individual PP and PS inversion, the values of least-squares objective function do not converge around a large neighborhood of chosen true model parameters. Fortunately for joint PP and PS inversion the values of the least-squares objective function show closed contours with single minima. Finally the power function fitting is used to provide a higher precision AVO attributes than traditional polynomial fitting. By using the four independent fitting attributes (two independent attributes for PP and PS respectively), the inversion of four ratio parameters (velocities and densities) would be estimated with less errors than that in traditional method.     

大陆架科学钻探CSDP-2井的垂直地震剖面测量

南黄海海相地层的地震波场特征和层位标定一直是困扰地震勘探的重要问题.为了近距离、高精度和高分辨率地观测井周围构造特征和岩石性质引起的波场变化,为地震资料的采集、处理与解释提供地震波衰减规律、速度与层位标定等信息,对大陆架科学钻探CSDP-2井实施了近零偏移距垂直地震剖面（VSP）观测.针对海相地层顶部强反射界面地震波穿透难的问题,采用了大容量气枪震源并设计了气枪阵列组合方式,提高了激发地震波的能量,获得了强反射界面之下清晰的PP、PS下行波和上行波信号.采用了三分量偏振合成、组合滤波和波场分离等处理方法,对VSP观测数据进行处理,获得了海相三叠系—志留系的精细的纵波、横波速度结构和地层吸收因子等物性数据,建立了钻井地层、测井、VSP上行波和多道地震剖面对应关系,实现了不同尺度的地质和地球物理属性资料的有效衔接,标定了钻井地质剖面上各深度地质体的地震反射特性,厘定了过井地震剖面上反射同相轴的地质属性.此次观测取得的纵波、横波速度信息,成为建立南黄海海相地层速度模型主要的资料来源,也是地震资料的岩性反演处理不可缺少的信息.     

YC地区转换波数据处理及裂隙预测

裂隙介质的横波响应能提供对裂隙走向与密度一种直接测量.为了探测YC地区裂隙型储层特征,在该区进行了纵波源三分量数据采集.经过精细地处理,YC地区转换波成像质量得到了改善.以此为基础,对该区的裂隙特征进行了预测.预测结果与本区的应力场分析和钻井揭示的裂隙特征比较吻合.本文所介绍的一些关键转换波资料处理技术、解释方法和分析结论对今后陆地转换波勘探有一定借鉴作用.