3D wavepath migration

3D Kirchhoff migration (KM) smears a trace's time sample along a quasi-ellipsoid in the model space. This is a costly and sometimes noisy process as reflection energy is smeared far away from the actual reflector position, introducing far-field migration artefacts. As a reduced form of 3D KM, 3D wavepath migration (WM) smears a picked reflection arrival to a small Fresnel zone portion centred about the specular reflection point, leading to fewer migration artefacts and reduced computation time. Both the traveltime and the angle of incidence are required by WM for locating the specular reflection point. Our results with 3D prestack synthetic data show that WM generates fewer migration artefacts and can sometimes define complex structure better than KM. Our results with 3D prestack field data show that WM can mostly suppress migration artefacts and can sometimes resolve reflection interfaces better than KM. The CPU comparison shows that, for both the synthetic and field data examples, WM can be more than an order of magnitude faster than KM. The limitation with 3D WM is that the angle of incidence calculation is sensitive to the recording geometry and the signal-to-noise (S/N) ratio, which can lead to blurred images.     

叠前逆时深度偏移中的激发时间成像条件

与其他偏移方法相比,逆时偏移基于精确的波动方程而不是对其近似,用时间外推来代替深度外推.因此,它具有良好的精度,不受地下构造倾角和介质横向速度变化的限制.激发时间成像条件的求取是叠前逆时偏移的难点之一,本文采用求解程函方程的方法得到地下各点的初至波走时,以此作为叠前逆时偏移的成像条件.基于任意矩形网格和局部平面波前近似的有限差分初至波走时计算方法精度较高并适用于强纵横向变速的复杂介质.试算结果表明,在复杂介质模型中利用叠前逆时深度偏移收到了很好的成像效果.     

Double-square-root one-way wave equation prestack tau migration in heterogeneous media

In this paper, source‐receiver migration based on the double‐square‐root one‐way wave equation is modified to operate in the two‐way vertical traveltime (τ) domain. This tau migration method includes reasonable treatment for media with lateral inhomogeneity. It is implemented by recursive wavefield extrapolation with a frequency‐wavenumber domain phase shift in a constant background medium, followed by a phase correction in the frequency‐space domain, which accommodates moderate lateral velocity variations. More advanced τ‐domain double‐square‐root wave propagators have been conceptually discussed in this paper for migration in media with stronger lateral velocity variations. To address the problems that the full 3D double‐square‐root equation prestack tau migration could meet in practical applications, we present a method for downward continuing common‐azimuth data, which is based on a stationary‐phase approximation of the full 3D migration operator in the theoretical frame of prestack tau migration of cross‐line constant offset data. Migrations of synthetic data sets show that our tau migration approach has good performance in strong contrast media. The real data example demonstrates that common‐azimuth prestack tau migration has improved the delineation of the geological structures and stratigraphic configurations in a complex fault area. Prestack tau migration has some inherent robust characteristics usually associated with prestack time migration. It follows a velocity‐independent anti‐aliasing criterion that generally leads to reduction of the computation cost for typical vertical velocity variations. Moreover, this τ‐domain source‐receiver migration method has features that could be of help to speed up the convergence of the velocity estimation.     

苏北大陆科学钻探靶区深反射地震的叠前深度偏移

由于深反射地震数据具有信噪比低和记录长度长等特点,叠前深度偏移方法的应用有许多困难．为此,我们研究了一种适合于深反射地震的叠前深度偏移方法;包括：用逆风有限差分方法计算程函方程;在常规速度扫描的基础上,用协方差控制提高速度分析精度;用联合反演算法计算层速度,再插值后得到初始速度模型;用Ｋｉｒｃｈｈｏｆｆ法作为偏移速度分析工具,求得最终的速度模型;最终的速度模型作为有限差分深度偏移的输入,求得最终的偏移结果．用该方法对“中国大陆科学深钻工程”东海二维深反射地震数据ＤＨ－４线进行了叠前深度偏移,取得了良好的效果。     

弹性矢量波成像域联合层析反演

在地震弹性矢量波场框架下,推导了多波联合层析速度反演方程以及走时残差与角道集剩余曲率的转换关系式,提出了一种利用成像域角道集更新P波、S波速度的走时层析反演方法.其实现过程可以概括为:将弹性波多分量数据作为输入,基于高斯束实现矢量波场成像并提取角道集,利用层析反演方程求解慢度更新量,最终获得多波联合反演结果.模型试算和实际资料处理验证了该方法的反演效果,能够为弹性矢量波联合深度偏移提供高质量的叠前速度场.     

非零偏VSP弹性波叠前逆时深度偏移技术探讨

非零偏VSP地震资料是一种多分量资料,处理非零偏VSP资料,弹性波叠前逆时深度偏移技术无疑是最适合的处理技术.本文从二维各向同性介质的弹性波波动方程出发,研究了对非零偏VSP资料进行叠前逆时深度偏移的偏移算法,讨论了逆时传播过程中的边值问题和数值频散问题及其相应的解决方案;采用求解程函方程计算得到地下各点的地震波初至时间作为成像时间,实现了非零偏VSP资料的叠前逆时深度偏移.最后进行了模型试算和非零偏VSP地震资料的试处理,结果表明该方法不受地层倾角限制,较适用于高陡构造地区或介质横向速度变化较大地区的非零偏VSP地震资料处理.     

Prestack Kirchhoff time migration of 3D coal seismic data from mining zones

Conventional seismic data processing methods based on post‐stack time migration have been playing an important role in coal exploration for decades. However, post‐stack time migration processing often results in low‐quality images in complex geological environments. In order to obtain high‐quality images, we present a strategy that applies the Kirchhoff prestack time migration (PSTM) method to coal seismic data. In this paper, we describe the implementation of Kirchhoff PSTM to a 3D coal seam. Meanwhile we derive the workflow of 3D Kirchhoff PSTM processing based on coal seismic data. The processing sequence of 3D Kirchhoff PSTM includes two major steps: 1) the estimation of the 3D root‐mean‐square (RMS) velocity field; 2) Kirchhoff prestack time migration processing. During the construction of a 3D velocity model, dip moveout velocity is served as an initial migration velocity field. We combine 3D Kirchhoff PSTM with the continuous adjustment of a 3D RMS velocity field by the criteria of flattened common reflection point gathers. In comparison with post‐stack time migration, the application of 3D Kirchhoff PSTM to coal seismic data produces better images of the coal seam reflections.     

Second-order interpolation of later-arrival traveltimes

The performance of a 3D prestack migration of the Kirchhoff type can be significantly enhanced if the computation of the required stacking surface is replaced by an efficient and accurate method for the interpolation of diffraction traveltimes. Thus, input traveltimes need only be computed and stored on coarse grids, leading to considerable savings in CPU time and computer storage. However, interpolation methods based on a local approximation of the traveltime functions fail in the presence of triplications of the wavefront or later arrivals. This paper suggests a strategy to overcome this problem by employing the coefficients of a hyperbolic traveltime expansion to locate triplications and correct for the resulting errors in the interpolated traveltime tables of first and later arrivals.     

Secondorder interpolation of traveltimes

To carry out a 3D prestack migration of the Kirchhoff type is still a task of enormous computational effort. Its efficiency can be significantly enhanced by employing a fast traveltime interpolation algorithm. High accuracy can be achieved if secondorder spatial derivatives of traveltimes are included in order to account for the curvature of the wavefront. We suggest a hyperbolic traveltime interpolation scheme that permits the determination of the hyperbolic coefficients directly from traveltimes sampled on a coarse grid, thus reducing the requirements in data storage. This approach is closely related to the paraxial ray approximation and corresponds to an extension of the wellknown     method to arbitrary heterogeneous and complex media in 3D. Application to various velocity models, including a 3D version of the Marmousi model, confirms the superiority of our method over the popular trilinear interpolation. This is especially true for regions with strong curvature of the local wavefront. In contrast to trilinear interpolation, our method also provides the possibility of interpolating source positions, and it is 56 times faster than the calculation of traveltime tables using a fast finitedifference eikonal solver.     

三维叠前深度偏移的准三维算法研究

介绍了目前叠前偏移方法的研究状况，引入准三维算法的概念并对其进行了讨论，同时对共方位角偏移算子作了详细的介绍，最后，给出了运用共方位角偏移算子在实际中的应用结果，认为共方位角方法具有运算速度快，适应范围大的优点，有较好的研究开发前景。     

数据域特征波反射走时反演

地震波的运动学信息(走时、斜率等)通常用于宏观速度建模.针对走时反演方法,一个基本问题是走时拾取或反射时差的估计.对于成像域反演方法,可以通过成像道集的剩余深度差近似计算反射波时差.在数据域中,反射地震观测数据是有限频带信号,如果不能准确地确定子波的起跳时间,难以精确地确定反射波的到达时间.另一方面,如果缺乏关于模型的先验信息,则很难精确测量自地下同一个反射界面的观测数据同相轴和模拟数据同相轴之间的时差.针对走时定义及时差测量问题,首先从叠前地震数据的稀疏表达出发,利用特征波场分解方法,提取反射子波并估计局部平面波的入射和出射射线参数.进一步,为了实现自动和稳定的走时拾取,用震相的包络极值对应的时间定义反射波的到达时,实现了立体数据中间的自动生成.理论上讲,利用包络极值定义的走时大于真实的反射波走时,除非观测信号具有无限带宽(即delta脉冲).然而,走时反演的目的是估计中-大尺度的背景速度结构,因此走时误差导致的速度误差仍然在可以接受的误差范围内.利用局部化传播算子及特征波聚焦成像条件将特征波数据直接投影到地下虚拟反射点,提出了一种新的反射时差估计方法.既避免了周期跳跃现象以及串层等可能性,又消除了振幅因素对时差测量的影响.最后,在上述工作基础之上,提出了一种基于特征波场分解的新型全自动反射走时反演方法(CWRTI).通过对泛函梯度的线性化近似,并用全变差正则化方法提取梯度的低波数部分,实现了背景速度迭代反演.在理论上,无需长偏移距观测数据或低频信息、对初始模型依赖性低且计算效率高,可以为后续的全波形反演提供可靠的初始速度模型.理论和实际资料的测试结果证明了本文方法的有效性.     

基于走时的保幅偏移方法

振幅随偏移距变化是描述储层特征的重要方法之一,保幅偏移方法就是使偏移剖面能够反映出振幅随偏移距的变化.本论文中的保幅偏移是以走时为基础,主要的方法是采用走时的双曲线展开法,通过走时的二阶空间导数来确定波前曲率.该方法通过建立在大网格上的走时表来确定插值系数,将大网格插值成为较为精细的网格,这样就节省了数据的存储空间.对于相同的网格密度,通过插值来计算走时表比采用程函方程有限差分法直接计算走时要节省5至6倍的时间.走时的插值系数还可以用来计算几何扩散因子、权函数,不仅提高了成像质量,还大大节省了计算时间.     

Stereotomography assisted by migration of attributes

Depth velocity model building remains a difficult step within the seismic depth imaging sequence. Stereotomography provides an efficient solution to this problem but was limited until now to a picking of seismic data in the prestack time un-migrated domain. We propose here a method for stereotomographic data picking in the depth migrated domain. Picking in the depth migrated domain exhibits the advantage of a better signal-to-noise ratio and of a more regular distribution of picked events in the model, leading to a better constrained tomographic inverse problem. Moreover, any improvement on the velocity model will improve the migrated results, again leading to improved picking. Our strategy for obtaining a stereotomographic dataset from a prestack depth migration is based on migration of attributes (and not on a kinematic demigration approach!). For any locally coherent event in the migrated image, migration of attributes allows one to compute ray parameter attributes corresponding to the specular reflection angle and dip. For application to stereotomography, the necessary attributes are the source/receiver locations, the traveltime and the data slopes. For the data slope, when the migration velocity model is erroneous, some additional corrections have to be applied to the result of migration of the attributes. Applying these corrections, our picking method is theoretically valid whatever the quality of the migration velocity model. We first present the theoretical aspects of the method and then validate it on 2D synthetic and real seismic reflection data sets.     

Event-oriented velocity estimation based on prestack data in time or depth domain1

A 2D reflection tomographic method is described, for the purpose of estimating an improved macrovelocity field for prestack depth migration. An event-oriented local approach of the ‘layer-stripping’ type has been developed, where each input event is defined by its traveltime and a traveltime derivative, taken with respect to one of four coordinates in the source/receiver and midpoint half-offset systems. Recent work has shown that the results of reflection tomography may be improved by performing event picking in a prestack depth domain. We adopt this approach and allow events to be picked either before or after prestack depth migration. Hence, if events have been picked in a depth domain, such as the common-shot depth domain or the common-offset depth domain, then a depth-time transformation is required before velocity estimation. The event transformation may, for example, be done by conventional kinematic ray tracingr and with respect to the original depth-migration velocity field. By this means, we expect the input events for velocity updating to become less sensitive to migration velocity errors. For the purpose of velocity estimation, events are subdivided into two categories; reference horizon events and individual events. The reference horizon events correspond to a fixed offset in order to provide basic information about reflector geometry, whereas individual events, corresponding to any offset, are supposed to provide the additional information needed for velocity estimation. An iterative updating approach is used, based on calculation of derivatives of event reflection points with respect to velocity. The event reflection points are obtained by ray-theoretical depth conversion, and reflection-point derivatives are calculated accurately and efficiently from information pertaining to single rays. A number of reference horizon events and a single individual event constitute the minimum information required to update the velocity locally, and the iterations proceed until the individual event reflection point is consistent with those of the reference horizon events. Normally, three to four iterations are sufficient to attain convergence. As a by-product of the process, we obtain so-called uncertainty amplification factors, which relate a picking error to the corresponding error in the estimated velocity or depth horizon position. The vector formulation of the updating relationship makes it applicable to smooth horizons having arbitrary dips and by applying velocity updating in combination with a flexible model-builder, very general macro-model structures can be obtained. As a first step in the evaluation of the new method, error-free traveltime events were generated by applying forward ray tracing within given macrovelocity models. When using such ‘perfect’ observations, the velocity estimation algorithm gave consistent reconstructions of macro-models containing interfaces with differential dip and curvature, a low-velocity layer and a layer with a laterally varying velocity function.     

人工源宽角地震实验的谱元法模拟和逆时偏移成像方法研究

人工源宽角反射(折射)地震资料具有偏移距较大、信噪比较低等特点,通常用于地震波走时反演重建地壳速度结构。逆时偏移成像方法作为勘探地震学领域获取地下构造形态的有效手段之一,可以有效弥补走时反演方法的不足。本文针对大偏移距宽角反射(折射)地震实验,利用四边形网格谱元法进行波场模拟,结合了有限元法的灵活性和谱方法的指数收敛性,高效且高精度获取模型合成地震记录,后采用逆时偏移成像方法将合成地震记录偏移归位,获取地壳几何结构,验证了逆时偏移成像方法在宽角地震资料处理及结果解释中的适用性,为后期实际地震资料的偏移成像提供了理论依据和支持。     

基于Dreamlet变换的地震数据压缩理论与方法

为达到更加有效地表示地震数据的目的,仅仅将地震数据当作普通的图像数据处理是远远不够的,地震数据中蕴含的地震波的运动学特性也应作为重要因素而被考虑到.本文讨论了利用Dreamlet变换方法实现地震数据压缩的方法,并针对地震数据本身所蕴含的频散关系特性进一步提出了多尺度Dreamlet变换压缩方法.Dreamlet变换由2个一维局部谐波变换的张量积构成,它在提供地震波场时间-空间局部化性质的同时可以保留波场的运动学特性.通过对二维SEG/EAGE叠前、叠后数据的算例说明了Dreamlet变换用于地震数据压缩的有效性.利用压缩后的数据进行成像的结果更表明,与Curvelet变换方法相比,Dreamlet与多尺度Dreamlet方法可以提供更高的压缩比;在相同压缩比的条件下,使用Dreamlet与多尺度Dreamlet方法压缩重建后的数据进行成像能更好地保留成像结果中的重要结构.     

盐丘模型 推覆体模型 波动方程 叠前深度偏移

三维波动方程叠前深度偏移是复杂介质中进行构造成像、弹性参数反演的重要环节.由于其技术实现不仅涉及波场延拓理论的创新，而且需要大规模计算，因而研究难度较大. 本文以实验效果的取得为目的，完整地实现了SEG/EAEG盐丘和推覆体模型的三维波动方程辛几何算法的叠前深度偏移成像计算. 文中详细考察了所研制的波动方程三维叠前深度偏移软件系统及其对复杂地质构造的成像能力. 具体包括：1）对于盐丘模型，文中讨论了成像参数的选择、地震子波对成像精度的影响、完成二维及三维叠前深度偏移的比较；2）对推覆体模型，文中进行了脉冲响应测试；3）由两个模型的成像结果可见本文的波动方程三维叠前深度偏移软件系统已具有适应强速度横向变化、复杂构造的成像能力.     

叠前时间偏移技术在淮北某煤矿采区三维地震勘探中的应用与效果

淮北某煤矿采区三维地震勘探地质条件极为复杂,区内褶皱、断层构造发育,常规叠后时间偏移处理很难准确成像,给后期资料解释工作带来不利影响.针对这一问题,采用叠前时间偏移技术,结合煤田地震勘探特点,对影响叠前偏移处理质量的关键步骤（预处理、静校正、叠前去噪、时间域速度模型建立）进行了研究,取得了显著效果.利用叠前偏移数据体解释,最终圆满完成了地质任务.本文通过应用实例,说明了煤田地震勘探资料处理利用叠前时间偏移技术对复杂构造成像效果较好,信噪比提高,断点解释更可靠.     

多步法快速射线追踪与圆台型高精度走时外插计算

为更好地适应复杂构造的地震偏移成像,本文提出了一套快速射线追踪算法和一种高精度的走时外插计算方法.采用线性多步法的预测-校正公式求解射线追踪方程组,与传统的四阶Runge-Kutta法相比,提高了计算效率.在网格节点上的走时计算中,应用一种基于圆台的外插方法,该方法以射线的方向为轴确定圆台,将轴上的走时外插到圆台内的网格节点上.与传统的矩形体外插方法相比,圆台走时外插方法提高了计算精度,且具有更好的稳定性.另外,该方法利用稀疏分布的射线即可获得高精度的走时表,节省计算量,对复杂构造的偏移成像非常有利,尤其是三维偏移.最后通过逆散射偏移成像算例,验证了算法的有效性和适用性.     

用共方位角偏移实现叠前二维资料三维化成像

波动方程偏移保持了波场动力学特征,依此本文应用共方位角叠前深度偏移技术来实现二维资料三维化处理.本方法能使共反射点偏移成像与叠前数据插值一步完成,较好地解决二维资料处理中的成像点不能准确归位、侧面反射波难处理、主测线与联络线不闭合等问题.