INTEGRATED INTERPRETATION, 3D MAP MIGRATION AND VSP MODELLING PROJECT,NORTHERN U.K. SOUTHERN GAS BASIN1

Depth conversion in the northern part of the U.K. Southern Gas Basin is complicated by the presence of Zechstein (Permian) salt swells and diapirs. In addition, the post-Zechstein (post-Permian) section displays large lateral velocity variations. The primary agents which control the velocity of this stratigraphic section are: (1) depth of burial, (2) lithological variation within individual formations, and (3) the effects of subsequent tectonic inversion. An integrated approach which combines well velocity, seismic velocity and seismic interpretation is required for accurate depth estimation. In 1988 Mobil and partners drilled an exploratory well in the northern part of the U.K. Southern Gas Basin. This well was located near the crest of a Zechstein salt diapir. Over 2000 m of Zechstein was encountered in the well. The Permian Rotliegendes objective was penetrated at a depth of over 3700 m. The initial delineation of the objective structure was based on the results of 3D map migration of the seismic time interpretation. Spatially-variant interval velocity functions were used to depth convert through five of the six mapped horizons. Both well and model-based seismic interval velocity analysis information was used to construct these functions. A moving-source well seismic survey was conducted. The survey was run in two critical directions. In conjunction with presurvey modelling, it was possible to confirm immediately the structural configuration as mapped to a distance of 7 km from the well. Post-survey 3D map migration and modelling was employed to further refine the structural interpretation. Although the question of stratigraphic anisotropy was considered in the evaluation of the long offset modelling, no evidence was found in the field data to support a significant effect. Finally, comparisons were made of: curved-ray versus straight-ray migration/modelling, midpoint-depth velocity versus (depth-dependant instantaneous velocity functions, and Hubral- versus Fermat-based map depth migration algorithms. Significant differences in the results were observed for structural dips exceeding 15^o and/or offsets exceeding 6 km. Map depth migration algorithms which employed both curved rays and spatially-variant instantaneous velocity functions were found to best approximate the ‘true’ geological velocity field in the study area.     

Velocity imaging by tau–p transformation of refracted seismic traveltimes

We present a new method for producing a ‘brute’ velocity image rapidly and automatically from traveltimes picked from densely sampled refraction data. The procedure involves imaging by data transformation from the time–offset domain into the tau–p (intercept–slope) domain, and does not include conventional modelling steps. Differences in apparent velocity and tau along reciprocal paths in the up- and downdip directions allow the estimation of the true velocity and geometrical position of the ray turning points. The tau–velocity–turningpoint (τ–ν–x) map distributes phases automatically on the basis of geometry and velocity to give a two-dimensional representation of subsurface structure. This map may be converted simply to depth and two-way-time images. Such images have potential for direct geological interpretation, for use as a starting model for seismic inversion, for superimposition on to conventional reflection images, or for input into prestack depth migration and other processing routines.     

地震偏移成像不确定性分析初探

勘探地球物理领域存在大量宏观动力学过程的物理数学表达及尚待深入探索的不确定性事实,其中一类是源自对偶量的不可交换性或不可对易性.本文从地震偏移成像中的不确定性分析入手,探讨偏移成像中受到波场延拓过程不确定性的影响.文中给出了若干深度偏移算法对Marmousi模型的偏移不确定度分布表达及图像显示;文中还对若干波场延拓方法,从不确定性估计的角度进行了比较.笔者认为对应用技术领域中的宏观不确定性分析,将有益于深化对宏观动力学过程的理性认识,也为偏移算法评价提供一种途径;重要的是,进行不确定性分析将有利于把握所用偏移算法的不确定度分布,继而有针对性地采取相应的改善措施,以期提高算法的整体确定性程度.     

Utilization of multiple scattering: the next big step forward in seismic imaging

Surface removal and internal multiple removal are explained by recursively separating the primary and multiple responses at each depth level with the aid of wavefield prediction error filtering. This causal removal process is referred to as “data linearization.” The linearized output (primaries only) is suitable for linear migration algorithms. Next, a summary is given on the migration of full wavefields (primaries + multiples) by using the concept of secondary sources in each subsurface gridpoint. These secondary sources are two‐way and contain the gridpoint reflection and the gridpoint transmission properties. In full wavefield migration, a local inversion process replaces the traditional linear imaging conditions. Finally, Marchenko redatuming is explained by iteratively separating the full wavefield response from above a new datum and the full wavefield response from below a new datum. The redatuming output is available for linear migration (Marchenko imaging) or, even better, for full wavefield migration. Linear migration, full wavefield migration, and Marchenko imaging are compared with each other. The principal conclusion of this essay is that multiples should not be removed, but they should be utilized, yielding two major advantages: (i) illumination is enhanced, particularly in the situation of low signal‐to‐noise primaries; and (ii) both the upper side and the lower side of reflectors are imaged. It is also concluded that multiple scattering algorithms are more transparent if they are formulated in a recursive depth manner. In addition to transparency, a recursive depth algorithm has the flexibility to enrich the imaging process by inserting prior geological knowledge or by removing numerical artefacts at each depth level. Finally, it is concluded that nonlinear migration algorithms must have a closed‐loop architecture to allow successful imaging of incomplete seismic data volumes (reality of field data).     

Multiwave velocity analysis based on Gaussian beam prestack depth migration

Prestack depth migration of multicomponent seismic data improves the imaging accuracy of subsurface complex geological structures. An accurate velocity field is critical to accurate imaging. Gaussian beam migration was used to perform multicomponent migration velocity analysis of PP- and PS-waves. First, PP- and PS-wave Gaussian beam prestack depth migration algorithms that operate on common-offset gathers are presented to extract offset-domain common-image gathers of PP- and PS-waves. Second, based on the residual moveout equation, the migration velocity fields of P- and S-waves are updated. Depth matching is used to ensure that the depth of the target layers in the PP- and PS-wave migration profiles are consistent, and high-precision P- and S-wave velocities are obtained. Finally, synthetic and field seismic data suggest that the method can be used effectively in multiwave migration velocity analysis.     

地震叠前深度偏移方法流程及应用

针对复杂介质的成像问题，提出了一套地震资料叠前深度偏移方法流程，主要包括三部分：（１）地震资料精细预处理；（２）速度－深度模型建立；（３）叠前深度偏移成像．以Ｋｉｒｃｈｈｏｆｆ偏移理论为基础，强调地质与地球物理的综合以及地震处理与解释的一体化．在ＺＸ地区成功地实现了二维地震资料叠前深度偏移，所获得的ＮＥ２０６叠前深度偏移剖面揭示了复杂的ＺＸ古潜山及其内部构造，并清楚地展示了逆掩断层的存在．     

地震相干偏移与数据自参照偏移的关系

地震相干偏移和数据自参照偏移是最近几年发展起来的地震成像新方法,并且已在地震勘探研究领域开始应用.该方法在VSP（垂直地震剖面）中的应用尤其值得关注,因为它部分地解决了一些在地面反射地震方法中难以解决的问题.地震相干偏移是用地震数据的互相关处理后的数据进行偏移,而数据自参照偏移则不必在偏移前对数据进行互相关处理,而是通过记录数据中的两个不同部分波场的延拓来互相参照进行偏移.二者的优点都是在偏移运算中可以不考虑原始震源位置、激发时间以及地震波初始传播路径.本文从偏移成像的理论出发分析了这两类算法,发现二者在成像原理上是一致的.本文还通过一个二维VSP多次波偏移的数值模拟实验,比较和归纳了这两类算法在实际应用中的特点.     

TI介质局部角度域射线追踪与叠前深度偏移成像

研究与实践表明,对于长偏移距、宽方位地震数据,忽略各向异性会明显降低成像质量,影响储层预测与描述的精度.针对典型的横向各向同性(TI)介质,本文面向深度域构造成像与偏移速度分析的需要,研究基于射线理论的局部角度域叠前深度偏移成像方法.它除了像传统Kirchhoff叠前深度偏移那样输出成像剖面和炮检距域的共成像点道集,还遵循地震波在成像点处的局部方向特征、基于扩展的脉冲响应叠加原理获得入射角度域和照明角度域的成像结果.为了方便快捷地实现TI介质射线走时与局部角度信息的计算,文中讨论和对比了两种改进的射线追踪方法:一种采用从经典各向异性介质射线方程演变而来的由相速度表征的简便形式;另一种采用由对称轴垂直的TI(即VTI)介质声学近似qP波波动方程推导出来的射线方程.文中通过坐标旋转将其扩展到了对称轴倾斜的TI(即TTI)介质.国际上通用的理论模型合成数据偏移试验表明,本文方法既适用于复杂构造成像,又可为TI介质深度域偏移速度分析与模型建立提供高效的偏移引擎.     

Three-dimensional kinematic depth migration of converted waves: application to the 2002 Molise aftershock sequence (southern Italy)

Migration techniques, currently used in seismic exploration, are still scarcely applied in earthquake seismology due to the poor source knowledge and sparse, irregular acquisition geometries. At the crustal scale, classical seismological studies often perform inversions based on the arrival time of primary phases (P- and S-waves) but seldom exploit other information included in seismic records. Here we show how migration techniques can be adapted to earthquake seismology for converted wave analysis. As an example, we used data recorded by a dense local seismic network during the 2002 Molise aftershock sequence. In October and November 2002, two moderate magnitude earthquakes struck the Molise region (southern Italy), followed by an aftershock sequence lasting for about one month. Local earthquake tomography has provided earthquake hypocenter locations and three-dimensional models of P and S velocity fields. Strong secondary signals have been detected between first-arrivals of P- and S-waves and identified as SP transmitted waves. In order to analyse these waves, we apply a prestack depth migration scheme based on the Kirchhoff summation technique. Since source parameters are unknown, seismograms are equalized and only kinematic aspects of the migration process are considered. Converted wave traveltimes are calculated in the three-dimensional (3D) tomographic models using a finite-difference eikonal solver and back ray tracing. In the migrated images, the area of dominant energy conversion corresponds to a strong seismic horizon that we interpreted as the top of the Apulia Carbonate Platform and whose geometry and position at depth is consistent with current structural models from existing commercial seismic profiles, gravimetric and well data.     

Anisotropic reverse-time migration for tilted TI media

Seismic anisotropy in dipping shales results in imaging and positioning problems for underlying structures. We develop an anisotropic reverse‐time depth migration approach for P‐wave and SV‐wave seismic data in transversely isotropic (TI) media with a tilted axis of symmetry normal to bedding. Based on an accurate phase velocity formula and dispersion relationships for weak anisotropy, we derive the wave equation for P‐wave and SV‐wave propagation in tilted transversely isotropic (TTI) media. The accuracy of the P‐wave equation and the SV‐wave equation is analyzed and compared with other acoustic wave equations for TTI media. Using this analysis and the pseudo‐spectral method, we apply reverse‐time migration to numerical and physical‐model data. According to the comparison between the isotropic and anisotropic migration results, the anisotropic reverse‐time depth migration offers significant improvements in positioning and reflector continuity over those obtained using isotropic algorithms.     

三维叠前深度偏移速度分析及蒙特卡洛自动层速度拾取

是否能够正确地建立深度域三维速度模型是三维叠前深度偏移成败的关键 .本文根据Deregowski循环 ,利用叠前深度域地震成像对速度模型变化的敏感性 ,采用偏移迭代逐次逼近最佳成像速度 ,研究开发了一套快捷有效的三维叠前深度偏移深度域速度模型建立技术 .借鉴时间域CDP(共深度点 )道集上常规叠加速度分析的策略 ,在深度域CRP(共反射点 )道集上 ,提出剩余慢度平方谱的概念并建立相应的实现技术 .导出深度域中均方根速度与层速度之间的关系 ;按照串级偏移原理确定偏移循环过程中初始速度、剩余速度及修改后速度之间的关系 ;采用蒙特卡洛非线性优化算法实现从剩余慢度平方谱中自动拾取层速度 ,讨论了其地质速度约束条件和蒙特卡洛非线性优化的收敛准则 ,使得所拾取的层速度模型具有合理的地质意义并获得最佳偏移成像效果 .SEG EAGE理论模型数值试算验证了方法的有效性 ,在海拉尔盆地霍多莫尔工区 ,5 8km2 三维资料的速度模型建立并获得满意的三维叠前深度偏移成像 .     

DEPTH IMAGING OF OFFSET VERTICAL SEISMIC PROFILE DATA1

Depth migration consists of two different steps: wavefield extrapolation and imaging. The wave propagation is firmly founded on a mathematical frame-work, and is simulated by solving different types of wave equations, dependent on the physical model under investigation. In contrast, the imaging part of migration is usually based on ad hoc‘principles’, rather than on a physical model with an associated mathematical expression. The imaging is usually performed using the U/D concept of Claerbout (1971), which states that reflectors exist at points in the subsurface where the first arrival of the downgoing wave is time-coincident with the upgoing wave. Inversion can, as with migration, be divided into the two steps of wavefield extrapolation and imaging. In contrast to the imaging principle in migration, imaging in inversion follows from the mathematical formulation of the problem. The image with respect to the bulk modulus (or velocity) perturbations is proportional to the correlation between the time derivatives of a forward-propagated field and a backward-propagated residual field (Lailly 1984; Tarantola 1984). We assume a physical model in which the wave propagation is governed by the 2D acoustic wave equation. The wave equation is solved numerically using an efficient finite-difference scheme, making simulations in realistically sized models feasible. The two imaging concepts of migration and inversion are tested and compared in depth imaging from a synthetic offset vertical seismic profile section. In order to test the velocity sensitivity of the algorithms, two erroneous input velocity models are tested. We find that the algorithm founded on inverse theory is less sensitive to velocity errors than depth migration using the more ad hoc U/D imaging principle.     

Estimation of geological dip and curvature from time‐migrated zero‐offset reflections in heterogeneous anisotropic media

Starting from a given time‐migrated zero‐offset data volume and time‐migration velocity, recent literature has shown that it is possible to simultaneously trace image rays in depth and reconstruct the depth‐velocity model along them. This, in turn, allows image‐ray migration, namely to map time‐migrated reflections into depth by tracing the image ray until half of the reflection time is consumed. As known since the 1980s, image‐ray migration can be made more complete if, besides reflection time, also estimates of its first and second derivatives with respect to the time‐migration datum coordinates are available. Such information provides, in addition to the location and dip of the reflectors in depth, also an estimation of their curvature. The expressions explicitly relate geological dip and curvature to first and second derivatives of reflection time with respect to time‐migration datum coordinates. Such quantitative relationships can provide useful constraints for improved construction of reflectors at depth in the presence of uncertainty. Furthermore, the results of image‐ray migration can be used to verify and improve time‐migration algorithms and can therefore be considered complementary to those of normal‐ray migration. So far, image‐ray migration algorithms have been restricted to layered models with isotropic smooth velocities within the layers. Using the methodology of surface‐to‐surface paraxial matrices, we obtain a natural extension to smooth or layered anisotropic media.     

Prestack shot-gather depth migration by a rigid flow of Gaussian wave packets

We introduce a new method for prestack depth migration of seismic common-shot gathers. The computational procedure follows standard steps of the reverse-time migration, i.e., downward continuation of the source and the receiver wavefields, followed by application of an imaging condition (e.g. zero-lag cross-correlation of these fields). In our method we first find a sparse data representation with a small number of Gaussian wave packets. We then approximate the downward wavefield propagation (for the source and the receiver fields) by a rigid flow of these wave packets along seismic rays. In this case, the wave packets are simply translated and rotated according to the ray geometry. One advantage of using Gaussian wave packets is that analytic formulas can be used for translation, rotation, and the application of the cross-correlation imaging condition. Moreover, they allow more sparse representations than competing methods. Finally we formulate a computationally and memory efficient migration procedure, as only few rays have to be traced, and since it is cheap to compute the cross-correlation for the intersecting rays.     

东辛油田地震勘探历程——一个复杂断块构造的三维地震解释实例

东辛油田是一个典型的断块复杂构造。在这个油田上,地震精查的方法在搞清断裂系统方面,取得了较好的成效。本文简述了在该油田早期所采用的一套三维地震解释（立体归位）方法,其中有些方法对当前搞三维地震勘探,及断层面的地质解释方面都还有着参考价值。近年来,又用数字地震方法,在此浅层的复杂构造的下面,发现了比较简单的深层构造,在几张附图中展示了深浅层构造之间的关系。     

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.     

断陷盆地陡坡带高精度地震勘探技术应用

本文以河南泌阳凹陷为例,从散射波波动方程正演着手,分析大断面地震波散射场的物理机制和特点,结合叠前偏移处理需要进行观测系统论证。采集上主要运用模型约束正演技术优选采集参数,并采取了主要解决山前激发能量问题的技术措施;资料处理上主要应用了基于精细速度建模的叠前深度偏移成像处理方法;解释上通过可视化构造成图分析陡坡砂砾岩体的分布规律和发育期次,并运用沟扇对应理论,结合地震相分析、层拉平水平切片分析、属性分析等方法,预测砂砾岩体分布区带和层位。部署探井并取得了成功,相继在凹陷南部陡坡带发现了两个富集含油区块。     

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.     

A few case histories of subsurface imaging with EMAP as an aid to seismic processing and interpretation1

The electromagnetic array profiling (EMAP) exploration method can be combined with a direct one-dimensional inversion process for conversion to depth to produce a subsurface resistivity cross-section. This cross-section may then be interpreted in parallel with a seismic cross-section to enhance the prediction of rock type and structure. In complex thrust environments and areas of shallow carbonate rocks, the EMAP method is often used to provide additional data either to help the seismic processor and/or to aid the seismic interpretation. In particular, the electromagnetic (EM) data can be used to build an independent seismic velocity file for depth migration. Three EMAP test areas in the western United States are used to demonstrate such a use of EMAP as an expioration tool. The first shows how a velocity file is estimated from resistivity data for seismic depth migration processing in a complex thrust environment. In the second example, the method is applied in layer-cake geology with high seismic velocity rocks at the earth's surface. The third example is another complex thrust environment, but in this case the velocity file derived from the resistivity data is used for stacking the seismic data.