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1.
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.  相似文献   

2.
波动方程法共成像点道集偏移速度建模   总被引:15,自引:1,他引:15       下载免费PDF全文
叠前深度偏移的成像效果对偏移速度场相当敏感,建立正确的偏移速度场是实现高质量叠前深度偏移成像的关键。首先应用成像精度高的波动方程法叠前深度偏移抽取共成像点道集;然后基于摄动法通过参数化速度函数和改进的剩余曲率分析建立偏移速度误差和成像深度误差的定量关系;最后采用单参数/多参数联合迭代反演实现偏移速度建模。对Marmousi模型的试算结果表明:该方法对复杂地质体具有较强的适应性和较好的建模和成像效果,一般只需分析和控制主要反射层,通过3-4次近代就可以满足精度要求。  相似文献   

3.
For successful prestack depth migration an accurate velocity model is needed. One method for model updating is based on image gather analysis. In an image gather all reflectors line up horizontally if the correct velocities are used for the depth migration. This is also true for dipping reflectors, as all traces of an image gather belong to the same surface coordinate. The images of the reflector in an image gather curve upwards if the velocity used for the migration is too low, or downwards if the velocity is too high. This deviation can be used for model updating. Curves which depend on depth, offset and a parameter which relates the estimated to the true model are fitted to the image. By calculating the coherence along the deviation curves, this parameter can be estimated and hence an update can be calculated. Formulae are derived for the deviation curves and the update of the velocity depth model for a multilayered model for both shot and common-offset migrated data, with and without gradients. The method is tested on synthetic data with satisfactory results.  相似文献   

4.
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.  相似文献   

5.
叠前深度偏移已成为近年来地震偏移成像领域广泛应用的技术之一,其中复杂盐丘下方目标区域结构的成像始终是偏移成像中较难解决的问题.局部角度域波场分解可以提供在空间和方向上的双重局部化信息,因而这一技术被广泛地应用在方向照明分析、成像振幅校正等方面.本文在采用局部指数标架小波束进行角度域方向照明分析的基础上,研究采集系统对复杂盐丘下部层状结构成像质量的影响;同时通过分析目标区对应于相对地表采集系统分布的照明能量,确定叠前炮集数据中对盐下目标区成像起重要贡献的部分,并将这些数据用来对目标区成像,从而达到提高盐下结构成像质量的目的.本文将二维SEG-EAGE盐丘模型中盐下反射体作为目标区,分别计算其照明分析和由照明能量分布确定的炮集数据对目标区的偏移成像结果,同时通过与成像振幅校正前后的成像结果对比,说明了该方法对提高目标区结构成像质量的有效性.  相似文献   

6.
Interval velocity analysis using post‐stack data has always been a desire, mainly for 3D data sets. In this study we present a method that uses the unique characteristics of migrated diffractions to enable interval velocity analysis from three‐dimensional zero‐offset time data. The idea is to perform a standard three‐dimensional prestack depth migration on stack cubes and generate three‐dimensional common image gathers that show great sensitivity to velocity errors. An efficient ‘top‐down’ scheme for updating the velocity is used to build the model. The effectiveness of the method is related to the incorporation of wave equation based post‐stack datuming in the model building process. The proposed method relies on the ability to identify diffractions along redatumed zero‐offset data and to analyse their flatness in the migrated local angle domain. The method can be considered as an additional tool for a complete, prestack depth migration based interval velocity analysis.  相似文献   

7.
We use Kirchhoff prestack depth migration to calculate migrated sections in 3-D simple anisotropic homogeneous velocity models in order to demonstrate the impact of anisotropy on migrated images. The recorded wave field is generated in models composed of two homogeneous layers separated by one either non-inclined or inclined curved interface. The anisotropy in the upper layer is triclinic. We apply Kirchhoff prestack depth migration to velocity models with different types of anisotropy: a triclinic anisotropic medium, an isotropic medium, transversely isotropic media with a horizontal (HTI) and vertical (VTI) symmetry axis. We observe asymmetry in migration caused by triclinic anisotropy and we show the errors of the migrated interface caused by inaccurate velocity models used for migration. The study is limited to P-waves.  相似文献   

8.
Prestack image volumes may be decomposed into specular and non‐specular parts by filters defined in the dip‐angle domain. For space‐shift extended image volumes, the dip‐angle decomposition is derived via local Radon transform in depth and midpoint coordinates, followed by an averaging over space‐shifts. We propose to employ prestack space‐shift extended reverse‐time migration and dip‐angle decomposition for imaging small‐scale structural elements, considered as seismic diffractors, in models with arbitrary complexity. A suitable design of a specularity filter in the dip‐angle domain rejects the dominant reflectors and enhances diffractors and other non‐specular image content. The filter exploits a clear discrimination in dip between specular reflections and diffractions. The former are stationary at the specular dip, whereas the latter are non‐stationary without a preferred dip direction. While the filtered image volume features other than the diffractor images (for example, noise and truncation artefacts are also present), synthetic and field data examples suggest that diffractors tend to dominate and are readily recognisable. Averaging over space‐shifts in the filter construction makes the reflectors? rejection robust against migration velocity errors. Another consequence of the space‐shift extension and its angle‐domain transforms is the possibility of exploring the image in a multiple set of common‐image gathers. The filtered diffractions may be analysed simultaneously in space‐shift, scattering‐angle, and dip‐angle image gathers by means of a single migration job. The deliverables of our method obviously enrich the processed material on the interpreter's desk. We expect them to further supplement our understanding of the Earth's interior.  相似文献   

9.
Iterative migration velocity analysis is computationally expensive, where most of the computation time is used for generating prestack depth images. By using a reduced form of Kirchhoff migration, denoted as wave path migration, we can significantly speed up the depth imaging process and reduce the entire velocity analysis expense accordingly. Our results with 2D synthetic and field data show that wave path migration velocity analysis can efficiently improve the velocity model and the wave path migration velocity analysis updated velocity correlates well with that from the Kirchhoff migration velocity analysis. The central processing unit comparison shows that, for a 2D synthetic and field data set, wave path migration velocity analysis is six times faster than Kirchhoff migration velocity analysis. This efficiency should be even greater for 3D data.  相似文献   

10.
盐丘模型 推覆体模型 波动方程 叠前深度偏移   总被引:4,自引:0,他引:4  
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.  相似文献   

11.
While seismic imaging for crustal and mantle structures has traditionally relied on surface wave and refraction data, the use of reflection data for crustal-scale targets has been largely limited to the common midpoint (CMP) stack techniques. The rapid increase in the number of seismograph array deployments in recent years in crustal and mantle seismology has reached a level such that a re-examination of the imaging techniques is becoming necessary. In this paper we show the advantage of prestack depth imaging for crustal reflection studies, based on data from two reflection surveys of the Los Angeles Regional Seismic Experiment (LARSE) to map faults and crustal-scale structures. Our analysis indicates that the quality of the previous images of these surveys is limited by the CMP stack technique. For comparison, we present here depth images of the same LARSE data using wave equation prestack depth imaging and a tomographic velocity model based on first arrivals of the LARSE surveys and local earthquakes. Our new images are considerably improved over previous images in terms of resolution and reflector continuity. The new images show reflectors throughout the crust and suggest truncations in the Moho associated with the San Andreas Fault. A series of bright reflector segments, which are associated with the San Gabriel and San Andreas faults have been identified and might represent reflections from the fault zones. Our results suggest that the presence of high noise level, strong lateral velocity heterogeneity and wide angle geometry argue for, rather than against, the use of prestack depth imaging over the simple CMP stack techniques. As demonstrated in this study, it is now viable to conduct prestack depth imaging of crustal reflection data using a velocity model based on earthquake first arrivals thanks to the dense acquisition deployment.  相似文献   

12.
Migration velocity analysis and waveform inversion   总被引:3,自引:0,他引:3  
Least‐squares inversion of seismic reflection waveform data can reconstruct remarkably detailed models of subsurface structure and take into account essentially any physics of seismic wave propagation that can be modelled. However, the waveform inversion objective has many spurious local minima, hence convergence of descent methods (mandatory because of problem size) to useful Earth models requires accurate initial estimates of long‐scale velocity structure. Migration velocity analysis, on the other hand, is capable of correcting substantially erroneous initial estimates of velocity at long scales. Migration velocity analysis is based on prestack depth migration, which is in turn based on linearized acoustic modelling (Born or single‐scattering approximation). Two major variants of prestack depth migration, using binning of surface data and Claerbout's survey‐sinking concept respectively, are in widespread use. Each type of prestack migration produces an image volume depending on redundant parameters and supplies a condition on the image volume, which expresses consistency between data and velocity model and is hence a basis for velocity analysis. The survey‐sinking (depth‐oriented) approach to prestack migration is less subject to kinematic artefacts than is the binning‐based (surface‐oriented) approach. Because kinematic artefacts strongly violate the consistency or semblance conditions, this observation suggests that velocity analysis based on depth‐oriented prestack migration may be more appropriate in kinematically complex areas. Appropriate choice of objective (differential semblance) turns either form of migration velocity analysis into an optimization problem, for which Newton‐like methods exhibit little tendency to stagnate at nonglobal minima. The extended modelling concept links migration velocity analysis to the apparently unrelated waveform inversion approach to estimation of Earth structure: from this point of view, migration velocity analysis is a solution method for the linearized waveform inversion problem. Extended modelling also provides a basis for a nonlinear generalization of migration velocity analysis. Preliminary numerical evidence suggests a new approach to nonlinear waveform inversion, which may combine the global convergence of velocity analysis with the physical fidelity of model‐based data fitting.  相似文献   

13.
苏北大陆科学钻探靶区深反射地震的叠前深度偏移   总被引:4,自引:2,他引:2  
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。  相似文献   

14.
Tilted transversely isotropic formations cause serious imaging distortions in active tectonic areas (e.g., fold‐and‐thrust belts) and in subsalt exploration. Here, we introduce a methodology for P‐wave prestack depth imaging in tilted transversely isotropic media that properly accounts for the tilt of the symmetry axis as well as for spatial velocity variations. For purposes of migration velocity analysis, the model is divided into blocks with constant values of the anisotropy parameters ε and δ and linearly varying symmetry‐direction velocity VP0 controlled by the vertical (kz) and lateral (kx) gradients. Since determination of tilt from P‐wave data is generally unstable, the symmetry axis is kept orthogonal to the reflectors in all trial velocity models. It is also assumed that the velocity VP0 is either known at the top of each block or remains continuous in the vertical direction. The velocity analysis algorithm estimates the velocity gradients kz and kx and the anisotropy parameters ε and δ in the layer‐stripping mode using a generalized version of the method introduced by Sarkar and Tsvankin for factorized transverse isotropy with a vertical symmetry axis. Synthetic tests for several models typical in exploration (a syncline, uptilted shale layers near a salt dome and a bending shale layer) confirm that if the symmetry‐axis direction is fixed and VP0 is known, the parameters kz, kx, ε and δ can be resolved from reflection data. It should be emphasized that estimation of ε in tilted transversely isotropic media requires using nonhyperbolic moveout for long offsets reaching at least twice the reflector depth. We also demonstrate that application of processing algorithms designed for a vertical symmetry axis to data from tilted transversely isotropic media may lead to significant misfocusing of reflectors and errors in parameter estimation, even when the tilt is moderate (30°). The ability of our velocity analysis algorithm to separate the anisotropy parameters from the velocity gradients can be also used in lithology discrimination and geologic interpretation of seismic data in complex areas.  相似文献   

15.
Although it is widely recognized that anisotropy can have a significant influence on the focusing and positioning of migrated reflection events, conventional depth imaging methods still operate with isotropic velocity fields. Here, we present an application of a 2D migration velocity analysis (MVA) algorithm, designed for factorized v(x, z) VTI (transversely isotropic with a vertical symmetry axis) media, to an offshore data set from West Africa. By approximating the subsurface with factorized VTI blocks, it is possible to decouple the spatial variations in the vertical velocity from the anisotropic parameters with minimal a priori information. Since our method accounts for lateral velocity variation, it produces more accurate estimates of the anisotropic parameters than those previously obtained with time‐domain techniques. The values of the anellipticity parameter η found for the massive shales exceed 0.2, which confirms that ignoring anisotropy in the study area can lead to substantial imaging distortions, such as mis‐stacking and mispositioning of dipping events. While some of these distortions can be removed by using anisotropic time processing, further marked improvement in image quality is achieved by prestack depth migration with the estimated factorized VTI model. In particular, many fault planes, including antithetic faults in the shallow part of the section, are better focused by the anisotropic depth‐migration algorithm and appear more continuous. Anisotropic depth migration facilitates structural interpretation by eliminating false dips at the bottom of the section and improving the images of a number of gently dipping features. One of the main difficulties in anisotropic MVA is the need to use a priori information for constraining the vertical velocity. In this case study, we successfully reconstructed the time–depth curve from reflection data by assuming that the vertical velocity is a continuous function of depth and estimating the vertical and lateral velocity gradients in each factorized block. If the subsurface contains strong boundaries with jumps in velocity, knowledge of the vertical velocity at a single point in a layer is sufficient for our algorithm to determine all relevant layer parameters.  相似文献   

16.
基于波动方程的广义屏叠前深度偏移   总被引:15,自引:7,他引:15       下载免费PDF全文
地震波传播算子的计算效率和精度是制约三维叠前深度偏移的关键因素. 广义屏传播算子(GSP, Generalized Screen Propagator)是一种在双域中实现的广角单程波传播算子. 这一方法略去了在非均匀体之间发生的交混回响,但它可以正确处理包括聚焦、衍射、折射和干涉在内的各种多次前向散射现象. 通过背景速度下的相移和扰动速度下的陡倾角校正,广义屏算子能够适应地层速度的强烈横向变化. 这种算子可以直接应用于炮集叠前偏移,通过将广义屏算子作用于双平方根方程,还可以获得一种高效率、高精度的炮检距域叠前深度偏移方法,用于二维共炮检距道集和三维共方位角道集的深度域成像. 本文首先简述了炮检距域广义屏传播算子的理论,进而讨论了共照射角成像(CAI, Common Angle Imaging)条件,由此给出各个不同照射角(炮检距射线参数)下的成像结果,进而得到共照射角像集. 由于照射角和炮检距的对应关系,共照射角像集又为偏移速度分析和AVO(振幅随炮检距变化)分析等提供了有力工具.  相似文献   

17.
Migration velocity analysis by depth image-wave remigration: first results   总被引:2,自引:0,他引:2  
The image‐wave equation for depth remigration is a partial differential equation that is similar to the acoustic wave equation. In this work, we study its finite‐difference solution and possible applications. The conditions for stability, dispersion and dissipation exhibit a strong wavenumber dependence. Where higher horizontal than vertical wavenumbers are present in the data to be remigrated, stability may be difficult to achieve. Grid dispersion and dissipation can only be reduced to acceptable levels by the choice of very small grid intervals. Numerical tests demonstrate that, upon reaching the true medium velocity, remigrated images of curved reflectors propagate to the correct depth and those of diffractions collapse to single points. The latter property points towards the method's potential for use as a tool for migration velocity analysis. A first application to inhomogeneous media shows that in a horizontally layered medium, the reflector images reach their true depth when the remigration velocity equals the inverse of the mean medium slowness.  相似文献   

18.
The development of cost-effective and environmentally acceptable geophysical methods for the exploration of mineral resources is a challenging task. Seismic methods have the potential to delineate the mineral deposits at greater depths with sufficiently high resolution. In hardrock environments, which typically host the majority of metallic mineral deposits, seismic depth-imaging workflows are challenged by steeply dipping structures, strong heterogeneity and the related wavefield scattering in the overburden as well as the often limited signal-to-noise ratio of the acquired data. In this study, we have developed a workflow for imaging a major iron-oxide deposit at its accurate position in depth domain while simultaneously characterizing the near-surface glacial overburden including surrounding structures like crossing faults at high resolution. Our workflow has successfully been showcased on a 2D surface seismic legacy data set from the Ludvika mining area in central Sweden acquired in 2016. We applied focusing prestack depth-imaging techniques to obtain a clear and well-resolved image of the mineralization down to over 1000 m depth. In order to account for the shallow low-velocity layer within the depth-imaging algorithm, we carefully derived a migration velocity model through an integrative approach. This comprised the incorporation of the tomographic near-surface model, the extension of the velocities down to the main reflectors based on borehole information and conventional semblance analysis. In the final step, the evaluation and update of the velocities by investigation of common image gathers for the main target reflectors were used. Although for our data set the reflections from the mineralization show a strong coherency and continuity in the seismic section, reflective structures in a hardrock environment are typically less continuous. In order to image the internal structure of the mineralization and decipher the surrounding structures, we applied the concept of reflection image spectroscopy to the data, which allows the imaging of wavelength-specific characteristics within the reflective body. As a result, conjugate crossing faults around the mineralization can directly be imaged in a low-frequency band while the internal structure was obtained within the high-frequency bands.  相似文献   

19.
An integrated multiscale seismic imaging flow is applied to dense onshore wide‐aperture seismic data recorded in a complex geological setting (thrust belt). An initial P‐wave velocity macromodel is first developed by first‐arrival traveltime tomography. This model is used as an initial guess for subsequent full‐waveform tomography, which leads to greatly improved spatial resolution of the P‐wave velocity model. However, the application of full‐waveform tomography to the high‐frequency part of the source bandwidth is difficult, due to the non‐linearity of this kind of method. Moreover, it is computationally expensive at high frequencies since a finite‐difference method is used to model the wave propagation. Hence, full‐waveform tomography was complemented by asymptotic prestack depth migration to process the full‐source bandwidth and develop a sharp image of the short wavelengths. The final traveltime tomography model and two smoothed versions of the final full‐waveform tomography model were used as a macromodel for the prestack depth migration. In this study, wide‐aperture multifold seismic data are used. After specific preprocessing of the data, 16 frequency components ranging from 5.4 Hz to 20 Hz were inverted in cascade by the full‐waveform tomography algorithm. The full‐waveform tomography successfully imaged SW‐dipping structures previously identified as high‐resistivity bodies. The relevance of the full‐waveform tomography models is demonstrated locally by comparison with a coincident vertical seismic profiling (VSP) log available on the profile. The prestack depth‐migrated images, inferred from the traveltime, and the smoothed full‐waveform tomography macromodels are shown to be, on the whole, consistent with the final full‐waveform tomography model. A more detailed analysis, based on common‐image gather computations, and local comparison with the VSP log revealed that the most accurate migrated sections are those obtained from the full‐waveform tomography macromodels. A resolution analysis suggests that the asymptotic prestack depth migration successfully migrated the wide‐aperture components of the data, allowing medium wavelengths in addition to the short wavelengths of the structure to be imaged. The processing flow that we applied to dense wide‐aperture seismic data is shown to provide a promising approach, complementary to more classical seismic reflection data processing, to quantitative imaging of complex geological structures.  相似文献   

20.
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.  相似文献   

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