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转换波叠前时间偏移方法研究
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摘要
偏移是地震数据处理三大技术(反褶积、叠加和偏移)之一。随着地震勘探技术的发展和勘探目标复杂程度的增大,对地震偏移成像的精度提出了更高的要求。在共中心点叠加基础上的叠后偏移已经不能很好的适应地质解释的要求,因此,叠前偏移成为地震资料处理必不可少的环节。叠前时间偏移方法以双平方根方程非零炮检距成像理论为基础,沿着非零炮检距的旅行时轨迹(Cheops金字塔)对振幅求和。相对于叠前深度偏移来说,叠前时间偏移在构造复杂、速度横向变化不剧烈的情况下也可以得到比较理想的成像效果,并且它对速度模型不是很敏感,也比较节省计算时间。另外,相对于基于有限差分法的叠前时间偏移来说,本文讨论的基于克希霍夫法的叠前时间偏移运算效率比较高,而且适应能力也比较强。
     随着转换波地震勘探技术的发展,转换波偏移成像技术成为研究的重点。转换波勘探提供了与纵波不同的信息,从而能获得地下含流体性、裂隙及弹性参数等信息。对转换波资料的分析与处理,在一定程度上能够弥补纵波勘探的不足,而转换波资料的偏移是其处理的重要环节。
     本论文在前人工作的基础上,研究并实现了三种实用的二维纵波叠前时间偏移方法,即等效偏移距偏移、非成像炮集偏移以及倾角时差校正加叠前成像方法,并对其各自不同的求和路径及形成的中间道集进行了分析,通过对这三种方法进行对比研究,说明了叠前时间偏移是对双平方根方程定义的旅行时曲面的绕射求和,这三种方法的区别就在于其定义的求和路径不同;实现了快速、有效的基于等效偏移距偏移方法的转换波叠前时间偏移,该方法在将转换波数据映射到共转换散射点道集时,去除了转换波射线路径的不对称性,简化了转换波的处理过程;作者编程实现了P-P、P-SV波叠前时间偏移程序,并用几组典型模型和实际资料进行了试算,验证了方法的有效性,结果表明,该几种方法可以对陡倾角地质体进行正确成像,对复杂构造以及小尺度地质体也可以成像,成像效果比常规成像方法的偏移结果更加接近真实地质体。
Migration is one of the three most important seismic data processing techniques which are deconvolution, stack and migration. With the development of seismic prospecting technique and the increase of complexity in seismic prospecting, higher demand for the precision of seismic migration is put forward. Post-stack migration based on the CMP stack can not adapt to the requirements of geological interpretation, so pre-stack migration become indispensable step in seismic data processing. Pre-stack time migration, whose basis is non-zero offset imaging theory of double square root equation, sum the amplitude along non-zero offset travel-time path. The pre-stack time migration can also get ideal imaging result when the structure is not very complex and the transverse variation of velocity is not severe. It is notable that the pre-stack time migration is insensitive to the velocity model and can save the computation time comparing with pre-stack depth migration. In addition, relative to the pre-stack time migration based on finite difference method, the pre-stack time migration based on the Kirchhoff method discussed in this paper has higher operation efficiency and better adaptability.
     With the development of P-SV wave seismic prospecting technique, the P-SV wave migration becomes the key problem on the research. The P-SV wave prospecting can provide information different from P wave, such as fluid and fissure in the media, elastic parameters and so on. So the analysis and processing for P-SV wave data can remedy the deficiency of P wave prospecting, in which P-SV wave migration is an important step for data processing.
     This paper studies and achieves three practical 2-D P wave pre-stack time migration methods on the basis of the previous researches. The three methods are Equivalent Offset Migration, Non-Imaging Shot-Record Migration, Dip Moveout and Prestack Imaging. Then this paper analyzes their different summation paths and middle step gathers respectively. The pre-stack time migration can be considered as diffraction summation over the travel-time surface defined by double square root equation and the differences of these three methods are that they define different summation paths by comparing and studying these three methods. This paper achieves the rapid and effective P-SV wave pre-stack time migration which is based on the Equivalent Offset Migration method. This method gets rid of the asymmetry of P-SV wave ray path and simplifies the P-SV wave processing step during P-SV wave data is mapped into the common converted scatter point gathers. The paper achieves P-P and P-SV pre-stack time migration programs, and the validity of these methods is verified by using several typical models and practical data. The results show that steep slope geological body, complex structure and small size geological body can be imaged correctly using these pre-stack time migration methods, and the imaging effect is more close to real geological body than conventional migration method.
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