Study On Converted Wave Pre-Stack Time Migration Method

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.