The Research Of Migration Velocity Analysis Technique

Today, the seismic migration over complex structures has become the majordevelopment direction of oil exploration. On account of much advantages of pre-stackdepth migration, it is essential to use it to get a correct earth image from seismic datacollected over complex structures. A necessary condition for obtaining a correctimage is that the pre-stack depth migration is done with an accurate velocity model. Forits three advantages, first, target oriented imaging, second, ideally suitable to seismicdata collected in any irregular survey, third, high efficiency in computation and imaging,Kirchhoff integral for 3-D pre-stack depth migration is the leading actor in this domain.And, it can create CRP gathers expediently. This is particularly useful for migrationvelocity analysis.Migration velocity analysis (MVA) is the key issue to 3-D pre-stack depthmigration, but its relative techniques are under developing. Nowadays no successfulmethods can be used directly. According to Deregowski loop, the best migrationvelocity can be approached by cascade migration based on the sensibility of seismicimaging to velocity variation. In this paper, a global optimize method, horizon basedtomography, was been used to update the interval velocity and depth in the processingof building the velocity-depth model,which finally has a higher precision and can beused to get a satisfying depth imaging effect, for a 3-D field data and a 2-D syntheticdata.The analysis of the 2-D synthetic model data and the pre-stack depth migration ofthe 3-D field data in this paper, indicate that the pre-stack depth migration is the bestimaging method when the complex structure and intense velocity change at present. Inthis paper, the high precision of the velocity model created by horizon basedtomography has been validated, and, the disturbing factors of the picking of residualdepth moveout have been discussed and suggestions been given, a distinct example ofthe aperture's influence on pre-stack depth migration has been given. All these can beused for reference in seismic data analysis.