Research And Application Of Velocity Model Building Technology For Pre-stack Depth Migration

At present,the focus of oil and gas exploration has shifted from conventional anticlinal traps to lithologic traps.More complex geological structure requires higher quality seismic data processing results.Complex geological structural belts have the characteristics of fast lateral lithology change,large dip angle and sharp velocity change,resulting in weak seismic response energy,low resolution and signal-to-noise ratio,making conventional time migration to obtain accurate underground structural information,thus restricting the exploration and development of oil and gas resources.Therefore,the application of accurate pre-stack depth migration imaging is particularly important.At present,the inaccurate velocity model often leads to poor migration imaging effect in actual production,which makes the advantage of high precision of prestack depth migration imaging not fully reflected.Therefore,high precision velocity model is the key to successful prestack depth migration.In fact,pre-stack depth migration velocity modeling includes two steps: the initial velocity model and the velocity model modification.The establishment of initial velocity model is mainly to describe the model structure and velocity variation of underground media by using velocity information obtained by various routes and combining exact solution with interactive analysis.Velocity model updating is to use pre-stack depth migration as a tool,analyze the accuracy of velocity model based on imaging results,extract velocity error and correct velocity model.Firstly,this paper introduces the velocity modeling criteria and some common methods,as well as some basic principles and development process of migration.Then the velocity modeling method is analyzed and evaluated on the basis of forward pre-stack gathers of theoretical models with different complexity.In the initial model building stage,the conventional Dix formula conversion method and the three-dimensional constrained Dix inversion method are used to convert the interval velocity,and the conversion results and the first pre-stack depth migration results are used to evaluate the advantages and disadvantages of the two methods.In the model modification stage,linear inversion and nonlinear inversion are used to update the initial model iteratively,and the characteristics and rules of the two methods are evaluated according to the migration imaging quality and computational efficiency.By analyzing the influencing factors and applicable conditions of different velocity modeling methods in each link,the advantages and disadvantages of each method are summarized in detail,and the better modeling methods are selected to optimize the combination,and the velocity modeling process adapted to complex geological conditions is proposed.Finally,the process is applied to the actual 3D seismic area,and a good migration effect is achieved.