The Research Of Inversion Based On Gaussian Beam Tomography

Structure of Southern and Western Oil and Gas Areas of China are high,steep and abnormal developed,at the same time,lateral velocity variation is high and subsurface structure is complex,so as to these areas,it is a huge challenge in velocity modeling and seismic imaging.Now,the prestack depth migration has developed very well and got its popularity for complex structure imaging,such as Kirchhoff migration,One-way wave equation migration and following developed Gaussian beam migration.Compared with migration algorithm,the development of velocity modeling method is relatively backward,which cannot satisfy the needs of the imaging methods for velocity model.Now,travel time tomography inversion basing on ray is one of the most popular velocity inversion method that can offer a relatively accurate travel time and macro velocity model.However,the spatial resolution of this method merely applies to Fresnel zone.For complex model,it is easy to see to shadow,caustic and other problems appear in the transmission of conventional ray.On the other hand,only grid points on the ray are taken into consideration during the transmission period,so the Frechet matrix of this structure is sparse and ill-posed.As a result,the inversion is unstable and low accuracy and cannot meet current demand for velocity mode.Actually,wave-equation tomography and full wave inversion can offer the highest resolution of inversion results theoretically.However,these methods have many practical problems as their perfect data assumption.For solving the problems mentioned above,this paper studied the Gaussian beam tomography inversion method that is a compromised inversion method lies between conventional tomography and waveform tomography.The Gaussian beam tomography inversion method retains high computing efficiency of conventional tomographic methods,and using the Frechet matrix built with Gaussian beams' kernels to improve stability of inversion at the same time.Basing on tomographic theory,this paper firstly deduced conventional tomographic methods.Then proposed Gaussian beam tomographic inversion method and deduced formula of Gaussian beam kernel function.The Gaussian beam initial value selection principle and Kernel function boundary conditions defining methods were also provided.Gaussian beam tomographic inversion method can be applied in many tomographic inversions,such as first breaks tomographic inversion?slope tomography?migration velocity analysis and so on.This paper mainly discussed migration velocity analysis.As Gaussian beam prestack depth migration not only has great advantage in dealing with high and steep structure,but can also acquire angle-domain common imaging gathers(ADCIGs)with high resolution directly.Putting the ADCIGS into Gaussian beam tomographic inversion can get an accurate migration velocity model.To avoid dangers of manual picking-up in high and steep areas,this paper proposed the automatic piking-up technology that can acquire coordinates of reflection points and angles of underlying structureLastly,this paper compared the results of Gaussian beam tomography with the conventional tomography.It can be seen that Gaussian beam tomography has higher accuracy and stability.The practical data computing was also conducted in this paper and the results shows that Gaussian beam tomographic method also perform well in complex detective areas.In a conclusion,this paper proposed a high accuracy velocity inversion method.