The Application Of Gaussian Beam Tomography In Dingshan Area

At present,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,in most cases,there is a lack of low-frequency long offset data,thus facing various problems in actual productionFor 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 improve stability of inversion.Under the assumption of the Born and Rytov approximation of wave equation,we derived the linear relation of the traveltime perturbation and velocity perturbation in the imaging domain,by which we construct the explicit expression of kernel function for the wave equation traveltime tomography and establish the wavepath traveltime tomography equation.Through the high frequency approximation of wave equation in the ray center coordinate system,we obtain the dynamic ray tracing equation which is different from the one in the Cartesian coordinate system,and by which we construct the Gaussian beam propagation operator.The operator is applied to the subsequent prestack depth migration and tomography.Making use of the Gaussian beam propagation operator,we compute the kernel function for the wave equation traveltime tomography and implement the wavepath traveltime tomography in the imaging domain.To avoid dangers ofmanual 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.At the same time,the method is applied in dingshan area.In a conclusion,this paper proposed a high accuracy velocity inversion method.