Reflection Travel Time Inversion Based On Fast Marching Ray Tracing

Seismic travel time tomography can get the velocity structure of underground media by analyzing and inversion of seismic travel time data.Seismic travel time tomography inversion is mainly divided into forward modeling and inversion.Ray tracing of forward modeling is a popular technology in recent years.There are many kinds of ray tracing methods,such as the traditional ray tracing methods based on ray theory,shooting method and bending method;the finite difference method based on the expansion of the grid element,the shortest path method,the fast marching method based on the eikonal equation;the wavefront construction method based on the above two theories,etc.The fast marching method based on eikonomic equation has been proved to be unconditionally stable,accurate and efficient by previous studies and demonstrations for many years,so it has been widely used in the field of ray tracing by many scholars at home and abroad.Since the inversion of first break wave only involves the velocity of the formation surface,and it can not realize the inversion of the formation interface shape,it can not meet the requirements in most exploration processes.In contrast,the travel time of reflection wave carries more abundant underground information,and it is more sensitive to the formation interface,so it can accurately reflect the velocity information of underground media.Therefore,this paper uses the fast marching method based on eikonal equation to complete the calculation of the incident wave field and the reflected wave field.In addition,it also realizes the tracking of the reflected wave and the calculation of the ray path.This paper first introduces the research status of travel time tomography,ray tracing and inversion methods at home and abroad,then introduces the basic theory of seismic wave propagation,and then introduces the solution of eikonomics equation and fast marching method,as well as the cubic B-spline interpolation,source near thinning and other problems used to improve the accuracy.The back tracking method is used to realize the ray tracing.The basic theory of subspace algorithm and the calculation of Frechet derivative are introduced.Finally,a theoretical model is designed for numerical simulation and analysis by combining fast marching method and subspace inversion method.