Calculating Traveltime In VTI Medium With Hybrid Meshes Under Rugged Interface Conditions

Seismic wave field numerical simulation is not only an important way to recognize and study the propagation of seismic waves in underground media,but is also an effective method to study the collection,processing and interpretation of seismic data in complex regional.Seismic exploration under the conditions of loess plateau,dune and mountainous environment in the west of china,due to the strong ups and downs of the near surface topography and the complexity of the subsurface structure,make it difficult to accurately calculate the static correction amount,objectively identify the seismic wave,and even achieve accurate target seismic imaging.Therefore,it has some practical significance to study the numerical simulation of seismic wave fields that applicable to complex media with complex structures.Seismic wave traveltime tomography is an important inversion algorithm that takes the observed traveltime and ray paths of seismic waves as known information and uses model reconstruction algorithms to retrieve important information such as subsurface media structure,velocity distribution and elastic parameters.First-break has not only obvious advantages in velocity analysis of cross-hole or near surface and complex surface static move-out because it has the characteristics of easy identification and tracking,but also carries a wealth of geological information of shallow surface,which can reflect the distribution of tectonic and velocity parameters at different depths to a certain extent.Therefore,seismic first-wave traveling time tomography can provide more efficient and accurate near-surface seismic velocity for high-precision seismic imaging,which will improve the accuracy of seismic exploration.This paper focuses on the issues of seismic wave field forward modeling in complex media with fluctuating surface and first-break tomography under fluctuating surface conditions.In order to provide reliable velocity information for high-precision seismic imaging in complex areas.In order to improve the accuracy and efficiency to calculate traveltimes under complex structures with rugged topographical conditions,the author presents a new method of LTI based on hybrid meshes,that consisting of rectangle meshes and irregular quadrilateral meshes instead of which on single rectangle meshes?gives the new formulas to calculate the local traveltimes in hybrid meshes and proves the formula is steady,in this paper.Then,the author promotes this new method into multi-stage LTI method base on hybrid meshes.The results of numerical computation show that this new method can not only deal with the sharply rugged topography and velocity interface with complex constructs neatly but also keep higher calculation precision in traveltimes and ray paths.In order to calculate the seismic wave traveltime in the VTI medium under the condition of rugged topography or interface,the model is divided by the hybrid mesh composed by the rectangular mesh and the irregular quadrilateral mesh to make the mesh fit the rugged topography or interface better.In the hybrid mesh,the relationship between group velocity and group angle is transformed into the relationship between group velocity and interpolated point coordinates to find the local traveltime,and then combined with the multi-step partitioning technique,The multi-step LTI ray tracing algorithm is extended to the VTI medium to realize the traveltime calculation of the seismic waves(first arrival,reflection,multiple reflection,multiple transmission conversion and multiple reflection conversion)in VTI medium.The author compares the calculated results of this method with the results of the finite difference method,FMM method and ISPM method and numerical simulation on the model with complex structure.the results show that the algorithm is accurate and has good adaptability to the model with complex structure.The first-break traveltime calculated by the algorithm is used as the observation value and using the LSQR algorithm to iteratively solve the equation to obtain the slowness vector of the entire model.Thus,the shallow surface velocity imaging in the complex region is achieved,through several sets of theoretical model test and result analysis,it is verified that the proposed algorithm has some practicality.