Study On The Computation Method Of Seismic Traveltimes Including Surface Topography

A great quantity of geophysical prospecting work is carried out in complex surface areas in western china, compared with the plain areas, seismic exploration in complex surface areas is of a lot of problems, such as low signal to noise ratio, many types of interference of the collecting seismic data and the difficulties of imaging in data processing. Aiming at these problems, we should first solve the problems of the seismic wave field's structure and propagation characteristics completely, and then research the collecting, processing and interpretation techniques of seismic data.The research about the problems of the seismic wave field's structure and propagation characteristics can start with many ways, and the seismic wave field forward modeling is a very effective way. The forward modeling based on ray theory has advantages of high efficiency and intuitive wave field. As a forward modeling tomography based on ray theory, the computation about the seismic traveltimes can be applied to tomography, Kirchhoff migration, velocity analysis, seismic modeling, and other fields and it will play a very important role in solving complex surface problems. In all, the computation of seismic traveltimes is developing to the direction of high accuracy, high efficiency, good flexibility and good stability, and a lot of computation technologies had developed relatively mature. However, so far, there is no the relevant research for complex surface seismic waves traveltimes computation which limits the existing technology applied in the mountains, hills and other complex surface areas.The purpose of the study related to the content of this paper is to propose and implement seismic waves traveltimes computation program under the conditions of complex surface. In order to achieve this objective, we propose the following research ideas: Firstly, review the existing computation technologies of seismic traveltimes and select a optimal computation method (FMM) as the foundation of other follow-up methods; Secondly, study the FMM carefully and improve it in each aspect; Finally, integrate the improved FMM and the complex surface problems, we propose the implementation strategy of studying on the computation of seismic traveltimes in the condition of surface topography. Under the guidance of these ideas, we did the following three aspects of work:â‘ We review various traveltimes algorithms: First, classify various traveltimes algorithms; Second, state the basic principle and implementation strategy of various traveltimes algorithms; Third, compare these algorithms, we know the present situation and development trend of traveltimes algorithms by reviewing them; finally, we draw a conclusion: The finite-difference method is an optimal algorithm to calculating seismic traveltimes and FMM is an optimal finite-difference algorithm. So, we select FMM as the basic algorithm to study the computation of seismic traveltimes in the condition of surface topography.â‘¡We further research the FMM: First, discuss the origin and development, basic principles and implementation, unconditional stability, computational accuracy and efficiency, et al of FMM in detail, and get the following conclude: The order of difference format is an important factor impacting the calculating accuracy of FFM, the order of difference scheme is higher, and the calculating accuracy is higher. The calculating accuracy of second-order scheme is higher than first-order scheme's. Grid space is another important factor impacting calculating accuracy, the grid space is larger, the calculating accuracy is less, and vice versa. The heap sort technology is the most important factor impacting the calculating efficiency of FMM; Second, according to improving the algorithm of FMM form the local difference scheme, greatly enhance the computation accuracy of the algorithm; then start with improving the heap sort technology, greatly enhance the computation efficiency of the algorithm. Finally, a large number of examples about the computation of the FMM verify that the various measures for improving the effective and the results are very good.â‘¢The implementation of the computing seismic traveltimes in the condition of surface topography: First, establish a reasonable, complex physical model of the surface corresponding to the mathematical model; Second, the model is complete, introduce FMM to solve complex surface problems and propose the implementation strategy about the seismic waves traveltimes computation in the complex surface condition. and set up implementation process framework; Third, through processing the velocity model and making interpolation processing near interface and surface, compute the traveltimes on interface and surface more reasonably; Finally, a large number of examples of computation and analysis confirmed that the implementation strategy about the seismic time computation which is presented in this paper is feasible, the results of computation can well describe propagating process of seismic wave at complicated surface area.The main innovation of results of the research as follows:â‘ Through changing the partial algorithm, we enhance the accuracy of the FMM;â‘¡Through developing the heap sort technology, we greatly enhance the computation efficiency of FMM;â‘¢We first present a technical scheme of the seismic traveltimes computation in surface topography condition, set up implementation process framework and compily complete program code.The main results of the research have great significance to study on the structure and propagation characteristics of seismic wave field under the condition of complex surface topography, lay the theoretical foundation for the research of seismic data collecting, processing and interpreting in complex surface technology condition and have a very important theoretical significance for the hydrocarbon exploration in complex surface area(especially in the western mountains).