Research On3-D Forward Algorithm Of Seismic Wave Traveltime And Inversion Technology

Calculation of forward algorithm is the basis and crucial step of seismic tomography and its precision is directly related to the imaging of the inversion which can have a direct effect on the resolution of seismic imaging. Seismic forward numerical modeling is an important tool to accurately simulate wave propagation in various media and record the situation of all the grid points underground at each moment, and it exploit the process of the seismic wave propagate underground directly, which is the most widely used numerical method in exploration seismology. This paper includes three parts:the first part is seismic first arrival ray tracing algorithm; the second part is multiple (reflected, transmitted, and converted) arrivals in three dimensional complex layer media; the last is the technology of traveltime tomography process.The3-D seismic first arrival ray tracing algorithm is the first part in this paper. We adopt the linear traveltime interpolation method (LTI) to calculate the first arrival traveltime. Successive partitioning of the cell interfaces scheme is used in the forward step based on the LTI. By comparing the new method with the shortest path ray tracing method (SPM) and the finite-difference method (FD), we assess the advantages and disadvantages of each method. Then we introduce three ways to calculate the traveltime in the forward process (the traditional way, the circulation way, and the wave front expansion). In the backward process, we trace the ray path by following the gradient of travel time field from the receiver back to the source and calculate the gradient of traveltime of the tracing point by cubic B-spline interpolation. In the complex3-D media, we adopt a local ray tracing method to avoid the error caused by the B-spline interpolation in the area where the velocity changes dramatically. Numerical experiments are designed to compare the new method with the tradition LTI method. These comparisons show obvious improvements to compute travetimes and ray paths, both in precision and computational efficiency.The second part is the simulation of multiple arrivals in3-D model. The first arrival wave is the ray with the minimum traveltime which may contain direct wave, reflected wave and refracted wave. As reflected wave and refracted wave are not necessarily the first arrival, they cannot be calculated as the first arrival. In the practical seismic exploration project, the problem is how to calculate multiple arrivals, especially in the complex three dimensional models. Based on the research status and direction of multiple wave ray tracing algorithm development,the multi-stage LTI method is proposed in this paper combining the improved LTI method with the multi-stage calculation technology, which can trace any multiple or single transmission wave as well as converted wave and reflection wave in three dimensional media. The stability and precision of this method is checked by model tests and error analysis. In order to trace multiple value seismic ray paths in complex structures, we develop another ray tracing method, called the extreme value algorithm based on the multistage MMTQ which is suitable for the multi-value ray path tracing in the complex velocity structure.For the indeterminate coefficient matrix in the traveltime tomography inversion caused by uneven distribution of ray path in the model, we present have improved some technologies such as multi-directional search interpolation, dynamic smoothing and edge recursive in the inversion process. In the forward process we use the modified maximum traveltime gradient ray tracing method (MMTG) and adopte LSQR as inversion method, we verify the effectiveness of the new strategy by actually observing systems under different units of the grid size and the initial model settings.