Studying On Forward And Inversion Modeling Methods Of Shear Wave Velocity Under Non-linear Condition

The shear wave velocity of rock and soil layers plays an important role in engineering field, it has being used for site classification, determining the thickness of soil layer,judging sand or powder soil seismic liquefaction of engineering site, calculating the soil dynamics parameters, and dividing the earthquake areas. At present, the method of calculating the shear wave velocity is mainly by using wave velocity situ testing means to pick up seismic wave travel time data, then according to the travel time data to calculate the rock and soil layer shear wave velocity. For example, puckering method, cross hole method and surface wave method. When calculating the shear wave velocity, these methods assume that the seismic wave propagate along a straight line, ignoring the effect of uneven distribution of geotechnical layer, as a result, these methods not only need a large number of complex calculation, but also the precision is low. The purpose of this study in paper is by combing geophysical forward modeling with inversion method, and using a computer programming technique to obtain a high precision, convenient, fast solution method of shear wave velocity. This article about the method of calculating the shear wave velocity is on the basis of predecessors' research which give their method further improvement.The core content of this article: one is the forward solution the other is the inversion calculation. Forward modeling method use the shortest path ray tracing method(SPR), it is a ray tracing method of network technique which can simulate the seismic wave propagation path better. The inversion calculation method is simultaneous iterative reconstruction method(SIRT), it is a kind of quick inversion method of solving the large linear equations, characterized by the good algorithm with stable and convergent results. In this paper, the theory of the two methods are combined together, and compiled into MATLAB algorithm, it can work out shear wave velocity of rock and soil layer in engineering site by running the program. The following aspects are carried out in this paper revolving around the method of calculating the shear wave velocity.1. The article describes the forward and inversion theory development, and elicit the method which uses forward and inversion techniques to solve shear wave velocity under the conditions of nonlinear, it also instructs the solving theory and process design in detail.2. This paper discusses the application value and research significance of shear wave velocity. At the same time, shear wave velocity testing technique and its data analysiswhich widely used in our country was introduced in detail. The article makes a contrast on these shear wave velocity test methods and points out the advantages and disadvantages of each methods.3.This paper briefly introduces the several principles which seismic wave propagation observed and shows propagation law of direct wave, reflection wave and transmission wave, refraction wave, inflexion wave.4. This paper introduces the developing course of ray tracing, it also gives the ray tracing method which was put forward in various stages of development and list its advantages and disadvantages. The shortest path ray tracing method adopted in this paper was introduced in detail, it made a in-depth introduction to the theoretical basis, model building and algorithm implementation of shortest path ray tracing method in turns. This paper also gives a ray tracing algorithm procedures and specifications it uses model to verify the validity and stability of the program as well.5. The articles make an introduction of the geophysical inversion theory, and the five major problems in inversion theory to be solved, namely, the existence of solutions, the structure of the model, the non uniqueness of the solution, the stability of the solution, and the evaluation of the results are briefly introduced. This paper introduces the theory of the inversion method in detail, and gives the algorithm program and its instructions.6. This paper applies the algorithm program to three specific engineering example,and it compare the result of shear wave velocity obtained from the article with shear wave velocity gained from test method. Simultaneously, it verify the feasibility of the method to solve the shear wave velocity used in this article, meanwhile, it prove the effectiveness and stability of the algorithm program as well. At the end of the paper, the advantages and disadvantages of this method are explained.