Joint Reflection And Refraction Traveltime Tomography

Seismic tomography is a technique which deduces and draws the physical properties of underground structure from the inversion of seismic data. Its main purpose is to determine the fine structure of the Earth's interior and local inhormogeneity ( Wencai Yang, 1993 ). Tomography is originally used in medical research. With the increasing development of geological tomography, the research on geological tomography has achieved great successes and received increasing acceptability and expansion.According to the differences in the used seismic data, seismic tomography can fall into four types-transmission tomography, reflection tomography, refraction tomography and surface wave tomography. Transmission tomography, which conducts observation in cross-wells, is studied deeper than the other types. It is followed by refraction tomography, of which the advantage is that it utilizes long offset records. Research on reflection tomography is far away from maturity. Although reflection tomography studies deeper subsurface structure than refraction tomography does, it has lower inversion accuracy. In order to improve the inversion accuracy of seismic tomography, researchers begin to carry out simultaneous inversion of two or more types of seismic data.This paper outlines the development and the basic principles of seismic tomography, and analyzes the advantages and disadvantages of various ray tracing methods and inversion algorithms. This paper also deduces simultaneous tomography equation from reflection tomography equation and refraction tomography equation. The inversion accuracy of simultaneous tomography, in theory, can be improved, since it utilizes both reflection data and refraction data and this combination can increase observation angle. Then this paper improves the distribution of grid nodes in Jun Korenaga's joint reflection and refraction tomography program, and then carries out numerical simulation with two models. The results show that the improved program has higher inversion accuracy and simultaneous tomography with wide-angle seismic data has better results than other kinds of seismic data. In addition, a comparison experiment is conducted and its result shows that seismic data of intermediate scale and large scale (the length of seismic arrangement is more than several kilometers) is more appropriate for simultaneous tomography than small scale data (the length of seismic arrangement is less than several kilometers).