Velocity plays an important role in exploration seismology. It is widely used in various areas in seismic data processing (such as multifold seismic data stacking and migration) and seismic stratigraphic interpretation, as well as determination of information regarding structural geology and lithology. Seismic velocity is also used to obtain information about anisotropy and heterogeneity of the elastic medium.
Recently, considerable research in inversion of seismic tomographic data for velocity structure has been accomplished (Ivansson, 1985; Chiu and Stewart, 1987; Iverson, 1988; Zhu and McMechan, 1988; Abdalla and Stewart, 1990; Lines and Tan, 1990; Stewart, 1990). A different method for velocity analysis is proposed in this project; it is similar to those commonly used in conventional velocity analysis of CMP gathers. This method is to derive velocity information from the first breaks of crosswell data. Common shot or receiver gathers of either P- or S-wave data can be used for velocity analysis by the method described here. Assuming that the subsurface is a continuous isotropic medium in which the velocity varies linearly as the depth increases, a familiar formula is derived. To obtain the velocities which best fit the subsurface, a semblence analysis is made. Based on this, a computer program is then written to implement this algorithm. Both synthetic and real crosswell shot or receiver gathers will be used to test the validity of the method.
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