| Compared with surface seismic profile which is derived from the receiver on the surface, vertical seismic profile is derived from the receiver underground. The waves' motive and dynamic characteristics are more conspicuous because it is by observing the wave field distribution in the vertical direction to study the geological section of the vertical changes. VSP not only has higher signal/noise ratio and resolution, but also can record up going and down going wave and can provide a more accurate processing and interpretation of time-depth conversion and velocity model for surface seismic processing and reliable identification of seismic reflection layer geologic position for calibration Compared with the surface seismic technology. VSP data to extract more pure seismic wavelet can be used on the surface seismic data deconvolution to improve resolution of surface seismic data. Offset VSP can also well away from the well of the ground around the structure and a higher vertical and horizontal imagination through the VSP P-wave and converted wave of research can effectively obtain the changes of lithology parameters. This article focus on the imagination of offset VSP processing.Seismic modeling is a very effective seismic data processing aids, elastic wave equation can be fully and accurately describe the dynamic characteristics of seismic waves and kinematics of offset VSP to do is wave equation modeling, is Mass media about elastic wave field characteristics and propagation of an important method, to design and develop a more rational approach and process. In the staggered grid of the discrete differential equations, elastic wave equation is derived time difference of second order accuracy, order accuracy finite difference scheme for differential space; with perfectly matched layer (PML) absorbing boundary condition to solve the truncated wave boundary layer reflection problem, the numerical accuracy of the synthetic records calculated.Three-component offset VSP recorded observation wave field is very complex. The various wave fields are often mixed together, to separate the work of a great deal of inconvenience. This article chooses a kind of two-step method for wave field separation by the polarization characteristics and apparent velocity characteristics of the wave field of three-component offset VSP data. the first scanning through the polarization filter based on angle, the original records in the complex wave field separation into simple wave field, and then a simple wave field as the velocity filter, separating the up going and under going wave field, in order to the separation of the various waves.Because the VSP data contains a wealth of seismic wave velocity, inversion of seismic wave in the real rate of spread, the result is more direct and true inversion, VSP seismic interval velocity inversion has a considerable advantage. In this paper, ray tracing method to find the interval velocity. The first arrival time of down going wave is used to strike the interval velocity of the layer in the above of the receivers; the up going wave is used to strike the interval velocity of the layer under receivers, for the reverse time migration interval velocity to provide a more accurate model.Offset VSP elastic wave reverse time migration mainly composed of three parts: computation of the imaging conditions, reverse time extrapolation of the recorded data, and application of the imaging condition at each time step during the extrapolation. This article derived a high order finite difference reverse time extrapolation scheme in the staggered grid for two dimensional isotropic media, and a perfectly matched layer(PML) absorbing boundary conditions applicable to reverse-time migration of elastic waves is also derived in the paper. The imaging condition is computed by solving the eikonal equations. On the basis of above techniques, a prestack reverse-time depth migration algorithm of multi-component offset VSP data is developed. |
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