Research On Converted Shear Wave Splitting Analysis And Correction Technique

In recent years, multi-wave and multi-component technology has made great progress, People have made more understanding for anisotropy, Multi-wave data contain more information than single P-wave, Utilization for P-wave and S-wave data can get more detailed and reliable formation parameters. When S-wave is not parallel nor perpendicular through the anisotropic medium (fracture), S-wave splitting occurs .When the polarization direction along the fracture, the propagation speed is faster, named as fast S-wave, when the shear wave polarization direction perpendicular to the fracture, the propagation speed is slow, named as slow S-wave.Only when the wave with the fracture orientation at some angle, S-wave will split, and the strength of splitting is closely related to fracture intensity, and we can get the fracture orientation and density from the analysis of S-wave splitting. These parameters can be used to predict fracture more accurately. When P-waves was uesd to explore, the downgoing and upgoing wave meet the interface will be converted into S-waves. When upgoing S-wave encounters fracture, splitting also occurs. Thus, the use of fast and slow wave splitted from converted S-wave has become into smost direct and reliable method to study the development fracture and its direction.Arrange the radial and transverse components of converted wave together in accordance with the azimuth order, due to the impact of anisotropy, there is a big azimuth time-delay in radial component azimuths gathers, 0 degrees to 360 degrees azimuths datas show form of sine wave; there is strong energy in transverse component, and also show a great azimuths time delay. In the radial component, peak of the sine wave said direction of anisotropy, while trough side the direction perpendicular to the anisotropy; study found that in the transverse component of 90-degree intervals, the polarity reversal occurs, very reversal point represents the change in the direction of anisotropy. Strong anisotropy in the data, to get better imaging section, anisotropy must be corrected.Alford rotation method is mainly used for four-component VSP data by dual-source exploration, according to the SV and SH wave we got the fast and slow wave information and then realize the separation of fast wave and slow wave. This thesis based on converted shear wave splitting theory, using ground azimuth converted wave data, by building mutually perpendicular radial and transverse wave components on which the Alford rotation method used successfully, so as to realize the separation of fast wave and slow wave in surface data. At the same time, by using converted wave azimuth radial and transverse component data, we completed the analysis of multi-layer fracture media. On this basis, we have completed the correction of converted wave azimuthal anisotropy, which greatly improved the azimuth radial component of converted wave imaging quality.Azimuth converted shear wave splitting anisotropy correction contain the next main steps:First, according to the radial and transverse azimuthal data to determine a shallow analysis window, use Alord rotation in the analysis window to seprate fast and slow wave; If there are multiple azimuth data, you need to strike a different direction, respectively, and then accumulate.Secondly, because fast and slow wave are similar, we can get cross-correlation values, and get the anisotropy direction and shear wave time delay by explaining the angle-delay spectrum.Then, the time-delay was applied to slow wave to compensate time-delay.Finally, according to anisotropy azimuth, use anti-rotation technique to make fast wave and slow wave after compensating time-delay to the original radial and transverse direction, then we can get radial and transverse component after anisotropy corrected.These can only complete the anisotropy in first fracture, get the fracture direction in first layer,fracture density, the compensated radial component and other data. The data of fast and slow wave were jointed weighted in first and second analysis window. And so on, to do the deeper data analysis and processing after complete shallow layer analysis.In this paper, we forward modeling shear wave splitting according to the reflection ratio method, not only using modeling data to separate the fast and slow wave, but also do the wave anisotropy correction, and the results show that the method is correct. The research method of converted shear wave splitting anisotropy correction was applied to process 3D3C wide-azimuth seismic data in the XC gas fields in West of Sichuan. Not only the fracture parameters was obtained, but also the azimuthal anisotropy of converted waves was eliminated, and the image quality was increased greatly.