نوع مقاله : مقاله پژوهشی
نویسندگان
1 گروه مهندسی نفت، مواد و معدن ، واحد تهران مرکزی ،دانشگاه آزاد اسلامی،تهران، ایران
2 گروه فیزیک زمین، موسسه ژئوفیزیک دانشگاه تهران، تهران، ایران
چکیده
کلیدواژهها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
A medium is isotropic if its elastic properties do not change with direction. The usual meaning of seismic anisotropy is variation of seismic velocity, which itself depends on the elastic properties of the medium, with the direction in which it is measured.
In sedimentary rock sequences, the anisotropy may be caused by preferred orientation of anisotropic mineral grains, preferred orientation of cracks or thin bedding of layers. Anisotropy parameters that defined by Thomsen are ε and δ. Thomsen’s anisotropic parameters are estimated by well and seismic data. However, one difficulty in addressing anisotropy lies not in the algorithms, but in the reliable estimation of anisotropic parameters.
Most of seismic data analysis assume isotropic behavior for subsurface while the earth is often anisotropic and seismic velocity isn’t constant in different directions. So, this analysis must consider anisotropic assumption. One of the most common anisotropy-related phenomena is seismic imaging.
Output of migration is the section that is similar to the geological model. Depth migration leads to correct image when velocity changes laterally and vertically in the subsurface. Generally, isotropic Pre-Stack Depth Migration (PSDM) corrects only for lateral velocity heterogeneity; however, anisotropic PSDM algorithms also correct for velocity changing with direction. Anisotropic PSDM corrects for vertical shifts, correctly positions events in depth, and properly focuses diffraction energy. Thomsen’s anisotropic parameters, ε and δ, are two main parameters required in a velocity model for PSDM imaging.
Accurate estimation of travel time is very essential in seismic imaging and velocity analysis. Inaccurate approximation of travel time leads to migration errors. Several equations have been developed for nonhyperbolic travel time approximation in transversely isotropic media with vertical symmetry axis (vertical transverse isotropy, VTI).
In this study, 2D seismic line and well data are used for doing isotropic and anisotropic PSDM. Conventional processing sequences are performed on seismic data. Initial velocity model was created and then 3 horizons were picked on seismic section. Anisotropy parameters are calculated using true depth of horizons in seismic and well data and then PSDM is performed with and without considering anisotropic parameters.
Results of applying anisotropic PSDM in seismic data show that this method moves the events to correct positions and can significantly reduce seismic-to-well mis ties, hence, providing more accurate structural images in depth domain.
In addition, anisotropic PSDM attenuates the hockey stick events on CDP gathers and improves the quality of the subsurface image in terms of reflector continuity.
کلیدواژهها [English]
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