A Method of Anisotropic Velocity Modeling for HTI Medium in the Surface of Piedmont Gravel Zone
There are widespread alluvial fans with extremely thick gravel deposits in the surface of the piedmont zones in western China.These fans exhibit strong azimuth anisotropy.The tomographic inversion velocity differs greatly from the vertical velocity,severely impact-ing the accuracy of static correction value of 3D seismic data and the imaging of shallow-to-medium layer in pre-stack depth migration.Based on the HTI medium theory,a method for determining the symmetry axis and constructing an anisotropic model through azimuthal to-mography fitting using first-arrival forward modeling was proposed.First,based on anisotropy,the first-arrival time from both micro-logging forward modeling and actual data is examined and compared to identify the symmetry axis characteristics.Then,elliptical fitting is per-formed on the azimuthal first-arrival tomography inversion model to derive the initial fast and slow velocities and the symmetry axis orienta-tion.Next,by using 2D VTI anisotropic forward modeling and tomography inversion,the correction coefficients of the velocity in the direc-tion of the symmetry axis are obtained.Finally,an anisotropic velocity model is established for the piedmont gravel zone.
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