山前砾石区表层HTI介质速度各向异性建模方法

A Method of Anisotropic Velocity Modeling for HTI Medium in the Surface of Piedmont Gravel Zone

中国西部山前带表层普遍发育巨厚砾石沉积的冲积扇,这些冲积扇存在较强的方位各向异性,层析反演模型与垂向速度相差较大,导致三维资料静校正量计算精度和叠前深度偏移的中—浅层成像均受到较大影响。基于HTI介质理论,提出初至正演确定对称轴和分方位层析拟合建立各向异性模型的方法。首先通过各向异性调查微测井的表层结构正演初至时间和实际资料的初至时间对比确定各向异性对称轴特征,再对分方位初至层析反演模型进行椭圆拟合,求取初始快速度、慢速度和对称轴方位,使用二维VTI各向异性正演模拟和层析反演,求取对称轴方向速度的校正系数校正对称轴方位速度,进而建立山前砾石覆盖区的各向异性模型。

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 impacting the accuracy of static correction value of 3D seismic data and the imaging of shallow⁃tomedium 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 performed on the azimuthal first⁃arrival tomography inversion model to derive the initial fast and slow velocities and the symmetry axis orientation.Next,by using 2D VTI anisotropic forward modeling and tomography inversion,the correction coefficients of the velocity in the direction of the symmetry axis are obtained.Finally,an anisotropic velocity model is established for the piedmont gravel zone.