Abstract

Converted-wave (C-wave) splitting estimation and compensation (SEAC) estimates and removes the effects of shear-wave splitting from C-wave data. A locally 1D earth is assumed where a priori rotation of the field data to radial-transverse coordinates is valid. Subsurface fractures (horizontal transverse isotropy [HTI] layers are assumed) polarize C-wave reflection energy onto the transverse component, and introduce azimuth-dependent traveltime variations to the radial component. SEAC estimates the fast principal direction of the fractures, and the amount of traveltime splitting, from input radial and transverse azimuth-sectored stacks. SEAC also produces a splitting-compensated radial component, and a data misfit transverse component. Local fracture variations not accounted for in the coarse-interval inversion may be interpreted in the data misfit. Synthetic data generated by anisotropic reflectivity modeling for a model containing two HTI layers having different principal directions was used to illustrate SEAC. The field data example used was from a large3-C, 3D Vectorseis survey from onshore China. Preprocessed C-wave data (radial and transverse components) were prestack time migrated into offset planes within 10-degree azimuth sectors. These data were then corrected for residual moveout (azimuth-independent correction), and stacked over offset to produce azimuth-sectored stack gathers that were input to SEAC. SEAC estimated the azimuth of the fast principal direction ϕfast and the amount of traveltime splitting Δtsplit that describe the overburden anisotropy. Spatially variable parameter estimates for the entire 3D data set, (ϕfast=90°±20° and Δtsplit=28±20ms), produce significantly reduced energy on the transverse component at all record times after inversion. Azimuth-dependent traveltime variations on the input radial data were also significantly reduced at all record times, resulting in a postinversion radial full-azimuth stack having improved reflection continuity and temporal bandwidth. The data misfit (transverse component after inversion) potentially revealed local variations in shear-wave splitting not accounted for by the overburden layer-stripping correction.

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