Converted-wave splitting estimation and compensation

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 [Formula: see text] and the amount of traveltime splitting [Formula: see text] that describe the overburden anisotropy. Spatially variable parameter estimates for the entire 3D data set, ([Formula: see text] and [Formula: see text]), 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.