Joint velocity building and imaging of compressional and shear waves: application to gas cloud imaging

Abstract The Cenozoic formations in the Sanhu area from the Qaidam Basin bear enormous biogas resources. Subsurface accumulation of gas causes severe P-wave distortion (gas clouds). A multi-component seismic survey was performed to resolve the “gas clouds” issue. We present a case study of joint depth-domain imaging using PP- and SH–SH-waves. First, a joint static correction is performed by using tomographic inversion of PP- and SH–SH-waves. By considering the relatively weak influence of thin layers on long wavelength waves, band-pass filtering solved the discontinuity issue of SH–SH first breaks. Second, different types of noise are observed in both wave datasets. We applied linear-noise suppression and multi-window, multi-channel median filtering to significantly attenuate surface waves and their associated noise in the PP-wave dataset; while the Radon transform is used to attenuate multiples in the SH–SH-wave dataset. Third, we proposed a joint velocity-building approach to improve imaging consistency between the PP- and SH–SH-waves. Specifically, for PP-waves, the initial velocity model was established by integrating the first-break tomographic velocity, transformed P-wave velocity from tomographic S-wave velocity, and the interval velocity converted from root mean square velocity. Reflection travel-time tomography generated the final velocity models. Finally, true-surface Kirchhoff pre-stack depth migration delivers structural consistent PP- and SH–SH-wave images, providing reliable data for further comprehensive interpretation and inversion in this region. The case study demonstrates that the proposed joint PP- and SH–SH-wave velocity building and imaging workflow is effective and efficient in imaging the gas cloud area, which cannot be imaged by PP-wave alone. Therefore, further investigation into multi-wave, multi-component seismic exploration is a valuable research direction.