Advanced surface-wave analysis for 3D ocean bottom cable data to detect localized heterogeneity in shallow geological formation of a CO₂ storage site

To characterize heterogeneous geological formation with high-resolution, we propose a surface-wave analysis method for estimation of the spatial variation of S-wave velocity and attenuation coefficients. We applied the proposed method to the 3D ocean bottom cable (OBC) seismic data acquired at the Tomakomai CO₂ storage site in Japan. By inversion of the observed dispersion curves of surface-wave (Scholte-wave in offshore survey), we successfully estimated the high-resolution pseudo-3D S-wave velocity structure for the shallow geological formation. We observed the dominant higher modes of the surface-wave, generated by the complex velocity structure such as inclusions of low-velocity layers. The interpretation based on the dispersion curves (or inverted S-wave velocities) and attenuation coefficients identified the lithological boundary in shallow formation across the 3D seismic survey area, but no fractures were detected. Our proposed method to estimate pseudo-3D S-wave velocity and attenuation structure can provide vital information for the CO₂ geological storage, as it provides useful data for evaluation of CO₂ leakage paths and estimation of permeability heterogeneity used in reservoir simulation. Moreover, it improves the accuracy of reflection seismic profile for deep structural analysis, and enhances estimations of lithology strength used for geomechanical modeling.