Application of horizontal-to-vertical spectral ratios in active-source OBN multi-component Scholte waves for S-wave velocity inversion in shallow seabed sediments

The Shear (S)-wave velocity structure of seabed sediments is crucial for various marine applications, including marine multi-component seismic exploration, marine engineering, and prevention of submarine geological disasters. The horizontal-to-vertical spectral ratios (HVSR) method offers some advantages such as efficiency, strong interference robustness, and sensitivity to S-wave velocity of sediments, etc. We introduce an application of the HVSR method to multi-component seismic data from active-source ocean bottom nodes (OBN) in the East China Sea. Given the unique acquisition characteristics of OBNs, preprocessing of multi-component OBN data is required, including relocation, azimuth rotation, filtering, and other procedures. As higher-order mode Scholte waves influence HVSR values, the methodology involves initially separating multi-mode Scholte waves from the inline and vertical velocity components of OBN data to obtain fundamental mode Scholte waves seismic records. Subsequently, the HVSR of the fundamental mode Scholte waves is calculated. Finally, Monte Carlo stochastic inversion method is conducted to invert the S-wave velocity of shallow sediments beneath the OBN survey line. The inversion results show low S-wave velocity values from 200 to 750 m/s and few lateral variations within the top 0.5 km of shallow marine sediments. Additionally, the S-wave velocity inverted using the HVSR method is slightly greater than that inverted using the Scholte wave dispersion method, but it is closer to the S-wave velocity inverted using converted wave travel-time inversion. The successful application of the active-source OBN data in East China Sea proves the effectiveness and practicality of the HVSR method for shallow subsurface S-wave velocity inversion beneath the seafloor.