Seismic imaging of S-wave structures of shallow sediments in the East China Sea using OBN multicomponent Scholte-wave data

Abstract

The S-wave structures of shallow marine sediments are important for offshore geotechnical studies, deep crustal S-wave imaging, multicomponent seismic exploration, and underwater acoustics studies. We apply multi-component Scholte-wave analysis technique to an active-source shallow marine seismic profile in the East China Sea. Scholte waves were excited by a 5450 inch3 air-gun array shots and recorded at the seafloor using Ocean Bottom Nodes (OBNs). First, we extract the common receiver gathers (CRGs) and correct for the time drift simultaneously using a forward and inverse Fast Fourier Transform resampling algorithm. Three CRGs of seismic sensors were used for Scholte-wave analysis. Raw sensors CRGs were rotated to the inline, crossline and vertical coordinate system. The rotated tilt and roll angle were directed using the inner electric compass log value, and the shot inline azimuth was estimated using the particle motion method. The velocity spectra were then calculated from the inline and vertical components using the phase-shift method. Higher Scholte-wave modes dominated on horizontal components, while the stronger fundamental mode dominated on the vertical component. The multi-component velocity spectrum stacking (MVSS) method was adopted to produce the final dispersion energy image. Up to four modes of dispersion curves were retrieved within the 1.1–4.3 Hz frequency band. Multi-mode dispersion curves inversion was constructed for imaging the shallow sediments. The results suggest low Vs of 180–650 m/s, and little lateral variations within the top 0.5 km of shallow marine sediments in the East China Sea. This model can provide an important reference for offshore geotechnical investigations, especially for OBN multi-component seismic exploration data processing. The use of OBNs shows high feasibility in Vs imaging for shallow-marine sediments when combined with the Scholte-wave dispersion curve inversion.

Imaging the Full Dispersive Modes of Scholte waves by Adding Independent SGP Processing to Eliminate the Truncation Effects of Dispersive Guided-P waves

“…Most importantly, the maximum number of higher modes involved in the inversion can effectively improve the inversion accuracy (Figure S1a), increase the resolution of the models (Luo et al, 2007), significantly improve the nonuniqueness and convergence of the inversions (Wu et al, 2020), and "see" deeper (Xia et al, 2003). Wang et al (2020) demonstrate that four-mode Scholte wave dispersion curve inversion can decrease errors by 4.1 times and increase inversion depth by 0.7 times compared with twomode inversion for the same OBN station. Similarly, the wider frequency range of dispersion curves for a single mode was included in the inversion, and the shallow marine imaging results were improved (Figure S1b).…”

Estimating the Shear-Wave Velocities of Shallow Sediments in the Yellow Sea Using Ocean-Bottom-Seismometer Multicomponent Scholte-Wave Data

“…During the iterations, only v s is considered as a free parameter, v p and h are fixed, and ρ is related to v s via the empirical relation ρ (g/cm 3 ) 0.8 log(v s ) + 0.23, with units of m/s (Herrmann, 2013). This inversion strategy has been proven reliable for the inversion of Scholte-wave dispersion curves (Wang et al, 2016;Wang et al, 2020). Wang et al (2016) gave details on how the initialized model parameters affect the final fitting results.…”

“…Imaging suboceanic shear-wave velocity (V s ) structures is fundamental in the geophysical investigations of marginal seas (Ewing et al, 1992;Klein et al, 2005;Kugler et al, 2007;Wang et al, 2016;Wang et al, 2020). The V s for shallow marine sediment is treated as an important parameter in many marine engineering construction activities, such as offshore platforms, wind parks, and pipelines.…”

“…The single vertical component of OBS active source Scholtewave data was used to investigate the V s structure of shallow marine sediments (Shtivelman, 2003;Shtivelman, 2004;Bohlen et al, 2004;Klein et al, 2005;Kugler et al, 2005;Kugler et al, 2007;Nguyen et al, 2009;Socco et al, 2011;Vanneste et al, 2011;Dong et al, 2012;Wang et al, 2016). Wang et al (2020) noted that the three-component Scholte-wave dispersive analysis can extract additional higher-order modes compared with using a single vertical component. Compared with the fundamentalmode inversion, joint higher-order modes can effectively increase the inversion depth and resolution (Xia et al, 2003;Luo et al, 2007).…”

“…Compared with the fundamentalmode inversion, joint higher-order modes can effectively increase the inversion depth and resolution (Xia et al, 2003;Luo et al, 2007). Wang et al (2020) demonstrated that the joint inversion of the four-mode dispersion curves can decrease the maximum inverted error by a factor of 16 compared with one fundamental mode inversion. However, the possibility of and method to image higher-order dispersive modes from the fourcomponent (4C with three seismic components and one hydrophone component) OBS Scholte-wave data remain unknown and are worth exploring.…”