Shear-wave velocity structure of the shallow sediments in the Bohai Sea from an ocean-bottom-seismometer survey

Abstract

To investigate the seismic velocity structure of the shallow sediments in the Bohai Sea of China, we conducted a shear-wave velocity inversion of the surface-wave dispersion data from a survey of 12 ocean-bottom seismometers (OBSs) and 377 shots of a [Formula: see text] air gun. With OBS station spacing of approximately 5 km and air-gun shot spacing of approximately 190 m, high-quality Scholte-wave data were recorded by the OBSs within 0.4–5 km offset. We retrieved the Scholte-wave phase-velocity dispersion for the fundamental mode and first overtone in the frequency band of 0.9–3.0 Hz with the phase-shift method and inverted for the shear-wave velocity structure of the shallow sediments with a damped iterative least-squares algorithm. Pseudo-2D shear-wave velocity profiles with a depth of approximately 400 m revealed coherent features of relatively weak lateral velocity variation. We also estimated the uncertainty in shear-wave velocity structure based on the pseudo-2D profiles from six trial inversions with different initial models, which suggested a velocity uncertainty less than [Formula: see text] for most parts of the 2D profiles. The layered structure with little lateral variation may be attributable to the continuous sedimentary environment in the Cenozoic sedimentary basin of the Bohai Bay basin. The shear-wave velocity of 200–300 [Formula: see text] in the top 100 m of the Bohai seafloor may provide important information for offshore site response studies in earthquake engineering. Furthermore, the very low shear-wave velocity structure (150–600 [Formula: see text]) down to 400 m depth could produce a significant traveltime delay of approximately 1 s in the shear-wave arrivals, which needs to be considered to avoid serious bias in shear-wave traveltime tomographic models.

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.…”

“…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. The initial geophysical model is constructed as an isotropic homogenous water-sedimentary layered model with 40 total layers.…”

“…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.…”

Electroseismic Scholte‐wave analysis: A potential method for estimating shear‐wave velocity structure of shallow‐water seabed sediments

“…In addition, reliable shallow shear‐wave velocities can improve the imaging of the deep crust and upper mantle structures (e.g. Li et al., 2014; Wang et al., 2016). An effective tool for evaluating high‐resolution shear‐wave velocity structure is Scholte‐wave analysis.…”