Abstract
Scholte-wave dispersion analysis is effective at imaging the relatively low shear-wave velocity of shallow marine sediments in marginal seas. The combination of a four-component ocean-bottom-seismometer (OBS) and a towed air-gun source can economically and effectively acquire the marine dispersive seismic data. Extracting higher-order dispersive Scholte wave modes is the most critical problem in the dispersion analysis method. The extremely low shear-wave velocity and severe attenuation in the top hundreds of meters of marginal sea sediment provide an uneven dispersive energy distribution for the four components of the Scholte wave data. The fundamental mode dispersive energy dominates in the vertical component and higher-order modes dominate in the horizontal component. We developed the method of the four-component OBS Scholte velocity-spectra stacking, which can effectively, rapidly, and robustly extract higher-order modes. We imaged the shear-wave velocity structure of complicated shallow marine sediment in the North Yellow Sea using an active OBS seismic profile with a large-volume air-gun array. The fourth higher-order Scholte wave mode can be imaged with the four-component velocity-spectra stacking method with a lower frequency range of 1.0–7.0 Hz. Only the second-order mode can be recognized from the dispersion energy image of the single vertical component. The joint inversion of multimode dispersion curves can provide more accuracy and deeper constraints for the inverted model; thus, the constraint depth with five modes increases by a factor of 1.9 compared with single fundamental mode inversion. The inverted profile suggests a low shear-wave velocity of 123–670 m/s and strong lateral variations within 350 m. The main regional geological structures are shown by the inverted shear-wave velocity structure.
Highlights
Imaging suboceanic shear-wave velocity (Vs) 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)
4C OBS Scholte-Wave Dispersion Analysis maximum of five modes, five groups extracted a maximum of four modes, 13 groups extracted a maximum of three modes, four groups extracted a maximum of two modes, and one group extracted a maximum of one mode
All 26 of the 1D shear-wave velocity structures were inverted from the multimode dispersion curves
Summary
Imaging suboceanic shear-wave velocity (Vs) 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). 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. Compared with the single-vertical-component DEI method, this procedure either increases the number of recognizable higherorder modes or widens the selectable frequency range of the dispersion curves for higher-order modes. To determine the deepest constraint depth during inversion, we calculate the sensitivity kernels for the five modes of the Scholte-wave dispersion curves based on the initial geophysical model. To illustrate how much the Scholte-wave phase velocity changes due to the density alone, another group of predicted dispersion curves (green solid line in Figure 10C) were calculated based on the inverted density and the initial vs profile. These simple statistical results demonstrate that for the same frequency, the constraint depth of the fourth higher-order mode is 2.94 times that of the
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