Very-low-frequency (VLF) sound has significant potential for underwater detection and estimation of geoacoustic models of the ocean bottom structure. In marine settings, one type of VLF sound is the interface wave. These waves, trapped near the fluid–solid interface, are called Scholte waves, and this is the subject of this study. A field experiment was carried out in the South China Sea with the objective of exciting Scholte waves and investigating the propagation. The data were acquired by an ocean bottom seismometer, deployed on the seafloor. A large volume airgun array near the sea surface provided the sound source. The fundamental and three higher-order mode Scholte waves were excited. The Scholte waves are investigated by seismograms and a phase velocity inversion. The observed frequencies are in the range of 1.0–2.9 Hz. The energy attenuation is proportional to 1/r at the peak frequency 1.4 Hz. The shear wave speed structure, down to 600 m beneath the seafloor, is revealed from the dispersion curves by a least-squares inversion algorithm. The inversion result shows that the shear wave speed is below 300 m/s in the uppermost layer, which explains well the weak excitation of Scholte waves in this experiment.

For an acoustic receiver deployed at the bottom of the direct arrival zone of a submerged source at short horizontal ranges in deep ocean, the interference pattern of the direct and surface-reflected acoustic arrivals shows periodic modulation, which is directly related to the source depth, source frequency, and vertical arrival angle. In this work, the interference cycle presented in the frequency domain is used to extract the broadband source depth, with the vertical arrival angle obtained from the ratio of vertical acoustic intensity and horizontal acoustic intensity from the signal recorded by a single vector sensor. Experimental results demonstrate the source depth estimation without requiring knowledge of the ocean environment.

A B S T R A C TThe phase-shift-plus-interpolation and extended-split-step-Fourier methods are wavefield-continuation algorithms for seismic migration imaging. These two methods can be applied to regions with complex geological structures. Based on their unified separable formulas, we show that these two methods have the same kinematic characteristics by using the theory of pseudodifferential operators. Numerical tests on a Marmousi model demonstrate this conclusion. Another important aspect of these two methods is the selection of reference velocities and we explore the influence of the selection of reference velocities by comparing the geometric progression method and the statistical method. We show that the geometric progression method is simple but does not take into account the velocity distribution while the statistical approach is relatively complex but reflects the velocity distribution.