Recent studies have illustrated that the Multichannel Analysis of Surface Waves (MASW) method is an effective geoacoustic parameter inversion tool. This particular tool employs the dispersion property of broadband Scholte-type surface wave signals, which propagate along the interface between the sea water and seafloor. It is of critical importance to establish the theoretical Scholte wave dispersion curve computation model. In this typical study, the stiffness matrix method is introduced to compute the phase speed of the Scholte wave in a layered ocean environment with an elastic bottom. By computing the phase velocity in environments with a typical complexly varying seabed, it is observed that the coupling phenomenon occurs among Scholte waves corresponding to the fundamental mode and the first higher-order mode for the model with a low shear-velocity layer. Afterwards, few differences are highlighted, which should be taken into consideration while applying the MASW method in the seabed. Finally, based on the ingeniously developed nonlinear Bayesian inversion theory, the seafloor shear wave velocity profile in the southern Yellow Sea of China is inverted by employing multi-order Scholte wave dispersion curves. These inversion results illustrate that the shear wave speed is below 700 m/s in the upper layers of bottom sediments. Due to the alternation of argillaceous layers and sandy layers in the experimental area, there are several low-shear-wave-velocity layers in the inversion profile.

In research into various hydrophysical and hydroacoustic wave processes, it is extremely important to know the regularities of their propagation in the sea at decreasing depths, especially in the shelf areas, and also to know the regularities of their transformation into seismoacoustic processes in the earth crust. In the course of the processing and analysis of the experimental data of our complex experiment, in this paper we investigate these regularities. In our experiment, we used a low-frequency hydroacoustic transmitter that generated harmonic oscillations at the frequency of 22 Hz and received hydroacoustic systems with a shore laser strainmeter. It was established that hydroacoustic waves, propagating at the shelf of decreasing depth, transform into seismoacoustic waves at the depth of the sea equal to or less than a half-length of the hydroacoustic wave. A comparison of the results of this work with earlier-obtained results allows us to state that such regularities should be inherent to all hydrophysical and hydroacoustic processes.

This paper presents a modal-based geoacoustic inversion method adapted for a very-low-frequency leaky waveguide. It is applied to air gun data collected by a seismic streamer during the multi-channel seismic exploration experiment in the South Yellow Sea. The inversion is carried out by filtering the waterborne and bottom-trapped mode pairs from the received signal and comparing the modal interference features (waveguide invariant) to replica fields. The effective seabed models are inferred at two positions, and the two-way-travel time of basement interface reflected waves calculated using these models exhibit good agreement with geological exploration results.

Very-low-frequency (VLF) (in the range 1–100 Hz) sound propagation is significantly influenced by the properties of the seafloor, especially the presence of low velocity bottom (LVB) layers. During a VLF sound propagation experiment in the northwestern sub-basin of the South China Sea (about 3800 m), a distinctive acoustic multipath excited by a source near the sea surface was observed for the first time. This phenomenon is characterized by the arrival of low-frequency components preceding high-frequency components with bandpass effects around 30–60 Hz, as recorded by an ocean-bottom-seismometer that received signals from broadband explosive sources deployed at approximately 200 m. Numerical simulations reveal that this multipath is formed by the sediment borne mode in the LVB layer, which corresponds to the distinctive acoustic path that refracts in the water column and interacts with the LVB layer at a small grazing angle. Unlike previous studies in an isovelocity profile, the sediment borne mode oscillates with a certain amplitude distribution rather than exponentially decaying in the water column under the incomplete sound channel, leading to the observation of distinctive multipath even when the source is near the surface. Furthermore, through geoacoustic inversion, the sound velocity of the LVB layer aligns well with previous in situ measurements.

It is difficult to separate and estimate the intersected group velocity dispersion curves for different normal modes when the frequency is lower than the cutoff frequency of water column. To address this issue, an estimation method based on the joint processing of sound pressure (P) and vertical particle velocity (Vz) is proposed in this paper. Theoretical analysis shows that the amplitudes of P and Vz corresponding to the nth normal mode exhibit a complementary relationship in the certain frequency band, providing a theoretical basis for the method. The feasibility of the method was verified using sea trial data.

To investigate low-frequency acoustic propagation in a wedge-shaped ocean, two ocean bottom seismometers (OBSs) were deployed on the seabed and a land-based seismometer (LS) was positioned near the coastline. A broadband acoustic source (airgun) generated signals at a standoff distance. The OBSs captured forward-direct waves and backscattered waves, while the LS detected shore-coupled forward-direct arrivals. Spectral element modeling revealed a frequency-dependent propagation mechanism: High-frequency components (>200 Hz) of the normal modes exhibited strong backscattering from the seabed slope, while low-frequency components (<200 Hz) of the first normal mode coupled into the seabed sediment and propagated onshore as geoacoustic waves.