Scholte Wave Dispersion Modeling and Subsequent Application in Seabed Shear-Wave Velocity Profile Inversion

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.

Scholte wave inversion and passive source imaging with ocean-bottom DAS

Geotechnical characterization of marine sediments remains an outstanding challenge for offshore energy development, including foundation design and site selection of wind turbines and offshore platforms. We demonstrate that passive distributed acoustic sensing (DAS) surveys offer a new solution for shallow offshore geotechnical investigation where seafloor power or communications cables with fiber-optic links are available. We analyze Scholte waves recorded by DAS on a 42 km power cable in the Belgian offshore area of the southern North Sea. Ambient noise crosscorrelations converge acceptably with just over one hour of data, permitting multimodal Scholte wave dispersion measurement and shear-wave velocity inversion along the cable. We identify anomalous off-axis Scholte wave arrivals in noise crosscorrelations at high frequencies. Using a simple passive source imaging approach, we associate these arrivals with individual wind turbines, which suggests they are generated by structural vibrations. While many technological barriers must be overcome before ocean-bottom DAS can be applied to global seismic monitoring in the deep oceans, high-frequency passive surveys for high-resolution geotechnical characterization and monitoring in coastal regions are easily achievable today.

Parabolic Equation Modeling of Scholte Waves and Other Effects Along Sloping Fluid-Solid Interfaces

Several methods for handling sloping fluid–solid interfaces with the elastic parabolic equation are tested. A single-scattering approach that is modified for the fluid–solid case is accurate for some problems but breaks down when the contrast across the interface is sufficiently large and when there is a Scholte wave. An approximate condition for conserving energy breaks down when a Scholte wave propagates along a sloping interface but otherwise performs well for a large class of problems involving gradual slopes, a wide range of sediment parameters, and ice cover. An approach based on treating part of the fluid layer as a solid with low shear speed is developed and found to handle Scholte waves and a wide range of sediment parameters accurately, but this approach needs further development. The variable rotated parabolic equation is not effective for problems involving frequent or continuous changes in slope, but it provides a high level of accuracy for most of the test cases, which have regions of constant slope. Approaches based on a coordinate mapping and on using a film of solid material with low shear speed on the rises of the stair steps that approximate a sloping interface are also tested and found to produce accurate results for some cases.

An Investigation of Silica Aerogel to Reduce Acoustic Crosstalk in CMUT Arrays

This paper presents a novel technique to reduce acoustic crosstalk in capacitive micromachined ultrasonic transducer (CMUT) arrays. The technique involves fabricating a thin layer of diisocyanate enhanced silica aerogel on the top surface of a CMUT array. The silica aerogel layer introduces a highly nanoporous permeable layer to reduce the intensity of the Scholte wave at the CMUT-fluid interface. 3D finite element analysis (FEA) simulation in COMSOL shows that the developed technique can provide a 31.5% improvement in crosstalk reduction for the first neighboring element in a 7.5 MHz CMUT array. The average improvement of crosstalk level over the −6 dB fractional bandwidth was 22.1%, which is approximately 5 dB lower than that without an aerogel layer. The results are in excellent agreement with published experimental results to validate the efficacy of the new technique.

Analytical solution to Scholte’s secular equation for isotropic elastic media

In terms of a method based on Cauchy integrals, we have obtained a robust analytic expression to predict a unique physical solution for the Scholte slowness in all range of possible elastic and isotropic media. Proper analysis of the discontinuities of the secular Scholte equation allows the identification of the velocity of the evanescent wave in one of three possible regimes. When the liquid phase tends to vanish, it was observed: a) the Rayleigh wave solution or the free surface limit, and b) the rarefied fluid medium limit, where there exists a gradual extinction of the Scholte wave as both the density and velocity of the fluid decrease. In general terms, the results show that the propagation speed of a Scholte wave is less than or equal to that of a Rayleigh wave.

IDENTIFICATION OF NORMAL WAVES FORMING A SOUND FIELD IN A SHALLOW SEA AT THE INFRASONIC FREQUENCY RANGE. Part II

Анализируются результаты экспериментальных исследований звукового поля, зарегистрированного комбинированными приемниками, образующими вертикально ориентированную двухэлементную антенну. Звуковое поле формировалось дискретными составляющими вально-лопастного звукоряда шумового сигнала НИС «Юрий Молоков» в инфразвуковом диапазоне частот 2–20 Гц, а также буксируемым низкочастотным излучателем полигармонического сигнала в диапазоне частот 30–60 Гц. Глубина моря и рабочий диапазон частот 2–20 Гц исключали возможность возбуждения нормальных волн дискретного спектра в модельном волноводе Пекериса в этом диапазоне частот. По результатам спектрального анализа шумового сигнала получена оценка потенциальной помехоустойчивости комбинированного приемника при использовании полного набора информативных параметров, характеризующих энергетическую структуру звукового поля. По результатам анализа вертикальной структуры звукового поля в инфразвуковом диапазоне частот был сделан вывод о том, что звуковое поле сформировано неоднородными нормальными волнами Шолте, регулярной и обобщенной (гибридной). В дальней зоне источника доминирует регулярная волна Шолте, локализованная на границы раздела вода – морское дно. В ближней зоне источника возрастает роль обобщенной волны Шолте, локализованной на горизонте источника, а звуковое поле формируется парой волн Шолте, регулярной и обобщенной. The results of experimental studies of the sound field recorded by combined receivers forming a vertically oriented two-element antenna are analyzed. The sound field was formed by discrete components of the vane-blade scale of the noise signal of the science ship «Yuri Molokov» in the infrasonic frequency range of 2–20 Hz, as well as by a towed low-frequency emitter of a polyharmonic signal in the frequency range 30–60 Hz. The depth of the sea and the operating frequency range of 2–20 Hz excluded the possibility of exciting normal waves of the discrete spectrum in the model Pekeris waveguide in this frequency range. Based on the results of spectral analysis of the noise signal, an estimate of the potential noise immunity of the combined receiver was obtained using a full set of informative parameters characterizing the energy structure of the sound field. Based on the results of the analysis of the vertical structure of the sound field in the infrasonic frequency range, it was concluded that the sound field is formed by inhomogeneous normal Scholte waves, regular and generalized (hybrid). In the far zone of the source, a regular Scholte wave dominates, localized at the water – seabed interface. In the near-field zone of the source, the role of the generalized Scholte wave localized at the source horizon increases, and the sound field is formed by a pair of Scholte waves, regular and generalized.

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

The shear-wave (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 have applied the multicomponent Scholte-wave analysis technique to an active-source shallow marine seismic profile in the East China Sea. Scholte waves have been excited by shots from a 5450 inch3 air-gun array and their recordings have been conducted 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 are used for Scholte-wave analysis. Raw sensor CRGs are rotated to the inline, crossline, and vertical coordinate system. The rotated tilt and roll angle are directed using the inner electric compass log value, and the shot inline azimuth is estimated using the particle motion method. Then, the velocity spectra are calculated from the inline and vertical components using the phase-shift method. Higher Scholte-wave modes dominate on the horizontal components, whereas the stronger fundamental mode dominates on the vertical components. The multicomponent velocity spectrum stacking method is adopted to produce the final dispersion energy image. Up to four modes of dispersion curves are retrieved within the 1.1–4.3 Hz frequency band. The multimode dispersion curve inversion is constructed for imaging the shallow sediments. The results suggest a low [Formula: see text] of 180–650 m/s and few 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 multicomponent seismic exploration data processing. The use of OBNs has high feasibility in [Formula: see text] imaging for shallow marine sediments when combined with the Scholte-wave dispersion-curve inversion.