Scholte Waves Research Articles

The sounds of submarine volcanoes

When submarine volcanoes erupt, several processes can create sounds in the ocean, mostly at low frequencies <100 Hz. Explosions may occur directly in the water column, while earthquakes and other seismicity may produce seismic waves that convert into hydroacoustic waves or boundary (Scholte) waves. Volcanic sounds can propagate large distances through the SOFAR channel. During its 2014 eruption, Ahyi seamount, Northern Mariana Islands produced repetitive signals for approximately 2 weeks at a high rate. These likely explosions were widely recorded on seismometers throughout the region and on hydrophone arrays as far as Chile, ∼12,000 km distant. Bogoslof volcano, a shallow submarine volcano in the Aleutian Islands, Alaska, began erupting in December 2016. Many of the detected earthquakes associated with this eruption have large amplitude hydroacoustic phases, likely Scholte waves. A few earthquake swarms were recorded as converted hydroacoustic waves by seismometers on Tanaga volcano, ∼700 km away. A few eruption sequences were also detected on the Tanaga stations, one of which included a monochromatic glide, suggesting efficient transmission of energy into the water column. A single hydrophone deployed near Bogoslof several months after the eruption began may add evidence for how eruptions of submarine volcanoes contribute to the underwater soundscape.

Bayesian inference and model selection to investigate seabed shear-wave velocity profiles via interface-wave dispersion inversion

This paper applies a general geoacoustic profile parameterization based on Bernstein polynomial basis functions to consider the form of seabed shear-wave velocity profiles via Bayesian inversion of interface (Scholte) wave dispersion. Theory and observations indicate that the shear-wave velocity profile of seabed sediments of uniform composition and density often corresponds (approximately) to a power law function of depth. Hence, past inversions have often been based on a power-law parameterization without considering independently if this form is actually required by the data. The Bernstein parameterization, based on a linear combination of Bernstein-polynomial basis functions with the polynomial order determined by the Bayesian information criterion, is general and allows the form of the profile to be determined by the data, rather than by a subjective prior choice. In this paper, measured-data inversions are compared for power-law, Bernstein-polynomial, and layered trans-dimensional parameterizations to investigate the shear-wave velocity profile form.

Seismoelectric responses to an explosive source in a fluid above a fluid‐saturated porous medium

AbstractWe numerically compute seismoelectric wavefields generated at a fluid/porous medium interface by an explosive source in the fluid. Our numerical experiments show that electromagnetic (EM) signals accompanying the P, S, and interface waves can be observed at receivers located in the fluid regions near the interface. Such accompanying EM signals are produced by the inhomogeneous EM waves that are generated by the seismic waves at the interface and their amplitudes decrease with the distance from interface. Under the excitation of an explosive source whose strength is within the capability of industry air guns, electric and magnetic fields that accompany the Scholte wave are on the order of 1 μV/m and 0.01 nT, respectively. This means that the EM signals arising from the electrokinetic effect at an ocean bottom are detectable and suggest that it is possible to measure the EM signals during marine seismic explorations to study the properties of the seafloor material. EM signals that accompany the P, S, and interface waves are also observed in the porous medium region near the interface. Component analysis shows that they contain contributions from multiple modes of waves, among which the slow compressional wave contributes significantly to the vertical electric field, leading to a much stronger vertical electric field than the horizontal electric field during the passage of a seismic wave along the interface.

Interaction between lithotripsy-induced surface acoustic waves and pre-existing cracks

The interaction between pre-existing cracks and surface acoustic waves (SAW) in lithotripsy is investigated. Surface acoustic waves are generated at a water-glass interface by an incident shock wave produced by the spark discharge of a nano pulse lithotripsy (NPL) device or an electromagnetic shock wave lithotripsy (SWL) source. Evidence of SAW, including leaky Rayleigh wave and Scholte wave, will be presented based on photoelastic imaging and numerical stimulations using COMSOL. A clear correlation between SAW and the location of the maximum tensile stress produced on the glass boundary has been identified, which can lead to ring-like fractures on a flat glass surface exposed to NPL-generated spherically divergent shock waves. To simulate cavitation-induced surface pitting in SWL, pre-existing crack will be introduced on the glass surface by microindentation using a Vickers or Knoop indenter. The interaction of SAW with the pre-existing cracks will be examined to characterize crack extension and branching as a function of their location and orientation to the incident shock wave.

Radial anisotropy in Valhall: ambient noise-based studies of Scholte and Love waves

Radial anisotropy in Valhall: ambient noise-based studies of Scholte and Love waves

Design, fabrication and testing of surface micromachined CMUTs for surface and interface waves

Interdigital CMUTs weredesigned and fabricated in transmit/receive pairs. Each transmitter or receiverhas 20 double-phase finger pairs with 146 micrometer pitch and 3 millimeter aperture.The wave transmission distance between the receiver and transmitter is 9millimeters. Devices were fabricated on the highly doped silicon wafers by thesurface micromachining technology using silicon nitride as the structuralmaterial and chromium as the sacrificial material. Interface (Scholte type)waves were transmitted and received at 100 V bias and 10 V pp 10 MHz 20 periodharmonic burst excitation. The wave propagation velocity in water andisopropanol were measured by the time-of-flight and spectral analysis methods.The last gave 1376 m/s and 1157 m/s phase velocity of the Scholte waves for water andisopropanol, correspondingly. At the same time, CMUT structural resonance inimmersion was 8.9 MHz for water and 9.3 MHz for isopropanol. This led to theconclusion about better informativity of the Scholte type waves. DOI: http://dx.doi.org/10.5755/j01.eie.22.5.7123

Tsunami generation and associated waves in the water column and seabed due to an asymmetric earthquake motion within an anisotropic substratum

AbstractIn this paper, closed‐form integral expressions are derived to describe how surface gravity waves (tsunamis) are generated when general asymmetric ground displacement (due to earthquake rupturing), involving both horizontal and vertical components of motion, occurs at arbitrary depth within the interior of an anisotropic subsea solid beneath the ocean. In addition, we compute the resultant hydrodynamic pressure within the seawater and the elastic wavefield within the seabed at any position. The method of potential functions and an integral transform approach, accompanied by a special contour integration scheme, are adopted to handle the equations of motion and produce the numerical results. The formulation accounts for any number of possible acoustic‐gravity modes and is valid for both shallow and deep water situations as well as for any focal depth of the earthquake source. Phase and group velocity dispersion curves are developed for surface gravity (tsunami mode), acoustic‐gravity, Rayleigh, and Scholte waves. Several asymptotic cases which arise from the general analysis are discussed and compared to existing solutions. The role of effective parameters such as hypocenter location and frequency of excitation is examined and illustrated through several figures which show the propagation pattern in the vertical and horizontal directions. Attention is directed to the unexpected contribution from the horizontal ground motion. The results have important application in several fields such as tsunami hazard prediction, marine seismology, and offshore and coastal engineering. In a companion paper, we examine the effect of ocean stratification on the appearance and character of internal and surface gravity waves.

Generation of different types of surface acoustic waves by shock wave-Stone interaction

The generation of different types of surface acoustic waves (SAWs) in lithotripsy is investigated by numerical simulations using COMSOL, considering either the case of a weakly focused shock wave as an incident plane wave (in the focal plane) in shock wave lithotripsy (SWL) or a spherically divergent shock wave produced by a spark discharge in Nano Pulse Lithotripsy (NPL). The interaction of these two types of shock waves with an artificial kidney stone immersed in water may generate three different types of SAWs: namely, Scholte wave, acoustic surface evanescent wave (ASEW), and leaky Rayleigh wave (LRW). In particular, we examined the generation of SAWs on a flat, cylindrical, or spherical surface under different incident angles. It was found that the geometry and acoustic properties of the stone could significantly influence the types of SAWs generated and the resultant peak tensile stress produced at the stone-water boundary. Comparison with experimental measurements (i.e., Schlieren and photoelastic imaging) and stone fracture will also be made. [Work supported by NIH through 5R37DK052985-20.]

Acoustic and seismic time series analysis using ensemble empirical mode decomposition

Empirical mode decomposition (EMD) is an adaptive or data-driven time series analysis technique ideally suited to investigate non-stationary signals. EMD decomposes the signal into fast and slow oscillations called intrinsic mode functions (IMFs). Ensemble EMD (EEMD) methods have been developed to alleviate the mode-mixing phenomenon present in the EMD technique. Various real signals will be analyzed using these methods to investigate whether the IMFs correlate to acoustic modes. These real signals include underwater acoustic signals from broadband sources, Scholte and Rayleigh wave signals and music signals. These underwater acoustic signals and the seismic signals exhibit multi-modal structure. In addition these modes are also dispersive in nature. Accurate resolution of these modes in the time-frequency plane is critical for the estimation of medium properties via inverse schemes. Finally, the EEMD technique will be used to investigate the complex dynamic structure of the pitching structure in South Indian Classical music. In all these cases investigated the mapping of the signal into the time-frequency plane reveal distinct features representing the modal dispersion and pitching structure, respectively. EEMD will be used to extract these features accurately with high resolution. [Work supported by Office of Naval Research.]

Some new observations on the underwater sound field from impact pile driving

The underwater sound field from impact pile driving has been studied in recent years, motivated largely by the high levels and impulsive character of the sound and potential impacts on marine life. Here we report on new observations made on a vertical line array (VLA) and neutrally buoyant geophone (vector sensor) both positioned 120 m from the pile, to inform future modeling and mitigation efforts. These observations include: (1) a ground-wave precursor of amplitude 200 Pa, or 10% of peak pressure (2) delayed arrival structure in the impulsive time series due to injection of airborne sound from the hammering process, and (3) properties of the Scholte wave as a function of receiver depth. The study originates from Puget Sound, WA, and involves a 30-cm diameter steel pile installed with impact hammer energy ~200 kJ. Water depth at pile was 9 m, gradually increasing to 17 m at the VLA range. Pile depth was recorded over the sequence of 300 pile strikes, and this knowledge is essential to interpret these observations as the pile was driven 12 m into the sediment. [Research supported by Washington State Dept. of Transportation with partial support from ONR.]

Scholte-wave propagation for near-surface soil stiffness profiling

This paper presents the feasibility of using Scholte waves by spectral-analysis-of-surface-waves (SASW) method for near-surface soil stiffness profiling. Scholte-wave measurement does not require ground coupling thus fast SASW can be performed. Scholte wave propagation at the air-to-soil interface was analyzed first by Finite Element (FE) simulation. In the FE model, a coupled acoustic-structural model was employed to simulate Scholte-wave propagation and acoustic wave contamination were controlled by sensor insulation. Field testing at several compaction sites based on FE results was performed. We found that the dispersion curve of Scholte waves agrees well with Rayleigh-wave dispersion curve if sensors are perfectly insulated. Higher phase velocity is observed when Scholte waves are interrupted by acoustic wave. However, 1 cm opening at the bottom of insulated microphone can be considered as perfect insulation.

Estimation of shear wave properties using Scholte wave inversions

Shear speeds in semi-consolidated and consolidated shallow water sediments can significantly impact compressional wave attenuation and arrival times of acoustic normal modes. One of the most promising approaches to estimate shear speed is to invert the shear speed profile using the dispersion of interface waves (Scholte waves). The propagation speed of the Scholte waves is closely related to the shear wave speed over a depth of 1–2 wavelengths into the seabed. A geophone system for the measurement of these interface waves, along with an inversion scheme that inverts the Scholte wave dispersion data for sediment shear speed profiles and shear attenuation has been developed. A forward model based on the dynamic stiffness method has been developed and implemented in the inversion. A new Interface Wave Sediment Profiler (iWaSP) is being currently developed for the measurement of bottom properties such as shear speed and attenuation in the top 1–2 m of a variety of sediment types (including mud) with a wideban...

Lamb-type waves generated by a cylindrical bubble oscillating between two planar elastic walls

The volume oscillation of a cylindrical bubble in a microfluidic channel with planar elastic walls is studied. Analytical solutions are found for the bulk scattered wave propagating in the fluid gap and the surface waves of Lamb-type propagating at the fluid-solid interfaces. This type of surface wave has not yet been described theoretically. A dispersion equation for the Lamb-type waves is derived, which allows one to evaluate the wave speed for different values of the channel height h. It is shown that for h<λt, where λt is the wavelength of the transverse wave in the walls, the speed of the Lamb-type waves decreases with decreasing h, while for h on the order of or greater than λt, their speed tends to the Scholte wave speed. The solutions for the wave fields in the elastic walls and in the fluid are derived using the Hankel transforms. Numerical simulations are carried out to study the effect of the surface waves on the dynamics of a bubble confined between two elastic walls. It is shown that its resonance frequency can be up to 50% higher than the resonance frequency of a similar bubble confined between two rigid walls.

Dealing with spatial sampling sparseness and irregularity in 3D OBC seismic data offshore Abu Dhabi

Due mainly to commercial and operational constraints, seismic data are often sparsely and irregularly sampled, leading to several challenges in processing of 3D OBC seismic data offshore Abu Dhabi. Conventional linear noise attenuation techniques based on multi-channel filters are not effective with Scholte waves as they are usually aliased with typical sampling intervals in 3D OBC seismic data (e.g., 25 m source and receiver point intervals), and sometimes scattered because of near-surface heterogeneity. To address them, we apply model-based surface wave attenuation, Surface Wave Analysis Modelling and Inversion (SWAMI), which enables an estimate of local near-surface properties by analysing dispersion curves. Thus, both direct and scattered Scholte waves are effectively modelled and attenuated without suffering from under-sampling. A data interpolation and regularization technique called Matching Pursuit Fourier Interpolation (MPFI) is then applied to enhance spatial sampling. MPFI employs an anti-aliasing capability so optimum data reconstruction can be performed for any frequency range. In addition to the regularization aspect, MPFI with a 5D implementation (4 spatial coordinates and time) is targeted to densify receiver line interval and extend source lines, which consequently enhances fold, offset and azimuth distributions of the data.The implementation of the two techniques successfully addresses processing challenges in sparsely and irregularly sampled OBC seismic data.

Study of laser-generated interfacial waves and their interaction with a defect at solid-fluid interface

The analysis of the pulsed laser generation of interfacial waves propagating along a plane solid-fluid interface and their interaction with a defect (notch) on the interface are presented. The solid-fluid interface motion due to laser-generated interfacial waves including leaky Rayleigh and Scholte wave is simulated by finite element method for two configurations, one is “hard solid-fluid” interface, like steel-water, the other one is “soft solid-fluid” interface, like Plexiglas-water. And then their interaction with a notch-like defect in the interface has also been studied. Two series defects are introduced into the interface. One is with same width but different depth, and the other one with the same depth but different width. For hard and soft solid-fluid configurations, leaky Rayleigh and Scholte wave is severely affected by the notch, respectively. In contrast, Scholte and leaky Rayleigh wave experiences almost no influence for hard and soft solid-fluid interface. The results may lead to an application for defect identification and nondestructive materials characterization.

The effect of sediment content to fluid - solid interface time-domain waveform

A model was established for the fluid - solid interface of semi-infinite medium, and the characteristic equation was established using the potential function. Applying line source at the fluid-solid interface, transform solution of displacement could be obtained, and time-domain solution could be evaluated through the inverse Fourier transform. Two kinds of ultrasound suspension model, UA and HT, were used to study and discuss the effect of sediment content to fluid-solid interface time-domain waveform. The results of two models have a good consistency. The sediment content have a great impact on the Scholte wave. With the increase of the sediment content, for Scholte wave, the velocity increases and the attenuation decreases.

Innovative Processing Approaches to Overcome Sampling Sparseness and Irregularity in 3D OBC Seismic Data Offshore Abu Dhabi

Due mainly to commercial and operational constraints, seismic data are often sparsely and irregularly sampled, leading to several challenges in processing of 3D OBC seismic data offshore Abu Dhabi. Conventional linear noise attenuation techniques are not effective with Scholte waves as they are usually aliased with typical sampling interval in 3D OBC seismic data, and sometimes scattered because of near-surface heterogeneity. To address this, we apply model-based surface wave attenuation, Surface Wave Analysis Modeling and Inversion (SWAMI), which enables an estimate of local near-surface properties by analysing dispersion curves. Thus, both direct and scattered Scholte waves are effectively modelled and attenuated without suffering a lack of spatial sampling. We also highlight the shortcomings of the application of interferometry to scattered noise attenuation for sparse acquisition geometry. Matching Pursuit Fourier Interpolation (MPFI) is then implemented to deal with insufficient sampling in crossline direction caused by acquisition geometry. MPFI employs anti-aliasing capability so optimum data reconstruction can be performed for any frequency ranges. In addition to regularization aspect, MPFI with 5D implementation (4 spatial coordinates and time) is targeted to densify receiver line interval and extend source lines, which consequently enhances fold, offset and azimuth distributions of the data.

Ambient seismic noise tomography at Ekofisk

We have studied an approximately 40-hr recording from the Life of Field Seismic array installed over Ekofisk field to assess whether the recorded ambient seismic energy was suitable for passive seismic surface-wave interferometry. Passive seismic interferometry aims to retrieve virtual seismic sources by crosscorrelation. We have found that the noise recorded by the pressure sensors between 0.4 and 1.4 Hz consisted predominantly of Scholte-wave microseism energy. The noise incidence directions have an almost uniform distribution over the azimuth, enabling the synthesis of nearly symmetric estimated Green’s functions between all stations in the array that formed virtual seismic sources. These sources were dominated by Scholte waves traveling along the seafloor, and we sought to determine which features in the subsurface can be imaged by straight-ray group-velocity tomography. We located a high-velocity anomaly in the center of Ekofisk’s production-induced subsidence bowl, surrounded by lower velocities. This pattern seemed to result from production-induced seafloor subsidence that altered the near-surface shear strengths. A dispersion analysis showed that the Scholte-wave virtual seismic source exhibited an approximate penetration depth to 600 m below the seafloor. These results are significant because they demonstrated that recordings made at the ocean-bottom cable array at Ekofisk field in the absence of seismic shooting can be used to image and monitor the near surface.

Modeling explosion generated Scholte waves in sandy sediments with power law dependent shear wave speed.

Experimental measurements of Scholte waves from underwater explosions collected off the coast of Virginia Beach, VA in shallow water are presented. It is shown here that the dispersion of these explosion-generated Scholte waves traveling in the sandy seabed can be modeled using a power-law dependent shear wave speed profile and an empirical source model that determines the pressure time-series at 1 m from the source as a function of TNT-equivalent charge weight.

Near‐surface Scholte wave velocities at Ekofisk from short noise recordings by seismic noise gradiometry

AbstractWe propose a new approach for imaging the subsurface using a stochastic wavefield of interface waves present in the ambient seismic field. Unlike seismic interferometry, our technique does not rely on cross correlations to obtain the Green's function between two seismic receivers. Rather, it relies on the local measurements of phase velocity obtained directly from the ratio between second‐order temporal and spatial derivatives of the wavefield. We process 10 min of ambient seismic noise recording made using a large and dense array installed over Ekofisk. We image a subsidence‐induced geomechanical imprint on the Scholte wave phase velocities in the near surface. This resulting phase velocity pattern is verified by comparison to results from a seismic‐noise cross‐correlation tomography.