Scholte Waves Research Articles

Distributed sensing of microseisms and teleseisms with submarine dark fibers

Sparse seismic instrumentation in the oceans limits our understanding of deep Earth dynamics and submarine earthquakes. Distributed acoustic sensing (DAS), an emerging technology that converts optical fiber to seismic sensors, allows us to leverage pre-existing submarine telecommunication cables for seismic monitoring. Here we report observations of microseism, local surface gravity waves, and a teleseismic earthquake along a 4192-sensor ocean-bottom DAS array offshore Belgium. We observe in-situ how opposing groups of ocean surface gravity waves generate double-frequency seismic Scholte waves, as described by the Longuet-Higgins theory of microseism generation. We also extract P- and S-wave phases from the 2018-08-19 {M}_{w}8.2 Fiji deep earthquake in the 0.01-1 Hz frequency band, though waveform fidelity is low at high frequencies. These results suggest significant potential of DAS in next-generation submarine seismic networks.

Improving Scholte-Wave Vibration Signal Recognition Based on Polarization Characteristics in Coastal Waters

Zhao, S.; Zhang, J.; Wang, Z.; Huang, H.; Hu, J., and Guo, C., 2020. Improving Scholte-wave vibration signal recognition based on polarization characteristics in coastal waters. Journal of Coastal Research, 36(2), 382–392. Coconut Creek (Florida), ISSN 0749-0208.Scholte waves on the underwater interface provide a lot of information, which can be employed to identify the target in coastal water and speculate the coastal underwater medium. However, as the practical acquired signals are the superposition of multiple wave fields, it is necessary to separate the Scholte wave and to improve its signal-to-noise ratio. The LS-DYNA software was used to simulate the underwater acoustic field to analyze the excitation mechanism and particle trajectory law of Scholte waves and to obtain the propagation characteristic of Scholte waves. The results showed that Scholte waves attenuate according to r–0.5 on the underwater interface and according to exponent in the depth direction. The amplitude of Scholte waves mainly concentrates below 25 Hz, and the corresponding particle trajectory makes a counterclockwise elliptical shape by filtering the acquired signals of particle vibration velocity below 25 Hz on and below the underwater interface. This method was applied to separate Scholte waves from the acquired complex signals of the wave field in the lake experiment to obtain the pure signals of the Scholte wave. This research can provide significant knowledge to better take advantage of resourceful information provided by Scholte waves in the coastal environment.

Theoretical and numerical analyses of a half space with overlying liquid subjected to an internal line load

The dynamic response of a 2D half space with overlying liquid under an internal line load is analytically and numerically studied, as a first attempt toward simulating undersea tunnels with moving loads. Firstly, the wave equations in partial differential form are derived from the equations of motion of the solid and liquid in rectangular coordinates. By the Fourier transformation, the wave equations are converted to the ordinary differential form. Then, a formulation is presented for the Scholte waves existing at the interface between the liquid and solid, with related properties unveiled. Numerically, the 2D half space is modeled by finite and infinite elements for the near and far fields, respectively, and the liquid of infinite depth as equivalent nodal loads. The present analytical solutions compare well with Ewing's, with minor deviations owing to different load definitions. Also, the computed results are consistent with, but more reliable than the analytical ones, since the singularity effect near the loading point has been circumvented. Shallow liquid has little effect on the response of the solid. The displacement of the solid decreases as the liquid depth increases. It fluctuates with decay along the horizontal axis and reaches a limit for liquid depth approaching infinite.

On the Dispersive Characteristics of Shallow Water Seismic Waves Excited by Air Gun Sound Source in Shallow Water

In order to study the dispersion characteristics of seabed seismic waves excited by acoustic sources in shallow water, the sound field at liquid-solid interface is derived based on the wave equation firstly, which results demonstrate that the longitudinal wave and transversal wave are not dispersive, while the Scholte wave is normal dispersive. Then, the numerical simulation is carried out on the sound field at the liquid-solid interface excited by the air gun sound source based on high-order staggered grid finite difference method, which results demonstrate that the dispersion phenomenon of Scholte waves is observed and the theoretical analysis results were verified. Finally, the data of experiment in the sea are analyzed based on the transformation, and the dispersion curve of Scholte wave is finely extracted.

Tomography of crust and lithosphere in the western Indian Ocean from noise cross-correlations of land and ocean bottom seismometers

SUMMARYWe use seismic noise cross-correlations to obtain a 3-D tomography model of SV-wave velocities beneath the western Indian Ocean, in the depth range of the oceanic crust and uppermost mantle. The study area covers 2000 × 2000 km2 between Madagascar and the three spreading ridges of the Indian Ocean, centred on the volcanic hotspot of La Réunion. We use seismograms from 38 ocean bottom seismometers (OBSs) deployed by the RHUM-RUM project and 10 island stations on La Réunion, Madagascar, Mauritius, Rodrigues, and Tromelin. Phase cross-correlations are calculated for 1119 OBS-to-OBS, land-to-OBS, and land-to-land station pairs, and a phase-weighted stacking algorithm yields robust group velocity measurements in the period range of 3–50 s. We demonstrate that OBS correlations across large interstation distances of >2000 km are of sufficiently high quality for large-scale tomography of ocean basins. Many OBSs yielded similarly good group velocity measurements as land stations. Besides Rayleigh waves, the noise correlations contain a low-velocity wave type propagating at 0.8–1.5 km s−1 over distances exceeding 1000 km, presumably Scholte waves travelling through seafloor sediments. The 100 highest-quality group velocity curves are selected for tomographic inversion at crustal and lithospheric depths. The inversion is executed jointly with a data set of longer-period, Rayleigh-wave phase and group velocity measurements from earthquakes, which had previously yielded a 3-D model of Indian Ocean lithosphere and asthenosphere. Robust resolution tests and plausible structural findings in the upper 30 km validate the use of noise-derived OBS correlations for adding crustal structure to earthquake-derived tomography of the oceanic mantle. Relative to crustal reference model CRUST1.0, our new shear-velocity model tends to enhance both slow and fast anomalies. It reveals slow anomalies at 20 km depth beneath La Réunion, Mauritius, Rodrigues Ridge, Madagascar Rise, and beneath the Central Indian spreading ridge. These structures can clearly be associated with increased crustal thickness and/or volcanic activity. Locally thickened crust beneath La Réunion and Mauritius is probably related to magmatic underplating by the hotspot. In addition, these islands are characterized by a thickened lithosphere that may reflect the depleted, dehydrated mantle regions from which the crustal melts where sourced. Our tomography model is available as electronic supplement.

Study on fluctuant element and attenuation rules of seismo-acoustic wave

In order to study the propagation characteristics and attenuation rules of seismo-acoustic wave, in this article, the wave equation and displacement function of the seismo-acoustic wave were established, the propagation process of seismo-acoustic wave was three-dimensional simulated by the finite element software LS-DYNA, and the propagation characteristics and attenuation rules of the seismo-acoustic wave were obtained. The NI data acquisition system was used for data acquisition of the underwater test. The simulation and test results are processed by wavelet transform. Theoretical derivation, simulation calculation, and comparison of test results showed that the seismo-acoustic wave includes first break, spherical compressional wave and shear wave, and cylindrical Scholte wave. At the fluid–solid interface, the compressional wave and shear wave propagation attenuated as 1/ r2, and Scholte wave propagation attenuated as 1/ r0.5. In the depth direction, Scholte wave propagation decays exponentially. Scholte wave is mainly concentrated below 25 Hz. The research results can provide important reference value for the research of using seismo-acoustic wave target recognition.

Seismic on floating ice: data acquisition versus flexural wave noise

ABSTRACTGeophysical surveying of the Arctic will become increasingly important in future prospecting and monitoring of the terrestrial and adjacent areas in this hemisphere. Seismic data acquired on floating ice are hampered with extensive noise due to ice vibrations related to highly dispersive ice flexural waves generated by the seismic source. Several experiments have been conducted on floating ice in van Mijenfjorden in Svalbard in the Norwegian Arctic to specifically analyse the extent of flexural waves recorded with various seismic receivers and sources deployed both on top of ice and in the water below. The data show that flexural waves are severely damped at 5 m or deeper below the ice and hydrophone data suffer less from these vibrations compared with data recorded on the ice. Aliasing of single receiver hydrophone data can to some extent be suppressed using an in‐line line source of detonating cord. Experiments on ice on shallow water show prominent guided wave modes often referred to as Scholte waves propagating along the seabed. In this case, both flexural and Scholte waves interfere and make a complicated pattern of coherent noise. On shallow water, the positioning and type of the seismic source must be evaluated with respect to the coherent noise generated by these waves. Geophone strings of 25 m effectively suppress both flexural and Scholte waves due to their relative short wavelengths. An airgun generates relative more low‐frequency energy than a surface source of detonating cord. Accordingly, seismic mapping of deep seismic horizons seem to be best achieved using geophone strings of such length and an airgun source. For shallow targets, the use of hydrophones in combination with detonating cord is an appropriate solution. Seismic surveying in the Arctic always have to follow environmental restrictions of not disturbing or harming wildlife and not causing permanent footprints into the vulnerable tundra, which implies that the choice of seismic acquisition strategy might occur as a trade‐off between optimum data quality and environmental constraints.

Seismic on floating ice on shallow water: Observations and modeling of guided wave modes

Seismic mapping of the shallow, coastal areas of the Arctic is best facilitated in periods when the sea is covered with solid, floating ice. Data from three seismic acquisition campaigns on sea ice floating on shallow water reveal how coherent noise related to guided waves is differently exposed for various source and receiver systems placed on and below the ice. The main coherent noise is due to interference of ice flexural and Scholte waves. The experimental data were overall successfully modeled using a wavenumber integration technique. A seismic source at or near the ice generates high-amplitude, slowly propagating, and highly dispersive flexural waves. Their amplitudes are severely reduced when recorded at hydrophones deployed 5 m or more below the sea ice. The extent of flexural waves generated using an air gun below the ice similarly reduces as the depth of the air gun increases, but then the amplitudes of the seabed Scholte waves increase. Our experiments indicate that an inline line source of detonating cord on the ice combined with hydrophones deployed at the appropriate depth below the ice constitute an efficient setup for reducing the imprints of the ice flexural and Scholte waves on seismic data.

Characterization of bottom parameters in the New England Mud Patch using geophone and hydrophone measurements

Measurements of acoustic pressure and particle velocity were made during the Seabed Characterization Experiment (SCEx) in the New England Mud Patch south of Cape Cod in about 70 meters of water. The University of Rhode Island and Woods Hole Oceanographic Institution deployed the “geosled” with a four-element geophone array, a tetrahedral array of four hydrophones, and several hydrophone receive units (SHRUs). In addition, a new low frequency source, interface Wave Sediment Profiler (iWaSP) was deployed to excite interface waves (Scholte waves). The iWaSP system consists of a source to generate the interface wave and a four-element accelerometer receive array. Modal arrivals from broadband sources (SUS and CSS) on geophones and hydrophones will be presented and compared. The individual arrivals on the sensors will be identified though seismo-acoustic modeling. Based on the propagation paths of the various waves (compressional, shear and Scholte waves), geoacoustic parameters will be estimated. Seismic data collected at the location will be used to constrain the model parameters in the inversion. Sediment data from cores, other in-situ measurements and inversions using other types of data will be used to compare and validate our inversions. [Work supported by Office of Naval Research.]

The study on dispersion characteristics of Scholte wave in elastic media with variable parameters

In the real ocean environment, due to the reflection of the water-sediment interface, the elastic sediment will generate inhomogeneous compressional wave and inhomogeneous shear wave, and the interaction between them will form the Scholte wave. Besides, the compressional and shear wave velocities in an elastic sediment layer are changing with depth leading to the coupling of compressional and shear waves. As the dispersion characteristics of the Scholte wave is affected by the coupling effect, in this paper, a study on the dispersion characteristics of Scholte wave in an environment with variable parameters elastic sediment layer is given. Specific environmental examples are simulated and the dispersion curves are compared with the KRAKENC program. In addition, the influence of the parameters of elastic sediment layer on the Scholte wave dispersion curves is analyzed. Comparing the dispersion curve of the Scholte wave in the uniform sediment layer and in elastic medium with variable parameters, it is found that some characteristics of the dispersion curve in the uniform layer will no longer fit the elastic layer with variable parameters.

Seismic wave propagation in floating ice sheets – a comparison of numerical approaches and forward modelling

Seismic data processing collected from arctic sea ice is often challenging. Large amplitudes of coherent surface waves interfere with the reflected seismic waves and can make the reflection data virtually unusable. In order to understand this noise, forward modelling of the full wavefield is a useful approach. I present and test a forward modelling workflow by comparing the modelling results with common numerical approaches. The main differences between numerical curves and modelling results depend on simplifications in the assumptions of the numerical approaches. Forward modelling is less dependent on rough assumptions and can give a more accurate understanding of the surface wave propagation and its interaction with reflected waves. Furthermore, a thin elastic layer with high velocity located between two low velocity layers, like sea ice, generates guided waves. The numerical approaches focus only on the flexural waves and ignore other wave types. The presented results demonstrate that forward modelling can provide improved detail in the generated wavefield. In addition, the forward modelling results in this work show that leaky Rayleigh and Scholte waves can exist simultaneously.

A Method for Estimating the Distance of Near-Ocean-Bottom Sources by Combining VLF Underwater Acoustic Field and Scholte Wave Field

In order to solve the problems of passive detection of very-low-frequency (VLF) underwater sound sources, this paper proposes a method of estimating sound source distance based on combining underwater sound field and Scholte wave field. VLF sources near the ocean bottom generate the sound field in the water, then seismic waves are excited by the of near-field effect. The surface wave propagating along the ocean bottom interface is the Scholte wave. By deploying a hydroacoustic&seismic-wave joint detection system on the ocean bottom, the acoustic field and the Scholte wave field excited by the source can be collected. Due to the different physical properties of water acoustics and Scholte waves, the Scholte waves can be distinguished by analysing the polarization characteristics of the captured signals. Due to the different velocity between the underwater acoustics and Scholte waves, the propagation delays to the joint detection system are also different, and the delay difference can be obtained by the correlation method. Combined with the surrounding parameters, the distance of the target source can be determined. Theoretical analysis and finite element simulation experiments show that this method can estimate the position information of near-ocean-bottom VLF underwater sources in the ideal case.

Peculiarities of Rayleigh wave displacement in a transversely isotropic half space

We consider a transversely isotropic medium with vertical axis of symmetry (VTI). Rayleigh wave displacement components in a homogeneous VTI medium contain exp(±krjz), where z is the vertical coordinate, k is the wave number, and j = 1, 2; rj may be considered as depth-decay factor. In a VTI medium, rj can be a real or imaginary as in an isotropic medium, or it can be a complex depending on the elastic parameters of the VTI medium; if complex, r1 and r2 are complex conjugates. In a homogeneous VTI half space, Rayleigh wave displacement is significantly modified with a phase shift when rj changes from real to complex with variation of VTI parameters; at the transition, the displacement becomes zero when r1 = r2. In a liquid layer over a VTI half space, Rayleigh waves are dispersive. Here, also Rayleigh wave displacement significantly modified with a phase shift when rj changes from real to complex with a decrease of period. At very low period, phase velocity of Rayleigh waves becomes less than P-wave velocity in the liquid layer giving rise to Scholte waves (interface waves). The amplitudes of Scholte waves with a VTI half space are found to be significantly larger than those with an isotropic half space.

Study of Scholte wave dispersion curves and modal energy distribution using a wavefield numerical simulation method

Surface waves travelling along the subsurface interface between water and sediment are called Scholte (or generalised Rayleigh) waves. In this paper, we examine the characteristics of Scholte wave dispersion curves and their modal energy distribution through synthetic simulation tests. Analytical solutions of the dispersion equation for a homogeneous isotropic layered medium describe the propagation characteristics of the fundamental and higher modes, but do not provide information about the relative energy distributions of those wave modes. In real situations, the dispersion curves with different modes have different energies corresponding to different frequency bands. It is instructive to study and analyse the behaviours of Scholte wave dispersion curves by using joint dispersion equation solutions and numerical simulations results. Dispersion equation solutions are used to validate the numerical results, which in turn are used to assess energy distribution for both the fundamental and the higher modes. We observe that the thickness of the overlying water layer and the elastic properties of the ocean bottom play important roles in the dispersion characteristics of Scholte waves. The dispersion of the soft ocean-bottom model (whose S-wave velocity of the first solid layer is lower than the P-wave velocity of water) is dominated by the fundamental mode and appears to be less dependent on the thickness of the overlying water layer. In contrast, the dispersion of the hard ocean-bottom model contains significant energy in the higher modes. Higher modes will shift to lower frequencies and will be spatially closer (in the frequency direction) when the water layer thickness increases. The dispersion characteristics of the real data are in a good agreement with the conclusion of a soft ocean-bottom model. Our modelling approach of combining analytical solutions of the dispersion equation with numerical modelling can facilitate Scholte wave exploration in water-covered areas.

Computation of interface wave motions by reciprocity considerations

The current theoretical study deals with computation of Stoneley waves along a solid–solid interface and Scholte waves (also called Scholte-Gogoladze) along a solid–liquid interface by reciprocity considerations. Closed-form solutions of the wave motions generated by a time-harmonic line load applied in two bonded elastic half-spaces of different material properties are derived in a simple manner. In order to perform direct applications of reciprocity theorems, we introduce in this article new expressions for the displacements of free interface waves. Reciprocity relations between an actual state, interface wave motion generated by a time-harmonic line load, and a virtual state, an appropriately chosen free wave traveling along the interface, are derived. Scattered amplitudes of Stoneley waves and Scholte waves due to the load are thus computed. To show application of the obtained results, scattering of Stoneley wave by a delamination at the interface is then studied.

Inversions for sediment parameters using geophone and tetrahedral array measurements in the New England Mud Patch

Measurements of acoustic pressure and particle velocity were made during the Seabed Characterization Experiment (SCEx) in the New England Mud Patch south of Cape Cod in about 70 meters of water. The University of Rhode Island and Wood Hole Oceanographic Institution deployed the “geosled” with a four-element geophone array, a tetrahedral array of four hydrophones, and several hydrophone receive units (SHRUs). In addition, a new low frequency source, interface Wave Sediment Profiler (iWaSP) was deployed to excite interface waves (Scholte waves). The iWaSP system consists of a source to generate the interface wave and a four-element accelerometer receive array. Results of inversions for geoacoustic parameters will be presented. Arrival time dispersion from broadband sources (SUS and CSS) and iWaSP will be used for these inversions. Both p-wave and Scholte wave arrivals will be investigated using the geophone and hydrophone data. Seismic data collected at the location will be used to constrain the model parameters in the inversion. Sediment data from cores, other in-situ measurements and inversions using other types of data will be used to compare and validate our inversions. [Work supported by Office of Naval Research.]

Analytical solution for the transient wave propagation of a buried cylindrical P-wave line source in a semi-infinite elastic medium with a fluid surface layer

This article develops an analytical solution for the transient wave propagation of a cylindrical P-wave line source in a semi-infinite elastic solid with a fluid layer. The analytical solution is presented in a simple closed form in which each term represents a transient physical wave. The Scholte equation is derived, through which the Scholte wave velocity can be determined. The Scholte wave is the wave that propagates along the interface between the fluid and solid. To develop the analytical solution, the wave fields in the fluid and solid are defined, their analytical solutions in the Laplace domain are derived using the boundary and interface conditions, and the solutions are then decomposed into series form according to the power series expansion method. Each item of the series solution has a clear physical meaning and represents a transient wave path. Finally, by applying Cagniard's method and the convolution theorem, the analytical solutions are transformed into the time domain. Numerical examples are provided to illustrate some interesting features in the fluid layer, the interface and the semi-infinite solid. When the P-wave velocity in the fluid is higher than that in the solid, two head waves in the solid, one head wave in the fluid and a Scholte wave at the interface are observed for the cylindrical P-wave line source.

Effect of surface wave propagation in a four-layered oceanic crust model

Dispersion of Rayleigh type surface wave propagation has been discussed in four-layered oceanic crust. It includes a sandy layer over a crystalline elastic half-space and over it there are two more layers—on the top inhomogeneous liquid layer and under it a liquid-saturated porous layer. Frequency equation is obtained in the form of determinant. The effects of the width of different layers as well as the inhomogeneity of liquid layer, sandiness of sandy layer on surface waves are depicted and shown graphically by considering all possible case of the particular model. Some special cases have been deduced, few special cases give the dispersion equation of Scholte wave and Stoneley wave, some of which have already been discussed elsewhere.

Correction of OBS clock errors using Scholte waves retrieved from cross-correlating hydrophone recordings

Correction of OBS clock errors using Scholte waves retrieved from cross-correlating hydrophone recordings

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.