Low-frequency Surface Waves Research Articles

Generation of quasi-cylindrical waves during inhomogeneous heating of a metal by a focused laser pulse

The generation of low-frequency quasi-cylindrical and surface waves generated by a long pulse of laser radiation focused into a narrow strip on the metal surface is studied. Waves are generated by non-linear currents arising from inhomogeneous heating of electrons. It is shown that the total low-frequency field is mainly determined by the quasi-cylindrical wave field. The angular and frequency distributions of the generated radiation energy have been studied. The range of angles in which it is necessary to take into account the near-surface structure of quasi-cylindrical waves is established.

Study on the crustal velocity structure of the Dunhua-Mishan fault based on the dense seismic array by ambient noise tomography

The Dunhua-Mishan fault is a component of the Tanlu fault zone, and it is also a major regional fault structure in Liaoning area. The study area is located in Fushun area of the Dunhua-Mishan fault. In order to study the crustal velocity structure in this region, we used 60 sets of three-component short-period seismographs and lay out two temporary profiles in Fushun, Liaoning from Nov. 3 to 25, 2019. Each temporary profiles is about 12km long that is composed of 30 seismometers, which nearly vertically cross the Dunhua-Mishan fault where the survey lines are 6km apart. The installation stations spacing distance from 200 to 400m use the external GPS time service to record frequency range of 0.2 to 150Hz in the instrument. The crustal structure characteristics of the fault zone are studied by ambient noise cross-correlation method. In order to efficiently extract high and low frequency surface wave dispersion signals at the same time, we adopt the Extended Range Phase Shift (ERPS) method. Finally, we use ERPS method to process the collected data and invert the S-wave velocity structure. We initially obtained the S-wave velocity structure of the upper crust below 5 km in the area, which matches well with the local geological data. The S-wave velocity structure has significant lateral inhomogeneity of the Dunhua-Mishan fault in Fushun area. The S-wave velocity is obviously lower on the Dunhua-Mishan fault, while with the existence of ancient mixed granite the velocity on both sides of the fault is obviously higher.

ModellingPwaves in seismic noise correlations: advancing fault monitoring using train traffic sources

SUMMARYThe theory of Green’s function retrieval essentially requires homogeneously distributed noise sources. Even though these conditions are not fulfilled in nature, low-frequency (<1 Hz) surface waves generated by ocean–crust interactions have been used successfully to image the crust with unprecedented spatial resolution. In contrast to low-frequency surface waves, high-frequency (>1 Hz) body waves have a sharper, more localized sensitivity to velocity contrasts and temporal changes at depth. In general, their retrieval using seismic interferometry is challenging, and recent studies focus on powerful, localized noise sources. They have proven to be a promising alternative but break the assumptions of Green’s function retrieval. In this study, we present an approach to model correlations between P waves for these scenarios and analyse their sensitivity to 3-D Earth structure. We perform a series of numerical experiments to advance our understanding of these signals and prepare for an application to fault monitoring. In the considered cases, the character of the signals strongly diverges from Green’s function retrieval, and the sensitivity to structure has significant contributions in the source direction. An accurate description of the underlying physics allows us to reproduce observations made in the context of monitoring the San Jacinto Fault in California using train-generated seismic waves. This approach provides new perspectives for detecting and localizing temporal velocity changes previously unnoticed by commonly exploited surface-wave reconstructions.

In Situ Oil Separation and Collection from Water under Surface Wave Condition.

Removal of oil from water is strongly desired due to environmental pollution, and related studies are mainly limited to the material itself. Here, we propose an oil-water separation device, called a floating well, which includes a container for oil collection and a modified mesh screen for oil separation. The mesh screen is superhydrophilic to oil and superhydrophobic to water. The oil removal experiment was performed in a basin. Under the calm surface condition, the oil collection efficiency is shown to be 42-69%. The inadequate oil collection is explained by the breakdown of a thin oil film above water to form an oil-free area, preventing direct contact between the separator and the oil. Sustained by a wave generator, we are surprised to find that with a low-frequency surface wave, the oil collection efficiency is increased to 98%, and the collection speed reaches 2.5 times that under the calm surface condition. The almost complete collection is due to the sustained contact between the separator and the oil, under which the surface wave continuously drives the oil film toward the separator from elsewhere, thus the oil-free area cannot be formed. Our work presents a new clue for large-scale in situ applications, in which the nature wave energy of river/sea can be the driving force for continuous oil separation and collection.

Retrieval of shallow S-wave profiles from seismic reflection surveying and traffic-induced noise

In urban subsurface exploration, seismic surveys are mostly conducted along roads where seismic vibrators can be extensively used to generate strong seismic energy due to economic and environmental constraints. Generally, Rayleigh waves also are excited by the compressional wave profiling process. Shear-wave (S-wave) velocities can be inferred using Rayleigh waves to complement near-surface characterization. Most vibrators cannot excite seismic energy at lower frequencies (<5 Hz) to map greater depths during surface-wave analysis in areas with low S-wave velocities, but low-frequency surface waves ([Formula: see text]) can be extracted from traffic-induced noise, which can be easily obtained at marginal additional cost. We have implemented synthetic tests to evaluate the velocity deviation caused by offline sources, finding a reasonably small relative bias of surface-wave dispersion curves due to vehicle sources on roads. Using a 2D reflection survey and traffic-induced noise from the central North China Plain, we apply seismic interferometry to a series of 10.0 s segments of passive data. Then, each segment is selectively stacked on the acausal-to-causal ratio of the mean signal-to-noise ratio to generate virtual shot gathers with better dispersion energy images. We next use the dispersion curves derived by combining controlled source surveying with vehicle noise to retrieve the shallow S-wave velocity structure. A maximum exploration depth of 90 m is achieved, and the inverted S-wave profile and interval S-wave velocity model obtained from reflection processing appear consistent. The data set demonstrates that using surface waves derived from seismic reflection surveying and traffic-induced noise provides an efficient supplementary technique for delineating shallow structures in areas featuring thick Quaternary overburden. Additionally, the field test indicates that traffic noise can be created using vehicles or vibrators to capture surface waves within a reliable frequency band of 2–25 Hz if no vehicles are moving along the survey line.

PROPAGATION OF LINEAR WAVES IN MULTILAYERED STRUCTURAL - INHOMOGENEOUS CYLINDRICAL SHELLS

In this paper, axisymmetric viscoelastic waves in an extended cylindrical shell of circular cross section are considered. The study of axisymmetric viscoelastic waves in multilayer cylindrical systems with the maximum possible use of analytical methods is relevant for the development of a new generation of sounding techniques for wells. The aim of the work is to study the propagation of axisymmetric viscoelastic waves in extended multilayer cylindrical structures, to determine the volumetric structure variant from the responses of one-way sounding using a priori information about the possible structure of the system. The paper uses classical methods of mathematical physics, applied to solving boundary value problems in a cylindrical coordinate system, the spatial Fourier transform, the method of complex amplitudes for the variable components of the displacement vector and the stress tensor. The low-frequency resonances of a cylindrical shell are investigated. The physical laws of the formation of the pitch of its fundamental tone are determined sounding. The impossibility of propagation along the casing of wells is proved low-frequency surface waves, as well as SH-polarization waves at axisymmetric excitation. The thin-walled cylindrical shell in the low-frequency region has two fundamental resonance associated with the resonance of the shell as a whole and resonance of each individual section as a ring. Beats of these frequencies the basic tone of the sound of an extended shell of this type is determined. At frequencies below the cutoff of higher modes, all the features of the dispersion natural waves in the wall of a circular cylinder shell are described approximate algebraic equation of the third degree. One the real root of this equation corresponds to the zero mode antisymmetric Lamb wave, and one of the other two complex conjugate, describes the zero mode of the symmetric wave.

Propagation of ion-acoustic surface waves in a turbulent quantum plasma with quantum recoil: transverse truncation method approach

The effect of quantum recoil on the low-frequency electrostatic surface waves on a semi-bounded quantum turbulent plasma is investigated based on the kinetic dielectric permittivity of a plasma where the wave frequency is in the range of ion-acoustic wave. The dispersion relation and the damping rate of the surface wave are derived by employing the specular reflection boundary condition and the transverse truncation method. It is shown that the frequency of surface wave is enhanced by the quantum recoil for both non-degenerate and degenerate cases. The magnitude of damping rate is also increased by the quantum effect. The quantum recoil effect on the wave propagation is significantly reduced as the wave number becomes small. In the range of such small wave numbers, the diffusional dissipation becomes more important for the wave propagation.

Automated Data Selection in the Tau–p Domain: Application to Passive Surface Wave Imaging

In the recent decades, passive surface wave methods have gained much attention in the near-surface community due to their ability to retrieve low-frequency surface wave information. Temporal averaging over a sufficiently long period of time is a crucial step in the workflow to fulfill the randomization requirement of the stationary source distribution. Because of logistical constraints, passive seismic acquisition in urban areas is mostly limited to short recording periods. Due to insufficient temporal averaging, contributions from non-stationary sources can smear the stacked dispersion measurements, especially for the low-frequency band. We formulate a criterion in the tau–p domain for selective stacking of dispersion measurements from passive surface waves and apply it to high-frequency (> 1 Hz) traffic noise. The criterion is based on the automated detection of input data with a high signal-to-noise ratio in a desired velocity range. Modeling tests demonstrate the ability of the proposed criterion to capture the contributions from the non-stationary sources and classify the passive surface wave data. A real-world application shows that the proposed data selection approach improves the dispersion measurements by extending the frequency band below 5 Hz and attenuating the distortion between 6 and 13 Hz. Our results indicate that significant improvements can be obtained by considering tau–p-based data selection in the workflow of passive surface wave processing and interpretation.

Erratum: “Low-frequency surface waves on semi-bounded magnetized quantum plasma” [Phys. Plasmas 23, 084501 (2016)

Erratum: “Low-frequency surface waves on semi-bounded magnetized quantum plasma” [Phys. Plasmas <b>23</b>, 084501 (2016)

Using surface wave amplitude spectra to estimate the source depth of Sichuan Jiuzhaigou &amp;lt;italic&amp;gt;M&amp;lt;/italic&amp;gt;7.0 earthquake

The accurate source depth of earthquake is important for understanding plate-tectonic processes and structure of seismic faults, identifying natural earthquake and non-natural earthquake. The amplitude formula of seismic surface wave shows that the amplitude of surface wave is closely related to the focal mechanism, the azimuth from epicenter to station and the source depth. When the focal mechanism and the observation position are determined, the excitation energy of surface wave is sensitive to source depth. The surface wave amplitude spectrum can be applied to determine the focal depth. The amplitude energy of shallow earthquake shows the “notch” phenomenon in band of frequency between 0.01 and 0.08 Hz in amplitude spectra, i.e. the attenuation of amplitude energy. The position of the frequency-dependent “notch” is related to the source depth. In this study, the relationship between the position of the “notch” and the source depth is analyzed by theoretical seismogram. We obtain the amplitude spectrums of the far-field at different source depths and then locate the source depth by means of comparing with observed seismic waveforms. By the amplitude spectrum of theoretical surface wave, small changes in the source depth can cause the variation of the amplitude spectrum so that the source depth of shallow earthquake can be accurately determined. The focal mechanism is coupled with the source depth. In order to analyze the influence of the disturbance of source mechanism to source depth determination, the focal mechanism of Jiuzhaigou earthquake was inversed, with disturbance of 10 degrees to strike, dip and rake. The results show only rather small changes in locating depth. In order to eliminate the influence of low frequency noise, shear wave and surface wave of short-period on amplitude spectra, this study adopts the AK135 velocity model to remove the dispersion. In this study, the seismic source depth of the Jiuzhaigou M 7.0 earthquake of 8 August 2017 in Sichuan province, China, was determined at 8.2 km by the energy attenuation characteristics of Rayleigh amplitude spectrum.

Broken degeneracy of low frequency surface waves in semi-bounded quantum plasmas including the quantum recoil effect

We present a derivation of the dispersion relation for electrostatic waves propagating at the interface of semi-bounded quantum plasma in which degenerate electrons are governed by the Wigner–Poisson system, while non-degenerate ions follow the classical fluid equations. We consider parameters for metallic plasmas in terms of the ratio of plasmon energy to Fermi energy. The dispersion relation is solved numerically and analyzed for various plasmon energies. The result shows that two-mode of waves can be possible: high- and low-mode. We have found that the degeneracy for high-mode wave would be broken when the plasmon energy is larger than the Fermi energy. We also discuss the characteristics of group velocities for high- and low-mode waves.

Attenuation of Lg waves in the New Madrid seismic zone of the central United States using the coda normalization method

Unique properties of coda waves are employed to evaluate the frequency dependent quality factor of Lg waves using the coda normalization method in the New Madrid seismic zone of the central United States. Instrument and site responses are eliminated and source functions are isolated to construct the inversion problem. For this purpose, we used 121 seismograms from 37 events with moment magnitudes, M, ranging from 2.5 to 5.2 and hypocentral distances from 120 to 440km recorded by 11 broadband stations.A singular value decomposition (SVD) algorithm is used to extract Q values from the data, while the geometric spreading exponent is assumed to be a constant. Inversion results are then fitted with a power law equation from 3 to 12Hz to derive the frequency dependent quality factor function. The final results of the analysis are QVLg (f)=(410±38) f0.49±0.05 for the vertical component and QHLg (f)=(390±26) f0.56±0.04 for the horizontal component, where the term after ± sign represents one standard error. For stations within the Mississippi embayment with an average sediment depth of 1 km around the Memphis metropolitan area, estimation of quality factor using the coda normalization method is not well-constrained at low frequencies (f<3Hz). There may be several reasons contributing to this issue, such as low frequency surface wave contamination, site effects, or even a change in coda wave scattering regime which can exacerbate the scatter of the data.

Exchange interaction effects on low frequency surface waves in a quantum plasma slab

The propagation characteristics of low-frequency (in comparison with the electron cyclotron frequency) surface (LFS) plasma waves propagating at the interface of a quantum plasma slab are studied in the presence of a uniform external magnetic field. A quantum hydrodynamic model is used, and the effects of the Fermi pressure, the quantum force (as a gradient of the Bohm potential), as well as the Coulomb exchange interaction force, associated with the spin polarization of electrons, are considered to derive the dispersion relation for LFS waves. It is found that the dispersion properties of such LFS waves are significantly modified by this new quantum effect. It is also shown that when the spin polarization effect is increased, the contribution of the Coulomb exchange potential becomes higher than those of the Fermi-pressure and the particle dispersion (Bohm potential). Furthermore, the frequency of the surface wave is seen to be down-shifted by the influence of the Coulomb exchange interaction force.

Low-frequency surface waves on semi-bounded magnetized quantum plasma

The propagation of low-frequency electrostatic surface waves on the interface between a vacuum and an electron-ion quantum plasma is studied in the direction perpendicular to an external static magnetic field which is parallel to the interface. A new dispersion equation is derived by employing both the quantum magnetohydrodynamic and Poisson equations. It is shown that the dispersion equations for forward and backward-going surface waves are different from each other.

Estimation of surface-wave group velocity using slant stack in the generalized S-transform domain

We have developed the slant stacking approach in the generalized S-transform (GST) domain of seismic data for the estimation of the group velocity of the multimodal surface wave. We used two versions of the GST. The constant-scale GST uses a constant scaling factor in the Gaussian window to control the spectral time-frequency localization. We found that a smaller scaling factor should be chosen for the low-frequency surface wave, whereas for higher frequencies, a larger scaling factor should be chosen. Therefore, the transform exhibits a trade-off in the group velocity resolution of low frequencies with small values of the scaling factor and the group velocity resolution of high frequencies with large values of the scaling factor. The modified S-transform (MST), another version of the GST used in this study, enhanced the time-frequency resolution by projecting the frequency into a linear frequency function in the Gaussian window. This property allowed us to estimate the group velocity for a broad range of frequencies. We demonstrated the robustness of the MST for the estimation of the group velocity by synthetic and real data examples.

Magnetopause Waves Controlling the Dynamics of Earth's Magnetosphere

Earth's magnetopause separating the fast and often turbulent magnetosheath and the relatively stagnant magnetosphere provides various forms of free energy that generate low-frequency surface waves. The source mechanism of this energy includes current-driven kinetic physical processes such as magnetic reconnection on the dayside magnetopause and flux transfer events drifting along the magnetopause, and velocity shear-driven (Kelvin-Helmholtz instability) or density/pressure gradient-driven (Rayleigh-Taylor instability) magnetohydro-dynamics (MHD) instabilities. The solar wind external perturbations (impulsive transient pressure pulses or quasi-periodic dynamic pressure variations) act as seed fluctuations for the magnetopause waves and trigger ULF pulsations inside the magnetosphere via global modes or mode conversion at the magnetopause. The magnetopause waves thus play an important role in the solar wind-magnetosphere coupling, which is the key to space weather. This paper presents recent findings regarding the generation of surface waves (e.g., Kelvin-Helmholtz waves) at the Earth's magnetopause and analytic and observational studies accountable for the linking of the magnetopause waves and inner magnetospheric ULF pulsations, and the impacts of magnetopause waves on the dynamics of the magnetopause and on the inner magnetosphere.

Shallow seismic detection of the fault zone associated with a high scarp in southwestern Montana

We collected shallow reflection data in southwestern Montana, USA, across a 5.4-m-high tectonic scarp. The goal was to image the normal fault associated with the scarp, observed in an adjacent trench. Processing of the data was challenging because the height of the scarp was comparable to the depths of the reflectors of interest. To find out how to proceed, we processed synthetic data generated using velocity models derived in part from actual shot gathers. The actual data are dominated by large-amplitude low-frequency surface waves, but clear high-frequency reflections are seen in the more distant geophones. Common-offset gathers for the raw and high-pass filtered data reveal sharp discontinuities in arrival times and a strong decrease in amplitudes, respectively, under the scarp. These changes in the wavefield are indicative of lateral variations in elastic properties and are consistent with the presence of a fault zone seen in the trench. The actual data were stacked after the surface waves were removed with a narrow f-k filter. Severe muting was applied to isolate the reflections seen in the high-pass filtered data. The stacked data reveal a clear and fairly continuous horizontal reflector on the downthrown side of the fault and more disrupted reflectors on the upthrown side, with truncated reflections and changes in amplitude roughly across the projection of the fault mapped in the trench. These observations are consistent with faulting and would be difficult to explain if the scarp were an erosional feature.

Ground effect due to rough and resonant surfaces

Particularly if the ground surface between noise source and receiver would otherwise be smooth and acoustically hard, structured low-rise ground roughness can be used as an alternative to conventional noise barriers. The techniques of periodic-spacing, absorptive covering, and local resonance can be used, as when broadening metamaterial stop bands, to achieve a broadband ground effect. This has been demonstrated both numerically and through laboratory experiments. Computations have employed multiple scattering theory, the Finite Element Method and the Boundary Element Method. The experiments have involved measurements over cylindrical and rectangular roughness elements and over their resonant counterparts created by incorporating slit-like openings. Resonant elements with slit openings have been found numerically and experimentally to add a destructive interference below the first roughness-induced destructive interference and thereby mitigate the adverse effects of the low-frequency surface waves generated by the presence of roughness elements. A nested configuration of slotted hollow roughness elements is predicted to produce multiple resonances and this idea has been validated through laboratory experiments.

Generation of surface waves and low-frequency radiation under exposure of a conductor to a laser pulse focused by a cylindrical lens

We have developed a theory of generation of low-frequency radiation and surface waves under the pondermotive action of a femtosecond laser pulse irradiating a conductor along the normal and focused by a cylindrical lens. It is shown that for the chosen focusing method and specified values of laser pulse duration and flux density it is possible to significantly increase the total energy of both surface waves and low-frequency radiation.

Undulations from amplified low frequency surface waves

We study the linear scattering of gravity waves in longitudinal inhomogeneous stationary flows. When the flow becomes supercritical, it is known that counterflow propagating shallow waves are blocked and converted into deep waves. Here we show that in the zero-frequency limit, the reflected waves are amplified in such a way that the free surface develops an undulation, i.e., a zero-frequency wave of large amplitude with nodes located at specific places. This amplification involves negative energy waves and implies that flat surfaces are unstable against incoming perturbations of arbitrary small amplitude. The relation between this instability and black hole radiation (the Hawking effect) is established.