Rayleigh Wave Velocity Research Articles

Effects of Al Content on Elastic Parameters of AlxGa1-xAs (0 ≤ x ≤ 1) Alloys

Elastic parameters of AlxGa1-xAs (0 ≤ x ≤ 1) alloys are numerically determined and analyzed on the basis of scanning acoustic microscopy technique. Thus, the dependence of Al concentrations, x, on all features of reflection coefficient and acoustic materials signatures (critical angles of reflected modes, spatial periods, peaks of FFT spectra and their corresponding wave velocities) has been considered, analyzed and discussed. It is found that as Al content increases several behaviors are obtained: (i) all critical angles of longitudinal, transverse and Rayleigh waves, decrease, (ii) all spatial periods increase and (ii) both Rayleigh and longitudinal wave velocities increase. Moreover, the variations of these parameters, P, were quantified and semi-empirical formulas were found to be of the form: P = c + ax + bx2; from these formulas, valuable information can be derived and may be useful for AlxGa1-xAs compositional characterization.

Seismic Velocity Variations at Different Depths Reveal the Dynamic Evolution Associated With the 2018 Kilauea Eruption

AbstractAdvances in noise‐based velocity change measurements have revealed anomalies before volcanic eruptions. Still, it is difficult to assign them to a certain depth based on the traditional coda wave method. Here we used volcanic tremor (1–2.5 s) and oceanic noise (3 and 3.5 s) as passive sources to calculate the Rayleigh wave velocity changes associated with the 2018 Kilauea eruption. Our simulation show that the velocity decreased at least by 5% at 1–2 km depth beneath the East Rift zone in the co‐eruption time. Before the eruption, we observed a decrease (1%) at the same depth, whereas, at the depths of 0–1 and 2–8 km, the velocity increased by 0.2%–1%. Combined with microearthquakes, The Global Positioning System data, and numerical simulation, our observations suggest that magma intrusion may open the magma dike at 1–2 km depth but induce higher compressive stress (about 3.6–14.4 MPa) in the surrounding areas.

FEM Simulation of a High-Performance 128°Y-X LiNbO3/SiO2/Si Functional Substrate for Surface Acoustic Wave Gyroscopes.

To obtain a high-performance surface acoustic wave (SAW) gyroscope substrate, the propagation characteristics and gyroscopic effect of Rayleigh waves in a 128°Y–X LiNbO3/SiO2/Si (LNOI) functional substrate were investigated with a three-dimensional finite element method. The influence of LNOI structural parameters on Rayleigh wave characteristics, including the phase velocity (vp), electromechanical coupling coefficient (K2) and temperature coefficient of frequency (TCF), were analyzed. The results demonstrate that the SiO2 layer compensates for the negative TCF of 128°Y–X LiNbO3 and enhances the K2 of the LNOI substrate. The Rayleigh wave velocity change of the LNOI substrate after rotations in different directions was studied. The gyroscope gain factor (η) represents the strength of the gyroscopic effect in the differential traveling wave SAW gyroscope and is defined. The ηy and ηz of the LNOI substrate with different structural parameters were investigated. Finally, an LNOI substrate with an hLN/λ of 0.2 and an hSiO2/λ of 0.05 was obtained by balancing the characteristic parameters, with a K2 of 3.96%, TCF of −18.75 ppm/°C and ηy of 0.26. The LNOI substrate has a better gyroscopic effect and temperature stability than the 128°Y–X LiNbO3 crystal. The LNOI substrate meets device miniaturization and integration needs.

Effect of snowfall on changes in relative seismic velocity measured by ambient noise correlation

Abstract. In mountainous, cold temperate and polar sites, the presence of snow cover can affect relative seismic velocity changes (dV/V) derived from ambient noise correlation, but this relation is relatively poorly documented and ambiguous. In this study, we analyzed raw seismic recordings from a snowy flat field site located above Davos (Switzerland), during one entire winter season (from December 2018 to June 2019). We identified three snowfall events with a substantial response of dV/V measurements (drops of several percent between 15 and 25 Hz), suggesting a detectable change in elastic properties of the medium due to the additional fresh snow. To better interpret the measurements, we used a physical model to compute frequency-dependent changes in the Rayleigh wave velocity computed before and after the events. Elastic parameters of the ground subsurface were obtained from a seismic refraction survey, whereas snow cover properties were obtained from the snow cover model SNOWPACK. The decrease in dV/V due to a snowfall was well reproduced, with the same order of magnitude as observed values, confirming the importance of the effect of fresh and dry snow on seismic measurements. We also observed a decrease in dV/V with snowmelt periods, but we were not able to reproduce those changes with our model. Overall, our results highlight the effect of the snow cover on seismic measurements, but more work is needed to accurately model this response, in particular for the presence of liquid water in the snowpack.

Field measurements, laboratory tests and empirical relations for investigating the solid-to-fluid transition of a rapid earthflow

Earthflows are widespread phenomena in the Mediterranean area. These landslides involve fine-grained soils and clay-bearing rocks, and despite their low speed are responsible for significant economic losses in vast areas. The dynamics of these landslides is still relatively obscure thus reducing our ability to forecast and mitigate their effects. In this study, we present a methodological approach for the characterization of active earthflows based on the combination of geophysical surveys, laboratory tests, and empirical formulas. Geophysical surveys consist of periodic measurements of Rayleigh wave velocities repeated over time to evaluate the change of stiffness with time of the landslide material. Laboratory tests combine Atterberg limits, fall cone and oedometric tests and allow to constrain the empirical correlations between geophysical and geotechnical properties. The method is designed to obtain relevant data when direct methods like boreholes or geotechnical soundings are not possible for safety reasons, with the aim of investigating the solid-to-fluid transition that can occur in rapid earthflows. We applied this approach to study the Montevecchio landslide (Northern Apennines of Italy), an active earthflow in young marine clays which was affected by multiple reactivations in the recent years. Results show that after a surge the earthflow material is very soft (shear wave velocity in the order of 50–60 m/s) and characterized by a high water content. However, the exact value of the Liquidity Index remains unknown due to the lack of direct measurements and to the uncertainty dictated by the empirical relationships.

Design and experiment of array Rayleigh wave-EMAT for plane stress measurement

The Rayleigh wave excited by the electromagnetic acoustic transducer (EMAT) is an effective selection for surface plane stress measurement. However, the propagation velocity of Rayleigh wave on the metal surface is easily affected by the original rolling process. Besides, the direction of the plane stress state is usually unknown, which means that the propagation velocity cannot be expressed linearly by the stress. As a result, the traditional measurement model of one transmitter and one receiver can only realize the decouple of plane stress components by rotating method, which not only brings position error but also low measurement efficiency. Therefore, this paper focuses on a novel Rayleigh wave-EMAT for plane stress ultrasonic measurement. Firstly, the Rayleigh wave measurement model is established based on the acoustoelastic equation and displacement expression. Furthermore, an array Rayleigh wave-EMATintegrating three transmitters and three receivers is designed. Finally, the typical plane stress state of 5052 aluminum alloy plate after friction stir welding (FSW) is measured. The experimental results show good agreement compared with the hole-drilling method, which verifies the effectiveness of proposed method and designed EMAT.

Wave dispersion analysis in auxetic rods

Very few works have dealt with the wave propagation in auxetic rods. They mainly considered the effect of density changes on the longitudinal wave velocity. To this goal, the elementary one-dimensional wave propagation equation has been modified to include the high variation in material density that occurs in auxetic materials. In this work, we are mainly interested in analysing wave dispersion in auxetic rods. To this purpose, the three-dimensional Pochhammer-Chree equation is solved, for a Poisson’s ratio ν ranging from −1 to 0.5, using the regula-falsi method. We mainly show that the Poisson’s ratio controls the wave dispersion in rods through two effects: radial inertia and shear stiffness. Increasing the module of the Poisson’s ratio leads to an increase in radial inertia thus reducing wave velocity. This happens at low frequencies or large wavelengths. Consequently, the wave velocity within this range is mainly governed by the Poisson’s ratio module. On the opposite, in the high frequencies range (short wavelengths range) the wave velocity is rather governed by the shear stiffness of the auxetic rod. The wave velocity at vanishing wavelengths converges to the velocity of Rayleigh waves, which increases as the Poisson’s ratio decreases. In the high frequencies range, wave velocity is sensitive to the algebraic value of the Poisson’s ratio. In this work, we have also dealt with higher modes (higher solutions). Namely, the Pochhammer-Chree equation have been solved for the second and third modes; and analytical approximations were proposed for both low and high wavenumber ranges.

On propagation characteristics of Rayleigh wave in saturated porous media based on the strain gradient nonlocal Biot theory

Based on the strain gradient nonlocal Biot theory, the semi-analytical solution of Rayleigh wave fields is obtained using the wave function expansion method. The influence of nonlocal pore size (characterized by nonlocal parameter) and heterogeneity structure effect (characterized by scale factor) on the propagation of Rayleigh wave are numerically implemented and then discussed in detail. The results show that the nonlocal parameter and scale factor have an ignorable influence on the propagation of Rayleigh wave in low frequency region while a significant influence within high frequency range by comparing with the results obtained from the classical Biot theory. A critical ratio of scale factor to nonlocal parameter defining the transition of negative dispersion to positive dispersion is observed. The Rayleigh wave velocity in saturated porous media increases with increasing nonlocal parameter and decreases with the increase of the scale factor. The length of the principal axis and minor axis of the ellipse formed by the displacement fields strongly depends on the ratio of nonlocal parameter to scale factor and the depth of the observation point. Furthermore, the displacement amplitude is intensified by increasing the nonlocal parameter and weakened by increasing the scale factor.

Global reference seismological data sets: multimode surface wave dispersion

SUMMARYGlobal variations in the propagation of fundamental-mode and overtone surface waves provide unique constraints on the low-frequency source properties and structure of the Earth’s upper mantle, transition zone and mid mantle. We construct a reference data set of multimode dispersion measurements by reconciling large and diverse catalogues of Love-wave (49.65 million) and Rayleigh-wave dispersion (177.66 million) from eight groups worldwide. The reference data set summarizes measurements of dispersion of fundamental-mode surface waves and up to six overtone branches from 44 871 earthquakes recorded on 12 222 globally distributed seismographic stations. Dispersion curves are specified at a set of reference periods between 25 and 250 s to determine propagation-phase anomalies with respect to a reference Earth model. Our procedures for reconciling data sets include: (1) controlling quality and salvaging missing metadata; (2) identifying discrepant measurements and reasons for discrepancies; (3) equalizing geographic coverage by constructing summary rays for travel-time observations and (4) constructing phase velocity maps at various wavelengths with combination of data types to evaluate inter-dataset consistency. We retrieved missing station and earthquake metadata in several legacy compilations and codified scalable formats to facilitate reproducibility, easy storage and fast input/output on high-performance-computing systems. Outliers can be attributed to cycle skipping, station polarity issues or overtone interference at specific epicentral distances. By assessing inter-dataset consistency across similar paths, we empirically quantified uncertainties in traveltime measurements. More than 95 per cent measurements of fundamental-mode dispersion are internally consistent, but agreement deteriorates for overtones especially branches 5 and 6. Systematic discrepancies between raw phase anomalies from various techniques can be attributed to discrepant theoretical approximations, reference Earth models and processing schemes. Phase-velocity variations yielded by the inversion of the summary data set are highly correlated (R ≥ 0.8) with those from the quality-controlled contributing data sets. Long-wavelength variations in fundamental-mode dispersion (50–100 s) are largely independent of the measurement technique with high correlations extending up to degree ∼25. Agreement degrades with increasing branch number and period; highly correlated structure is found only up to degree ∼10 at longer periods (T > 150 s) and up to degree ∼8 for overtones. Only 2ζ azimuthal variations in phase velocity of fundamental-mode Rayleigh waves were required by the reference data set; maps of 2ζ azimuthal variations are highly consistent between catalogues ( R = 0.6–0.8). Reference data with uncertainties are useful for improving existing measurement techniques, validating models of interior structure, calculating teleseismic data corrections in local or multiscale investigations and developing a 3-D reference Earth model.

Ambient Seismic Noise Cross - correlation of Ambon Island and Surrounding Area, Eastern Indonesia: Preliminary Result

The island of Ambon lies on complex tectonics, part of Banda Arc which is driven by the Australia – Eurasia collision. Historical earthquake data show that an earthquake resulting the greatest tsunami in Indonesia had occurred at Ambon Island. On 26 September 2019, Ambon was shaken by an M 6.5 earthquake at a depth of 10 km (BMKG). In this study, we use ambient noise data from 11 temporary stations deployed by ITB and 4 permanent stations owned BMKG which are recorded from October until December 2019. Here, we purely use the vertical component of seismogram to retrieve the Empirical Green’s Function of Rayleigh waves. Cross-correlations were obtained from the daily data series and stacked the day-by-day cross-correlation data into one inter-station cross-correlation. The Empirical Green’s Function is seen at the band period 1-15 s. As a part of our study, we analyze the Green’s Function with frequency-time analysis (FTAN) to get Rayleigh wave group velocity. The group velocity of Rayleigh waves varies from 1.04 km/s – 3.75 km/s. Low group velocity might be indicated the presence of sediment or volcanic deposits and high group velocity might be indicated metamorphic rocks. The result of this study might give a finer velocity model of the shallow crustal beneath Ambon Island and the surrounding area.

A note on the effect of rigidity and initial stress on the propagation of Rayleigh waves in pre-stressed orthotropic elastic layered medium

This paper contains the secular equation of Rayleigh waves in a prestressed semi-infinite space of orthotropic material which is covered by a prestressed elastic layer of orthotropic material. The semi-infinite space and layer are treated as in spring contact to each other. This paper treats the surface of layer as rigid. The effect of tensile and compressional stress on velocity curve is depicted in form of graphs. Particular cases for perfect and sliding interface is discussed and secular equation is derived. Comparison of velocity curve for welded and spring contacts has been examined graphically. MATLAB programming language is used to plot the graphs. Effect of stiffness constants on velocity of Rayleigh waves has been shown graphically.

Extending the formulation of the spatial autocorrelation (SPAC) method to strain, rotation and tilt

SUMMARYThe spatial autocorrelation (SPAC) method has been applied to ambient seismic noise measured by arrays of translational seismometers for inverting phase-velocity dispersion curves of Rayleigh or Love waves for shallow S-wave velocity structure. Recently, it is becoming possible to observe wave spatial gradients such as strain, rotation and tilt owing to the development of dense seismic networks and improving measurement technologies. Therefore, it is desirable to extend the formulation of the SPAC method to strain, rotation and tilt. This study presents analytical expressions of cross-spectra and coherence of the strain, rotation and tilt components that are measured on the free surface. According to the results, both Rayleigh and Love waves contribute to most components of strains. The exceptions are the areal strain and the vertical axial strain (ezz) on the free surface that are affected by only Rayleigh waves. Only Rayleigh waves contribute to the tilts and rotations around the horizontal axes on the free surface, too. On the other hand, only Love waves contribute to the rotation around the vertical axis. Therefore, different kinds of wave spatial gradients are helpful to separate Rayleigh and Love waves correctly. For practical applications, the analytical expression for the radial axial strain (err) component will be applied directly to distributed acoustic sensing data measured with straight sections of a fibre-optic cable. On the other hand, dense observations of rotation and tilt may still be difficult to carry out at present. However, an application of analytical formulations in this study to arrays of at least several three-component rotational seismometers is attractive because it enables us to separately estimate the phase velocities of Rayleigh and Love waves.

Influence of gravity and initial stress on Rayleigh wave propagation in magneto-thermoelastic medium

The main aim of present study is to show the effect of gravity, initial stress and magnetic field on the Rayleigh waves propagation in homogeneous orthotropic magneto-thermoelastic medium in the context of Three Phase Lag (TPL) model at two temperature. The governing equations of thermoelasticity have been solved by normal mode technique to deduce the frequency equation for Rayleigh wave with relevant boundary conditions. Special cases have been derived for isothermal and thermally insulated surfaces. Computer simulation is used for numerical discussion to show the effects of various parameters on phase velocity of Rayleigh waves. The variation in phase velocity corresponding to wave number has been demonstrated graphically in the presence of gravity, initial stress and magnetic field.

Basic performance of a spatial autocorrelation method for determining phase velocities of Rayleigh waves from microtremors, with special reference to the zero-crossing method for quick surveys with mobile seismic arrays

SUMMARY We theoretically and empirically demonstrate the usability of the zero-crossing method for quick microtremor surveys in earthquake engineering (i.e. microtremor array surveys), namely shallow (< a few kilometres) surveys with small-scale (< 1 kilometre in radius) mobile seismic arrays with a short observation time (< a few hours). The zero-crossing method is a type of spatial autocorrelation (SPAC) method that determines phase velocities based on multiple frequencies at which the SPAC coefficient curve crosses zero. It is theoretically shown that the zero-crossing method is robust against incoherent noise and that the use of the first zero crossings (i.e. those at the lowest frequencies) is more robust against inadequate conditions of the microtremor wavefield than the use of later zero crossings (i.e. those at higher frequencies). We used microtremor array data with maximum array radii and observation durations of 400 m and 120 min on average, respectively, at 445 observation sites in the Kanto Plain, Japan, for validating the practicality of using the first zero crossings. As an illustration of the robustness against low signal-to-noise ratios (SNRs), we show that with the zero-crossing method, low-sensitivity (i.e. low-SNR) seismometers provide the same analysis results as those obtained with high-sensitivity seismometers, even when the power spectral densities for the low-sensitivity seismometers are close to the self-noise level. We then show that a reference phase velocity dispersion curve (RPVDC), created mainly based on the first zero crossings at each site, has a spatial distribution that well corresponds to the geology and topography and is consistent with that obtained in a previous study. We inverted five RPVDCs to model 1-D S-wave profiles and validated them using S-wave profiles obtained from velocity logs at nearby deep (e.g. hundreds of metres) boring wells. The accuracy of phase velocities at the later zero crossings for three-sensor/four-sensor arrays and all zero crossings for two-sensor arrays are statistically examined (maximum of 9805 data) based on a comparison with the RPVDCs. The disadvantage of the zero-crossing method is that it can only provide information on phase velocities at discrete wavelengths up to a maximum wavelength of 2.6r (i.e. corresponding to the first zero-crossing point), where r is the radius of a seismic array. Therefore, the RPVDCs were then used to examine the upper limit of the analysable wavelength ranges for the conventional SPAC method for microtremor array surveys. Based on a few hundred three-sensor/four-sensor arrays, it was found that for arrays with radii larger than several tens of metres, three-quarters of the upper limit wavelengths (ULWs) stayed within 5r. For arrays with radii smaller than this value, the ULWs strongly depended on the array radius; the ULWs dramatically increased with decreasing array radius. For example, for arrays with an r value of 0.6 m, half of 336 data ranged between 26r and 54r, and the maximum ULW reached 186r. This strong size dependence can be explained by differences in SNR.

Analysis of the Effect of Sio2 Film on the Characteristics of Rayleigh Wave in Zno/Sic Structure

The influence of SiO2 film on the phase velocity and the electromechanical coupling coefficient of ZnO/SiC structure was analyzed by finite element method. The results show that the middle SiO2 film layer reduces the phase velocity of Rayleigh wave and significantly increases the electromechanical coupling coefficient. When the top SiO2 film layer thickness is larger than the ZnO film thickness, the Rayleigh wave phase velocity increases. Otherwise, the Rayleigh wave phase velocity decreases. The electromechanical coupling coefficient increases first and then decreases. These conclusions can guide the development of surface acoustic wave devices with high phase velocity and high electromechanical coupling coefficient.

Thermoelastic Rayleigh wave: explicit expression for complex velocity

Propagation of Rayleigh waves is considered in an isotropic thermoelastic semi-infinite medium with isothermal or insulated boundary. This requires to solve an irrational complex equation for an implicit complex velocity. A complex analysis technique is used to solve this implicit equation so as to derive an explicit expression for the velocity of the Rayleigh wave in thermoelastic materials. The same expression allows calculation of the velocity of Rayleigh wave in thermo-viscoelastic medium as well.

High speed crack propagation characteristics of functionally graded brittle materials under ultra-high loading rate

Functionally graded materials are considered to be a special composite material. By continuously changing the composition and structure of the material, the interface and delamination problems of the material can be eliminated, which causes the material properties to change slowly with the change of the composition and structure of the material. In this paper, a lattice spring model, which can accurately show the mechanical properties of brittle materials, is used to study the evolution law and mechanism of the crack path and crack velocity in functionally graded brittle materials under ultra-high loading rate. This model uses the quantitative parameter mapping method proposed by Gusev to solve the spring stiffness coefficient of the model accurately, and the fracture criterion of the model based on the energy balance principle of Griffith is set up, using the Fortran software construct the lattice spring model for functionally graded brittle materials and carry out the numerical simulation, the evolution of crack propagation path and velocity in functionally graded brittle materials under ultra-high loading rate is studied. The simulation results show that the propagation path of high speed cracks in functionally graded brittle materials under the action of ultra-high loading rate has two stages: single crack and bifurcation crack. Once the crack reaches the critical condition of crack initiation, it will start to crack and grow at a high speed. The crack propagation velocity suddenly jumps from 0 to more than 0.5 times the Rayleigh wave velocity(CR), followed by a process of speed decline and recovery; after the initial drastic change, the growth speed gradually reaches a stable platform. Thereafter, the growth rate slowly climbed and exceeded 0.69CR; due to the stress relaxation caused by the crack initiation, the maximum tensile stress at the crack tip also showed a process of declining and slowly rising. Through comparison, it is found that the change law of the crack tip has a significant correlation with the change law of crack growth rate (high-speed growth stage), which is helpful to reveal the internal mechanism that determines the crack growth rate. The study of crack propagation under different loading rates can predict the crack propagation speed, direction and distance in functionally graded brittle materials.

Connection of microstructure and propagation velocities of transverse and Rayleigh waves in steel bars 45

Connection of microstructure and propagation velocities of transverse and Rayleigh waves in steel bars 45

ОБ АКУСТИЧЕСКИХ ПАРАМЕТРАХ МЕТАЛЛА КОНСТРУКЦИИ ПРИ НАКОПЛЕНИИ ПОВРЕЖДЕНИЙ

In the work with the use of nondestructive control methods, the issues of determining acceptable durability or safe resource are considered. It is shown that the design monitoring procedure can be presented as follows: determines the original state zero zone, where the material of the product was subjected to low operational loads; further, with the use of nondestructive control methods, the acoustic parameter is measured (without disassembling the product)., such as the speed of elastic acoustic waves, nonlinear acoustic parameter, difference of velocities with twofrequency sensing, etc.; acoustic scanning of the product's metal is performed, in areas where there have been significant loads, signoff voltages, leading to an intensive accumulation of damage (the destruction of metal leading to cracks); defined zone “N” where the metal state parameter, for which the value is taken, characterizing the difference of the acoustic parameter (the speed of elastic waves, nonlinear acoustic parameter, the difference in velocity in twofrequency sensing) relative to the same parameter in the zone of zero exceeds the established level. The regularities established in the work linking the presence of plastic deformation with the difference in the delays (velocity) of elastic Rayleigh waves at different sounding frequencies at a fixed base between the emitter and the receiver of elastic waves, as well as the behavior of a nonlinear acoustic parameter during the safe resource time, suggest the possibility of using the observd fact as a principle for controlling the limiting state of the material due to plastic deformations on industrial structures. Based on the proposed approach, an engineering methodology for determining the technical condition of the material of the structures of production facilities is proposed, which allows to establish three stages of operation: the reliable operation mode; the controlled operation mode and the critical operation mode.

Изменение скорости ультразвука в процессе растяжения сплава Fe-Cr-Ni при температурах 180-318 К

Investigations of the mechanical characteristics and changes in the propagation velocity of ultrasound (Rayleigh waves) during plastic deformation of the Fe-Ni-Cr alloy in the temperature range 180 ≤ T ≤ 318 K. The implementation of the method for measuring the velocity of Rayleigh waves consisted in the periodic generation of rectangular pulses with a duration of 100 ns at the input of the radiating piezoelectric transducer and registration of the wave passed through the sample by means of a receiving piezoelectric transducer connected to a digital oscilloscope. It was found that a decrease in the temperature of the alloy under study changes not only the type of the deformation curve under uniaxial tension, but also changes the character of the dependence of the ultrasound velocity on deformation and stresses associated with the growth of the martensitic α'-phase formed as a result of γ-α'- phase transformation.