Elastic Surface Waves Guided by Thin Films

Estimation of near-surface shear-wave velocities and quality factors using multichannel analysis of surface-wave methods

Comparative analysis of Rayleigh and Love waves detected propagating in the Nobi and Kanto basins during the 2004-, 2007- Chuetsu and 2011 Tohoku earthquakes

The surface wave structure is investigated by using acoustic spectro-microscopy for various coarse-grained nickel crystals with low symmetry. Experimentally measured angular distributions of the surface wave velocity are in fairly good agreement with the theoretically calculated ones. Although the measured surface wave velocity dispersions are independent of the ultrasonic frequency within a range from 40 MHz to 140 MHz, the surface wave attenuation has significant dependency. It may be noted in our methodology that, not only the Rayleigh surface wave velocity distributions but also the pseudo-surface wave velocity distributions can be determined for linking the elastic anisotropy with crystallographic orientations. A systematic examination through direct measurement and evaluation of the surface wave structure reveals that the angular dispersion of pseudo-surface wave velocity is more sensitive to crystallographic orientations than that of Rayleigh surface wave velocity.

The gelation process of tungstic acid was investigated by observing surface waves at two points on a sample surface. We used a deflected-light detection system, in which the surface waves were excited with a PZT bimorph vibrator. From the propagation time (δt) between the two isolated measuring points, surface wave velocity was calculated. We also estimated attenuation of the surface wave from the ratio of the observed amplitudes at the measuring points. Around the gelation time (t g), which was determined by the tilting test, a marked increase in the surface wave velocity was observed. The attenuation coefficient showed frequency dispersion in the frequency range from 20 to 60 Hz, which reached a maximum value around t g.

For an isotropic linearly elastic half-space whose elastic moduli and mass density depend on the distance from the free surface, the surface wave motion radiating from a time-harmonic point load applied normal to the surface of the half-space is considered in this paper. The surface wave velocity, which depends on the wave number, together with expressions for the displacements and stresses of free surface waves, are obtained in a high-frequency approximation. Application of the reciprocity theorem using general expressions for the surface wave radiated by the oscillating point load, together with a virtual free surface wave, yields relatively simple expressions for the amplitude factor of the radiated surface wave. For a few examples, results show the velocity of surface waves and the amplitude factor of the surface waves as a function of the wave number.

Dispersive characteristic measurement of multi-layer cement mortar slabs using SASW method and neural network

The dispersive properties of surface waves on an isotropic elastic body with elastic moduli and mass density that depend on depth have been analyzed in the high frequency range, for the case of axially symmetric surface waves, which are of interest for point loading of a body. The method of approach requires some simplifications, but the final analysis yields simple expressions for the displacements, for the case that the two elastic moduli and the mass density each have different dependencies on depth. In a high-frequency approximation expressions are obtained for the displacements and the stresses. The condition that the surface tractions vanish at the free surface yields the dispersion equation which relates the surface wave velocity to the wavenumber. Conditions have been derived for a class of examples for which this equation yields a real valued surface wave velocity, and the displacement amplitudes decay exponentially with depth. Results for the surface wave velocity as a function of the wavenumber have been compared with numerical results which were obtained when the continuous inhomogeneity with depth is replaced by an equivalent layering. For some typical cases of increasing and decreasing material properties with depth, excellent agreement has been obtained between analytical and numerical results.

We present a shear velocity model of the crust and uppermost mantle under the Aegean region by simultaneous inversion of Rayleigh and Love waves. The database consists of regional earthquakes recorded by portable broadband three-component digital stations that were installed for a period of 6 months in the broader Aegean region. For each epicentre–station ray path group velocity dispersion curves are measured using appropriate frequency time analysis (FTAN). The dispersion measurements for more than 600 Love wave paths have been used. We have also incorporated previous results for c . 700 Rayleigh wave paths for the study area. The single-path dispersion curves of both waves were inverted to regional group velocity maps for different values of period (6–32 s) via a tomographic method. The local dispersion curves of discrete grid points for both surface waves were inverted nonlinearly to construct 1D models of shear-wave velocity v. depth. In most cases the joint inversion of Rayleigh and Love waves resulted in a single model (from the multiple models compatible with the data) that could interpret both Rayleigh and Love wave data. Around 60 local dispersion curves for both Rayleigh and Love waves were finally jointly inverted. As expected, because of the complex tectonic environment of the Aegean region the results show strong lateral variations of the S-wave velocities for the crust and uppermost mantle. Our results confirm the presence of a thin crust typically less than 28–30 km in the whole Aegean Sea, which in some parts of the southern and central Aegean Sea becomes significantly thinner (20–22 km). In contrast, a large crustal thickness of about 40–45 km exists in western Greece, and the remaining part of continental Greece is characterized by a mean crustal thickness of about 35 km. A significant sub-Moho upper mantle low-velocity zone (LVLmantle) with velocities as low as 3.7 km s −1 , is clearly identified in the southern and central Aegean Sea, correlated with the high heat flow in the mantle wedge above the subducted slab and the related active volcanism in the region. The results obtained results are compared with independent body-wave tomographic information on the velocity structure of the study area and exhibit a generally good agreement, although significant small-scale differences are also identified.

S U M M A R Y Thin plate flexure theory provides an accurate model for the response of the lithosphere to vertical loads on horizontal length scales ranging from tens to hundreds of kilometres. Examples include flexure at seamounts, fracture zones, sedimentary basins and subduction zones. When applying this theory to real world situations, most studies assume a locally uniform plate thickness to enable simple Fourier transform solutions. However, in cases where the amplitude of the flexure is prominent, such as subduction zones, or there are rapid variations in seafloor age, such as fracture zones, these models are inadequate. Here we present a computationally efficient algorithm for solving the thin plate flexure equation for non-uniform plate thickness and arbitrary vertical load. The iterative scheme takes advantage of the 2-D fast Fourier transform to perform calculations in both the spatial and spectral domains, resulting in an accurate and computationally efficient solution. We illustrate the accuracy of the method through comparisons with known analytic solutions. Finally, we present results from three simple models demonstrating the differences in trench outer rise flexure when 2-D variations in plate rigidity and applied bending moment are taken into account. Although we focus our analysis on ocean trench flexure, the method is applicable to other 2-D flexure problems having spatial rigidity variations such as seamount loading of a thermally eroded lithosphere or flexure across the continental–oceanic crust boundary.

In this study, the influence of axial stress on surface wave propagation along cylindrical surfaces is investigated, with particular emphasis on quantifying curvature effects on acoustoelastic coefficients. The classical planar surface wave acoustoelastic formulation is first adopted as a reference. Three-dimensional transient finite element simulations are then performed to model surface wave excitation, propagation, and reception on aluminum cylinders with different radii and excitation frequencies. Stress-free simulations are used to extract surface wave velocities and reference time signals, while prestressed simulations provide stress-induced time delays, from which effective acoustoelastic coefficients are determined. The results indicate that both the surface wave velocity and the acoustoelastic coefficient exhibit clear dependencies on cylinder radius and excitation frequency. Curvature effects are especially pronounced at low frequencies, whereas at higher frequencies the coefficients corresponding to different radii tend to converge. These findings demonstrate that planar surface wave theory may lead to non-negligible errors when applied to cylindrical geometries and provide quantitative guidance for curvature-aware stress evaluation.

The characterization of the embankment can be done by constructing the shear wave velocity in subsurface. The investigation of near surface wave velocity using Rayleigh wave dispersion analysis is popular. This method is very fast and effective to construct the vertical wave velocity, but has no unique solution. As a result, the inverse modelling is often trapped in local optimums. Therefore, we used Quantum-behaved PSO to get the best solution (global optimum). Inversion method using Quantum-behaved PSO was performed on synthetic and field measurement data. Field data acquisition was executed at P80-81 using both active and passive surface waves methods. The inversion result of Rayleigh wave dispersion is shear wave velocity structure. Shear wave velocity imaging resulted by this method was used to characterize the embankment of mud flow dam in Sidoarjo (LUSI) and be compared to the resistivity measurement data. The subsurface structure which is constructed from the inversion of Rayleigh wave gives the same result as the resistivity.

Approximately 2 yr (2010–2011) of continuous seismic records from a subset of the Antarctic component of the Polar Earth Observing Network (POLENET-ANET) seismic network deployed in West Antarctica are used to compute the nine components of the correlation tensor between each pair of stations in the network. Rayleigh wave velocity information from the vertical and radial components was extracted in the form of group and phase velocity dispersion curves, whereas the transverse component provided complimentary Love wave velocity information. The multicomponent Rayleigh wave measurements (ZZ, RR, ZR, and RZ) were averaged and used to infer the measurement uncertainties. The Rayleigh and Love wave group and phase velocities were then regionalized in space using a 2D deterministic tomography. A transect that spans the West Antarctic rift system was extracted from the tomography at individual periods between 7 and 60 s for the four types of surface wave velocities (i.e., Rayleigh and Love phase and group velocities). A transdimensional Bayesian joint inversion algorithm was used to invert these four datasets for a 1D model of isotropic shear-wave velocity versus depth at each point along the transect. In this way, surface wave dispersion curves from multicomponent noise correlations were used to build a 2D isotropic shear-wave velocity model down to ∼55 km depth. In this model, the top of the large low-velocity zone beneath Marie Byrd Land was imaged (up to a 5% decrease in velocity at ∼50 km depth), which provides further evidence for a mantle hot spot beneath the crust that supports the high topography in this region. We also observed a large velocity contrast in the lower crust beneath Marie Byrd Land at a depth where previous long-period seismicity has been observed. This strong contrast occurs more shallow than in previous crustal models, which compared to our model identify a deeper Moho (∼5–10 km deeper) beneath Marie Byrd Land. This new model has implications for interpreting earthquake locations in this region and perhaps necessitates that we revisit past hypocenter estimation studies using updated velocity models for the region.

In this paper, we investigated the propagation of surface waves in a nonhomogeneous rotating fibre-reinforced viscoelastic anisotropic media of higher order of nth order including time rate of strain. The general surface wave speed is derived to study the effect of rotation on surface waves. Particular cases for Stoneley, Love and Rayleigh waves are discussed. The results obtained in this investigation are more general in the sense that some earlier published results are obtained from our result as special cases. Also results for homogeneous media can be deduced from this investigation. For order zero our results are well agreed to fibre-reinforced materials. Also by neglecting the reinforced elastic parameters, the results reduce to well known isotropic medium. It is also observed that, surface waves cannot propagate in a fast rotating medium. Comparison was made with the results obtained in the presence and absence of rotation and parameters for fibre-reinforced of the material medium Numerical results are given and illustrated graphically. The results indicate that the effect of rotation and parameters for fibre-reinforced of the material are very pronounced.

On the influence of a flexural biasing state on the velocity of piezoelectric surface waves

The primary objective of this study is to investigate the feasibility of an innovative surface-mount sensor, made of a piezoelectric disc (PZT sensor), as a consistent source for surface wave velocity and transmission measurements in concrete structures. To this end, one concrete slab with lateral dimensions of 1500 by 1500 mm and a thickness of 200 mm was prepared in the laboratory. The concrete slab had a notch-type, surface-breaking crack at its center, with depths increasing from 0 to 100 mm at stepwise intervals of 10 mm. A PZT sensor was attached to the concrete surface and used to generate incident surface waves for surface wave measurements. Two accelerometers were used to measure the surface waves. Signals generated by the PZT sensors show a broad bandwidth with a center frequency around 40 kHz, and very good signal consistency in the frequency range from 0 to 100 kHz. Furthermore, repeatability of the surface wave velocity and transmission measurements is significantly improved compared to that obtained using manual impact sources. In addition, the PZT sensors are demonstrated to be effective for monitoring an actual surface-breaking crack in a concrete beam specimen subjected to various external loadings (compressive and flexural loading with stepwise increases). The findings in this study demonstrate that the surface mount sensor has great potential as a consistent source for surface wave velocity and transmission measurements for automated health monitoring of concrete structures.