Velocity Dispersion Of Surface Waves Research Articles

Structure of the Martian Crust Below InSight From Surface Waves and Body Waves Generated by Nearby Meteoroid Impacts

AbstractWe measure group velocity dispersion of surface waves generated by two meteoroid impacts on Mars close to the lander of the InSight mission. This allows us to probe the crustal structure in the first few kilometers beneath the InSight lander. In combination with body wave arrival times from five impact events, we obtain direct seismic constraints on the seismic velocity of the crust in the vicinity of the InSight landing site. We confirm the existence of a uppermost low‐velocity layer with a mean thickness of ∼1.2 km, interpreted as layered volcanic materials, possibly interstratified with sedimentary and altered materials. Our joint inversion of surface and body waves shows a four‐layer model for the Martian crust, compatible with high‐ and low‐frequency P‐to‐S receiver functions estimated in previous studies.

Data‐Driven Velocity Model Evaluation Using K‐Means Clustering

AbstractWe develop a data‐driven clustering method to evaluate a velocity model using surface wave velocity dispersion. This is done by first computing theoretical dispersion curves for 1‐D velocity profiles of all the grid locations and then splitting the resulting dispersion curves into a certain number of groups via the K‐means clustering. The observed dispersion curves are also clustered following the same procedure and the velocity model is assessed by comparing the spatial patterns obtained for the observed and synthetic data sets. The method is applied to evaluate two community velocity models in southern California, CVM‐S4.26 and CVM‐H15.1, using phase velocity maps derived for 3–16 s Rayleigh waves. We found a good correlation in the spatial distribution of clusters between the result of CVM‐S4.26 and that of the observed data, suggesting that the CVM‐S4.26 fits the observed dispersion maps better than the CVM‐H15.1 in terms of features extracted from the clustering analysis.

What we can learn about slope response to earthquakes from ambient noise analysis: An overview

Earthquake induced slope failures are responsible for a significant amount of life loss and damage, and their effective mitigation requires further advancements in our comprehension of slope behaviour under seismic shaking. One source of uncertainty in seismic landslide susceptibility assessment is the phenomenon of enhanced amplification of ground motion along down slope directions. This implies a strength demand beyond that estimated by standard slope stability analysis. An extensive accelerometer monitoring of slope dynamic response in areas exposed to seismic landslide hazard is unfeasible. An alternative approach can take advantage of recent development of reconnaissance techniques based on the analysis of ambient noise recorded by portable instruments. The most popular technique, known as Nakamura or HVNR method, consists in analysing H/V spectral ratios between Horizontal and Vertical components of Noise Recording, and allows the recognition of site resonance frequencies. The application of HVNR to complex site conditions typical of marginally stable slopes is often difficult and requires the development of “ad hoc” procedures both for acquisition and analysis of noise recording. Tests in different geologic and geomorphic settings show that an analysis of azimuthal variation of spectral ratios can reveal the presence and orientation of directional resonance, whereas the recognition of main resonance frequencies requires a proper selection of signals to be analysed. Efforts to evaluate amplification factors currently rely on numerical simulations, which in turn require S-wave velocity of slope materials. Ambient noise analysis in terms of velocity models can contribute through the inversion of H/V spectral ratios and surface wave velocity dispersion curves derived from the processing of multiple simultaneous noise recordings. However these applications require a correct identification of the nature of surface waves present in the noise recording.

Compaction evolution observed via the HVSR of microtremors

The non-invasive measurement of horizontal to vertical spectral ratio (HVSR) of microtremors is proposed as a means to evaluate field compaction. The HVSR technique is a surface wave method that is quite distinct from the class of array-based surface wave techniques. This is because the basis of the former is the recording and fitting of the HVSR of microtremors, while the latter relies on the recording and fitting of the surface wave velocity dispersion curves of ground vibrations. This paper presents a controlled study of the evolution of compaction, showing it is possible to identify levels of field compaction by observing changes in HVSR data. The results from this study suggest that the fast and operationally simple HVSR technique could be used to assess near-surface ground compaction. It is further envisaged that the technique could be applied to ‘cover the gaps’ left from traditional and expensive invasive techniques (e.g. cone penetration test, standard penetration test, dilatometer test, etc.), particularly for extensive and deep-filled areas such as would occur in dynamically compacted sites.

Time–Frequency–Wavenumber Analysis of Surface Waves Using the Continuous Wavelet Transform

A modified approach to surface wave dispersion analysis using active sources is proposed. The method is based on continuous recordings, and uses the continuous wavelet transform to analyze the phase velocity dispersion of surface waves. This gives the possibility to accurately localize the phase information in time, and to isolate the most significant contribution of the surface waves. To extract the dispersion information, then, a hybrid technique is applied to the narrowband filtered seismic recordings. The technique combines the flexibility of the slant stack method in identifying waves that propagate in space and time, with the resolution of f–k approaches. This is particularly beneficial for higher mode identification in cases of high noise levels. To process the continuous wavelet transform, a new mother wavelet is presented and compared to the classical and widely used Morlet type. The proposed wavelet is obtained from a raised-cosine envelope function (Hanning type). The proposed approach is particularly suitable when using continuous recordings (e.g., from seismological-like equipment) since it does not require any hardware-based source triggering. This can be subsequently done with the proposed method. Estimation of the surface wave phase delay is performed in the frequency domain by means of a covariance matrix averaging procedure over successive wave field excitations. Thus, no record stacking is necessary in the time domain and a large number of consecutive shots can be used. This leads to a certain simplification of the field procedures. To demonstrate the effectiveness of the method, we tested it on synthetics as well on real field data. For the real case we also combine dispersion curves from ambient vibrations and active measurements.

Noncontact, nondestructive elasticity evaluation of sound and demineralized human dental enamel using a laser ultrasonic surface wave dispersion technique

Laser ultrasonic nondestructive evaluation (NDE) methods have been proposed to replace conventional in vivo dental clinical diagnosis tools that are either destructive or incapable of quantifying the elasticity of human dental enamel. In this work, a laser NDE system that can perform remote measurements on samples of small dimensions is presented. A focused laser line source is used to generate broadband surface acoustic wave impulses that are detected with a simplified optical fiber interferometer. The measured surface wave velocity dispersion spectrum is in turn used to characterize the elasticity of the specimen. The NDE system and the analysis technique are validated with measurements of different metal structures and then applied to evaluate human dental enamel. Artificial lesions are prepared on the samples to simulate different states of enamel elasticity. Measurement results for both sound and lesioned regions, as well as lesions of different severity, are clearly distinguishable from each other and fit well with physical expectations and theoretical value. This is the first time, to the best of our knowledge, that a laser-based surface wave velocity dispersion technique is successfully applied on human dental enamel, demonstrating the potential for noncontact, nondestructive in vivo detection of the development of carious lesions.

Recursive geometric integrators for wave propagation in a functionally graded multilayered elastic medium

The differential equations governing transfer and stiffness matrices and acoustic impedance for a functionally graded generally anisotropic magneto-electro-elastic medium have been obtained. It is shown that the transfer matrix satisfies a linear 1st order matrix differential equation, while the stiffness matrix satisfies a nonlinear Riccati equation. For a thin nonhomogeneous layer, approximate solutions with different levels of accuracy have been formulated in the form of a transfer matrix using a geometrical integration in the form of a Magnus expansion. This integration method preserves qualitative features of the exact solution of the differential equation, in particular energy conservation. The wave propagation solution for a thick layer or a multilayered structure of inhomogeneous layers is obtained recursively from the thin layer solutions. Since the transfer matrix solution becomes computationally unstable with increase of frequency or layer thickness, we reformulate the solution in the form of a stable stiffness-matrix solution which is obtained from the relation of the stiffness matrices to the transfer matrices. Using an efficient recursive algorithm, the stiffness matrices of the thin nonhomogeneous layer are combined to obtain the total stiffness matrix for an arbitrary functionally graded multilayered system. It is shown that the round-off error for the stiffness-matrix recursive algorithm is higher than that for the transfer matrices. To optimize the recursive procedure, a computationally stable hybrid method is proposed which first starts the recursive computation with the transfer matrices and then, as the thickness increases, transits to the stiffness matrix recursive algorithm. Numerical results show this solution to be stable and efficient. As an application example, we calculate the surface wave velocity dispersion for a functionally graded coating on a semispace.

Macroscopic and Microscopic Elastic Constant Measurements of Ceramic Matrix Composites Using Ultrasonic Waves

This paper presents the results of elastic constant determination of Ceramic Matrix Composites (CMC) using two types of ultrasonic measurement systems, and shows the possibility of the elastic constants determination for transversely isotropic materials not only in a macroscopic area but also in a microscopic one at the order of 1 mm. Macroscopic elastic constants were determined from the transmitted wave velocities measured by a double-through-transmission method. Microscopic elastic constants were inverted from the reflection coefficients at a small spot on the surface, which were measured using an ultrasonic micro-spectrometer. The inversion from the reflection coefficients was tried by two ways of analysis: (1) fitting to phase curves of reflection coefficients; (2) fitting to surface wave velocity dispersion. By comparison between these values and those measured by tensile tests, it was conformed that all elastic constants of a unidirectional CMC could be determined by ultrasonic waves appropriately not only in a macroscopic area but also in a microscopic one.

Evaluation of Mortar Layer Thickness on Concrete Boards Using the Surface Wave Velocity Dispersion Effect

A novel nondestructive test for the thickness of a mortar layer on the surface of a concrete building wall board is proposed in this paper. The method is based on the surface wave velocity dispersion effect in the mortar layer. The velocity of the surface wave varies from 1850 m/s to 1500 m/s at 77 kHz as a function of the thickness of the mortar layer from 10 mm to 30 mm.

Article

We first present a sea-based sedimentary mock-up designed in our acoustic laboratory. It is polymerized plate with a variable reticulation rate and thus with continuous (and linear) velocity profiles. In a plane geometry, the group velocity dispersion of surface waves, which propagate at the water-sediment interface, provides information on impedance profiles of the geophysical sedimentary structure. The dispersion equation is numerically solved from a multi-layer model. The experimental study of the dispersion requires a specific set-up to generate the interface wave (solid wedge method). The analysis of the received signals is carried out using a time-frequency method based on the Wigner-Ville distribution. Experimental and theoretical dispersion curves are compared

Deep structure of the Iberian Peninsula determined by Rayleigh wave velocity inversion

SUMMARY A rigorous study of velocity dispersion of surface waves generated by teleseismic events propagating across the Iberian Peninsula and traversing main geological units, has been carried out from a set of selected analogue data, as digital records have only become available recently. Dispersed seismic signals have been obtained over a period of 16 years, between 1967 and 1982, at the five Iberian stations having long-period instruments. In our study, we have considered many earthquakes thus obtaining a fairly good path coverage of most of the peninsula for two-station Rayleigh wave velocity measurements. In all cases, the approach azimuths of the wavefronts were carefully checked. Several digital filtering techniques have been employed to remove the effects of multipathing and modal contamination, and to isolate the fundamental mode from Rayleigh wavetrains. Thus, we have obtained good estimates for both phase and group velocities. A time-variable filter has reduced the influence of noise and removed higher mode interference. Multiple filtering is then used to compute group velocity. Frequency-domain Wiener deconvolution is used to compute the interstation phase velocity. The determined average Rayleigh wave velocities reveal differences in the propagation conditions of the seismic energy across the peninsula. A mapping of velocities for various periods of reference, together with a mapping of errors in velocity, are the basis for obtaining the Rayleigh wave velocity distribution in the peninsula. Theoretical 2-D layered earth models are obtained by joint inversion of phase and group velocity dispersion curves using the stochastic inverse operator. In our inversion scheme, we use velocities corrected for anelastic effects. Finally, a 3-D mapping of S velocity is performed. This study shows important regional features of the deep structure of Iberia; we see small lateral inhomogeneities and also two low-velocity layers: one with shear velocities usually ranging from 4.23 to 4.31 km s-1 directly under the Moho, and another, the asthenosphere, with a negative velocity gradient for depths between 81 and 181 km, terminated at the bottom by a sharp discontinuity.

Dispersion of acoustic surface waves by velocity gradients

The perturbation theory of Auld [Acoustic Fields and Waves in Solids (Wiley, New York, 1973), Vol. II, p. 294], which describes the effect of a subsurface gradient on the velocity dispersion of surface waves, has been modified to a simpler form by an approximation using a newly defined velocity gradient for the case of isotropic materials. The modified theory is applied to nitrogen implantation in AISI 4140 steel with a velocity gradient of Gaussian profile, and compared with dispersion data obtained by the ultrasonic right-angle technique in the frequency range from 2.4 to 14.8 MHz. The good agreement between experiments and our theory suggests that the compound layer in the subsurface region plays a dominant role in causing the dispersion of acoustic surface waves.