Near-surface Elastic Properties Research Articles

Investigation of acoustoelastic surface acoustic waves in prestressed media

Dispersion of broadband surface acoustic waves (SAWs) is often used for the nondestructive evaluation of near-surface elastic properties of materials. The accurate forward model of SAW propagation is essential for inversion. In this paper, we present a combining semi-analytical finite element (SAFE) and perfectly matched layer method for accurately predicting the phase velocity dispersion, attenuation and polarization modes of SAWs in any prestressed anisotropic layered half-space. Equations of motion are formulated based on the Biot's theory of small deformations influenced by initial stress, so it does not matter whether the residual stress results from hyperelastic transformations. The simulated results are firstly validated by the solutions from the exact three-dimensional elasticity theory-based method. The SAFE method is then used to discuss the effect of material anisotropy and residual stress on the SAWs. Finally, the method is applied to two practical examples. The differences and similarities of two common types of acoustoelastic theories are also discussed.

P/S-wave separation of multicomponent seismic data at the land surface based on deep learning

P/S-wave separation is a key step for data processing in multicomponent seismic exploration. The conventional methods rely on either the prior information of near-surface elastic properties or the carefully selected parameters to estimate the polarization directions of the P- and S-modes when arriving at the geophones. In the case of complex wave propagation or the near-surface elastic parameters being unavailable, the conventional methods fail to achieve a high-quality P/S separation. The P/S-wave separation of multicomponent seismic data recorded at the land surface can be regarded as a nonlinear problem of point-by-point image prediction and addressed by using the powerful ability for feature extraction of a deep neural network. To obtain an accurate, abundant, and representative labeled data set, a variety of elastic models are constructed through geologically reasonable data augmentation, followed by elastic forward modeling and Helmholtz decomposition. A convolutional neural network is trained with these labeled data to predict scalar P- and S-wave recordings from the multicomponent data in a data-driven manner. Eventually, a novel approach of P/S-wave separation is established for multicomponent seismic imaging with the help of deep learning, which can work without prior knowledge of the near-surface elastic parameters. Numerical examples indicate that, by constructing an augmented labeled data set for training, our method significantly improves the generalization of the deep neural network and thus enables the trained neural network to achieve more accurate P/S-wave separation than the conventional model-driven method for the target data.

Dynamic acoustoelastic testing (DAET) with a thermal strain pump for characterization of closed fatigue cracks

Dynamic acoustoelastic testing (DAET) is a nonlinear acoustic technique used to characterize the nonlinear elastic behavior of materials. To enable the in-situ implementation of DAET, we propose a modification to the conventional DAET setup by replacing the mechanical strain pump with a thermal strain pump. This study presents the feasibility of the proposed thermal-pump DAET setup for in-situ detection of closed fatigue cracks in an aluminum test specimen. Our setup employs a hot air blower to induce a small (∼4 °C) thermal gradient near the surface of the specimen. The surface temperature evolution is captured by an infrared camera allowing thermal strain evaluation on the surface. A pair of high frequency angle-beam Rayleigh surface wave transducers are used to probe the thermal strain-induced changes in near-surface elastic properties. The slope of the relative change in Rayleigh wave speed with the applied thermal strain is taken as a measure of nonlinearity (γ) evaluated at several locations across the specimen along the intact and cracked regions. Our results show that γ measures higher near the crack and decreases as the probe moves towards the crack tip, demonstrating the potential of thermal-pump DAET for in-situ evaluation of closed fatigue cracks.

Unsupervised machine learning algorithm for detecting and outlining surface waves on seismic shot gathers

Surface waves are useful signals in seismic inversions of near-surface elastic properties. However, they are considered as noise in reflection seismology when exploring for hydrocarbons. Detecting and outlining the area of surface waves in seismic gathers facilitates either their inversion for sub-surface structures or their noise removal. We propose an unsupervised machine-learning algorithm to automatically distinguish surface waves in the raw seismic data according to their common features, such as low frequency, low velocity, and high amplitude when compared to body waves. The local attributes of frequency, amplitude, and velocity, which are obtained by Gabor frequency and structure tensor, are used in K-means analyses to detect and outline surface waves on seismic shot gathers. The performance of this new algorithm is evaluated on several datasets, including two synthetic datasets with random noise and missing traces, and a field dataset. In these examples, the algorithm is stable and successfully outlines the surface waves.

Modeling micromechanical measurements of depth-varying properties with scanning acoustic microscopy

Scanning acoustic microscopy (SAM) has been applied to measure the near-surface elastic properties of materials. For many substrates, the near-surface property is not constant but varies with depth. In this paper, we aim to interpret the SAM data from such substrates by modeling the interaction of the focused ultrasonic field with a substrate having a near-surface graded layer. The focused ultrasonic field solutions were represented as spherical harmonic expansions while the substrate solutions were represented as plane wave expansions. The bridging of the two solutions was achieved through the decomposition of the ultrasonic pressure fields in their angular spectra. Parametric studies were performed, which showed that near-surface graded layers exhibit distinctive frequency dependence of their reflectance functions. This behavior is characteristic to the material property gradation profile as well as the extent of the property gradation. The developed model was used to explain the frequency-dependent reflection coefficients measured from an acid-etched dentin substrate. Based on the model calculations, the elastic property variations of the acid-etched dentin near-surface indicate that the topmost part of the etched layer is very soft (3–6 GPa) and transitions to the native dentin through a depth of 27 and 36 microns.

Acoustic field radiated into a transversely isotropic solid from a small aperture spherical surface

The acoustic field modelling reported in this paper finds application in the design of a scanning probe tip for measuring the near-surface elastic properties of solids and surface structures at high frequencies and with high spatial resolution. The underlying concept is for a longitudinally polarized pulse to be launched from a spherically-shaped portion of the upper surface of the pyramidal or conical shaped tip, and focused towards the narrow lower end. The change in the reflectivity when the narrow end is brought into contact with a solid will provide a measure of the local frequency dependent compliance of that solid. The calculations assume the material from which the tip is fabricated to be transversely isotropic, with symmetry axis coinciding with the axis of the tip. The main issue addressed in this paper is the role of the curvature of the radiating surface and anisotropy of the medium in determining the focal length and focal spread of the radiated field. Two complementary approaches are taken, firstly the discretization of the equations of motion on an irregular mesh of around 3 × 10 5 triangular elements and solution using the commercial FE package ABAQUS/Explicit, and secondly an analytical approach based on ray tracing and a Green’s function method exploiting the angular spectrum method and stationary phase approximation in its evaluation. Consistency is achieved between these approaches regarding the characteristics of the focal region. With the combination of the two approaches it is thus possible to model the wave field from low frequencies, where the FE method is computationally economical and able to handle complex geometries, to high frequencies, where advantage increasingly lies with ray tracing and the Green’s function method.

Theoretical analysis of acoustic waves propagating in materials with continuous variations of near-surface elastic constants

The near-surface elastic properties (NSEPs) of materials take on continuous distribution after surface modification such as surface polishing. In this paper, the influence of the continuous variations of NSEPs on the ultrasonic waves propagating in finite-thick and semi-infinite-thick plates is investigated. An orthogonal polynomial extended method is presented to study the acoustic waves propagating in metals with continuous distribution of NSEPs. By this method, the near-surface layer of the materials does not need to be discrete sublayers, which can avoid the presence of pseudoacoustic waves. The velocities of different ultrasonic wave modes in finite-thick and semi-infinite-thick plates with continuous distributions of NSEPs are obtained. In addition, the influence of the near-surface layer’s thickness and the distribution of the NSEPs on the relative velocity dispersion of Lamb waves and surface acoustic waves are discussed, providing further theoretical foundation for the inversion of near-surface properties.

Low-frequency vibroseis data with maximum displacement sweeps

Several recent advances in land-seismic acquisition equipment and processing algorithms have encouraged exploration geophysicists to reconsider the potential benefits of energetic low frequencies for geophysical applications, imaging beneath high-velocity and highly absorbing formations, deep imaging, acoustic impedance inversion, velocity model estimation in geophysically complex areas, and inversion of surface waves to characterize near-surface elastic properties.

Laser-induced ultrasonic waves in steels with gradient changes of elastic property

Taking account of the gradient changes of the near-surface elastic property, a model of multiple gradient layers on a semi-infinite substrate is established. A finite element method (FEM) model is developed to simulate the laser-induced thermo-elastic generation and propagation of acoustic waves in the system. The two-dimensional fast Fourier transform (2-D FFT) method has been used to analyze the ultrasonic signal. The phase velocity dispersion is obtained for a number of waves of different modes obtained and is analyzed in systems with different spatial variations of the material elastic properties and different affected layer depths. The model can be used to extract the elastic parameters of the near-surface regions in an effective way.

Numerical simulation of laser-generated ultrasonic waves in steel with gradient changes of near-surface elastic property

Numerical simulation of laser-generated ultrasonic waves in steel with gradient changes of near-surface elastic property

The study of guided waves in surfaces and thin supported films using surface Brillouin scattering and acoustic microscopy

This paper reviews the use of surface Brillouin scattering (SBS) and acoustic microscopy (AM) in studying the surface dynamics of solids in order to obtain information about the near-surface elastic properties of solids and thin supported films. The vibrational modes that are probed by these means include Rayleigh surface and pseudo-surface acoustic waves, longitudinal lateral waves (surface skimming bulk longitudinal waves) and various thin film guided modes, such as Sezawa and Love waves. SBS is the inelastic scattering of light, mediated by thermodynamic fluctuations in the surface elevation and near surface elastic strains. The scattering cross-section is conveniently expressed in terms of Fourier domain elastodynamic Green’s functions. AM depends on the insonification of a surface through a coupling fluid, and the resulting excitation and subsequent decay of the various surface modes. The complex reflectivity of the fluid-loaded surface, and the line and point force surface Green’s functions are invoked in the interpretation of different modalities of AM, yielding much the same information about the surface dynamics. The focus in this paper is on the Green’s function approach. A number of illustrative examples, drawn from the authors’ research, are provided.

Applications of scanning acoustic microscopy in analyzing wear and single-point abrasion damage

Wear science has historically relied heavily on observations of surface features to elucidate the fundamental nature of wear processes. A host of surface imaging techniques are currently available, each with its own set of strengths and limitations. This paper considers the application of scanning acoustic microscopy (SAM) for analysis of wear damage processes. Reflection-mode SAM images are produced by the interaction of focused, high-frequency (100 MHz-2 GHz) sound waves with solid surfaces, and contrast is produced by localized variations in near-surface elastic properties. A powerful advantage of SAM is its ability to probe subsurface regions where fractures and delaminations may be concealed from the view of traditional light-optical and electron-optical instruments. The current paper will describe the principles of SAM and illustrate several of its uses in wear surface analysis; specifically, as regards the imaging of elastic strain fields near single-point abrasion damage in silicon carbide, in elucidating the geometry of slip line fields produced during friction tests of an oriented single crystal of stainless steel, and in studying the highly deformed layers of a taper-sectioned intermetallic alloy of Ni 3Al. SAM images are compared with photomicrographs of the same worn areas to illustrate the strengths and limitations inherent in the technique.

The nanoindentation response of systems with thin hard carbon coatings

In order to further our interpretation of nanoindentation data from coated systems, and to learn more about the systems themselves, several hydrogenated amorphous carbon (a-C:H) coated systems have been investigated with systematic variations in coating thickness and substrate characteristics. Three different substrates have been used: a tool steel (M42), a semi-plastic ceramic single crystal (MgO) and a polymer (polytetrafluorethylene (PTFE)), each with chemically vapour-deposited a-C:H coatings of thickness 0.1–1 microm. Low load indentation experiments were performed with a Nano Indenter TM II with indenter displacements typically less than, or of the order of, the nominal coating thicknesses. The resultant data have been analysed in terms of load-displacement curves and various comparative parameters ( e.g. hardness, total displacements and elastic recovery) derived from them. High resolution scanning electron microscopy has also been used to examine the deformation structures at the indention sites, the occurrence of various crack types in the coating being of particular interest. Our results clearly show how the change in near-surface elastic and plastic properties conferred by the coating can be characterized for a given system. Generally, the coatings were found to support part of the load and to delay the elastic-plastic transition in surface deformation behaviour to higher loads. The MgO studies clearly demonstrate that “pop-ins” in the load-displacement curves can arise from dislocation generation in the substrate as well as crack propagation in the coatings. The PTFE studies demonstrate how the viscoelastic response of the substrate can be modified by the coating.