Acoustic Anisotropy Research Articles

Multiscale anisotropy controlled by folding: the example of the Chaudrons fold (Corbières, France)

In this paper, anisotropies developed in silicoclastic continental deposits during the building of the Chaudrons anticline (Corbières, France) are studied. A microstructural analysis of the deformational features in three different panels within the fold (crest, hinge, and forelimb, respectively) is reported and compared with early field observations (distribution and orientation of cleavage) and laboratory measurements (estimation of magnetic and acoustic anisotropies). The main finding of this investigation is the preservation of unwelded joints between grains of calcite promoted by the presence of quartz grains. These joints, which appear as discontinuities in a matrix of calcite, are analyzed in orientation and composition. In the three panels of the fold that are investigated, a range of dip angles is observed with at least two major generations of joints, the average orientation of which is found to be consistent with both macroscopic cleavage and magnetic and acoustic fabrics. To account for the multimodal distribution of the joints orientation, we suggest an original scenario in which they are successively generated by sets. Two processes have operated simultaneously during the development of the fold: (1) horizontal rock mass compaction inducing pressure solution and twinning in calcite; (2) preservation of unwelded calcite/calcite grain joints due to stress heterogeneities associated with quartz inclusions. From these results, we suggest that microstructural processes are the same before and during folding, ruling out a passive shearing of cleavage plane formed during a first step of layer parallel shortening.

Structural and elastic properties ofMgB2under high pressure

We have investigated the structures and the elastic constants of hcp $\mathrm{Mg}{\mathrm{B}}_{2}$ at high pressures using the full-potential linearized muffin-tin orbital (FP-LMTO) scheme within the generalized gradient approximation correction (GGA) in the frame of density functional theory (DFT). The obtained normalized volume dependence of the resulting pressure is in excellent agreement with experiment. The elastic constants and acoustic anisotropy as a function of the applied pressure are presented. An analysis for the calculated parameters has been made to reveal the anisotropy of $\mathrm{Mg}{\mathrm{B}}_{2}$.

Pre-drilling prediction of the tectonic stress field with geomechanical models

Knowledge of the tectonic stress field in a reservoir is essential to optimize drilling and production. Borehole stability, orientations of natural and hydraulically induced fractures, fluid flow anisotropies, among others, all depend critically on the present-day stress distribution. Several techniques ranging from dipmeter analysis of borehole breakouts to anelastic strain recovery and shear acoustic anisotropy analysis of core samples (e.g., Yale, 2003; Sperner et al., 2003) can be used to determine the in-situ stress orientations and relative magnitudes, but obviously this valuable information will only become available after the well has already been drilled. However, there are also numerous cases where the stress orientation should be known prior to drilling. For example, if multiple fracs in a horizontal well are planned, the stress field orientation needs to be known beforehand because for optimal frac design the horizontal well path must be aligned parallel to the orientation of the least principal stress axis σ3. Similarly, the planning of well trajectories with respect to borehole stability as well as the design of secondary and tertiary recovery measures (e.g., water injection, hydraulic fracture treatments) are significantly improved by a pre-drilling knowledge of the subsurface stress field. Information on the regional stress orientations can be derived from large-scale data collections like, for example, the world stress map project (Zoback, 1992; Sperner et al., 2003). The orientation and magnitude of the stress field in sedimentary basins, however, can be highly variable and, particularly near faults, the local stress orientations can differ by up to 90° from the regional trend (e.g., Yale, 2003). In such cases, inference of reservoir-scale in-situ stress orientations from regional scale maps would inevitably lead to an incorrect pre-drilling prediction. This paper uses a numerical modelling approach to determine the magnitude and orientation of the tectonic stresses in a reservoir and, particularly, the local stress perturbations near faults. The model is based on reservoir and fault geometries taken from seismic data and boundary conditions representing the regional stress field. Thus, this tool is also applicable to cases where well data are absent. Following a brief outline of the modelling approach, a case study is presented to assess the practical value of such geomechanical models for the pre-drilling prediction of the tectonic stress field in fault-controlled reservoirs.

Acoustic anisotropy of piezoelectric LiNbO3 crystals and its application in integrated acousto-optical tunable filters

We use Joness matrix method and the global optimal method to investigate the propagation characteristic of surface acoustic wave(SAW) for X-cut single crystal Lithium Niobate(LiNbO3). As an application, a novel integrated acousto-optical filter, which allows to minimize the dimension of AO cells and suppress side-lobes, was designed on the base of LiNbO3 crystal.

Pore fabric shape anisotropy in porous sandstones and its relation to elastic wave velocity and permeability anisotropy under hydrostatic pressure

To understand the relationship between pore space anisotropy and petrophysical properties, we developed a novel apparatus capable of simultaneously measuring permeability, porosity and ultrasonic velocities at hydrostatic pressures up to 100 MPa. First, we use magnetic susceptibilities and acoustic wave velocities to identify the principal anisotropy axes under ambient laboratory conditions. This directional anisotropy data is then used to guide experiments on two sandstones (Bentheim and Crab Orchard) under hydrostatic pressure from 5 to 90 MPa. We find the structural anisotropy formed by the void space is well described by velocity anisotropy in both cases. Under hydrostatic pressure, the acoustic anisotropy of Crab Orchard sandstone (COS) decreases from 3% and 7% at 5 MPa (P-wave and S-wave) to 1.5% and 1%, respectively, at effective pressures over 40 MPa; for Bentheim sandstone the decrease is considerably less. Permeability of COS is 125×10 −18 m 2, decreasing rapidly as effective pressure increases, with permeability parallel to bedding approximately twice that normal to bedding. In contrast, permeability of Bentheim sandstone is 0.86×10 −12 m 2, and varies little with effective pressure or coring direction. We relate many of our measurements made under hydrostatic pressure to the contrasting pore fabric between the two rock types, and infer that a critical pressure is required for the initiation of crack closure.

The effect of temperature on the seismic anisotropy of the perovskite and post-perovskite polymorphs of MgSiO3

Using elastic constants determined by ab initio molecular dynamics over a range of temperatures, we show that the seismic anisotropy of MgSiO3 perovskite is significantly temperature dependent. At 90 GPa, the direction of greatest shear wave splitting changes from [001] at 0 K to [010] at 3500 K. In addition, there is no shear wave splitting in the [100] direction at 0 K, but a significant amount of splitting is exhibited at 3500 K. We have also calculated the first set of high-temperature elastic constants of the new post-perovskite phase by ab initio molecular dynamics methods. We find that, in contrast to MgSiO3 perovskite, temperature has little effect on the acoustic anisotropy of this phase. We determine ∂ln VS/∂ln VP for the post-perovskite phase and find it to be very low, about 0.9. This may be an important parameter for mapping out the post-perovskite phase in the lowermost mantle.

Scattering from bubbles rising in a vertical line—Comparison between theory and experiments

In this paper, the experimental data from the sound field around similar sized air bubbles rising in a vertical chain are analyzed. The data reveal a strong anisotropy in the acoustic field. The transition from individual to continuum behavior in a bubble chain is not correctly described with classical theory especially when the bubbles are uniformly sized, discretely populated, and the frequency of interest is close to the natural frequency of the individual bubbles. Single compact scatterers initiated at frequencies near their resonant frequency in isolation act preliminarily as monopole sources, amplifying the local pressure field by a factor of order 1/(k*a), a being the scattering radius and k the wave number at the resonant frequency in the surrounding material [Tolstoy, 1986]. A laboratory investigation used air bubbles in fresh water and varied the bubble sizes and separation by carefully controlling bubble production rates. A linear coupled equation method was developed to explain the result. The model reproduced the acoustic pressure anisotropy along the chain and the change in pulse waveform along the chain. The results suggest that the enhancement of sound intensity along the chain can be explained by bubbles acting as resonant scatterers retransmitting the acoustic energy.

Influence of Dosed Traction on the Mechanical-Acoustic Properties of the Skin of Human Extremities

The acoustic anisotropy of the skin was studied in healthy children and adults, in patients with a congenitally short extremity or hand syndactyly, and in healthy patients who had their lower extremities lengthened for cosmetic purposes. The age of the subjects varied between 6 and 39 years. In healthy individuals of different ages, the acoustic anisotropy of the skin of the extremities was low. Its character changed after dosed traction of all structures of an extremity segment. A relation between the transition of investing tissue to the oriented and stressed–deformed state and the direction of the traction vector was established.

Acoustoelastic birefringence effect in wood III: ultrasonic stress determination of wood by acoustoelastic birefringence method

Stress conditions produced in wood were analyzed by means of the acoustoelastic birefringence method. Bending load was applied against a wood beam specimen. Under loading, ultrasonic shear waves were propagated through the breadth direction of the wood beam specimen. The velocities of shear waves polarized in the longitudinal or tangential direction of the wood beam specimen were measured with the sing-around method. Bending stresses were determined by dividing the difference between the acoustic anisotropy and the texture anisotropy by the acoustoelastic birefringence coefficient. Shear stresses were also determined. These stress distributions of the beam specimen were in good agreement with those obtained by the strain gauge method and mechanical calculation.

Acoustoelastic birefringence effect in wood I: effect of applied stresses on the velocities of ultrasonic shear waves propagating transversely to the stress direction

The velocity changes of ultrasonic shear waves propagating transversely to the applied stress direction in wood were investigated. The wave oscillation directions were parallel and normal to the uniaxially applied stress direction. The velocities of the shear waves for both oscillations decreased as the compressive load increased, and increased as the tensile load increased. The velocity of the normally oscillated shear wave showed smaller change against the stress applied than that of the parallel oscillated wave. The initial birefringence due to the orthotropy of wood was observed without any stress. Velocity changes in the two principally oscillated shear waves were proportional to the stress within the stress range tested. The acoustoelastic birefringence effect was obtained from the velocity difference between the two shear waves. The relative difference between the two velocities (called acoustic anisotropy) was given as a function of the applied stress. The acoustoelastic birefringence constants were obtained from the relationships between the acoustic anisotropy and the applied stress.

ACOUSTICAL ANISOTROPY AND NON-HOMOGENEITY OF ROLLED STEEL PLATES

Rolled steel plates were subjected to ultrasonic inspection in order to examine acoustical anisotropy and non-homogeneity along the thickness direction. Two vertical planes of the plate were observed under a microscope in order to investigate changes in microstructure. Shear wave velocities, which depend on both propagation and polarization directions, were measured for both 40 mm and 26 mm thick steel plates. Changes in the shear wave velocities VL are found along the thickness direction. Especially, strong local anisotropy is found due to elongated ferrite grains in the near-surface zone. A high angle beam probe should be used carefully for ultrasonic testing on a plate exhibiting strong anisotropy.

Evaluation of Matrix Cracks in FRP Laminates Using Angular Dependence of Lamb Wave Propagation

A temperature-insensitive damage evaluation parameter for composite laminates is proposed. Ultrasonic wave velocity is directly related to material stiffness, so that it is a quantitative parameter that gives information about the mechanical state of the material. However, ultrasonic velocities vary with temperature therefore environmental factors might cause non-negligible error in inspected results. In this work, the angular dependence of Lamb wave propagation, i.e., the acoustic anisotropy, is exclusively focused on and the change in the acoustic anisotropy is used as a damage evaluation parameter. It is experimentally confirmed that the acoustic anisotropy is significantly altered by matrix cracks formation although it is insensitive to temperature variation. It is also shown that the combination of different Lamb modes is a more promising method for predicting the effective stiffness of damaged laminates.

Focusing of surface-acoustic-wave fields on (100) GaAs surfaces

Focused surface-acoustic waves (SAWs) provide a way to reach intense acoustic fields for electro- and optoacoustic applications on semiconductors. We have investigated the focusing of SAWs by interdigital transducers (IDTs) deposited on (100)-oriented GaAs substrates. The focusing IDTs have curved fingers designed to account for the acoustic anisotropy of the substrate. Different factors that affect focusing, such as the aperture angle and the configuration of the IDT fingers, were systematically addressed. We show that the focusing performance can be considerably improved by appropriate choice of the IDT metal pads, which, under appropriate conditions, create an acoustic waveguide within the IDT. We demonstrate the generation of narrow (full width at half maximum of approx 15 μm), high-frequency (0.5 GHz), continuous SAW beams with vertical displacement as high as 4 nm collimated over distances that exceed 100 μm.

Relating pore fabric geometry to acoustic and permeability anisotropy in Crab Orchard Sandstone: A laboratory study using magnetic ferrofluid

Pore fabric anisotropy is a common feature of many sedimentary rocks. In this paper we report results from a comparative study on the anisotropy of a porous sandstone (Crab Orchard) using anisotropy of magnetic susceptibility (AMS), acoustic wave velocity and fluid permeability techniques. Initially, we characterise the anisotropic pore fabric geometry by impregnating the sandstone with magnetic ferro‐fluid and measuring its AMS. The results are used to guide subsequent measurements of the anisotropy of acoustic wave velocity and fluid permeability. These three independent measures of anisotropy are then directly compared. Results show strong positive correlation between the principal directions given from the AMS, velocity anisotropy and permeability anisotropy. Permeability parallel to the macroscopic crossbedding observed in the sandstone is 240% higher than that normal to it. P and S‐wave velocity anisotropy and AMS show mean values of 19.1%, 4.8% and 3.8% respectively, reflecting the disparate physical properties measured.

Prise en charge des lombalgiques

The quasicollinear geometry of acousto-optic (AO) diffraction is notable as makes it possible to achieve an extremely high AO interaction length and, consequently, an anomalously high spectral resolution for AO devices. This geometry is especially convenient for the implementation of multifrequency AO diffraction, which has found wide application for solving the problems related to the laser pulse shaping. Since acoustic beams propagate over long distances in quasicollinear AO devices, and optical radiation spectral components diffract in the acoustic field in different parts of the AO crystal, accurate calculation of the characteristics of such devices requires knowing the distribution of the acoustic field amplitude inside the AO cell. The acoustic beam structure is affected by several factors in quasicollinear AO cells: the dimensions of the piezoelectric transducer, the geometry of acoustic wave propagation in the AO cell, acoustic anisotropy and the acoustic energy absorption along the chosen direction in the crystalline material used. In this paper, we propose a generic method to measure the acoustic beam power distribution along the direction of its propagation in the quasicollinear AO cell in the presence of ultrasound power absorption and media acoustic absorption. The measurements were carried out for the ultrasound frequency range from 72 to 176 MHz, for the case when the wave vector of the acoustic beam is directed at an angle of 1.58∘ to the [110] axis in the (11¯0) plane of the paratellurite crystal. The ultrasound attenuation coefficients were obtained for frequency interval between 87 and 176 MHz and their linear dependence on ultrasound frequency was confirmed.

Subcollinear acousto-optic tunable filter based on the medium with a strong acoustic anisotropy

We design and investigate the properties of acousto-optical tunable filter (AOTF) based on tellurium dioxide (TeO 2) single crystal. We use so-called “subcollinear” geometry of anisotropic acousto-optic interaction that occurs when the wavevector of incident light is collinear to acoustic energy vector. We have obtained the analytical formula of spectrum spread function of proposed AOTF and have found the condition determining the shape of that characteristic. Also we have designed and fabricated the experimental prototype of subcollinear AOTF. In our design we have used the reflection of acoustic wave that allowed to use crystal’s media more effectively and reduce the AOTF size. The main advantage of filter based on proposed geometry is extremely low value of electrical driving power (20 mW) that makes it a strong candidate in many applications.

Cs2HgCl4 crystal as a new material for acoustooptical applications

Abstract The paper reports the basic acoustical (anisotropy of phase velocity Vi and obliquity angle θ), optical (transparency spectra, refractive indices ni and photoelastic constants pij) and acoustooptical (figure of merit M 2 =p ef 2 n 6 /ρV 3 ) parameters of Cs2HgCl4 crystals that are necessary for a design of acoustooptical devices. Substantial photoelastic effect (pij∼0.17–0.41 for i,j=1–3) and low ultrasonic velocities (Vi∼870–2500 m/s) in Cs2HgCl4 crystals determine relatively high figure of merit M2 for the isotropic acoustooptical diffraction. We have found that for certain geometries of acoustooptical interactions M2∼(85–114)×10−15 s3/kg. The new material may be considered, therefore, as a candidate for applications in acoustooptical devices.

Spatial-dispersion-induced acoustic anisotropy in semiconductor structures

We demonstrate that a Rayleigh surface acoustic wave (SAW) propagating on the surface of a (001)-oriented GaAs quantum well structure induces different optical properties for propagation directions along the [110] and the $[1\ifmmode\bar\else\textasciimacron\fi{}10]$ symmetry axes. The nonequivalence of the two directions is attributed to the lift of the fourfold rotation symmetry of the optical and acoustic properties around the [001] growth axis by the finite wave vector of the SAW, a spatial dispersion effect which is also expected for bulk modes in zinc blende semiconductors.

Effects of acoustic anisotropy and screening on the energy relaxation of hot electrons in heterojunctions and quantum wells

In this paper we present theoretical calculations of the angle and mode dependence of the phonon emission by hot two-dimensional electrons in gallium arsenide. We compare them with experimental results of heat-pulse studies of the acoustic-phonon emission in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ heterojunctions and GaAs quantum wells. It is shown that the ratio of the strengths of the longitudinal- and transverse-acoustic modes is correctly predicted only by a theoretical model that properly includes the effects of acoustic anisotropy on the electron-phonon matrix elements, screening, and the form of the confining potential. As a result of this work, we are finally able to account for why longitudinal phonon emission from a heterojunction is not seen in heat-pulse experiments despite the fact that earlier theories predicted that it should be the dominant mode.

The phonon-drag effect in low mobility gallium nitride epilayers

We have used phonon imaging to study the drag current induced by non-equilibrium acoustic phonons in a low-mobility n-type Gallium Nitride epilayer. Current densities up to j=4.7×10 3 A/m 2 were induced by transverse acoustic phonon drag of electrons in the structure. We also carried out numerical calculations of the phonon-drag current including the effects of phonon focussing in the sapphire substrate, acoustic anisotropy in the GaN and acoustic mismatch between the two materials. We show that the theory is able to account for the qualitative features of the drag image as well as the magnitude of the phonon-drag-induced current.