Showing papers on "Reflection (physics) published in 1998"

Variation of P-wave reflectivity with offset and azimuth in anisotropic media

Abstract: P-wave amplitudes may be sensitive even to relatively weak anisotropy of rock mass. Recent results on symmetry‐plane P-wave reflection coefficients in azimuthally anisotropic media are extended to observations at arbitrary azimuth, large incidence angles, and lower symmetry systems. The approximate P-wave reflection coefficient in transversely isotropic media with a horizontal axis of symmetry (HTI) (typical for a system of parallel vertical cracks embedded in an isotropic matrix) shows that the amplitude versus offset (AVO) gradient varies as a function of the squared cosine of the azimuthal angle. This change can be inverted for the symmetry‐plane directions and a combination of the shear‐wave splitting parameter γ and the anisotropy coefficient δ(V). The reflection coefficient study is also extended to media of orthorhombic symmetry that are believed to be more realistic models of fractured reservoirs. The study shows the orthorhombic and HTI reflection coefficients are very similar and the azimuthal v...

The Reflection of Guided Waves From Circumferential Notches in Pipes

Abstract: The reflection of the L(0, 2), axially symmetric guidea elastic wave from notches in pipes is examined, using laboratory experiments and finite element simulation The result show that the reflection coefficient of this mode is very close to a linear function of the circumferential extent of the notch, and is a stronger function of the through thickne depth of the notch. The motivation for the work was the development of a technique for inspecting chemical plant pipework, but the study addresses the nature of the reflection function and has general applicability.

Correlation between Compton reflection and X-ray slope in Seyferts and X-ray binaries

Abstract: We find a very strong correlation between the intrinsic spectral slope in X-rays and the amount of Compton reflection from a cold medium in Seyfert AGNs and in hard state of X-ray binaries with either black holes or weakly-magnetized neutron stars. Objects with soft intrinsic spectra show much stronger reflection than ones with hard spectra. We find that at a given spectral slope, black-hole binaries have similar or more reflection than Seyferts whereas neutron-star binaries in our sample have reflection consistent with that in Seyferts. The existence of the correlation implies a dominant role of the reflecting medium as a source of seed soft photons for thermal Comptonization in the primary X-ray source.

Surface light source device of side light type

Abstract: A surface light source device comprises a primary light source, a reflection sheet, a scattering transmission sheet, a guide plate and prism sheet functioning as a light control member. The scattering transmission sheet is provided with scattering power which is given through roughening both faces, uniform containing of fine particles or other means. Light leaked from a back face of the guide plate is reflected and return to the guide plate, being scattered by the scattering transmission sheet on the way. The reflection sheet is a reflection member of high reflectance. A fluorescent lamp emits illumination light which enters into the guide plate and propagates as repeating reflections between the back face and an emission face. Meanwhile, light components having incidence angles smaller than the critical angle with respect to the emission face is emitted from the emission face. The prism sheet and the back face of the guide plate are provided with projections (arrows B, C) which are repeatedly arranged to correct directivity toward the frontal direction in planes parallel and perpendicular with respect to an incidence face of the guide plate. The scattering transmission sheet may be removed, if the reflection sheet is provided with scattering power.

The Mode Conversion of a Guided Wave by a Part-Circumferential Notch in a Pipe

Abstract: A study of the reflection of mode-convertea guided waves from notches in pipes has been carried out. Measurements were made on a 76-mm bore diameter (nominal 3-inch ), 5.5-mm wall thickness pipe with circumferentially oriented through-thickness notches of various lengths. In parallel, a finite element model was used to simulate the experiments The axially symmetric L(0, 2) mode was incident on the notches and the L(0, 2), F( 1, 3), and F(2, 3) modes were received in reflection. The results showed excellent agreement between the measurements and the predictions for all three modes. They also showed that the F( 1, 3) mode reflects as strongly as the L(0, 2) mode when the notch length is short. Finally, it has been shown that a very simple analysis based on an assumed crack-opening profile may be used to make accurate predictions of the mode conversion.

Diffuse and Specular Reflectance from Rough Surfaces

Abstract: We present a reflection model for isotropic rough surfaces that have both specular and diffuse components. The surface is assumed to have a normal distribution of heights. Parameters of the model are the surface roughness given by the rms slope, the albedo, and the balance between diffuse and specular reflection. The effect of roughness on diffuse reflection is taken into account, instead of our modeling this component as a constant Lambertian term. The model includes geometrical effects such as masking and shadowing. The model is compared with experimental data obtained from goniophotometric measurements on samples of tiles and bricks. The model fits well to samples with very different reflection properties. Measurements of the sample profiles performed with a laser profilometer to determine the rms slope show that the assumed surface model is realistic. The model could therefore be used in machine vision and computer graphics to approximate reflection characteristics of surfaces. It could also be used to predict the texture of surfaces as a function of illumination and viewing angles.

Ultraviolet irradiation device of the optical path division type

Abstract: An ultraviolet irradiation device of the optical path division type for treating a workpiece which is often subject to deformations and color changes due to heat, and in which the distribution of radiance is good and the average irradiance on the surface irradiated with light can be increased which can be achieved with light emitted from a rod-shaped lamp and reflected by a trough=shaped cold mirror being incident in cold mirrors which split the optical path. This light is thus divided into two parts and is incident in total reflection mirrors. On the other hand, the direct light emitted by the rod-shaped lamp is incident in second optical path splitting cold mirrors which divides this light and causes it to be incident in the total reflection mirrors. The light reflected by the total reflection mirrors is incident in heat reflection filters, and is transmitted by the heat reflection filters so as to be radiated onto a workpiece. On the workpiece the light divided into two parts is radiated such that the two beams of light come to lie partially superimposed one on top of the other. This improves the radiance distribution. Furthermore, light shielding components can also be used instead of the second optical path splitting mirrors.

Wave Interactions with Vertical Slotted Barrier

Abstract: The present paper outlines the numerical calculation of wave interactions with a thin vertical slotted barrier extending from the water surface to some distance above the seabed, and describes laboratory tests undertaken to assess the numerical model. The numerical model is based on an eigenfunction expansion method and utilizes a boundary condition at the barrier surface that accounts for energy dissipation within the barrier. Numerical results compare well with previous predictions for the limiting cases of an impermeable barrier and a permeable barrier extending down to the seabed. Comparisons with experimental measurements of the transmission, reflection, and energy dissipation coefficients for a partially submerged slotted barrier show good agreement provided certain empirical coefficients of the model are suitably chosen, and indicate that the numerical method is able to account adequately for the energy dissipation by the barrier. The effects of porosity, relative wave length, wave steepness, and irregular waves are discussed and the choice of suitable parameters needed to model the permeability of the breakwater is described.

Avoiding pitfalls in shallow seismic reflection surveys

Abstract: Acquiring shallow reflection data requires the use of high frequencies, preferably accompanied by broad bandwidths. Problems that sometimes arise with this type of seismic information include spatial aliasing of ground roll, erroneous interpretation of processed airwaves and air‐coupled waves as reflected seismic waves, misinterpretation of refractions as reflections on stacked common‐midpoint (CMP) sections, and emergence of processing artifacts. Processing and interpreting near‐surface reflection data correctly often requires more than a simple scaling‐down of the methods used in oil and gas exploration or crustal studies. For example, even under favorable conditions, separating shallow reflections from shallow refractions during processing may prove difficult, if not impossible. Artifacts emanating from inadequate velocity analysis and inaccurate static corrections during processing are at least as troublesome when they emerge on shallow reflection sections as they are on sections typical of petroleum ...

Application of the PML Absorbing Layer Model to the Linear Elastodynamic Problem in Anisotropic Heteregeneous Media

Abstract: We present and analyze a perfectly matched absorbing layer model for the velocity-stress formulation of elastodynamics. This layer has the astonishing property of generating no reflection at the interface between the free medium and the artificial absorbing medium. This allows us to obtain very low spurious reflection even with very thin layers. Several experiments show the efficiency and the generality of the model.

Electro-optical glazing structures having reflection and transparent modes of operation

Abstract: Electro-optical glazing structures having total-reflection and semi-transparent and totally-transparent modes of operation which are electrically-switchable for use in dynamically controlling electromagnetic radiation flow in diverse applications.

On the Theory of Shock Waves for an Arbitrary Equation of State

Abstract: The fundamental equations of the hydrodynamic theory of one-dimensional shock waves — that is, the equations of conservation of mass, of momentum, and of energy — are developed. These are used to calculate the velocity, massvelocity, temperature, and pressure rise in shock waves in air and in water. With one additional equation, they suffice to permit a calculation of detonation velocities in gaseous and in solid explosives. Predictions of detonation velocity as a function of loading density are thereby achieved, accurate to a few percent. Pressures, temperatures, and mass-velocities inside the explosive are also computed. The question of rarefaction waves following the detonation front in the explosive is investigated. The initial velocity, pressure, and so forth, of the shock wave produced at the end of a stick of explosive are calculated successfully. The dying away of shock waves, problems of reflection, and so forth, are also discussed briefly.

Internal wave reflection and mixing at Fieberling Guyot

Abstract: The structure of internal wave reflection off a sloping bottom on the steep flank of a tall North Pacific Ocean seamount is observed in year-long moored array records to differ substantially from the form predicted by linear theory, although linear theory accounts for several qualitative features of the process. This study documents new features of wave reflection as described below. Wave reflection is detectable as far as 750 m above the bottom. Motions are dominated by a single empirical mode whose phase structure obeys linear internal wave dispersion but whose amplitude decays with a scale comparable to the wavenumber. While the dominant mode has scales appropriate to the reflection of a first baroclinic mode wave incident from the open ocean, its decay from the bottom is such that current vectors in the vertical plane rotate clockwise in time when viewed with shallow water to the right. This flow resembles the lower half of the deepest cell pattern predicted by linear reflection from a uniform slope. At the local internal wave critical frequency, the dominant mode has nearly a vanishing wavenumber rather than the infinite wavenumber predicted by linear reflection. Reflected waves are aligned parallel to the bottom slope measured on wave spatial scales, rather than shorter ones. Wave reflection causes large, frequent density overturns, implying mixing. The rate and strength of these overturns imply a local vertical eddy viscosity of 2–6 × 10−4 m2/s over the bottom few hundred meters. The contribution of bottom-intensified mixing to the open deep ocean is roughly equivalent to that found in situ, although reflection from gentler slopes or at lower latitude may produce greater contribution from internal wave-reflection-induced mixing.

PP-wave reflection coefficients in weakly anisotropic elastic media

Abstract: Approximate PP-wave reflection coefficients for weak contrast interfaces separating elastic, weakly transversely isotropic media have been derived recently by several authors. Application of these coefficients is limited because the axis of symmetry of transversely isotropic media must be either perpendicular or parallel to the reflector. In this paper, we remove this limitation by deriving a formula for the PP-wave reflection coefficient for weak contrast interfaces separating two weakly but arbitrarily anisotropic media. The formula is obtained by applying the first-order perturbation theory. The approximate coefficient consists of a sum of the PP-wave reflection coefficient for a weak contrast interface separating two background isotropic half-spaces and a perturbation attributable to the deviation of anisotropic half-spaces from their isotropic backgrounds. The coefficient depends linearly on differences of weak anisotropy parameters across the interface. This simplifies studies of sensitivity of such coefficients to the parameters of the surrounding structure, which represent a basic part of the amplitude-versus-offset (AVO) or amplitude-versus-azimuth (AVA) analysis. The reflection coefficient is reciprocal. In the same way, the formula for the PP-wave transmission coefficient can be derived. The generalization of the procedure presented for the derivation of coefficients of converted waves is also possible although slightly more complicated. Dependence of the reflection coefficient on the angle of incidence is expressed in terms of three factors, as in isotropic media. The first factor alone describes normal incidence reflection. The second yields the low-order angular variations. All three factors describe the coefficient in the whole region, in which the approximate formula is valid. In symmetry planes of weakly anisotropic media of higher symmetry, the approximate formula reduces to the formulas presented by other authors. The accuracy of the approximate formula for the PP reflection coefficient is illustrated on the model with an interface separating an isotropic half-space from a half-space filled by a transversely isotropic material with a horizontal axis of symmetry. The results show a very good fit with results of the exact formula, even in cases of strong anisotropy and strong velocity contrast.

Thin-film optical filters with diffractive elements and waveguides

Abstract: The chief principles and properties of optical reflection and transmission guided-mode resonance (GMR) filters are presented. These devices are based on GMR effects in dielectric structures comprising gratings and homogeneous thin films. Detailed characteristics are calculated using rigorous coupled-wave analysis for bandpass filters operating in reflection and transmission for TE- and TM-polarized incident waves. High resonance efficiency with narrow or wide linewidths is achievable with near-zero reflectance or transmittance sidebands over extended wavelength ranges. To illustrate the potential of this technology, example GMR reflection and transmission characteristics are presented for filters operating in the visible spectral region. Excellent reflection-filter features are found when antireflection conditions prevail away from the resonance wavelength. Furthermore, long-range, low sidebands are found to be obtainable for a single-layer GMR reflection filter with a TM-polarized plane wave incident at the Brewster angle. The transmission filter is optimized when the structure is highly reflective off resonance. GMR filter fabrication tolerances are discussed with examples illustrating the sensitivity of the filter center wavelength to variations in layer thickness, grating shape, and incident angle. GMR filters are found to exhibit loss-dependent wavelength shifts such that the reflection peak occurs at a different wavelength than the corresponding transmission notch. However, under antireflection conditions, the resonance location becomes insensitive to loss. Finally, reflective GMR thinfilm structures that support multiple waveguide modes are studied. These devices exhibit unique characteristic angular and spectral signatures.

Engineered omnidirectional external-reflectivity spectra from one-dimensional layered interference filters.

Abstract: Owing to optical refraction, external rays that are incident upon a high-refractive-index medium fall within a small internal cone angle. A one-dimensionally periodic Bragg structure can reflect over an angular acceptance range that is greater than the small internal refraction cone, if the internal refractive-index contrast is sufficient. Thus Winn [Opt. Lett. (to be published)] charted the range of refractive indices at which omnidirectional external reflection occurs. A wide spectral gap requires a high-index contrast. It is proposed that, by chirping or grading the periodicity of the structure, one can cover an arbitrarily wide spectral range with only a modest index contrast and, furthermore, that arbitrary spectral shapes can be produced. The graded-periodicity approach requires only a modest index contrast, provided that the average refractive index is >2.

Ultrasonic Guided Wave NDT for Hidden Corrosion Detection

Abstract: An experimental study of hidden corrosion detection by using ultrasonic guided waves is presented combined with a BEM numerical simulation. Both corrosion simulation specimens by machine cutting and real corrosion specimens by electrochemical processing were used in the investigation, with a range of corrosion depths from 0.02 to 0.4 mm (1.5 to 20% of the original plate thickness). Various wave modes were subsequently generated on these specimens to examine the implications of thinning on mode cutoff, group velocity, and transmission and reflection amplitudes. The transmission and reflection of guided waves upon entering the corrosion zone were simulated by a hybrid BEM calculation that combines a normal mode expansion technique of Lamb waves for far fields with the boundary element representation for the scattered near fields. A quantitative technique for hidden corrosion depth with guided waves is developed based on a frequency compensation concept. The estimated depth of the real corrosion by this method shows good agreement with that by an optical microscope.

Optical properties (bidirectional reflection distribution functions) of velvet.

Abstract: A detailed investigation has been made of the unusual characteristics of the angular distribution of surface scattering from velvet in the visual region. We present a novel method in which samples of velvet fabric are wrapped around a right-circular cylinder so that reemitted radiance can be measured by a digital CCD camera. This setup makes it relatively simple to acquire a large set of bidirectional reflection distribution function (BRDF) samples. The study reveals that, apart from the grazing specular lobe and an anisotropic backscattering peak near 50°, the overall BRDF’s are rather uniform across the whole angular span of observation. Attempts are made to relate these scattering characteristics to the physical and the geometrical structure of velvet.

Fully nonlinear 3-d Numerical Wave Tank simulation

Abstract: Transient and steady-state nonlinear free surface waves and wave-body interactions are investigated in a 3-dimensional Numerical Wave Tank (NWT) using an indirect Desingularized Boundary Integral Equation Method (DBIEM) and a Mixed Eulerian-Lagrangian (MEL) time marching scheme. The Laplace equation is solved at each time step by using Rankine sources distributed outside the solution domain, and the fully nonlinear free surface boundary conditions are integrated with time to update its position and boundary values. A regridding algorithm is devised to eliminate the possible instabilities without using artificial smoothing. The incident waves are generated either by a piston type wavemaker or by feeding analytic forms on the input boundary. The outgoing waves are sufficiently dissipated by using spatially varying artificial damping on the free surface of the damping zone before they reach the downstream wall boundary. In some cases, side beaches are also implemented to minimize the sidewall reflection. Computations are first performed for nonlinear long-crested regular waves in a 3-dimensional numerical wave tank without a body. Subsequently, the nonlinear diffractions by a bottom-mounted vertical cylinder and truncated vertical cylinders are investigated and the present NWT results are compared with available experimental results and second-order diffraction computations.

Analysis and experiments on stress waves in planar trusses

Abstract: The dynamics of planar trusses are investigated in terms of axial (longitudinal) stress waves, which propagate along structural members and scatter at the joints. The scattering coefficients representing the reflection and transmission of axial waves at each joint are derived from the dynamics and compatibility conditions of the joint. The complex multiple reflections of waves within the structure are evaluated in the frequency domain with a newly developed reverberation matrix, which is formulated from the scattering coefficients and propagating phase factors. Transient waves are then derived by Fourier synthesis, and evaluated by a Fast Fourier Transform algorithm. Experimental results of propagating broad band pulses are presented for a truss model excited by a step loading. Comparison between theoretical results and transient wave records indicate that the axial wave theory is valid only for the response at the very early time. The discrepancy is much reduced if the scattering coefficients are modifie...

Wave phase conjugation of ultrasonic beams

Abstract: The current state of the acoustic wave phase conjugation (PC) problem is reviewed. The generation of phase conjugate ultrasonic waves is discussed with emphasis on the parametric method using electromagnetic pumping in solids. The giant-amplification supercritical parametric PC mode is considered in detail. Ultrasonic PC with a gain in excess of 80 dB is demonstrated for a soft magnetic ceramics-based amplifier. The high quality of supercritical parametric PC sound is confirmed by acoustic-optical visualization. Acoustic PC effects, such as anomalous sound reflection at the PC mirror and the autofocusing and self-targeting of phase conjugate sound beams incident on a scatterer in a liquid, are shown. Recent experimental results demonstrating the potential of PC for applications are presented.

Numerical and experimental study of a micro-blast wave generated by pulsed-laser beam focusing

Abstract: This paper describes a numerical and experimental study of a micro-blast wave which is produced from the source of several tens microns in dia. and propagates in the length scale of a few centimeter in diameter. The micro-blast wave was generated by focusing a Nd:Glass pulsed-laser beam in ambient air. Its propagation and reflection were visualized by using double exposure holographic interferometry and simulated numerically using the dispersion-controlled scheme to solve the Euler and Navier-Stokes equations with initial conditions of a point-source explosion specified with the Taylor similarity law. Good agreement was obtained between numerical solutions and experimental results, and this spherical micro-blast wave was shown to be a handy model of blast waves created in large scale explosions.

Noise in optical low-coherence reflectometry

Abstract: It is theoretically and experimentally shown that Fresnel end reflection at a waveguide under test can degrade the sensitivity of an optical low-coherence reflectometer (OLCR) with a balanced detection scheme. Optical mixing of the local oscillator (LO) light and the end reflection produces beat noise whose current noise spectral density is represented by 2(1+P/sup 2/) //spl delta//spl nu/, where P and /spl delta//spl nu/ are the degree of polarization and the effective linewidth of the light, respectively, and and are the total mean photocurrents of the LO light and the end reflection at the balanced mixer, respectively. The balanced detection technique suppresses the intensity noise of the light and the beat noise becomes the dominant source of sensitivity degradation. The minimum detectable reflectivity is derived which includes the effect of sensitivity degradation caused by beat noise.

Pressure Waves in Microscopic Simulations of Laser Ablation Leonid

Abstract: Laser ablation of organic solids is a complex collective phenomenon that includes processes occurring at different length and time scales. A mesoscopic breathing sphere model developed recently for molecular dynamics simulation of laser ablation and damage of organic solids has significantly expanded the length-scale (up to hundreds of nanometers) and the time-scale (up to nanoseconds) of the simulations. The laser induced buildup of a high pressure within the absorbing volume and generation of the pressure waves propagating from the absorption region poses an additional challenge for molecular-level simulation. A new dynamic boundary condition is developed to minimize the effects of the reflection of the wave from the boundary of the computational cell. The boundary condition accounts for the laser induced pressure wave propagation as well as the direct laser energy deposition in the boundary region.

Reflection and transmission of Alfvén waves at the auroral acceleration region

Abstract: Alfven waves transfer field-aligned currents, momentum and energy from the source region in the equatorial magnetosphere along Earth's magnetic field lines to the polar ionosphere and play therefore an important role in auroral electrodynamics. The parallel potential drop of some kV in the auroral acceleration region (AAR) at altitudes of some 1000 km has considerable effect on the propagation of these waves. The purpose of this report is to investigate the reflection and transmission properties of the AAR in the frame of a thin sheet approximation. Combining key relations of auroral electrodynamics with the requirement of current continuity results in an easy-to-handle physical description of the problem. The model provides an opportunity to study different acceleration mechanisms. It turns out that the reflection properties of the auroral zone depends strongly on the perpendicular scale length of the incident wave.

Interactions of a horizontal flexible membrane with oblique incident waves

Abstract: The interaction of oblique monochromatic incident waves with a horizontal flexible membrane is investigated in the context of two-dimensional linear hydro-elastic theory. First, analytic diffraction and radiation solutions for a submerged impermeable horizontal membrane are obtained using an eigenfunction expansion method. Secondly, a multi-domain boundary element method (BEM) is developed to confirm the analytic solutions. The inner solution based on a discrete membrane dynamic model and simple-source distribution over the entire fluid boundaries is matched to the outer solution based on an eigenfunction expansion. The numerical solutions are in excellent agreement with the analytic solutions. The theoretical prediction was then compared to a series of experiments conducted in a two-dimensional wave tank at Texas A&M University. The measured reflection and transmission coefficients reasonably follow the trend of predicted values. Using the computer program developed, the performance of surface-mounted or submerged horizontal membrane wave barriers is tested with various system parameters and wave characteristics. It is found that the horizontal flexible membrane can be an effective wave barrier if properly designed.