Showing papers on "Reflection (physics) published in 2004"

Diffraction imaging by focusing‐defocusing: An outlook on seismic superresolution

Abstract: Diffractions always need more advertising. It is true that conventional seismic processing and migration are usually successful in using specular reflections to estimate subsurface velocities and reconstruct the geometry and strength of continuous and pronounced reflectors. However, correct identification of geological discontinuities, such as faults, pinch-outs, and small-size scattering objects, is one of the main objectives of seismic interpretation. The seismic response from these structural elements is encoded in diffractions, and diffractions are essentially lost during the conventional processing/migration sequence. Hence, we advocate a diffraction-based, data-oriented approach to enhance image resolution—as opposed to the traditional image-oriented techniques, which operate on the image after processing and migration. Even more: it can be shown that, at least in principle, processing of diffractions can lead to superresolution and the recovery of details smaller than the seismic wavelength. The so-called reflection stack is capable of effectively separating diffracted and reflected energy on a prestack shot gather by focusing the reflection to a point while the diffraction remains unfocused over a large area. Muting the reflection focus and defocusing the residual wavefield result in a shot gather that contains mostly diffractions. Diffraction imaging applies the classical (isotropic) diffraction stack to these diffraction shot gathers. This focusingmuting-defocusing approach can successfully image faults, small-size scattering objects, and diffracting edges. It can be implemented both in model-independent and modeldependent contexts. The resulting diffraction images can greatly assist the interpreter when used as a standard supplement to full-wave images.

The reflection of guided waves from notches in pipes: a guide for interpreting corrosion measurements

Abstract: Ultrasonic guided waves are used for the rapid screening of pipelines in service and simple, standard testing procedures are already defined. The implementation of the method enables the localization of the defects along the length of the pipe and offers a rough estimate of defect size. In this article we present a systematic analysis of the effect of pipe size, defect size, guided wave mode and frequency on the reflection from notches. The maximum and minimum value of the reflection coefficient at varying axial extent are identified and used for the purpose of defect sizing. Maps of reflection coefficient as a function of the circumferential extent and depth of the defect are presented for a 3 in. schedule 40 steel pipe. An approximate formula, which allows these results to be extrapolated to other pipe sizes, is proposed and evaluated.

Large positive and negative lateral optical beam displacements due to surface plasmon resonance

Abstract: We report abnormally large positive and negative lateral optical beam shifts at a metal–air interface when the surface plasmon resonance of the metal is excited. The optimal thickness for minimal resonant reflection is identified as the critical thickness above which a negative beam displacement is observed. Experimental results show good agreement with theoretical predictions and the large observed bidirectional beam displacements also indicate the existence of forward and backward surface propagating waves at the surface plasmon resonance of the metal.

Measurement of the refractive index of liquids at 1.3 and 1.5 micron using a fibre optic Fresnel ratio meter

Abstract: A fibre optic technique based on the Fresnel reflection from the fibre tip is used for measurements of the refractive indices of various liquids at wavelengths of 1310 and 1551 nm. Reflection signals from liquid?fibre interface are compared with reflection signals from air?fibre interface to obtain the refractive index. Values of refractive indices for distilled water measured by this technique compare very well with known values at both wavelengths only if the fibre effective waveguide index is used. Applying the double-pulse measurement technique, it is shown that a measurement resolution of about 2.5 ? 10?5 can be achieved.

Multiple reflection and transmission phases in complex layered media using a multistage fast marching method

Abstract: Traditional grid-based eikonal schemes for computing traveltimes are usually confined to obtaining first arrivals only. However, later arrivals can be numerous and of greater amplitude, making them a potentially valuable resource for practical applications such as seismic imaging. The aim of this paper is to introduce a grid-based method for tracking multivalued wavefronts composed of any number of reflection and refraction branches in layered media. A finite-difference eikonal solver known as the fast marching method (FMM) is used to propagate wavefronts from one interface to the next. By treating each layer that the wavefront enters as a separate computational domain, one obtains a refracted branch by reinitializing FMM in the adjacent layer and a reflected branch by reinitializing FMM in the incident layer. To improve accuracy, a local grid refinement scheme is used in the vicinity of the source where wavefront curvature is high. Several examples are presented which demonstrate the viability of the new method in highly complex layered media. Even in the presence of velocity variations as large as 8:1 and interfaces of high curvature, wavefronts composed of many reflection and transmission events are tracked rapidly and accurately. This is because the scheme retains the two desirable properties of a single-stage FMM: computational speed and stability. Local grid refinement about the source also can increase accuracy by an order of magnitude with little increase in computational cost.

Relations between reflection and transmission responses of three-dimensional inhomogeneous media

Abstract: SUMMARY Relations between reflection and transmission responses of horizontally layered media were formulated by Claerbout in 1968 and by many others. In this paper we derive similar relations for 3-D inhomogeneous media. As the starting point for these derivations, we make use of two types of propagation invariants, based on one-way reciprocity theorems of the convolution type and of the correlation type. We obtain relations between reflection and transmission responses, including their codas, due to internal multiple scattering. These relations can be used for deriving the reflection response from transmission measurements (which is useful for seismic imaging of the subsurface, using passive recordings of noise sources in the subsurface, also known as acoustic daylight imaging) as well as for deriving the transmission coda from the reflection measurements (which is useful for seismic imaging schemes that take internal multiple scattering into account). Furthermore, following the same approach, we obtain mutual relations between reflection responses with and without free-surface multiples. The convolution-type relations are similar to those used by Berkhout and others for surface-related multiple elimination, whereas the correlation-type relations resemble Schuster's relations for seismic interferometry. Last, but not least, we obtain expressions for the reflection response at a boundary below an inhomogeneous medium, which may be useful for imaging the medium ‘from below’. The main text of this paper deals with the acoustic situation; the Appendices provide extensions to the elastodynamic situation.

X-ray reflection in the NLS1 1H 0707-495

Abstract: We apply a reflection-dominated model to the second XMM-Newton observation of the Narrow Line Seyfert 1 galaxy 1H 0707-495. As in the first XMM-Newton observation a sharp spectral drop is detected with energy that has shifted from 7 keV to 7.5 keV in two years. The drop is interpreted in terms of relativistically blurred ionised reflection from the accretion disc, while the energy shift can be accounted for by changes in the ionisation state and, more importantly, emissivity profile on the disc. A flatter emissivity profile during the second higher flux observation reduces gravitational redshift effects, therefore shifting the edge to higher energy. Remarkably, ionised disc reflection and the associated power law continuum provide a reasonable description of the broadband spectrum, including the soft excess. Considering both observations, the spectral variability in 1H 0707-495 appears to be due to the interplay between these two spectral components. The reflection component in the second observation is significantly less variable than the power law. Changes of the emissivity profile, spectral shape and variability properties (such as the rms spectrum) within the two observations are all consistent with a recently proposed model in which relativistic effects in the very inner regions of the nucleus play a major role.

New type of vibration dampers utilising the effect of acoustic 'black holes'

Abstract: One of the well-known ways of damping resonant flexural vibrations of different engineering structures or their elements, e.g. finite plates or bars, is to reduce reflections of flexural waves from their free edges. In the present paper, a new efficient method of reducing edge reflections is described that utilises gradual change in thickness of a plate or a bar from the value corresponding to the thickness of the basic plate to almost zero. It is proposed to use specific power-law shapes of plates of variable thickness (wedges) that ideally provide zero reflection even for negligibly small material attenuation – the so-called ‘acoustic black hole effect’. In particular, for powers m ≥ 2 - in free wedges, and m ≥ 5/3 – in immersed wedges, incident flexural waves become trapped near the edge and do not reflect back. Since, because of ever-present edge truncations in real manufactured wedges, the corresponding reflection coefficients are always far from zero, to make up for real wedges and make the systems more efficient it is proposed to deposit absorbing thin layers on wedge surfaces. It is shown that the deposition of thin damping layers on the wedge surfaces can dramatically reduce the reflection coefficients. Thus, the combination of a wedge with power-law profile and of thin damping layers can utilise the acoustic ‘black hole’ effect resulting in very effective damping systems for flexural vibrations.

Modified theory of physical optics.

Abstract: A new procedure for calculating the scattered fields from a perfectly conducting body is introduced. The method is defined by considering three assumptions. The reflection angle is taken as a function of integral variables, a new unit vector, dividing the angle between incident and reflected rays into two equal parts is evaluated and the perfectly conducting (PEC) surface is considered with the aperture part, together. This integral is named as Modified Theory of Physical Optics (MTPO) integral. The method is applied to the reflection and edge diffraction from a perfectly conducting half plane problem. The reflected, reflected diffracted, incident and incident diffracted fields are evaluated by stationary phase method and edge point technique, asymptotically. MTPO integral is compared with the exact solution and PO integral for the problem of scattering from a perfectly conducting half plane, numerically. It is observed that MTPO integral gives the total field that agrees with the exact solution and the result is more reliable than that of classical PO integral.

Surface plasmons at nanoscale relief gratings between a metal and a dielectric medium with optical gain

Abstract: Surface plasmons at the interface between metal and a dielectric with strong optical amplification are analyzed theoretically. It is shown that proper choice of optical indices of the dielectric medium results in an infinitely large effective refractive index of surface waves. Such resonant plasmons have extremely low group velocity and are localized within a vanishingly small distance near the interface. The plasmon-related anomalies in the UV reflection spectra are predicted for nanoscale gratings on a surface of a silver film covered by a concentrated dye solution with high optical amplification.

Separating reflection components based on chromaticity and noise analysis

Abstract: Many algorithms in computer vision assume diffuse only reflections and deem specular reflections to be outliers. However, in the real world, the presence of specular reflections is inevitable since there are many dielectric inhomogeneous objects which have both diffuse and specular reflections. To resolve this problem, we present a method to separate the two reflection components. The method is principally based on the distribution of specular and diffuse points in a two-dimensional maximum chromaticity-intensity space. We found that, by utilizing the space and known illumination color, the problem of reflection component separation can be simplified into the problem of identifying diffuse maximum chromaticity. To be able to identity the diffuse maximum chromaticity correctly, an analysis of the noise is required since most real images suffer from it. Unlike existing methods, the proposed method can separate the reflection components robustly for any kind of surface roughness and light direction.

All-pass transmission or flattop reflection filters using a single photonic crystal slab

Abstract: We show that a single photonic crystal slab can function either as optical all-pass transmission or flattop reflection filter for normally incident light. Both filter functions are synthesized by designing the spectral properties of guided resonance in the slab. The structure is extremely compact along the vertical direction. We expect this device to be useful for optical communication systems.

Device for detecting three-dimensional shapes of elongated flexible body

Abstract: For detecting three-dimensional shapes of an elongated flexible body, a sensor cable to be placed in a passage or channel which is formed axially and coextensively within the elongated flexible body. The sensor cable has two pairs of fiber Bragg grating strands within a tubular carrier casing. A signal light beam containing Bragg wavelength bands is projected from a light source to input same to refractive index change portions in the fiber Bragg grating strands. Reflection diffraction light signals from the refractive index change portions are received by a signal processor to measure the degree of strain at each one of the respective refractive index change portions by comparing wavelengths of the reflection diffraction light signals with reference wavelength.

Constraining X-ray binary jet models via reflection

Abstract: Although thermal disk emission is suppressed or absent in the hard state of X-ray binaries, the presence of a cold, thin disk can be inferred from signatures of reprocessing in the ~2-50 keV band. The strength of this signature is dependent on the source spectrum and flux impinging on the disk surface and is thus very sensitive to the system geometry. The general weakness of this feature in the hard state has been attributed to either a truncation of the thin disk, large ionization, or beaming of the corona region away from the disk with β ~ 0.3. This latter velocity is comparable to jet nozzle velocities, so we explore whether a jet can account for the observed reflection fractions. It has been suggested that jets may contribute to the high-energy spectra of X-ray binaries, via either synchrotron from around (100-1000)rg along the jet axis or from inverse Compton (synchrotron self-Compton and/or external Compton) from near the base. Here we calculate the reflection fraction from jet models wherein either synchrotron or Compton processes dominate the emission. Using as a guide a data set for GX 339-4, for which the reflection fraction previously has been estimated as ~10%, we study the results for a jet model. We find that the synchrotron case gives less than 2% reflection, while a model with predominantly synchrotron self-Compton in the base gives ~10%-18%. This shows for the first time that an X-ray binary jet is capable of significant reflection fractions and that extreme values of the reflection may be used as a way of discerning the dominant contributions to the X-ray spectrum.

Highly efficient photonic crystal-based multichannel drop filters of three-port system with reflection feedback

Abstract: We have derived the general condition to achieve 100% drop efficiency in the resonant tunneling-based channel drop filters of a three-port system with reflection feedback. According to our theoretical modeling based on the coupled mode theory in time, the condition is that the Q-factor due to coupling to a bus port should be twice as large as the Q-factor due to coupling to a drop port and the phase retardation occurring in the round trip between a resonator and a reflector should be a multiple of 2π. The theoretical modeling also shows that the reflection feedback in the three-port channel drop filters brings about relaxed sensitivity to the design parameters, such as the ratio between those two Q-factors and the phase retardation in the reflection path. Based on the theoretical modeling, a five-channel drop filter has been designed in a two-dimensional photonic crystal, in which only a single reflector is placed at the end of the bus waveguide. The performance of the designed filter has been numerically calculated using the finite-difference time domain method. In the designed filter, drop efficiencies larger than 96% in all channels have been achieved.

Seismic reflection imaging of water mass boundaries in the Norwegian Sea

Abstract: [1] Results from the first joint temperature and seismic reflection study of the ocean demonstrate that water mass boundaries can be acoustically mapped. Multichannel seismic profiles collected in the Norwegian Sea show reflections between the Norwegian Atlantic Current and Norwegian Sea Deep Water. The images were corroborated with a dense array of expendable bathythermographs and expendable conductivity-temperature depth profiles delineating sharp temperature gradients over vertical distances of ∼5–15 m at depths over which reflections occur. Fine structure from both thermohaline intrusions and internal wave strains is imaged. Low-amplitude acoustic reflections correspond to temperature changes as small as 0.03°C implying that seismic reflection methods can image even weak fine structure.

Basin‐scale motion in stratified Upper Lake Constance

Abstract: This paper describes experimental and modeled wind-induced oscillations in Upper Lake Constance with an emphasis on a coherent understanding of the basin-scale internal dynamics in this example of a large and stratified lake. Data were collected with eight Lake Diagnostic Systems (LDSs) consisting of thermistor chains and wind anemometers. The isotherm displacements as measured by the LDSs were interpreted using the three-dimensional hydrodynamic Estuary and Lake Computer Model (ELCOM). Three types of basin-scale waves were found to dominate the wave motion: the vertical mode-one Kelvin wave that had an observed period around 90 h, two vertical mode-one Poincare waves that had periods near 8 h and 12 h, and a vertical mode-two Poincare wave that had a period near 14 h. After strong westerly winds, upwelling of cold bottom water was observed east of the Sill of Mainau, where the lake's two subbasins connect. The width and length ratios of the subbasins, spatial variations of the wind field, and rotational effects over the lake are shown to play critical roles in the details of the upwelling structure. A sudden fall of the isotherms in Lake Uberlingen formed a surge. The reflection of the surge from the northwestern boundary induced a vertical mode-two response leading to an intrusion in the metalimnion that caused a three-layer velocity structure in the smaller subbasin.

A spectral-element method for modelling cavitation in transient fluid-structure interaction

Abstract: In an underwater-shock environment, cavitation (boiling) occurs as a result of reflection of the shock wave from the free surface and/or wetted structure causing the pressure in the water to fall below its vapour pressure. If the explosion is sufficiently distant from the structure, the motion of the fluid surrounding the structure may be assumed small, which allows linearization of the governing fluid equations. In 1984, Felippa and DeRuntz developed the cavitating acoustic finite-element (CAFE) method for modelling this phenomenon. While their approach is robust, it is too expensive for realistic 3D simulations. In the work reported here, the efficiency and flexibility of the CAFE approach has been substantially improved by: (i) separating the total field into equilibrium, incident, and scattered components, (ii) replacing the bilinear CAFE basis functions with high-order Legendre-polynomial basis functions, which produces a cavitating acoustic spectral element (CASE) formulation, (iii) employing a simple, non-conformal coupling method for the structure and fluid finite-element models, and (iv) introducing structure–fluid time-step subcycling. Field separation provides flexibility, as it admits non-acoustic incident fields that propagate without numerical dispersion. The use of CASE affords a significant reduction in the number of fluid degrees of freedom required to reach a given level of accuracy. The combined use of subcycling and non-conformal coupling affords order-of-magnitude savings in computational effort. These benefits are illustrated with 1D and 3D canonical underwatershock problems. Copyright © 2004 John Wiley & Sons, Ltd.

Magneto-inductive waves supported by metamaterial elements: Components for a one-dimensional waveguide

Abstract: The propagation and manipulation of magneto-inductive waves is studied under conditions when there is reflection due to lack of matching, when power is coupled from one waveguide to another, when coupling causes directional properties and when tunnelling occurs between two waveguides. The relationships obtained are compared and contrasted with those occurring in traditional transmission lines.

The crustal structure of the NW-Moroccan Continental Margin for Wide-angle and Reflection Seismic Data

Abstract: SUMMARY The Atlantic margin off Morocco with its neighbouring Jurassic oceanic crust is one of the oldest on earth. It is conjugate to the Nova Scotia margin of North America. The SISMAR marine seismic survey acquired deep reflection seismic data as well as wide-angle seismic profiles in order to image the deep structure of the margin, characterize the nature of the crust in the transitional domain and define the geometry of the synrift basins. We present results from the combined interpretation of the reflection seismic, wide-angle seismic and gravity data along a 440-km-long profile perpendicular to the margin at 33‐34 ◦ N, extending from nearly normal oceanic crust in the vicinity of Coral Patch seamount to the coast at El Jadida and approximately 130 km inland. The shallow structure is well imaged by the reflection seismic data and shows a thick sedimentary cover that is locally perturbed by salt tectonics and reverse faulting. The sedimentary basin thickens from 1.5 km on the normal oceanic crust to a maximum thickness of 6 km at the base of the continental slope. Multichannel seismic (MCS) data image basement structures including a few tilted fault blocks and a transition zone to a thin crust. A strong discontinuous reflection at 12 s two-way travel-time (TWT) is interpreted as the Moho discontinuity. As a result of the good data quality, the deep crustal structure (depth and velocity field) is well constrained through the wide-angle seismic modelling. The crust thins from 35 km underneath the continent to approximately 7 km at the western end of the profile. The transitional region has a width of 150 km. Crustal velocities are lowest at the continental slope, probably as a result of faulting and fracturing of the upper crust. Uppermantle velocities could be well defined from the ocean bottom seismometer (OBS) and land station data throughout the model.

Sensing characteristics of plastic optical fibres measured by optical time-domain reflectometry

Abstract: A new method based on photon counting is utilized to investigate sensing characteristics of plastic optical fibres (POF) using the optical time-domain reflectometry technique. The responses of POF under various disturbances including small-radius bending, clamping, axial strain, etc, are measured and discussed. The sensing characteristics of POF are then expressed via reflection and transmission loss due to the disturbances. This provides basic data for developing a distributed sensor using POF. The result shown here implies the possibility of discriminating different types of disturbances using their specific responses.

Spiraphase-type reflectarrays based on loaded ring slot resonators

Abstract: Reflective periodic arrays based on loaded ring slot resonators are analyzed. A full-wave mathematical model is developed and numerical results are presented. It is proven that the reflection angle for the normally incident circularly polarized wave can be effectively controlled by proper positioning of reactive loads in ring slot resonators. Analysis of the reflection characteristics of the Ka band one-layer reflectarray results in a conclusion that the incident wave can be effectively redirected in the directions determined by elevation angles as high as 65/spl deg/ with conversion coefficient better than -1.5 dB. It is also shown that the usage of the multilayer reflectarray leads to a considerable improvement in the reflection characteristics when it is compared with the one-layer reflectarray. The method of the waveguide simulator has been used to verify the developed mathematical model.

A method for the measurement of hydrodynamic oil films using ultrasonic reflection

Abstract: The measurement of the thickness of an oil film in a lubricated component is essential information for performance monitoring and control. In this work, a new method for oil film thickness measurement, based on the reflection of ultrasound, is evaluated for use in fluid film journal bearing applications. An ultrasonic wave will be partially reflected when it strikes a thin layer between two solid media. The proportion of the wave reflected depends on the thickness of the layer and its acoustic properties. A simple quasi-static spring model shows how the reflection depends on the stiffness of the layer alone. This method has been first evaluated using flat plates separated by a film of oil, and then used in the measurement of oil films in a hydrodynamic journal bearing. A transducer is mounted on the outside of the journal and a pulse propagated through the shell. The pulse is reflected back at the oil film and received by the same transducer. The amplitude of the reflected wave is processed in the frequency domain. The spring model is then used to determine the oil film stiffness that can be readily converted to film thickness. Whilst the reflected amplitude of the wave is dependent on the frequency component, the measured film thickness is not; this indicates that the quasi-static assumption holds. Measurements of the lubricant film generated in a simple journal bearing have been taken over a range of loads and speeds. The results are compared with predictions from classical hydrodynamic lubrication theory. The technique has also been used to measure oil film thickness during transient loading events. The response time is rapid and film thickness variation due to step changes in load and oil feed pressure can be clearly observed.

Projection screen and projection system comprising the same

Abstract: A projection screen includes a cholesteric liquid crystalline, polarized-light selective reflection layer for selectively diffuse-reflecting a specific polarized-light component. The layer has, on its observation-side surface, a roughened part for controlling the direction of interfacial reflection of imaging light that is projected on the layer. Imaging light projected on the projection screen from a projector enters the layer and is diffuse-reflected inside this layer; this light is diffused as diffuse-reflected light, in directions included in an approximately constant range. On the other hand, part of the light projected on the projection screen from the projector is reflected, by interfacial reflection, from the inclined planes of the roughened part. The reflected light emerges from the layer in a direction different from the main direction in which the light diffuse-reflected from the layer emerges and returns to the observation side as light reflected by interfacial reflection.

Numerical simulation of bistatic scattering from a target at low altitude above rough sea surface under an EM-wave incidence at low grazing angle by using the finite element method

Abstract: To study bistatic scattering from a target at low altitude above two-dimensional (2-D) randomly rough sea surface under an electromagnetic (EM) wave incidence at low grazing angle (LGA), a numerical approach of the finite element method (FEM) is developed. The conformal perfectly matched layer (PML), as the truncation boundary of the FEM, is employed to reduce the reflection error of planar PML in conventional FEM. Numerical code of our FEM is examined by available solution of the FBM (forward backward iterative) method. Bistatic and back-scattering from composite model of a target above randomly rough sea surface generated by Monte Carlo realization, and functional dependence upon the sea surface wind speed, target altitude, incident and scattering angles etc. are numerically demonstrated and discussed. This study presents numerical description for the observation principle and physical insight associated with the coupling interactions of a complex volumetric target and randomly rough sea surface.

Reflection-type light modulating array element and exposure apparatus

Abstract: A reflection-type light modulating array element has: a substrate; a movable member provided with a beam body provided on the substrate through a first gap, a light reflector capable of rotational displacement by twisting of the beam body, and an electrically conducting part formed at least in a partial portion of the movable member; a lower electrode provided on a substrate side to face the movable member through the first gap, and an upper electrode provided on a side opposite to the lower electrode to face the movable member through a second gap, and thereby the movable member is between the lower electrode and the upper electrode, wherein a voltage is applied to the upper electrode, the lower electrode and the electrically conducting part to cause an rocking displacement of the light reflector and thereby deflect a reflection direction of a light.