Showing papers on "Reflection (physics) published in 1985"
TL;DR: In this article, an iterative Gauss-Newton algorithm that produces a velocity-depth model which minimizes the difference between traveltimes generated by tracing rays through the model and traveltimes measured from the data is used.
Abstract: Estimation of reflector depth and seismic velocity from seismic reflection data can be formulated as a general inverse problem. The method used to solve this problem is similar to tomographic techniques in medical diagnosis and we refer to it as seismic reflection tomography. Seismic tomography is formulated as an iterative Gauss‐Newton algorithm that produces a velocity‐depth model which minimizes the difference between traveltimes generated by tracing rays through the model and traveltimes measured from the data. The input to the process consists of traveltimes measured from selected events on unstacked seismic data and a first‐guess velocity‐depth model. Usually this first‐guess model has velocities which are laterally constant and is usually based on nearby well information and/or an analysis of the stacked section. The final model generated by the tomographic method yields traveltimes from ray tracing which differ from the measured values in recorded data by approximately 5 ms root‐mean‐square. The i...
516 citations
357 citations
TL;DR: In this paper, a theory that strong reflection can be induced by the sandbars themselves, once the so-called Bragg resonance condition is met, was presented. But this theory is limited to weak reflection and fails at resonance.
Abstract: One of the possible mechanisms of forming offshore sandbars parallel to a coast is the wave-induced mass transport in the boundary layer near the sea bottom. For this mechanism to be effective, sufficient reflection must be present so that the waves are partially standing. The main part of this paper is to explain a theory that strong reflection can be induced by the sandbars themselves, once the so-called Bragg resonance condition is met. For constant mean depth and simple harmonic waves this resonance has been studied by Davies (1982), whose theory, is however, limited to weak reflection and fails at resonance. Comparison of the strong reflection theory with Heathershaw's (1982) experiments is made. Furthermore, if the incident waves are slightly detuned or slowly modulated in time, the scattering process is found to depend critically on whether the modulational frequency lies above or below a threshold frequency. The effects of mean beach slope are also studied. In addition, it is found for periodically modulated wave groups that nonlinear effects can radiate long waves over the bars far beyond the reach of the short waves themselves. Finally it is argued that the breakpoint bar of ordinary size formed by plunging breakers can provide enough reflection to initiate the first few bars, thereby setting the stage for resonant reflection for more bars.
319 citations
TL;DR: In this paper, the exact Green's function of the grounded dielectric slab is used in a moment method procedure, so surface waves as well as space-wave radiation are included.
Abstract: A solution is presented for the characteristics of microstrip open-end and gap discontinuities on an infinite dielectric substrate. The exact Green's function of the grounded dielectric slab is used in a moment method procedure, so surface waves as well as space-wave radiation are included. The electric currents on the line are expanded in terms of longitudinal subsectional piecewise sinusoidal modes near the discontinuity, with entire domain traveling-wave modes used to represent incident, reflected, and, for the gap, transmitted waves away from the discontinuity. Results are given for the end admittance of an open-ended line, and the end conductance is compared with measurements. Results are also given for the reflection coefficient magnitude and surface-wave power generation of an open-ended line on substrates with various dielectric constants. Loss to surface and space waves is calculated for a representative gap discontinuity.
273 citations
TL;DR: In this paper, the zero-eth order diffraction efficiency of a three-layer dielectric grating is studied theoretically and experimentally, and the operation of the structure as a tunable narrow-band wavelength filter in a reflection regime is demonstrated.
242 citations
TL;DR: In this paper, it was shown that reflection of a plane monochromatic wave by the surface of a corrugated waveguide structure may excite a guided mode in one of the diffraction orders and thus alter greatly the reflection and transmission coefficients of the incident wave until total reflection or total transmission are achieved.
Abstract: It is shown that reflection of a plane monochromatic wave by the surface of a corrugated waveguide structure may excite a guided mode in one of the diffraction orders and thus alter greatly the reflection and transmission coefficients of the incident wave until total reflection or total transmission are achieved.
184 citations
TL;DR: In this article, the authors investigated the form of the nonwhiteness found in the reflection coefficients from a wide variety of rock sequences around the world, and the observed power spectra were shown to be consistent with a simple model for the logarithm of acoustic impedance consisting of a mixture of processes where the distribution of (time) scale parameters is reciprocal.
Abstract: This paper investigates the form of the nonwhiteness found in the reflection coefficients from a wide variety of rock sequences around the world. In all but one case densities are taken as constant due to the paucity of suitable density data. The reflection sequences are pseudo-white only above a corner frequency, below which their power spectrum falls away according to a power law fβ, where β is between 0.5 and 1.5. This spectrum can be adequately modelled in practice very simply with an ARMA (1, 1) process which acts on a white innovation sequence. The corollary of this is that before wavelet estimation methods are applied (all of which-except those based on synthetic seismograms—presuppose white reflection sequences) or deconvolution filters estimated, seismic traces should be filtered with the inverse of this process.
Interestingly, the estimated ARMA processes group themselves into two clearly differentiated categories, having very different indices of predictability (or, strictly, indices of linear determinism). The two categories apparently correlate precisely with two kinds of sedimentation: one which consists largely of sequences of rocks with repeating properties, called “repetitive” in this paper but perhaps loosely describable as “cyclic”, and the other which is randomly bedded with no apparent pattern of components. The former has indices of predictability which are two to four times as great as those of the latter. Another, probably related, property is that β for the repetitive sequences tends to be greater than that for non-repetitive rock columns.
The observed power spectra are shown to be consistent with a simple model for the logarithm of acoustic impedance consisting of a mixture of processes where the distribution of (time) scale parameters is reciprocal.
Detailed effects of block-averaging and sampling the logs are shown to depend on the type of sequence under examination.
176 citations
TL;DR: In this article, two-dimensional time-dependent numerical simulations have been performed to study the structure and propagation of self-sustained detonations and the evolution of the curvature of the transverse wave appears to be a crucial feature.
172 citations
TL;DR: In this paper, a unified approach to the study of reflection and refraction of elastic waves in general anisotropic media is presented, where the Christoffel equations and boundary conditions for both anisometric media in coordinate systems formed by incident and interface planes, rather than in crystallographic coordinates, are considered.
Abstract: A unified approach to the study of reflection and refraction of elastic waves in general anisotropic media is presented. Christoffel equations and boundary conditions for both anisotropic media in coordinate systems formed by incident and interface planes, rather than in crystallographic coordinates, are considered. Consideration of wave propagation in an acoustic‐axis direction is included in the general algorithm, so results can be obtained both generally and for planes of symmetry, including planes of isotropy. General features of the numerical results are discussed. Energy conversion coefficients are shown to satisfy reciprocity relations which are formulated. It is much more natural to consider intensity–conversion ratios, rather than amplitude–conversion ratios, showing the important role of ray (rather than wave‐vector) directions in describing phenomena such as grazing angles. In particular, it is shown that the incident wave vector for grazing incidence may be greater or less than 90°: The domain of incident wave‐vector angles can actually split into disjoint pieces. The reflection coefficient at grazing incidence is shown to be unity, as in the isotropic case. Critical‐angle phenomena are described naturally by this approach.
168 citations
Patent•
11 Jan 1985
TL;DR: In this article, a control apparatus for a dazzle-free reflection mirror of a vehicle is presented, which is provided with a rear light sensor and a circuit for driving the reflection mirror into a light-free condition.
Abstract: A control apparatus for a dazzle-free reflection mirror of a vehicle is disclosed. The control apparatus is provided with a rear light sensor and a circuit for driving the reflection mirror into a dazzle-free condition in accordance with an intensity of a rear light detected by said rear light sensor when a light switch is turned on. The control apparatus is further provided with a winker manipulation detecting switch, a reverse position detecting switch and a steering wheel detecting switch for detecting a change in the moving direction of the vehicle. When one of them detects the change in the vehicle moving direction, the dazzle-free operation of the reflection mirror is disabled even if intensive light is incident to the reflection mirror from the rear of the vehicle.
TL;DR: In this paper, a general theory for multimirror interferometers, featuring matrices analogous to the theory of multilayer thin films, is presented. But the parameters in the matrix elements are mirror reflection coefficients and spacings instead of the usual refractive indices and layer thicknesses of thin films.
Abstract: The addition of one or more partially reflecting mirrors to a traditional two-mirror Fabry–Perot interferometer results in a multimirror Fabry–Perot interferometer. A superposition of all possible multiple reflected beams is described with a general theory for multimirror interferometers, featuring matrices analogous to the theory of multilayer thin films. However, the parameters in the matrix elements are mirror reflection coefficients and spacings instead of the usual refractive indices and layer thicknesses of thin films. The transmission characteristics of two-, three-, and four-mirror Fabry–Perot optical filters are described. It is shown that a suitable choice of reflection coefficients results in transmission properties that can be described approximately with Butterworth profiles, which are known from network analysis of electrical circuits.
TL;DR: In this article, four methods for the determination of Q in marine sediments are compared: two traditional methods, i.e., the risetime and the spectral ratio method, and two newly established ones, the spectrum modeling and the wavelet modeling method.
Abstract: Four methods for the determination of Q in marine sediments are compared: two traditional methods, i.e. the risetime and the spectral ratio method, and two newly established ones, the spectrum modeling and the wavelet modeling method. In the latter one Q and the reflection time T are determined simultaneously, which gives a much better accuracy for T than reading it from the seismogram. The risetime and the spectral ratio methods are used for obtaining Q directly from the data. The principle of the modeling methods is to calculate the effect of absorption and dispersion on a reference wavelet or its spectrum for various values of Q, and the best fit between the observed and the calculated data leads to the optimum result. Numerical tests on synthetic data show that a precision of more than 25% for data containing noise or superposed arrivals can hardly be achieved; in any case, wavelet modeling is the superior method. Application to data from a vertical reflection profile in the Baltic Sea yields Q in the range of 15-100 for different layers, which is to be expected in the sedimentary environment of this area.
TL;DR: In this paper, a nonlinear integrodifferential equation was derived to derive the susceptibility kernel for a one-dimensional homogeneous slab to the reflection operator for the medium, which can be determined from reflection data.
Abstract: The dispersion of transient electromagnetic waves in a homogeneous medium can be characterized by expressing either the complex permittivity as a function of frequency or the susceptibility kernel as a function of time. In this paper, a time domain technique is used to derive a nonlinear integrodifferential equation which relates the susceptibility kernel for a one‐dimensional homogeneous slab to the reflection operator for the medium. Thus, the susceptibility kernel (which is a function of time) can be determined from reflection data. A numerical implementation of this technique is shown. The more general case of a medium consisting of a stack of homogeneous dispersive layers is also addressed.
TL;DR: In this paper, the mixed-boundary value problem for a typical strip is reduced to a singular integral equation, which is solved numerically by the use of Fourier series techniques, and a check of the accuracy of the computations is provided by the balance of rates of energies.
TL;DR: In this article, the relationship linking the reflection coefficient to the dielectric properties is obtained from a theoretical analysis of the electromagnetic field in the vicinity of the aperture, including the effects of both cross polarization and higher order modes.
Abstract: The measurement of reflection from an open-ended waveguide is a simple and nondestructive technique for determining the dielectric properties of materials. A flange-mounted waveguide is considered, the flange being pressed on an unknown material which may be of finite or infinite thickness. The relationship linking the reflection coefficient to the dielectric properties is obtained from a theoretical analysis of the electromagnetic field in the vicinity of the aperture. The theory includes the effects of both cross polarization and higher order modes. An integral equation is obtained, the kernel of which is the dyadic Green function in each medium. The method of characteristic modes is used for the numerical computation. The theoretical results are in good agreement with experimental measurements. Futhermore, a simple and handy technique for data inversion is provided.
Patent•
12 Jan 1985
TL;DR: In this article, the authors proposed to eliminate the use of an optical system which converts the form of luminous flux to reduce the weight of a reader and at the same time to facilitate positioning of a photodetector.
Abstract: PURPOSE:To eliminate the use of an optical system which converts the form of luminous flux to reduce the weight of a reader and at the same time to facilitate positioning of a photodetector, by obtaining a focusing error signal from the variation of reflection factor at a level close to a critical angle after combining a diffraction grating with a parallel plate. CONSTITUTION:When parallel beams 17 are made incident vertically on the 1st boundary surface 13 of a parallel plate 13, the beams 17 transmit through the 2nd boundary surface 13b of the plate 13 and are made incident on a diffraction grating 15 to be diffracted there. The diffracted beam 18 is made incident again on the surface 13b and travels straight to reach a point P. The constant of the grating 15 is decided so that the incident angle at the point P is equal to the critical angle of total reflection. Therefore the beam 18 is totally reflected at the point P and made incident on a photodetector 11. The beam 18 is totally reflected if the angle theta is larger than the critical angle thetaS; while the beam 18 transmits partially through the surface 13a if the angle theta is smaller. When a disk 7 gets close to an objective lens 6, the light made incident on the grating 15 is turned into the dispersed luminous fluxes. Then the luminous flux has a semicircular form and is made incident only on a photodetector 11b. While the incident light of the grating 15 is focused when the disk 7 goes further away from the lens 6. Then the luminous flux has a semicircular form and is made incident only on a photodetector 11a.
TL;DR: In this paper, it was shown that the role of the upper wave region may be to allow a wave flux through the critical level, and numerically that the effect of an upper-wave region can be mimicked by a region of localized damping which leads to over-reflection.
Abstract: We reexamine the related problems of instability of parallel shear flows and over-reflection of internal waves at a critical level, concentrating on the stratified case. Our primary aim is to delineate the specific aspects of a flow that permit overreflection and instability. A related and partly realized aim is to develop a mechanistic ‘picture’ of how over-reflection and instability work. In the course of this study we have also uncovered some new results concerning the instability of stratified shear flows – showing how regions of enhanced static stability and enhanced damping can destabilize otherwise stable flows.For the scattering of steady plane waves, we show that, of the conditions found by Lindzen & Tung (1978), in the unstratified case, only the existence of wave-propagation regions above and below the critical level is always necessary for over-reflection (at least in the absence of damping), although a trapping region around the critical level and a reflecting surface bounding the upper wave region may play crucial roles in some cases. Our results suggest that the role of the upper wave region may be to allow a wave flux through the critical level. Moreover, we show numerically that the effect of an upper wave region can be mimicked by a region of localized damping which leads to over-reflection as well.We also consider an initial-value problem, using numerical methods. When a wave is incident on the incident level, the reflection and transmission coefficients grow smoothly to their final values. The rate of growth depends on the flow parameters, but there is some evidence to suggest there is a characteristic timescale involved that depends only on the shear (and not on wave travel time). This fits a mechanistic picture of over-reflection and instability that we describe, in which the essential part is a kinematic interaction between wave and mean flow at the critical level, depending only on shear.
TL;DR: In this paper, a simple ray model is introduced to establish a physical picture of the processes involved in the V(z) response of the scanning acoustic microscope (s. a. m.) when used to examine specimens with lateral discontinuities.
Abstract: This paper discusses theoretical aspects of the V ( z ) response of the scanning acoustic microscope (s. a. m.) when used to examine specimens with lateral discontinuities. The problem is introduced in terms of a simple ray model to establish a physical picture of the processes involved. An approximate Green function is then developed which enables use of a modified form of the Fourier optical formulations to calculate V ( z ) for a cylindrical lens. These calculations explain two different types of contrast observed when imaging specimens in the reflection s. a. m., such as (i) the ability to image fine discontinuities and display them with enhanced contrast and an apparent width determined by the acoustic wavelength; and (ii) to give a quantitative account of the amplitude of periodic ripple often observed running parallel to cracks on acoustic micrographs. Both these types of contrast may be predicted by using the same model and arise naturally from variation of the reflection and transmission properties of the discontinuity, the relative value of these parameters determines which type of contrast predominates. At an interface between media with different elastic properties, the contrast is affected not only by the scattering properties of the boundary but also by the very fact that surface waves must propagate in media with different elastic properties. This effect alone can provide a powerful contrast mechanism which enables one to understand the light to dark contrast reversals often observed at grain boundaries in polycrystalline specimens at different values of defocus.
TL;DR: In this article, an exact elastic form of the Kirchhoff-Helmholtz (KH) integral is first derived, then various asymptotic approximations are used to convert this integral into one which can be rapidly evaluated to give the motion of a single generalized ray.
Abstract: Summary. In a medium consisting of elastic layers with irregular interfaces, Kirchhoff-Helmholtz (KH) theory can be extended to synthesize the motion due to various generalized rays. An exact elastic form of the KH integral is first derived, then various asymptotic approximations are used to convert this integral into one which can be rapidly evaluated to give the motion of a single generalized ray. The approximations used are those of geometrical optics, for propagation across layers, and the Kirchhoff or tangent-plane approximation for propagation across boundaries. It is shown how the KH method leads naturally to a generalization of our usual notion of elastic reflection and transmission coefficients. The new coefficients are functions of both angle of incidence and angle of reflection or transmission and they are derived so as to obtain coordinate-free formulae that show clearly their relation to the conventional Snell’s law coefficients. The elastic KH method is applied first to the problem of a single interface, where its performance is compared to that of the Gaussian beam and Maslov methods. (For synthesizing reflections from irregular interfaces the KH method is superior because it includes signals diffracted from corners. However, when the interface is very smooth on the scale of a wavelength the Maslov and Gaussian beam methods are superior because they do not break down when there is a caustic on the reflector.) KH theory is then applied to a multilayered elastic medium and it is shown how the effects of frequency-dependent attenuation and dispersion can be incorporated into the theory by taking advantage of the approximately logarithmic variation of slowness with frequency in most earth materials. The limitations of the KH theory are discussed and some recent attempts to overcome these difficulties are reviewed. A new method for overcoming the problem of a caustic on the reflector becomes apparent when the KH integral is regarded as a member of a larger family of equivalent 1-fold integrals all of which are derivable from the same multifold path integral. Refracted or diving rays can be treated within the same formalism with equal benefit. For velocity models that are independent of one spatial direction (strike) a method is given for approximately converting 2-D results into 3-D results.
TL;DR: In this paper, a pulsed near-infrared laser system was converted to determine laser height and reflection values in airborne operations, which can be used in discriminating vegetation type and density.
Abstract: A pulsed near-infrared laser system was converted to determine laser height and reflection values in airborne operations. The laser produces precise traces of the terrain and vegetation canopy and tree measurements can readily be made in open forests since the laser beam frequently penetrates openings in the canopy cover. The reflection measurements were useful in discriminating between vegetation type and density. Using the combined laser height, laser reflection and reflection variability parameters, it was possible to arrive at a simple semiautomated terrain classification which allowed a distinction between conifer and broadleaf forests and terrain with low-growing vegetation cover. The classification of individual trees into coniferous and broadleaf of different height classes is demonstrated. The data is in digital form and can be incorporated into geographic information systems. Considering that these measurements were made at a single wavelength (904 nm), it is clearly evident that high-r...
TL;DR: In this article, a Gaussian reflectivity profile and a complementary transmission has been designed for a nonabsorptive smooth mirror based on radially varying the thickness of a high-refractive-index dielectric deposited on a transparent substrate.
Abstract: A mirror with a Gaussian reflectivity profile and a complementary transmission has been designed. This nonabsorptive smooth mirror is based on radially varying the thickness of a high-refractive-index dielectric deposited on a transparent substrate. Prototypes have been fabricated for use around 10 μm. The reflection and transmission profiles of one of them have been measured together with the modifications of the reflected and transmitted wave fronts.
TL;DR: A theory that describes the resonant scattering of light from a randomly rough surface is developed, based upon the rearranged vector Rayleigh equation obtained in previous work.
Abstract: A theory that describes the resonant scattering of light from a randomly rough surface is developed, based upon the rearranged vector Rayleigh equation obtained in previous work. Flux conservation in the intermediate states is ensured by deriving a Ward identity for the self-energy and irreducible vertex function. Approximate closed-form expressions for the differential reflection coefficients of a rough surface for p- and s-polarized light are given, and the results of numerical calculations are compared with some recent experimental results.
TL;DR: In this paper, the problem of reflection of a Gaussian beam from a parallel-sided dielectric slab is studied from the angular-spectrum point of view, and it is shown that the path of the peak of the reflected profile undergoes three types of effects with reference to its expected geometrical path: a lateral displacement independent of the propagation distance, a focal shift, and an angular shift that results in a lateral shift proportional to the total propagation distance between the beam waist and the point of observation.
Abstract: The problem of reflection of a Gaussian beam from a parallel-sided dielectric slab is studied from the angular-spectrum point of view. It is shown that the path of the peak of the reflected profile undergoes three types of effects with reference to its expected geometrical path: a lateral displacement independent of the propagation distance, a focal shift, and an angular shift that results in a lateral shift proportional to the total propagation distance between the beam waist and the point of observation and independent of the location in between, where reflection takes place. These effects are illustrated by computer simulating some typical cases. The generality of the analysis and its applicability to many different situations are noted. The results of several authors on related problems are shown to be special cases of the general results. The relevance of the analysis and the results to an interesting series of recent experiments in the microwave region is noted.
Patent•
12 Feb 1985
TL;DR: In this article, a control apparatus for a dazzle-free reflection mirror of a vehicle is presented, which is provided with a rear light sensor and a circuit for driving the reflection mirror into a controllable operation in accordance with the intensity of the rear light detected by the rear sensor when a light switch is turned on.
Abstract: A control apparatus for a dazzle-free reflection mirror of a vehicle is disclosed. The control apparatus is provided with a rear light sensor and a circuit for driving the reflection mirror into a dazzle-free operation in accordance with the intensity of the rear light detected by the rear light sensor when a light switch is turned on. The control apparatus is further provided with a mirror adjusting device for adjusting the reflection angle of the reflection mirror and a detecting circuit for detecting the adjusting operation of the mirror adjusting device. When the detecting circuit detects the adjusting operation, the dazzle-free operation of the reflection mirror is disabled even if intensive light is incident to the reflection mirror from the rear of the vehicle.
TL;DR: In this paper, the influence of collisions and the Earth's magnetic field on the electron acceleration process is discussed and the extent of the acceleration-caused modification of the fast electron distribution function and the value of the flux at fast electrons escaping into the magnetosphere are calculated.
TL;DR: In this article, the Helmholtz Reciprocity Principle is used to determine reflection properties of materials or components, and their validity depends on their validity on the principle of reciprocity.
Abstract: Many measurements of reflection properties of materials or components depend for their validity on the Helmholtz Reciprocity Principle. Yet this principle is widely misunderstood, and is stated in ...
TL;DR: In this article, an algorithm was developed for using the reflection of solar radiation in the oxygen A-band to determine cloud-top altitude, which was applied to an aircraft field experiment in conjunction with CCOPE.
Abstract: An algorithm has been developed for using the reflection of solar radiation in the oxygen A-band to determine cloud-top altitude. Because of multiple scattering and molecular absorption inside the cloud, the reflection of clouds is substantially modified in comparison with a mirror cloud, which is assumed to have a 100 percent reflection. To infer true cloud-top altitude, therefore, it is necessary to accurately estimate the amount of 'photon penetration'. Theoretical calculations indicate that the amount of photon penetration depends on the altitude, the scaled volume scattering coefficient, and the scaled optical thickness of the cloud. Algorithms using the reflection in the oxygen A-band to determine the cloud-top pressure have been applied to an aircraft field experiment in conjunction with CCOPE, 1981. Results of this study are very encouraging, especially for extended clouds.
TL;DR: In this article, a simple theory for a phase step, taking into account the possibility of large phase changes, deviations of the angle of incidence from the Bragg angle and possible displacements of the objective aperture, showed good agreement with the contrast observed for steps on the surfaces of gold and platinum crystals.
TL;DR: In this article, the authors give a model for the force exerted on vertical walls by the reflection of water waves with an arbitrary angle of incidence, and show that the maximum force per unit length can be caused by obliquely-incident waves rather than standing waves.
Abstract: Formulae are given for the force exerted on vertical walls: by the reflection of water waves with an arbitrary angle of incidence. The variation of the loads with all design variables show a number of unusual features, including the fact that the maximum force per unit length can be caused by obliquely-incident waves rather than standing waves. It is important for design that the whole range of possible wave conditions be considered. A method is developed for the numerical solution of the problem, which unlike the theory on which the above-mentioned formulae are based, solves the stated problem exactly. Results from the approximate formulae are compared with those from the numerical method, and are found to be surprisingly accurate over a wide range of wave conditions.