Showing papers on "Point source published in 1974"
TL;DR: In this paper, the effects of elastic wave propagation from a point source located in a fluid-filled cylinder are investigated and detailed consideration is given to the natural mode wave propagation for either fluid or elastic solid surroundings of infinite extent.
Abstract: Phenomena of elastic wave propagation from a point source located in a fluid‐filled cylinder are investigated. Detailed consideration is given to the natural mode wave propagation for either fluid or elastic solid surroundings of infinite extent. Phase and group velocity dispersion curves are presented and the relative modal sensitivity, interference pattern, and decay rates are determined for typical sources having various dominant wavelengths. Curves showing actual waveforms including both axial and radial dependence are also presented for this coupled system.
68 citations
TL;DR: In this article, the static displacement and strain fields caused by the introduction of a shear dislocation point source into a layered elastic half space have been evaluated using a Thomson-Haskell matrix method (Singh).
Abstract: Summary The static displacement and strain fields caused by the introduction of a shear dislocation point source into a layered elastic half space have been evaluated using a Thomson-Haskell matrix method (Singh). The point source was found to be an adequate representation of a fault at distances greater than four fault lengths. Gross earth structure (oceanic, shield and tectonic models) cause fields that differ little from those of a uniform half space. However, significant departures from the uniform half-space fields are found to be caused by low rigidity layers, both at the surface (representing sediment cover) and at depth (representing possible zones of partial melt). Both of these features cause complexities in the strain fields that depend on the source orientation and source-receiver distance, and these may result in amplification or attenuation of the uniform halfspace fields by factors of up to 10.
58 citations
TL;DR: It is shown that the largest SNR advantage occurs for a point source, while for very large, uniform sources there may be a slight net disadvantage to using a zone plate, and that optical processing does not give the highest possible SNR.
Abstract: A Fresnel zone plate, used as a coded aperture, offers a great advantage in geometric collection efficiency over a conventional pinhole or collimator. We present a detailed analysis of the signal-to-noise ratio (SNR) of a quantum-limited zone plate camera. The magnitude and spatial distribution of the noise field and its dependence on the source distribution and the characteristics of the optical processing system are derived. It is shown that the largest SNR advantage occurs for a point source, while for very large, uniform sources there may be a slight net disadvantage to using a zone plate. It is also shown that optical processing does not give the highest possible SNR.
40 citations
TL;DR: In this article, the power spectrum and total power of a moving point source in a round, slug flow jet were calculated and the effect of non-axial lines of source convection was explored with the aid of a plane jet, line source model problem.
13 citations
12 citations
Patent•
11 Mar 1974
TL;DR: In this article, a method and apparatus for nondestructive testing of articles by holography in which light beams from a virtual point source of laser light and from an observation point are reflected by a reflector toward the same point on the object being checked.
Abstract: A method and apparatus for nondestructive testing of articles by holography in which light beams from a virtual point source of laser light and from an observation point are reflected by a reflector toward the same point on the object being checked. The light beams when extended beyond the reflector intersect on an ellipsoid having the point source and the observation point as foci. A plate having a hologram of the article is interposed between the observation point and the reflector and flaws in the object being tested show up on the plate as interference lines when the object is modified in shape by a mass influence such as a temperature change, or a change in pressure thereon, or by vibrations.
10 citations
TL;DR: In this paper, the authors derived the equation governing the propagation of the coherence function for a finite beam or source in an anisotropic random medium and solved it in terms of a definite integral over the horizontal spectrum of the index of refraction field.
Abstract: In a previous paper we solved the problem of propagation of a plane wave in an anisotropic random medium. The characteristic correlation distances in the horizontal propagation direction were assumed to be large compared to the radiation wavelength, while in the vertical direction these distances were assumed to be of the same order or smaller than the wavelength. For this medium, we derive here the equation governing the propagation of the coherence function for a finite beam or source. This equation is then averaged over the vertical direction and solved. The solution is given in terms of a definite integral over the horizontal spectrum of the index of refraction field. The point source solution and the plane wave case are given as special cases. Explicit solutions are given for the propagation of an acoustic field in the ocean under conditions where the spectrum is dominated by the effects of internal waves.
9 citations
TL;DR: In this article, a new approximation, of dipole character, based on Young's approximation for the sound field in water due to a source in air, is derived and for a scattering of source and field points is shown to be in good agreement with Waters expression, which neglects the lateral wave and surface roughness.
Abstract: Young's approximation for the sound field in air due to a point source in water is questioned regarding its presumed quadripole character. A new approximation, of dipole character, based on Young's approximation for the sound field in water due to a source in air, is derived and for a scattering of source and field points is shown to be in good agreement with Waters expression, which neglects the lateral wave and surface roughness. It is shown that Horton's graphs can be scaled to produce satisfactory estimates of Waters values. Finally, on examination, Horton's isobaric contours are found to be satisfactory.
5 citations
TL;DR: In this article, an illustrative directivity pattern is presented, for an array adjusted to select the first mode in a shallow water problem, and the equations for relating the actual source and the line source are given, and their solution is discussed.
Abstract: Standard normal‐mode analysis of underwater‐sound propagation assumes only an omnidirectional point source. Other writers have proposed that, for use in calculations, a virtual line‐array of discrete point sources, arranged vertically, can well approximate the directivity pattern, in the vertical plane, of actual sources. It seems better, however, to use an equivalent vertical line array with a continuous distribution of strength and phase. The equations for relating the actual source and the line source are given, and their solution is discussed. Identical methods allow the calculation of reception by a directional receiver. A related topic, use of a vertical array of discrete elements to stimulate or receive a single mode, is also discussed. An illustrative directivity pattern is presented, for an array adjusted to select the first mode in a shallow‐water problem.
5 citations
TL;DR: The correlation function and power spectrum of phase difference fluctuations and angle-of-arrival fluctuations are calculated for tracking a moving spherical wave optical source through atmospheric turbulence to show that, in order to freeze effectively the received wavefront, the scanning frequency must be essentially equal to the highest frequency contained in the power spectrum.
Abstract: The correlation function and power spectrum of phase difference fluctuations and angle-of-arrival fluctuations are calculated for tracking a moving spherical wave optical source through atmospheric turbulence. For Kolmogorov turbulence, it is shown that, in order to freeze effectively the received wavefront, the scanning frequency must be essentially equal to the highest frequency contained in the power spectrum.
4 citations
01 Dec 1974
TL;DR: In this paper, a technique for determining the near and far acoustic field of a jet through measurements of pressure-gradient correlations on an imaginary conical surface surrounding the jet is discussed, and necessary analytical developments are presented, and their feasibility is checked by using a point source as the sound generator.
Abstract: The technique for determining the near and far acoustic field of a jet through measurements of pressure-gradient correlations on an imaginary conical surface surrounding the jet is discussed. The necessary analytical developments are presented, and their feasibility is checked by using a point source as the sound generator. The distribution of the apparent sources on the cone, equivalent to the point source, is determined in terms of the pressure-gradient correlations.
01 Aug 1974
TL;DR: In this paper, the authors used a gas correlation spectrometer onboard the NIMBUS-G environmental monitoring satellite to measure the mass of a given pollutant within a gas volume.
Abstract: The NIMBUS-G environmental monitoring satellite has an instrument (a gas correlation spectrometer) onboard for measuring the mass of a given pollutant within a gas volume. The present paper treats the problem: How can this type measurement be used to estimate the distribution of pollutant levels in a metropolitan area. Estimation methods are used to develop this distribution. The pollution concentration caused by a point source is modeled as a Gaussian plume. The uncertainty in the measurements is used to determine the accuracy of estimating the source strength, the wind velocity, diffusion coefficients and source location.
TL;DR: In this article, an exact integral expression for the velocity potential produced by a harmonic point source located in a semi-infinite isovelocity space is given, where a transition layer separates the source region from a second constant velocity layer.
Abstract: An exact integral expression is given for the velocity potential produced by a harmonic point source located in a semi‐infinite isovelocity space. A transition layer separates the source region from a second constant‐velocity layer. A depth‐dependent sound speed with two adjustable parameters is employed for the transition region. The velocity potential for the lateral wave is obtained using the method of steepest descent. The results are in agreement with those for the special case of the linear transition layer.
Dissertation•
01 Jan 1974
TL;DR: In this article, an investigation into the behavior of artificially produced ions, released from a point source a few metres above the Earth's surface, is presented and it is shown both experimentally and theoretically that the ions form a plume downwind of the ion source and moreover, the nature of this plume is rather similar to that of a smoke plume.
Abstract: An investigation into the behaviour of artificially produced ions, released from a point source a few metres above the Earth’s surface, is presented It is shown both experimentally and theoretically that the ions form a plume downwind of the ion source and, moreover, that the nature of this plume is rather similar to that of a smoke plume Electric field and ion concentration measurements have been used to deduce certain features about ion plumes and distinct connexions between ion plume behaviour and the local meteorological conditions have been observed In particular, the use of bidirectional vane techniques has indicated that ions can be used quite successfully as a tracer in short range atmospheric diffusion experiments The technique does however, possess certain limitations, the details of which are included in the text
TL;DR: In this paper, the problems of Cagniard and Abramovici-Alterman, regarding propagation of seismic pulses in horizontally layered media, are solved by a direct method without involving integral transforms.
Abstract: The problems of Cagniard and Abramovici-Alterman, regarding propagation of seismic pulses in horizontally layered media, are solved by a direct method without involving integral transforms
TL;DR: In this article, singular perturbation techniques are used to obtain approximate concentration distributions that result when a recharge well discharges a finite volume of radioactive pollutant into an aquifer.
Abstract: Singular perturbation techniques are used to obtain approximate concentration distributions that result when a recharge well discharges a finite volume of radioactive pollutant into an aquifer. The approximate solutions have a relatively simple form and are obtained for both a line source and a point source discharging into a flow that is at rest at infinity.
TL;DR: In this paper, the diffusion of polarized resonance radiation is considered, and an explicit expression for the Green's function of the corresponding equation for an unbounded uniform space is given for the diffusion.
Abstract: The diffusion of polarized resonance radiation is considered. An explicit expression is given for the Green's function of the corresponding equation for an unbounded uniform space.
TL;DR: In this paper, a modified Lax-Wendroff numerical technique was used to calculate the radius of a hydrogen ice sphere exploding due to an energy deposition by slowed-down fast neutrons radiating from a strong point source.
TL;DR: In this article, the behavior of the Jacobian of the mapping between small regions surrounding two points on a ray path is studied in an inhomogeneous, anisotropic propagation environment.
Abstract: In some applications of ray tracing, it is useful to know the behavior of certain ray-related quantities under a transformation of the variable of path parameterization. One such quantity is the Jacobian of the mapping between small regions surrounding two points on a ray path. A narrow bundle of rays emanating from a point source in an inhomogeneous, anisotropic propagation environment is considered. The mapping between a small region surrounding a reference point on an interior ray of the bundle, and the corresponding region surrounding an observation point on the same ray, is investigated. The Jacobian of the mapping is studied as a function of an arbitrary path parameter, and a new relation expressing the behavior of the Jacobian under a transformation of the parameter is derived.