Showing papers on "Point source published in 1969"

Turbulent buoyant convection from a source in a confined region

Abstract: This paper considers the effect of continuous convection from small sources of buoyancy on the properties of the environment when the region of interest is bounded. The main assumptions are that the entrainment into the turbulent buoyant region is at a rate proportional to the local mean upward velocity, and that the buoyant elements spread out at the top of the region and become part of the non-turbulent environment at that level. Asymptotic solutions, valid at large times, are obtained for the cases of plumes from point and line sources and also periodically released thermals. These all have the properties that the environment is stably stratified, with the density profile fixed in shape, changing at a uniform rate in time at all levels, and everywhere descending (with ascending buoyant elements).The analysis is carried out in detail for the point source in an environment of constant cross-section. Laboratory experiments have been conducted for this case, and these verify the major predictions of the theory. It is then shown how the method can be extended to include more realistic starting conditions for the convection, and a general shape of bounded environment. Finally, the model is applied quantitatively to a variety of problems in engineering, the atmosphere and the ocean, and the limitations on its use are discussed.

A Quantitative Evaluation of Seismic Signals at Teleseismic Distances—I Radiation from Point Sources

Abstract: Summary The aim of the series of papers to be published under the above title is to follow up and improve Carpenter's (1966) attempt to construct realistic pulse shapes for teleseismic body waves from underground explosions. The present work deals with both explosions and shallow earthquakes and with the radiation of surface waves as well as body waves. The extension of Carpenter's theory to take crustal layering more accurately into account involves the matrix theory first introduced by Thomson (1950) and developed by Haskell. We shall use the notation of Haskell's (1964) paper in the following analysis and the initial theoretical work is concerned with some of the groundwork and one or two results which have not been covered by Haskell in his series of papers (1953, 1962, 1964). A point source can be represented either as a system of forces or as a discontinuity in the displacement or stress or their derivatives across an element of surface. We show here that a general source of either type is equivalent in the generation of elastic radiation to a discontinuity across a horizontal plane in the displacement and the stress acting on the plane. This means that any point source can be put into a form suitable for computations based on the Thomson-Haskell theory. The theory is applied to the construction of theoretical models of earthquake and explosive sources. Some of the more realistic models so far proposed are given in the later section of the paper.

Long-Period P Waveforms and the Source Mechanism of Intermediate Earthquakes

Abstract: Long-period P waveforms have been analyzed to interpret the source mechanism of four intermediate earthquakes with magnitudes of 6.0-6.5 and focal depths between 100 and 200km. The synthetic seismograms appropriate to each recording station have been constructed, to compare with observed records, on the basis of moving dislocation models with various parameters, including the fault length and width, the amount of dislocation, its time dependence and the fracture velocity, taking into account the combined effects of wave propagation in the earth and of a recording instrument. General features of the observed waveforms do not differ greatly from those for a double-couple point source, but the comparison with synthesized waveforms at a number of stations indicated seismic moment of order of 5-9×1026 dyne·cm, and also probable ranges for some other source parameters on tne assumed source. The bounds of the stress-strain drop, the released strain energy, and of efficiency of seismic wave radiation at the source were also discussed.

Evidence for a Point Source of High Energy Cosmic Gamma Rays

Abstract: High energy cosmic gamma radiation detection from point source in Sagittarius, using balloon mounted spark chamber with Cerenkov telescope

Propagators of atmospheric motions 2. Excitation by switch‐on sources

Abstract: The excitation of various atmospheric wave motions by switch-on sources is investigated. An asymptotic description of the propagation of the transient forerunners and the stationary forced motion contained within a Brillouin front is given. Three-dimensional Rossby waves generated in an unbounded atmosphere by a traveling point source and several kinds of oscillating point sources are discussed. Also given is the solution for the switch-on of a horizontally traveling gravity wave or vertically traveling atmospheric acoustic wave. Various one-dimensional switch-on Rossby wave problems in a periodic domain are solved in terms of image sources.

On the Resolution of Imaging Systems

Abstract: The angular resolution of imaging systems is defined as the capability of the system to estimate the position of a point source object which is observed in a background of gaussian noise. The application of statistical estimation theory gives the resolution in terms of the noise characteristics and the modulation transfer function of the system. The resolution is also considered as the ability to estimate the separation of two point sources. Examples of estimation of both position and source intensity are given for a gaussian-shaped spread function.

Propagation from a point source in a randomly refracting medium

Abstract: This paper considers the propagation of scalar (acoustic) waves from a single-frequency point source imbedded in a medium with random refractive index, in contrast with the usual plane-wave case in which the source is far removed from the medium. With the index being a statistically homogeneous and isotropic function of position, but not a function of time, the average complex field $u_{o}(r) = \langle u(r)\rangle$ and the spatial covariance $\langle u_{i}(r)u*_{i}(\rho;)\rangle$ of the fluctuation field u i (r) = u(r) - u o (r) are calculated. Beyond a few correlation lengths from the source, the average field can be approximated by a spherical wave with the same complex wavenumber found in the plane-wave case. A near-source wave number is also obtained. Under an improved far-field condition, the spatial covariance is reduced to spectral integration formulas for both transverse and longitudinal separation of the receiving points. These formulas reveal that correlation lengths are much longer in the point-source case than in the plane-wave case, even though the relative variances are the same. We illustrate this result with plots for an exponential index spectrum and for a constant spectrum.

High-dispersion, high-resolution x-ray spectrometer having means for detecting a two-dimensional spectral pattern

Abstract: Apparatus is disclosed for providing a two-dimensional spectral pattern of X-ray radiation. The radiation, which may be provided by a wide angle point source in the apparatus or may come from a cosmic X-ray source, is directed upon a bent crystal to provide spectral dispersion and focusing The rays reflected from the crystal are then detected, as upon a film or the like, at an area positioned away from the focal region of the crystal to produce the two-dimensional pattern. In one embodiment of the apparatus provision is made to feed information from the detector to a computer to yield simultaneous analysis of the radiation both as to the frequencies present therein and the intensity of each.

Ray Description for Point Source Excited Surface Waves in Stratified Anisotropic Media

Abstract: A ray-optical description for the surface wave contribution to the far fields radiated by an electromagnetic point source in the presence of a planar interface between two lossless, homogeneous, anisotropic media is formulated. The surface wave rays are viewed as extending radially along the interface from the normal projection of the source onto the interface. Locally, the ray fields are those of the modal surface waves carrying energy in the ray direction. The effect of anisotropy on the surface wave radiation pattern is also considered.

The acoustical source in a two-dimensional jet,

Abstract: : An acoustical point source moving with the local fluid in a two-dimensional jet of infinite length is studied. In particular, the effect of linear velocity profile shear layers separating the uniform jet from the external fluid is studied. Comparison of shear layer jets with jets bounded by velocity discontinuities can show significant differences in far field mean-square pressures. Possible causes of these differences are discussed. It is noted that, for the supersonic jet, far field (but not near field) effects are confined to a downstream Mach cone. Pressures on the Mach cone are calculated to be zero for the velocity discontinuity jet (and surmised to be zero for the shear layer jet) in contrast to the infinite pressure calculated for a moving source when there is no jet. It is noted that as the velocity discontinuity jet approaches zero thickness the potential of an acoustical source traveling with the jet fluid approaches that of a modified moving source different from the conventional moving source ordinarily referred to in the literature. Some speculations regarding 'real' jets (i.e., jets of finite length) are included. (Author)

The thin lens equation for optical parametric image conversion

Abstract: It is shown that the image upconverter can operate with point source pump-beams as well as the commonly used planar pump source. The major effect of the pump beam divergence is not to degrade the resolution but is to change the transverse and longitudinal magnification of the image. We demonstrate by a paraxial ray-tracing analysis that these changes in image dimensions and location obey relations quite similar to thin-lens formulae.

Seismic source in a layered sphere: Reflected and diffracted pulses

Abstract: An exact solution is obtained for the displacement of a surface caused by an explosive point source within a layered sphere that consists of a homogeneous solid mantle and a fluid core. For all modes of periods 4 seconds and longer, the variation of amplitude, phase changes, and range of existence of several reflected, refracted, and diffracted pulses are found and compared with observed and model experimental results.

On S. H. point source in heterogeneous medium

Abstract: A S.H. point source in heterogeneous elastic medium where the modulus of rigidity and density vary exponentially with depth has been considered. The form of disturbance due to such a point source in a semi-infinite medium has been obtained in integral form. The integrals are evaluated by contour integration.

The method of images for solving the equations of heat conduction in layered media

Abstract: One obtains the temperature field generated by an instantaneous point source in the layer. It is shown that the solution can be obtained by the method of images in the plane.

Explosion Generated Seismic Waves

Abstract: : The work done during the first year under the contract 'Explosion Generated Seismic Waves' is discussed The technique used in the interpretation of source mechanisms of nuclear explosions using Rayleigh and Love waves is described Theoretical calculations to determine the stress wave pulse shapes from an explosive source in a cavity are carried out for two and three dimensional geometry The generation and development of Rayleigh waves from a point source in a half-space are studied utilizing Cagniard-deHoop technique (Author)

MTF of a Gap

Abstract: Subject to reasonable assumptions of geometrical optics, the point spread function is calculated for a point source in a plane radiating to a nearby parallel plane. The calculation includes the case in which the gap between the planes attenuates the signal. The modulation-transfer function (MTF) is calculated from the point-spread function. If the source is lambertian, and the receiving plane (e.g., photographic emulsion) absorbs as a more or less typical diffuse absorber, and there is a response which can be expressed as a linear function of the absorbed input, the MTF can be expressed as an integral that can be evaluated numerically. The computed curves are normalized in terms of the gap width, which is taken to be unity. The method may be applicable to a fairly wide class of image-evaluation problems.

Improvements relating to Infra-Red and Optical Signal Detecting Devices with particular reference to Background Discrimination

Abstract: 1,157,951. Radiation detection. ELECTRIC & MUSICAL INDUSTRIES Ltd. 14 June, 1962 [20 June, 1961], No. 22349/61. Heading H4D. A signal detecting device comprises two or more strip-form detecting elements, means for focusing radiation in the form of a line image upon said elements in such a way that radiation from a particular source is focused upon only one detector at any one moment, whilst radiation from a diffuse source is focused upon both or all said detecting elements in substantially equal amounts. An infra-red detecting device is described with reference to Fig. 1 enabling the detection of a particular source even when the level of the signal is considerably less than the level of background radiation within the same frequency band. As shown the device comprises a lens 1 forming part of a toroid and focusing radiation from a point source 2 into a line image such as line 2a in the image plane of the lens. The lens 1 and two infra-red detectors D 1 , D 2 , situated in the image plane of the lens, are mounted in a vehicle in such a position that relative movement between the vehicle and the source causes the focused line image to be substantially parallel with the longer dimensions of the detectors and to move in a direction perpendicular to those dimensions. The outputs of the detectors are connected to the ends of a potentiometer P 1 (Fig. 2) and the potentiometer tapping is adjusted so that there is no signal when the detectors are equally energized by the background source and by the point source. However, on movement of the point source to position 3, for example, the image falls along line 3a and the detectors are unequally energized to produce a signal. In addition movement of the point source can be detected since this results in each detector being illuminated in turn and consecutive signals of opposite polarity being produced at the output.

X-ray Heating of Air

Abstract: We focus our attention on the heating produced by a short but very intense burst of X-rays from a point source. The total radiation energy usually lies above 1020 erg and nearly all the photons have energies above the K edges of O and N.