Showing papers on "Point source published in 1979"

Resistivity modelling for arbitrarily shaped two-dimensional structures

Abstract: A numerical technique is developed to solve the three-dimensional potential distribution about a point source of current located in or on the surface of a half-space containing arbitrary two-dimensional conductivity distribution. Finite difference equations are obtained for Poisson's equations by using point- as well as area-discretization of the subsurface. Potential distributions at all points in the set defining the half-space are simultaneously obtained for multiple point sources of current injection. The solution is obtained with direct explicit matrix inversion techniques. An empirical mixed boundary condition is used at the “infinitely distant” edges of the lower half-space. Accurate solutions using area-discretization method are obtained with significantly less attendant computational costs than with the relaxation, finite-element, or network solution techniques for models of comparable dimensions.

Numerical evaluation of the transient acoustic waveform due to a point source in a fluid-filled borehole

Abstract: A key measurement employed in oil well wireline logging is the acoustic wave traveltime over a specified formation interval, typically 1 ft. In the traditional measurement, only the compressional head wave is monitored, but for some time it has been obvious that there is significant additional information, such as the shear head wave arrival, in the received waveform. We describe two numerical methods for computing the profile and parameter dependence of the transient waveform based on a model of the acoustic logging problem consisting of a point source on the axis of a fluid-filled cylindrical borehole. The response to this excitation is determined at a distance from the source, generally on the borehole axis. In the first of the two numerical methods, called 'real axis integration', the complete acoustic waveform is obtained. The second method, called 'branch-cut integration', evaluates the first compressional and shear-pseudo-Rayleigh arrivals individually with much less computation time than the first method. The validity and accuracy of the two methods are demonstrated by their close agreement within appropriate time windows. It is also shown that the results from the ordinary asymptotic method that exist in the literature predict different behavior. The dependence of the transient arrivals on formation parameters is illustrated by various numerical results in both time and frequency domains.

A normal mode theory of acoustic Doppler effects in the oceanic waveguide

Abstract: This paper considers the problem of computing the acoustic field generated by a moving point source. In particular, the acoustic field is obtained in terms of the normal modes of a horizontally stratified ocean. The source motion is assumed to be uniform (unaccelerated), but is not restricted to a path radial to the receiver. The structure of the Fourier inversion integral is carefully analyzed and an evaluation is carried out by the method of stationary phase. The stationary phase point is explicitly computed as an expansion in powers of the ratio of the source speed to the mode group velocity. The resulting expression for the velocity potential is examined for Doppler effects for both instantaneous (modal) Doppler as well as Doppler determined by a finite bandwidth Fourier transform.

Einstein observations of the X-ray structure of Centaurus A: evidence for the radio-lobe energy source.

Abstract: The X-ray source at the center of the radio galaxy Centaurus A has been resolved into the following components with the imaging detectors on board the Einstein X-ray Observatory: (1) a point source coincident with the infrared nucleus; (2) diffuse X-ray emission coinciding with the inner radio lobes; (3) a 4-arcmin extended region of emission about the nucleus; and (4) an X-ray jet between the nucleus and the NE inner radio lobe. The 2 x 10 to the 39th ergs/s detected from the radio lobes probably arises from inverse Compton scattering of the microwave background. The average magnetic field in the SW lobe is determined to be not less than 4 microgauss. The extended region may be due to emission by a cloud of hot gas, cosmic-ray scattering, or stellar sources. The jet provides strong evidence for the continuous resupply of energy to the lobes from the nucleus.

Steady thermal convection from a concentrated source in a porous medium

Abstract: Solutions for the axisymmetric velocity and temperature fields associated with a point source of thermal energy in a fluid-saturated porous medium are obtained numerically through use of similarity transformations. The two cases considered are those of a point source located on the lower boundary of a semi-infinite region and a point source embedded in an infinite region. Tabulated results are presented from which complete descriptions of the velocity and temperature fields can be constructed for Rayleigh numbers of 0.1, 1, 10, and 100.

Multiple scattering of solar resonance radiation in the nearby interstellar medium. I

Abstract: The paper presents approximate solutions obtained for a radiative transfer problem that represents a highly-idealized description of the multiple scattering of solar resonance radiation in the nearby interstellar medium. The problem of a point source in the center of a spherically symmetric cavity imbedded in an infinite uniform medium is solved for a range of cavity radii. First and second-order scattering contributions are calculated, and the Eddington approximation is used to estimate the higher order components of the radiation field. It is shown that for coherent scattering at very large cavity radii, the backscattered intensity from the cavity approaches three times the value deduced from the optically thin solution, in agreement with previous work. It is concluded that an accurate analysis of sky background will require including not only the frequency redistribution, but also the correct spatial distribution of density.

Inhomogeneous waves and the plane‐wave reflection coefficient

Abstract: The importance of knowledge of the plane‐wave reflection coefficient R for a horizontally stratified medium at complex angles of incidence ϑ=π/2−iα (α≳0) is established. It is shown that for a point source, when the combined source/receiver height is less than one‐quarter wavelength, these inhomogeneous plane waves can make significant contributions to the reflected field. But irrespective of source/receiver height, they are important when normal modes are excited in slow speed regions of the bottom via inhomogeneous–pure wave conversion, thus giving rise to poles in the reflection coefficient. The theory of inhomogeneous plane‐wave reflection is examined within the context of conservation of energy, and an expression for the intensity of these waves is derived. It is shown that although ‖R‖ is bounded by unity for real incident angles, it can be unbounded for complex angles without violation of energy conservation. A general asymptotic result for R for large horizontal wavenumber is also derived. The computation of R for inhomogeneous waves is illustrated for three canonical bottom examples: (a) impenetrable, (b) isovelocity fluid, and (c) isovelocity fluid layer overlying an isovelocity fluid half‐space.

A review of techniques available for estimating short-term NO2 concentrations.

Abstract: The Environmental Protection Agency is reviewing the need for a short-term NO2 standard based on an averaging time of three hours or less. State Implementation plans and New Source Reviews will require air quality simulation techniques capable of estimating ambient NO2 concentrations. There is a need for multi-source (urban) models and for point source models. A review of currently available techniques for the estimation of NO2 concentrations resulting from NOx point sources is presented. The available methods include simple screening techniques and refined reactive plume models. The screening techniques first use a standard gaussian dispersion model to estimate the maximum 1 hr NOx concentration caused by the source. The second step involves estimating the fraction of this NO* concentration occurring as NO2. Reactive plume models numerically simulate the simultaneous effects of dispersion and chemistry on NO2 concentrations. Organic as well as inorganic reactions are incorporated. Reactive plume models s...

The asymmetric, broad hydrogen emission lines of Seyfert 1 galaxies.

Abstract: Hbeta emission-line profiles are calculated for various choices of parameters pertaining to a schematic Seyfert 1 galaxy model The basic model consists of a spherical system of discrete clouds which is expanding with a velocity of between 10/sup 3/ and 10/sup 4/ km s/sup -1/ A point source of ionizing radiation is assumed to be situated at the center of the expanding systemEach cloud is assumed to be spherical in shape and to be ionized only on the thin surface region which is exposed directly to the central source of ionizing radiation A continuous opacity source such as dust is assumed to be present in the neutral part of each cloud so that, to an observer located outside the expanding system, approaching clouds would appear fainter than receding clouds This would cause the blueward half of the resulting Hbeta line profile to be suppressed relative to the redward half However, shadowing of receding clouds by approaching clouds would have the opposite effect For the usually quoted value of the covering factor of roughly 10%, the former effect is much greater than the latterA broad, asymmetric line is produced in this manner The composite spectra produced by combining the broad, asymmetricmore » components of Hbeta for the schematic model described here with arbitrarily chosen intensities for the narrow component (deltalambda/lambdaapprox =10/sup -3/) of Hbeta and (O III) lambda4959 and lambda5007 have the general features of spectra observed for many Seyfert 1 galaxies and quasars« less

Omni-directional compound paraboloid-hyperboloid radiation device

Abstract: The invention is a device which receives radiation from a point source, from the sun, or from other means, said radiation being received on the surface of a mirrored paraboloid of revolution which is tracking the point source, though the source may be stationary, and reflects the radiation to the mirrored surface of a confocal hyperboloid of revolution whose shape is dependent on the distance to the desired second focus. The hyperboloid reflects radiation of high intensity to the second focus, where the flux is intensified, using a conical mirrored surface or a compound parabolic concentrator, to a flux intensity approaching the surface flux intensity of the sun. The radiation input to the device may also be from an array of point or distributed sources, such as from an array of lasers.

A study of the point spread function in scintillation camera collimators based on Fourier analysis.

Abstract: A new technique for evaluating the point spread function of parallel hole collimators in scintillation cameras is studied. The collimator function is developed into a Fourier series and the intensity distribution in the detector plane for a point source is given by a mathematical expression that depends on the object position and on the collimator parameters. The septal penetration effect is considered. The technique is applied to obtain the general formulae for collimators with hexagonal holes and the PSF is evaluated.

High-frequency surface fields excited by a point source on a concave perfectly conducting cylindrical boundary

Abstract: A previous analysis of the surface currents excited by a line source located on a perfectly conducting concave cylindrical boundary is here extended to point-source excitation. Alternative field representations in terms of geometric optical contributions, whispering gallery modes, and canonical integrals are explored, and their utility is assessed by extensive numerical computations. As for the two-dimensional case, it is found that a combination of rays and whispering gallery modes provides an efficient and physically appealing method for calculating the field. The results are applicable to all observation points lying outside a narrow angular region surrounding the axial direction as seen from the source point.

A point source in a finite sphere

Abstract: Exact analysis and the FN method are used to compute the radiation field due to a point source of radiation located at the center of a finite sphere.

The gravitational lens effect and pregalactic halo objects.

Abstract: The changes in flux, position, and size of a distant extended (galaxy, etc.) source that result from the gravitational lens action of a massive opaque object are discussed. The flux increase is described by a single function of two parameters. One of these parameters characterizes the strength of the gravitational lens, the other describes the alignment of source and lens object. This function also describes the relative intensity of the images formed by lens. ( A similar formalism is discussed by Bourassa et al. for a point source). The formalism is applied to the problem of the galactic halo. It appears that a massive (10/sup 1/2 M/sub sun/) spherical halo surrounding the visible part of the galaxy is consistent with the observable properties of extragalactic sources (distant galaxies, H II regions in distant galaxies, quasars).

The measurement of the depth of a point source of a radioisotope from gamma ray spectra

Abstract: The spectrum of the scattered radiation from a 'point' source of 99Tcm located within a slab of tissue-like material has been studied with a 19-phototube gamma camera, a 7.5 cm x 7.5 cm sodium iodide detector and a Ge(Li) counter. Both backscatter and forward scattered radiation contribute to the spectra observed; 'infinite' backscatter is achieved at a thickness of 8 cm. The scattered radiation can be used to estimate the thickness of the overlying material. The scatter from 133Xe and 51Cr was also measured. The depth of a source could usually be determined to within a few millimetres from the proportion of scatter in the spectrum; it was more accurate with the 51Cr (320 keV) than with the lower-energy photopeaks of the other two isotopes. The variation in the spectrum as the source moved across the field of the gamma camera was recorded.

Relations between point sources buoyant convection phenomena

Abstract: The gross convection velocities of various simple point source convective phenomena in an unstratified stationary fluid can be related to each other through empirical constants by using Turner’s model of the starting plume cap. Considered here are ’’weak’’ (Rayleigh number approaching zero), laminar, and turbulent regimes of the phenomena, the thermal, the starting plume cap, and the steady plume. Thus, by determining an empirical constant a, for either the thermal or the starting plume cap, the convection velocity of the other phenomenon can be estimated. Here, a is defined in terms of the thermal, a≡zt2/ktAm, where zt is the thermal height above the buoyancy source at time t, k is a molecular diffusivity, A is the effective Rayleigh number, and m=1/2, 1, or 2 depending on the flow regime. The dependence of a on the Prandtl number Pr is presently unknown. For experimental verification of the model, new measurements of the laminar and of the turbulent starting plume are presented. The constant a, determin...

Point source bubbler systems to suppress ice

Abstract: An analysis of a point source bubbler system used to induce local melting of an ice cover is presented. The analysis uses empirical results of bubbler plume experiments and impingement heat transfer results to determine the rate of melting at the underside of an ice cover. Through a simple energy budget analysis of the ice cover, the melting of the ice cover and resulting extent of open water are determined as a function of air temperatures, depth and air discharge of the source, and water temperature. The analysis leads to a numerical simulation and an example simulation is presented.

A multidimensional inverse problem in transport theory

Abstract: In this paper we deal with the multidimensional transport theory involving the inverse problem, which presents results applicable to radiative transfer and neutron transport theory for a homogeneous medium. The Legendre expansion is used to construct the scattering kernel (or phase function) from assumed known experimental data described herein. Two cases are considered, the interior problem in which the medium contains a monodirectional point source and the exterior problem in which the medium is free of sources but is subjected to a plane wave source at infinity. Use of the principles of invariance is made to relate results of measurements with different geometries. The paper is an extension of earlier work on the one-dimensional case with azimuthal symmetry by Kanal and Moses.

A new derivation of the displacement potentials for motion in a homogeneous isotropic elastic medium

Abstract: Summary. We give a derivation of the displacement potentials and the wave equations which they satisfy. The derivation is similar to one given by Richards but is more general and yields explicit formulas for the source terms. This generality is retained when the moment tensor representation of the source is used. Formulas for the source terms are given in both spherical and cylindrical coordinate systems and are evaluated for the particular case of a point source with second order moment tensor.

A high-frequency point source of oscillations near a concave mirror

Abstract: A high-frequency expansion of the problem in question is obtained by the boundary-layer method. This expansion describes the complex phenomenon of a large number of ray reflections from the surface of the mirror.

Transient Heat Conduction from Buried Underground Radioactive Nuclear Waste

Abstract: An exact analytical solution for transient heat conduction in the ground from buried radioactive nuclear waste has been derived. The heat sources are assumed to be an array of arbitrarily located point sources in an infinite ground. The solution accounts for orthotropic thermal conductivity effects in three dimensions and for the timevarying nature of the heat generation rate of each source due to radioactive decay. Owing to the point source nature of the assumed heat sources, the solution derived is valid only at intermediateand far-field distances from real finite-size sources. Numerical results are presented for several typical waste repository situations in shale, salt, and basalt.

Mirror surface check unit

Abstract: PURPOSE: To obtain a clear and good-resolution check image, by not only making the light flux from a light source into a light point source without chromatic aberration but also using a light flux which is made parallel without chromatic aberration. CONSTITUTION: The light flux from white light lamp 6 is converged by a pair of achromat condenser lenses 8 and is made into a light point source through slit plate 9 having a pinhole. This light flux becomes parallel light flux 11 due to concave mirror 10 and is irradiated to wafer 13, and the reflection light reaches screen 14 to project the check image of wafer 13 onto scrren 14. As a result, resolution of the check image becomes high and the image becomes clear because the light flux of the light source is made into a light point source. Further, since a pair of achromat condenser lenses 8 and concave mirror 10 are used, the clear check image can be obtained without generation of chromatic aberration. COPYRIGHT: (C)1980,JPO&Japio

Neutrinos from neutron stars

Abstract: A calculation of the flux of ultra-high energy neutrinos from galactic neutron stars is presented. The calculation is used to determine the number of point sources detectable at the sensitivity threshold of a proposed deep underwater muon and neutrino detector array. The detector array would have a point source detection threshold of about 100 eV/sq cm-sec. Analysis of neutrino luminosities and the number of detectable sources suggests that the deep underwater detector may make a few discoveries. In particular, a suspected neutron star in the Cyg X-3 source seems a promising target for the deep underwater array.