Showing papers on "Point source published in 1985"

Shear-wave polarizations on a curved wavefront at an isotropic free surface

Abstract: Summary. We present polarization diagrams of the particle motions at the free surface of an isotropic half-space generated by incident shear waves from a local buried point source. The reflectivity technique is used to calculate synthetic seismograms from which the particle motions are plotted. The particle motions are examined over a range of epicentral distances in a uniform isotropic half-space for different source frequencies and polarization angles, and for different Poisson's ratios. The particle motions due to a curved wavefront possess different characteristics from those generated by plane wavefronts at corresponding incidence angles. A curved wavefront generates a local SP-phase: a P-headwave which propagates along the free surface, and arrives shortly before the direct S-wave. These two arrivals give rise to cruciform particle motions in the sagittal and horizontal planes, which could be misinterpreted as anisotropy-induced shear-wave splitting. An examination of the particle motion in the transverse plane, mutually orthogonal to the sagittal and horizontal planes, can be used to discriminate between isotropic and anisotropic interpretations. The amplitude of the SP-phase is enhanced when it propagates in a low-velocity surface layer overlying the source layer, and may then become the dominant phase on radial-component seismograms. The presence of even a single surface layer may introduce considerable complexity into the seismogram, and we examine the effects of layer thickness, velocity contrast, and source depth on the corresponding polarization diagrams. Reliable information on the source and propagation path characteristics of shear waves from a buried local point source can only be obtained from free-surface records if they are recorded within a very limited epicentral distance range.

Cerenkov light images of EAS produced by primary gamma

Abstract: It is shown that it should be possible to distinguish very effectively between background hadronic showers and TeV gamma ray showers from a point source on the basis of the width, length and orientation of the Cerenkov light images of the shower, seen in the focal plane of a focusing mirror, even with a relatively coarse pixel size such as employed in the Mt Hopkins detector Predictions of the response of this system to air showers are presented

Radiation from a point source and scattering theory in a fluid-saturated porous solid

Abstract: The time harmonic Green function for a point load in an unbounded fluid‐saturated porous solid is derived in the context of Biot’s theory. The solution contains the two compressional waves and one transverse wave that are predicted by the theory and have been observed in experiments. At low frequency, the slow compressional wave is diffusive and only the fast compressional and transverse waves radiate energy. At high frequency, the slow wave radiates, but with a decay radius which is on the order of cm in rocks. The general problem of scattering by an obstacle is considered. The point load solution may be used to obtain scattered fields in terms of the fields on the obstacle. Explicit expressions are presented for the scattering amplitudes of the three waves. Simple reciprocity relations between the scattering amplitudes for plane‐wave incidence are also given. These hold under the interchange of incident and observation directions and are completely general results. Finally, the point source solution is Fourier transformed to get the solution for a load which is a delta function in time as well as space. We obtain a closed form expression when there is no damping. The three waves radiate from the source as distinct delta function pulses. With damping present, asymptotic approximations show the slow wave to be purely diffusive. The fast and transverse waves propagate as pulses. The pulses are Gaussian‐shaped, which broaden with increasing time or radial distance.

The 23:19 aftershock of the 15 October 1979 Imperial Valley earthquake: More evidence for an asperity

Abstract: The well-recorded strong ground motion data for the 23:19 aftershock of the 15 October 1979 Imperial Valley earthquake provide a good opportunity to study the high-frequency source characteristics and the path effects at near-source distances. The best-fitting point source model has a strike-slip mechanism, N40°W, which is nearly identical to the main event. The estimated stress drop is extremely high, roughly 500 bars, with a triangular time history (0.1, 0.1 sec) but with a moment of 1.0 × 10^(24) dyne-cm. A double-source model found by inversion fits the high-frequency data better but indicates complex faulting: the first source (with strike = N319°E, dip = 42°NE, and slip angle = 165°) has a moment of 0.7 × 10^(24) dyne-cm, the second source (with strike = N324°E, dip = 82°SW, and slip angle = 181°) lies about 0.5 km to the north and has a seismic moment twice that of the first source. Source dimensions appear very small for this amount of energy release. Many of the anomalous behaviors observed at certain stations for the main event are, also, present in the aftershock data. These features are examined in terms of path effects.

Anisotropy in the oceanic lithosphere — theory and observations from the Ngendei seismic refraction experiment in the south‐west Pacific

Abstract: Summary. P-wave travel-time data from a seismic refraction experiment on the 1983 Ngendei expedition to the south Pacific indicate anisotropy at two levels in the oceanic lithosphere. In the upper mantle, P-wave velocities vary between 8.0 and 8.5 km s-1 with the fast direction at N30°E. Crustal anisotropy within layer 2 is characterized by azimuthal P-wave velocity differences of about 0.2–0.4 km s-1, with the fast direction at N120°E, orthogonal to the upper mantle anisotropy. The observed anisotropy is consistent with a model in which aligned olivine crystals cause anisotropy in the upper mantle and aligned cracks within layer 2 cause anisotropy in the crust. We examine a model of P-wave propagation from a point source in an isotropic layer above an anisotropic half-space and show that under these conditions anisotropy can cause seismic ray-paths to deviate from the vertical plane connecting source and receiver. This deviation has a negligible effect on seismic travel times but must be taken into account in modelling P-wave polarization anomalies. P-wave polarization anomalies within the anisotropic half-space are the sum of the particle motion deviation from the ray-path, and ray-path deviation from the source-receiver azimuth, and are typically range-dependent. P-wave polarization anomalies will also occur in the overlying isotropic layer, purely as a result of the ray-path deviation, with greater deviations at long ranges. A simple model of upper mantle anisotropy (6kms-1 crust above 8.0–8.6 km s-1 anisotropic mantle) produces surface polarization anomalies of 1.5–3.5°. In practice, P-wave polarization anomalies are difficult to observe because they are small compared to observed scatter in polarization data and are sensitive to differences in horizontal seismometer gain levels. We examine the implications of anisotropic ray-path deviations on travel times and P-wave polarizations for the Ngendei data as well as some previous marine seismic refraction experiments.

Optics of photoretinoscopy: results from ray tracing

Abstract: Recent photographic studies have generated renewed interest in the optics of the retinoscopic (skiascopic) image. In photoretinoscopy the subject's eye is illuminated by a point source of light and the fundal image of this light at the plane of the subject's pupil is observed or photographed from a position near the source. The image so obtained is a function of the dioptric defocus of the eye relative to the camera, D, the distance of the source and camera from the eye, A, the radius of the subject's pupil, R, and the distance of the point source from the edge of the camera aperture, E. The fraction of the subject's pupil which is not filled with light is termed the "Dark Fraction" (DF) and may be computed from a simple ray tracing model of the eye. It is given in terms of the above parameters by the equation DF = E/(2ARD). The validity of this equation was tested with a photoretinoscope and artificial eye. The photoretinoscope, which is a novel modification of an isotropic photorefractor, is also described.

Finite-difference seismograms for SH waves

Abstract: The accuracy of the finite-difference method for generating synthetic seismograms of SH wave propagation in cylindrically symmetric media is discussed The finite-difference method has the advantage that arbitrary density and velocity fields in the medium may be specified A point source is generated by a simple transformation of a line source The accuracy of the finite-difference seismograms in flat- and dipping-layered media is confirmed by comparison with the generalized ray method A source radiation pattern is inserted by introducing a “near-field” which has permanent displacement near the source Strong motion synthetics are constructed with this new method for the 1968 Borrego Mountain earthquake as recorded at El Centro Good fits to the data are achieved using the laterally varying model determined by a detailed refraction survey and the source parameters determined by teleseismic waveform modeling Shallow faulting is no longer necessary to explain the long-period surface-wave development

The wave pattern produced by a point source on a rotating disk

Abstract: It is pointed out that the boundary layer on a rotating disk is important in stability theory because it provides a particularly simple way to study the important phenomenon of crossflow instability. This type of instability is responsible for early transition on sweptback wings. Mack and Kendall (1983) have studied the wave patterns formed by harmonic point sources in a Blasius boundary layer on the basis that the source uniformly excites all oblique normal modes of the source frequency. The calculation procedure for planar boundary layers was modified to fit the different geometry of the rotating disk and the lack of an axis of symmetry. Calculations were performed of the wave pattern produced by a zero-frequency point source located at the Reynolds number of the artificial roughness element in an experiment conducted by Wilkinson and Malik (1983). The results provided in the present investigation confirm that the experimental wave pattern is a superposition of the complete azimuthal wavenumber spectrum of zero-frequency normal modes with uniform initial amplitude and phase.

Point-Source Initiation of Lean Spherical Flames of Light Reactants: An Asymptotic Theory

Abstract: We study the dynamics of small spherical flame kernels whose evolution is triggered in a lean mixture of a light mobile reactant by a time-dependent point- source of energy. For simplicity, the analysis is conducted in the framework of a one-reactant flame model, with a one-step Arrhenius overall kinetics. Using the method of matched asymptotic expansion for large activation energies, we show that: The concentration and temperature fields split into quasi-steady regions, inside and around the flame kernel, and an unsteady far-field; in both regions convection effects may be neglected. Match ing the near - and far-fields furnishes a parameter-free, non-linear evolution equation for the flame radius ; in addition to memory effects of diffusive origin, it includes explicitly the functional form of the chemical rate and the instan taneous power of the point source of energy we used as ignition device. Through a numerical integration of this equation, flame front trajectories and critical energies are...

A singularity method for calculating hydrodynamic forces and particle velocities in low-Reynolds-number flows

Abstract: A numerical technique is presented which allows one to estimate hydrodynamic forces and torques or translational and angular velocities of particles in a general flow field. Particle–solid wall interactions can be readily included. The base functions used in the technique presented are singular fundamental solutions of the Stokes equation for a point force and a point force and a point source. The least-square approach is used preferentially in order to find the intensities of these singularities. Test calculations show that the results are self-consistent and in fairly good agreement with the exact solutions in a wide range of conditions. For example, for a spherical particle moving with no slip towards the solid wall, it has been shown that the method can provide good estimates of the resistance coefficient up to separations of the order of 5% of the particle radius. We believe that better agreement, for smaller separations, is within reach at the expense of increased computer costs. For spheroidal particles good results were obtained for aspect ratio in the range 0.5–2.0.

Development and Evaluation of the Offshore and Coastal Dispersion Model

Abstract: The Offshore and Coastal Dispersion (OCD) model has been developed for the Minerals Management Service (MMS) to determine the impact of offshore and onshore emissions from point sources on the air quality of coastal regions. Constructed on the framework of the EPA guideline model MPTER, the OCD model incorporates overwater plume transport and dispersion as well as changes that occur as the plume crosses the shoreline. Hourly meteorological data are needed from both offshore and onshore locations, including wind direction and speed, mixing height, overwater air temperature and relative humidity, and the sea surface temperature. Observed turbulence intensities are preferred by the model but are not mandatory. Building downwash and plume rise are incorporated into the OCD model similar to their use in the Buoyant Line and Point Source (BLP) model. Partial plume penetration into elevated inversions is treated using Briggs’ model. Dispersion coefficients are proportional to turbulence intensities. A virtual so...

Three Types of Disturbances from the Point Source in the Boundary Layer

Abstract: The results of the experimental investigations presented in this paper are connected with the study of excitation and the development of disturbances of different intensity from the point source in the boundary layer of a flat plate. In the boundary layer there appear qualitative and some quantitative characteristics of the three types of disturbances: a Tollmien-Schlichting wave, a nonlinear localized structure and a turbulent spot. If two of them — a Tollmien-Schlichting wave and a turbulent spot are known, the third one — a localized disturbance is observed for the first time.

Multidimensional Born velocity inversion: Single wideband point source

Abstract: The inverse scattering problem for an acoustic medium is considered within the approximate direct inversion framework. As opposed to iterative methods, the direct inversion approach gives an estimate of the medium velocities by operating on the observed scattered field without repeated solutions of the forward problem. Previous solutions to the multidimensional Born velocity inversion problem require either coincident source and receiver arrays or plane‐wave sources. In both cases an array of point sources is required to collect the proper data for inversion. In this paper, the solution to the single point source problem is derived. It is shown that by extrapolating and imaging the observed scattered field appropriately, the projections of the velocity function at all angles can be obtained. The velocities are, then, reconstructed by the inverse Radon transform method of tomography.

Optical processing and holography with polychromatic point source illumination.

Abstract: An achromatic Fourier transforming system is analyzed in a manner which gives new physical insight. A matched spatial filtering experiment is performed comparing the noise performance of a coherently produced matched filter and one made using broad spectrum light in an achromatic Fourier transforming system. The broad spectrum light is obtained by scanning a dye laser through its tuning range, thereby simulating a polychromatic point source. The system is applied to the construction of low noise holographic optical elements.

Seismic waves generated by an impulsive point source in a solid/fluid configuration with a plane boundary

Abstract: The space‐time acoustic wave motion generated by an impulsive point source in a solid/fluid configuration with a vertical plane boundary is calculated with the aid of the modified Cagniard method. Two types of sources are considered in detail, viz. (1) a point source of expansion (model for an explosive source), and (2) a point force parallel to the vertical interface (model for a mechanical vibrator). Numerical results are presented for the transmitted scalar traction in the fluid in those regions of space where head wave contributions occur. There is a marked difference in the time response observed for the two types of sources and for the different positions of the receiver in the fluid with respect to the position of the source in the solid. These waveform differences are important when the transmitted wave in the fluid is used to determine experimentally the elastic properties of the solid. Scholte waves are observed only when the source is close to the fluid/solid interface. As compared with the tra...

A buoyant source in the lower of two, homogeneous, stably stratified layers

Abstract: A point source of buoyancy is located at a specified elevation within the lower of two, homogeneous, stably stratified layers. A turbulent buoyant plume is formed above the source, and it impinges on the layers' interface. Depending on the strength of the source, its position below the interface and on the density difference of the two layers, it is conjectured that either: (1) a central portion of the impinging plume flow will penetrate and continue upward into the far field of the upper layer as a buoyant plume, the outer portion of the flow penetrating but then dropping down toward the interface because of negative relative buoyancy; or (2) none of the impinging plume flow will penetrate the upper layer (indepth) because of its being uniformly of negative relative buoyancy. Associated with these possible conditions will be an effective horizontal outflow of fluid at the interface. The paper derives and solves a set of model equations for these plume-interface interactions, and the results are applied to a generic heat transfer problem related to fires in enclosures.

Radio emission from the site of supernova 1961v in NGC 1058

Abstract: The VLA has been used at 20 cm to detect a 0.19 mJy point source at the site of the extraordinary supernova 1961v in NGC 1058. At the distance of NGC 1058, the absolute monochromatic luminosity of the source is comparable to that of Cas A. 20 references.

A WKBJ spectral method for computation of SV synthetic seismograms in a cylindrically symmetric medium

Abstract: Summary. A method of synthetic seismogram computation for teleseismic SV-waves is developed in order to treat quantitatively SV-waves in problems of body wave source inversion and source-receiver structure studies. The method employs WKBJ theory for a generalized ray in a vertically inhomogeneous half-space and the propagator matrix technique for waves in nearsurface homogeneous layers. Wavenumber integration is done along the real axis of the wavenumber plane and anelasticity is included by using complex velocity in all regions of the earth model. The near-surface source structure is taken into account in the computation for the case of the shallow source by allowing a point source to be located in the homogeneous layers. Source and receiver area structures are also allowed to differ. A general moment tensor point source is considered.

Source solutions and station residuals from long‐period waveform inversion of deep events

Abstract: Deep earthquakes of up to moderate size have been analyzed by a long-period waveform inversion of Global Digital Seismographic Network data where the use of a point source model proved to be adequate, but the use of a standard earth model was inadequate without modification. Modifications include calculations in an earth model based on PREM but with different source and receiver structures and the introduction of time delays which were determined together with the moment tensor components of the source by an iterative correlation method. The moment tensor components are in accordance With the pattern of stress release in other work, but results for the seismic moment differ somewhat and there is no compelling evidence for deviations from a double couple mechanism. The time delays were inverted to find the source centroid depths and time shifts and the station residuals. There is no baseline effect in the times of source duration which could be explained by a simple circular source model, but there is such an effect in the station residuals together with a distance trend especially for S. Part of the baselines may be an effect of bias in origin times, but the pattern of S residuals in particular suggests also possible modifications of the standard model.

Penetration of sound from a point source into a rigid porous medium

Abstract: A method of steepest‐descents integration suitable for use when a pole may approach near to one of the saddle points in the complex plane of integration is applied to the problem of predicting the field resulting from a point source near to the interface between two semi‐infinite media. It has been shown that it is possible to obtain asymptotic approximations for the field above or below the interface to any desired degree of accuracy in a way that is more straightforward and less algebraically cumbersome than that which is employed when using the subtraction of the pole technique. Approximation for the case where the receiving medium has a high refractive index produces the expected result whereby the field below the interface is related to that incident on the interface according to Snell’s law. Measurements are made of the phase and amplitude of the sound field below the surface of a 0.5‐m‐thick layer of glass fiber material, relative to those of the field at the surface due to the point source close t...

Development of Artificially Excited Disturbances in Supersonic Boundary Layer

Abstract: The present paper suggests a method of introducing the artificial regulable disturbances into supersonic boundary layer with the help of a point source of disturbances. While using the technique suggested the experimental study of development of a spatial wave packet of a small intensity was carried out in the boundary layer of a flat plate.

Point source for testing a gamma camera

Abstract: A point source for calibrating a gamma camera uses the nuclide cerium-141. The source is disposed in a shuttered, shielded housing. The shutter is incrementally advanceable to expose, over time, an increasing portion of the area of the source to maintain a relatively constant gamma ray flux therefrom.

LHG 83: Another Outstanding X-Ray Binary in the LMC

Abstract: The strongest X-ray point source, LHG 83, discovered in the EINSTEIN survey of the LMC and not being associated with a nearby coronal type stellar emitter or background AGN is identified with a faint blue variable object. Spectrophotometry reveals low mass X-ray binary characteristics at a mean velocity consistent with LMC membership. The He II A4686 emission exhibits a unique blue shifted component suggesting outflow velocities of several thousand km/s. Optical brightness changes by 0.3 mag in less than one hour are likely to be intrinsic to the source rather than induced by orbital motion. The low X-ray to optical flux ratio is probably due to the fact that the central X-ray source is blocked from direct view by the accretion disk.

Transient response from a planar acoustic interface by a new point-source decomposition into plane waves

Abstract: For the wave field of a point source in full space there currently exist two classical decompositions into plane waves. The wave field can be decomposed into either (a) homogeneous and horizontally propagating (vertically attenuated) inhomogeneous plane waves by using the so-called Sommerfeld-Weyl integral, or (b) upgoing and downgoing homogeneous plane waves only using the Whittaker integral. Transient representations of both integrals exist. We propose a new decomposition integral that has a greater flexibility than both classical decompositions. Solutions for the point-source reflection/transmission response from a planar interface, if based on the Sommerfeld-Weyl integral, have for instance inherently an infinite integration limit. With the new formula, by which the wave field of a transient point source is decomposed into upgoing and downgoing homogeneous as well as horizontally propagating inhomogeneous transient plane waves, the point-source response is directly obtained in the form of an integral with a finite integration limit. It can also be interpreted in terms of certain plane waves by which the point source is simulated in a new manner. For that matter, solutions based on the new integral readily reveal the 9evanescent9 or 9nonray9 character of the point source. The new formula may be considered an extension of the Sommerfeld-Weyl or Whittaker integral. It can be used to compute reflection/transmission responses in a compact form in situations where the Sommerfeld-Weyl integral was hitherto considered fundamental.

Numerical modeling of subsurface communication

Abstract: Techniques are described for numerical modeling of through-the-earth communication. The basic problem considered is evaluation of the field at a surface or airborne station due to an antenna buried in the earth. Equations are given for the field of a point source in a homogeneous or stratified earth. These expressions involve infinite integrals over wave number, sometimes known as Sommerfield integrals. Numerical techniques used for evaluating these integrals are outlined. The problem of determining the current on a real antenna in the earth, including the effect of insulation, is considered. Results are included for the fields of a point source in homogeneous and stratified earths and the field of a finite insulated dipole. The results are for electromagnetic propagation in the ELF-VLF range, but the codes also can address propagation problems at higher frequencies.