Showing papers on "Point source published in 1983"

The anisotropic reflectivity technique: theory

Abstract: Summary. The reflectivity technique for the calculation of synthetic seismograms from a point source in a horizontally stratified isotropic structure is extended to include weakly anisotropic layers. The formulation is in terms of displacement excitation factors rather than potential functions, which have not yet been specified for wave propagation in anisotropic media. Coupling between vertical, radial and transverse components of motion increases the number of plane-wave reflection and transmission coefficients which must be computed for any problem. These coefficients are calculated by extending Kennett’s iterative scheme for the computation of isotropic coefficients to

Underwater seismic testing

Abstract: A method of marine seismic investigation involves positioning or deploying at a predetermined point close to or within a multi-element seismic source array a calibration source which is an acoustic pressure wave point source which has a far-field seismic signature of broad bandwidth. The radiated pressure wave field of the calibration source (3) is weaker than that of the main source (2). The two sources are fired at separate instants within a predetermined time range, preferably with the calibration source (3) being fired about two seconds before the main source (2). The reflected response of the sub-surface strata is recorded and processed to generate seismic data. In one embodiment, the response is recorded with two hydrophone arrays - a short high-resolution array (4) and a conventional main array (5). The method can be used to determine the far-field signature of the main source array (2). It can also be used to obtain a high-resolution shallow seismic section simultaneously with a deep seismic section. Apparatus suitable for the method is described, and also a method of conducting an underwater seismic survey.

Augmented adiabatic mode theory for upslope propagation from a point source in variable‐depth shallow water overlying a fluid bottom

Abstract: A uniform asymptotic solution is presented for sound propagation from a constant frequency point source in shallow water whose depth H(r) decreases monotonically with cylindrical distance r. The water has constant sound speed c1 and density ρ1; the bottom fluid extends indefinitely in depth and has sound speed c2 and density ρ2, where c2>c1. The interface depth has constant value H0 up to range r0 and thereafter decreases linearly to zero. The solution appears as a sum of modal terms, each such mode eventually encountering a critical depth Hc(n) (at which modal phase velocity equals c2) at a critical range rc(n). A previously derived local solution for a modal term near its critical range is modified such that it automatically reduces to the adiabatic mode solution at nearer ranges and such that it is valid at arbitrary distances beyond the critical range. Bulk attenuation is incorporated into the model using an appropriate modal average over depth. Numerical results are compared with four parabolic equat...

A point source of heat in a stable salinity gradient

Abstract: When a salinity gradient is heated at a single point, a hot plume is formed with a series of layers around and above it in a ‘Christmas-tree’ flow pattern. Qualitative visual observations of the development of this system are reported. Three kinds of layers are observed: the first kind is formed above the basic plume from a hierarchy of secondary plumes on top of the basic one, the second develops around the upper part of the basic plume, and is driven by the same mechanism as the layers in sidewall-heating experiments, and the third, forming around the lower part of the basic plume, is of the same nature as the second type but also has something in common with double-diffusive intrusions. A characteristic feature for all the three kinds of layers is the existence of systematic shearing motions and vortices. Quantitative estimates for the plume height and the thickness of the layers are obtained.

Coherent response to a point source irradiating a rough plane

Abstract: We consider a point source of sound exciting a planar rough surface, and write the coherent response as the Sommerfeld‐Weyl integral in terms of the reflection coefficient (R) for correlated distributions of protuberances (bosses) on rigid or free base planes. The coefficient R and the associated angle‐dependent impedance are determined by the ensemble‐averaged multiple scattering amplitude (F) for one fixed boss of revolution, and F is specified by its single scattered value and the statistical‐mechanics radial distribution function (p) for impenetrable disks. Low‐frequency forms in terms of simple integrals of p delineate multipolo‐coupling and packing effects on propagation and attenuation (arising from incoherent scattering as well as absorption) as functions of the fraction (w) of base plane covered by bosses. Approximations of the response integral and corresponding plots are presented to exhibit the dependence on w and on other key parameters, and near‐grazing aspects are emphasized.

Analytical model of jet shielding

Abstract: An analytical model of the shielding of a stationary point noise source by a cylindrical jet is developed. The directivity function is derived which estimates the normalized sound pressure level at a far field receiver. The shielding model is compared to experimental data for a point noise source impinging on an unheated air jet and on a simulated hot air jet. The model compares favorably to measured shielding at receiver locations away from the jet axis. The trend of the estimated shielding diverges from the measured data as the jet axis is approached. Refinement of the model is discussed. STIMATION of aircraft generated noise includes identification not only of the sources of noise on the aircraft, but also of the propagation path between the source and receiver. One of the numerous factors affecting the noise transmission path is shielding of one jet by another. The shielding jet, because of the high temperature and flow speed with respect to the immediate surroundings, acts as a partial barrier between the source and the receiver. The resultant noise reduction not only affects the overall aircraft noise level, but also indicates the possibility of jet engine in- stallation as a means of aircraft noise control. The problem of reflection and transmission of sound by a moving medium has been addressed assuming a plane wave incident on a plane surface.1"4 Ray tracing techniques have been applied to two-dimensional jets5 and cold jets.6 The two- dimensional formulation, for a cylindrical noise source impinging on the shielding jet has been developed.7 The present study is an extension to three dimensions of the previous analysis.7 The model developed consists of the sound field emitted from a stationary, discrete frequency point source, which impinges on a cylinder of locally parallel flow. The temperature and velocity profiles are uniform in the jet at any location downstream of the nozzle. While this model is an idealization of the twin jet, it is felt to incorporate the basic elements essential for a realistic representation; not only of the source, but also of the shielding jet. Model Development Formulation of the Model The mechanisms by which shielding occurs are reflection of sound at the boundary between the jet and the surrounding air and by diffraction around the jet. The noise source is modeled by a stationary discrete frequency point source located at (r0, 0,0). The shielding jet is a cylinder of radius a, and is infinite in extent along the z axis. The temperature and flow velocity are uniform across the cylinder cross section. The model is illustrated in Fig. 1. The expression for acoustic velocity potential is written for two regions; region I is outside the jet, region II is within the jet. In region I (outside the flow) In region II (inside the flow)

Passive source localization from spatially correlated angle-of-arrival data

Abstract: Bias and variance equations are presented for two-dimensional location estimators of a nonmoving point source of radiation in an isotropic, stationary random medium. The estimators are calculated from spatially correlated angle-of-arrival data which are collected simultaneously at two sensor positions, and assumed to consist of true (unbiased) source angles plus zero-mean angular noise with equal variances at both sensors and negligible higher moments. Under these assumptions the square of the estimator bias is, in general, a quadratic function and the estimator variance a linear function of the spatial data correlation coefficient. However, for source ranges much larger than sensor separation, both the bias and the variance tend to increase linearly with decreasing correlation coefficient, whereas they tend to decrease with increasing sensor separation. The combined effect for a distant source in a stationary random medium, when evaluated for typical spatial wavefront autocorrelation functions, is a significant reduction in the estimator bias and variance dependence on sensor separation, as compared to the uncorrelated case. With minor modifications, the same results apply to the equivalent problem of using time-of-arrival data from three colinear sensor positions.

High-frequency approximation to the nongeometrical S* arrival

Abstract: When an explosive point source is buried less than one wavelength from the free surface in a perfectly elastic, homogeneous, isotropic half-space, a very strong non-geometric arrival, tentatively denoted as S * by Hron and Mikhailenko (1981), exists in the reflected shear wave field. It has all basic characteristics of an ordinary shear body wave (linear polarization, transverse particle motion, and shear wave velocity) and, as such, it can be also reflected or transmitted upon incidence at an interface according to the rules governing ordinary body waves. Because of its very strong amplitude which at most epicentral distances exceeds even the amplitude of a direct P wave, the S * arrival is undoubtedly responsible for a large part of the shear wave energy content seen in the seismic records in oil exploration, which deals almost exclusively with shallow explosive sources. In our paper, we develop a high-frequency approximation to the S * arrival applying a saddle-point method to the integral representing a shear wave reflected from the free surface due to the incidence of a spherical longitudinal wave radiated by the shallow point source. Numerical results presented in the paper show that our high-frequency approximation preserves all basic features of the S * arrival, thereby making it suitable for easy incorporation into any synthetic seismogram computation based on the ray approach.

Ray Tracing of a Concave-Curved-Crystal Spectrometer and Its Detailed Characteristics for X-Ray Spectroscopic Diagnostics of High-Temperature Plasmas

Abstract: A method of ray tracing is presented for a spectrometer with a concave-curved crystal to be used for X-ray spectroscopic diagnostics of high-temperature plasmas. Calculations have been carried out for an LiF(200) crystal with a lowland circle of radius 50 cm and a Gaussian rocking curve. With a linear X-ray source, the most suitable parameters of the spectrometer were accurately determined by use of the ray tracing, and the characteristics were investigated in detail. The ray tracing was also applied to a point source of X-rays, and the best conditions for geometric relations between the point source and the spectrometer were found. The spectral distortion is also described here analytically. The calculated line profile including the natural broadening of the X-ray source is compared with the experimental result for FeKα1. Good agreement is obtained.

The structure of the internal wave Mach front generated by a point source moving in a stratified fluid

Abstract: The height and velocity of an internal wave (IW) generated by a point fluid source moving horizontally in a stratified fluid is calculated at asymptotically large distances behind the source, in linear approximation. Particular emphasis is given to the region near the Mach front, defined by the off‐track angle θM =sin−1(cM/ V), where cM is the speed of the fastest IW and V is the source velocity. To elucidate the asymptotic IW profile in that regime, a novel uniform asymptotic expansion is developed which is valid on both sides of the Mach front. It is found that, for the fluid model employed, the asymptotic approximation for the IW height, as well as for all its derivatives, is continuous at that front, and that all these IW profiles are remarkably smooth and slowly varying functions of off‐track distance in that regime.

Point‐source x‐ray backlighting for high‐density plasma diagnostics

Abstract: We have developed a new backlighting method which uses a point x‐ray source. It has the advantages of a large magnification factor, high spatial resolution, and freedom from the source intensity nonuniformity. Initial test results and evaluation of this method in comparison with other methods are presented.

A passive target detector

Abstract: Passive target detectors which employ infra-red sensors may have difficulty in distinguishing between a cool target near the detector and a warmer one further away. A detector 1 detects a source of infra-red radiation and if its size indicates that it may be of interest, a point source detector 3 operates a gate 5 to allow information regarding the direction of the target to be transmitted along line 7 to a store 8 and to a calculator 10. This information is combined at the calculator 10 with directional information from the previous scan stored at 8 to give the range of the source. The real intensity of the source is then calculated by circuit 13 from its range and apparent intensity. If the target is of interest an alarm is operated at 14.

Seismic source decomposition

Abstract: We exploit the differences that exist between the radiation fields of a point source and an array to design a time‐separated marine seismic source array with desired power spectral and directional characteristics, whose far‐field time signature is known precisely from measurements. The desired power spectral characteristics are created by firing a predetermined series of point source units sequentially, such that their time signatures do not overlap. The effective power spectrum of the whole series of time‐distributed signatures can be made to approximate the sum of the power spectra of the individual signatures and can, therefore, be designed to suit the desired application by the appropriate choice of source units. The desired directional characteristics of the array can be created by arranging the source unit separations such that each source unit reaches the desired spatial position at the prescribed firing instant. The key to the subsequent processing of the recorded data is to measure the pressure w...

Monte Carlo estimates of specific absorbed fractions for an I‐125 point source in water

Abstract: Monte Carlo estimates of eight specific absorbed fractions associated with an I‐125 point source in water are presented for distances ranging from 1–10 cm.

The probable association of SN 1957d in NGC 5236(M83) with an unresolved radio source.

Abstract: Astrometric measurements of the position of SN 1957d in the spiral galaxy NGC 5236 using archival plates indicate that it is within 2 arcsec of the position of a point radio source of approximately Cas A strength on a VLA map made by Cowan and Branch (1982) This coincidence strongly suggests that the radio source originated from the supernova If further investigation shows this point source to indeed be a supernova remnant, then this would be the first reported detection of an intermediate-age supernova remnant

Reflection effect in close binaries. III. Distribution of radiation incident from an extended source

Abstract: We have studied the effects of irradiation from an extended surface of the secondary component on the atmosphere of the primary. We have considered an isothermal and purely scattering medium. The resultant radiation field due to irradiation from an extended surface and self-radiation is different from that due to irradiation from a point source and self-radiation. In the case of the point source the middle layers of the exposed part of the atmosphere show maximum reflection while in the former case the reflection gradually decreases from the centre of the component towards the surface of the outermost layers of the atmosphere. The reflection effect appears to be strongly dependent on the density distribution of the electrons.

X-Ray calibration system

Abstract: A method and apparatus for the radiometric calibration of an X-ray detector, such as a spectrometer, for wavelengths in the range of 2 to 200 angstroms. A hot filament X-ray diode for generating a point source of X-rays is disposed within a vacuum manifold and is surrounded by a housing having two ports equally spaced from the point source. A center line of each port passing through the point source forms substantially the same angle with respect to the line defined by the stream of electrons passing from the cathode to the anode of the X-ray diode so that the apparent intensity of the point source through each port is substantially identical. A reference detector is placed opposite one port to intercept a known portion of an X-ray beam emitted therefrom, while the detector to be calibrated is placed opposite the other port for interception of a known portion of an X-ray beam passing through that port. The X-ray diode includes an anode which is provided with a replaceable cap for control of the wavelength of radiation emitted. The output generated in the reference detector is used for calibration of the detector to be calibrated.

Discovery of a New Non-Geometrical S Arrival Generated at Free Interface

Abstract: A new and very prominent non-geometrical arrival, denoted as S*, has been detected in synthetic seismograms representing an exact solution to Lamb's problem for an explosive point source located close to the free surface. The new arrival displays a linear polarization and propagates with the speed of an ordinary shear wave. Seemingly, it originates on the free surface directly above the buried P source with an amplitude which decays exponentially with source depth. Under favorable circumstances, i.e. when the source depth is less than the predominant wavelength of the source pulse, the S* arrival may be stronger than any other body wave arriving at a given epicentral distance. The full understanding of its properties and nature will have a direct impact on the interpretation of field seismograms in oil exploration where, as a rule, explosive sources are always close to the surface.

Reflection effect in close binaries. II. Distribution of emergent radiation from the irradiated component along the line of sight

Abstract: We have calculated the effects of irradiation from a point source observed at infinity. Plane-parallel approximation and spherically-symmetric approximations are employed in calculating the self-radiation field for the sake of comparison. It is found that there are considerable changes in the radiation received at infinity between the approximation of plane-parallel stratification and spherical symmetry.

Energy radiation from a spherically symmetric homogeneous source

Abstract: General expressions for energy radiated from seismic sources in linear elastic bodies are illustrated and clarified by examining the energy radiated from a spherically symmetric source. Calculations with the spherical source demonstrate that the point source approximation for the radiated energy is not asymptotic in the sense that it does not approach the actual radiated energy for small source dimension. Comparison between the energies radiated from the spherical source for a ramp time function and for a modulated ramp demonstrate that the modulation is not effective in increasing the total radiated energy even though it does cause a peak in the spectrum of the far-field particle velocity at the characteristic frequency of the modulation. For a/cT > 2, where a is the radius of the spherical source, c is the wave speed, and T is the rise time, the strain energy change overestimates the radiated energy by less than a factor of two.

Characteristics of maximum concentrations

Abstract: Characteristics of maximum short-term ground level concentrations from an elevated point source, namely, the effective plume height, the critical wind speed, the distance to the point of maximum concentration, and the maximum concentration, are derived from the gaussian plume model. Both phases of plume development--before and after it has reached its final height--are considered. The plume rise treatment includes both thermal buoyancy and momentum effects. Certain limitations on critical wind speed are discussed. The dispersion model whose basis is established in this paper should be especially useful in application where on site meteorological data are unavailable.