Theoretical Ray Research Articles

Tomographic imaging of three-dimensional P-wave velocity structure beneath the Pannonian Basin

Tomographic P-wave velocity inversion has been performed for the Pannonian region using first arrival times of local and regional events from the bulletins of the International Seismological Center and the Hungarian Earthquake Bulletins. The input data consist of 4071 arrival times of 570 events. A three-dimensional velocity model has been gained by joint hypocenter-velocity inversion on a coarse and a fine non-uniform cartesian grid of nodes. The initial one-dimensional model was determined by genetic algorithm. A bending ray tracer has been used to calculate the theoretical travel times and ray paths. The reliability of the inverted velocity parameters were checked by the checkerboard method and by the analysis of the model resolution matrix. The results are generally in agreement with the known structural characteristics of the Pannonian Basin. An interesting high-velocity anomaly has been found in the uppermost mantle beneath the southern part of the Great Hungarian Plain.

Numerical experiments on three dimensional seismic tomography using graph theoretical ray tracing

Numerical experiments on 3-D seismic tomography implemented by graph theoretical ray tracing and conjugate gradient (CG) inversion method for the laterally inhomogeneous media is presented with emphasis on the technique of improving the resolution of the reconstructed image. Graph theoretical ray tracing is efficient and useful for calculating the traveltime of the first arrival seismic signal and the corresponding ray path. It can trace accurate and stable ray path and produce corresponding traveltime even in the complicated 3-D medium in which traditional ray tracing methods fails. In this study, two kinds of forward-star set, the spherical and the cubic, for the graph theoretical ray tracing were designed. The features of each forward-star set were examined. In the spherical forward-star, the angle between two adjacent forward-star nodes is constant as is in the cubic forward-star but the number of forward-star node is about two third of that of the cubic when the same forward-star level is selected. Thus the computation time of ray tracing is reduced using the spherical forward-star set. We employ the CG method for inversion, because it is fast and has a good convergence rate compared with other inversion schemes. In the tomographic process with some source-receiver configurations, the reconstructed images are distorted and elongated when only one direction of source-receiver configuration is used. However, more accurate image was obtained with two directions of source-receiver configuration, especially in orthogonal direction. The numerical simulations with iterative operations of ray tracing and CG inversion for structured models of an spheroidal body and an elongated sedimentary basin show that the technique presented in this paper is useful to reconstruct subsurface image of 3-D inhomogeneous media with small number of iteration.

Abyssal Penetration and Bottom Reflection of Internal Tidal Energy in the Bay of Biscay

Abstract This paper describes field observations in the Bay of Biscay, and presents convincing evidence for the existence of a broad beam of internal tidal energy propagating downward from a source region on the upper continental slopes, which, after penetrating the deep ocean interior along a theoretical ray path, reaches the abyssal plain, at a depth of about 4.2 km, 58 km from the source. The results concentrate on the region between 50 and 60 km from the generating area, where the beam of energy is below a depth of 2 km. A marked change in phase of the beam (4 hours) was observed over a distance of 12 km at a depth of about 3 km. In addition, semidiurnal currents were noticeably increased near the sea floor. These results are interpreted as demonstrating the occurrence of partial bottom reflection of the internal tidal energy, in agreement with theoretical predictions, at a distance of about 58 km from the generating region on the upper slope, and extend our previous results farther into the ocean.

Downward propagation of internal tidal energy into the Bay of Biscay

The generation and propagation of internal tidal energy from the continental shelf break is determined by the gradient of the topography and the static stability of the water column. This paper describes field observations in the Bay of Biscay, and after discussing the above parameters, presents convincing evidence for the existence of a beam of internal tidal energy propagating downward, from a source region on the upper continental slopes, into the deep ocean interior along a theoretical ray path. Although this phenomenon has been reported previously in the laboratory, and described by theoretical models, it has never before, to our knowledge, been observed in nature. The observations are compared with the results from a numerical model, and good agreement is obtained. This gives confidence in the model as well as in the interpretation of the observations.

Ultrasonic beam profiles and beam propagation in an austenitic weld using a theoretical ray tracing model

A model for ultrasonic wave propagation in anisotropic and inhomogeneous materials is applied to the case of ultrasonic inspection of an austenitic V-butt weld manufactured by the downhand Manual Metal Arc technique. We examine the propagation behaviour of waves within the weld region and, in addition, model beam divergence behaviour. From this work we predict directions of low inspection sensitivity and also identify regions of material to which no ultrasound penetrates. The relative merits of the three different wave modes are examined, showing clearly the advantages of horizontally polarized shear waves for austenitic steel inspection. Vertically polarized shear waves are shown to be the least effective for such inspections. We discuss the relevance of this work to the ultrasonic non-destructive testing of austenitic steel components, concluding that care is needed over the choice of wave modes and angles, to ensure sensitive inspection of the whole weld material.

A ray model for predicting sound attenuation by double barriers

The results of J. Keller's Geometrical Theory of Diffraction and A. Pierce's single and double edge diffraction theories are used to develop a theoretical ray model for predicting noise attenuations by double barriers. Acoustical scale model experiments were conducted to test the validity of the theory. The experimental results are compared with the attenuations calculated using a computer model. It is shown that the double barrier attenuation (above and beyond that due to geometrical spreading) should be calculated by cascading the attenuation caused by each of the two barriers. This contradicts the generally accepted assumption that given two widely spaced barriers located between a source and receiver, only one of them significantly reduces the noise reaching the receiver. [Work supported by NSF and MIT; thesis advisor: Prof. Richard H. Lyon.]

Three-Dimensional Propagations of Electron Bernstein Waves

Electron Bernstein waves were found to propagate over a wide (ray) range of directions with respect to a localized launcher, although the wave propagation is directed nearly perpendicular to the magnetic field. The observed wave fronts of the electron Bernstein waves were in accord with the theoretical wave fronts in which the effect of the magnetic field of the earth was included. The theoretical ray velocity surfaces of the electron Bernstein waves were confirmed experimentally and theoretically.

Radiation of electrostatic plasma waves near the lower hybrid frequency

In an investigation of ray velocities (phase velocities in the direction of the group velocity) with an electrostatic dispersion relation in a hot magnetized plasma, the lower hybrid wave and the ion acoustic wave were found to coexist. Using a small launcher in a magnetized plasma near the lower hybrid frequency, we could observe the lower hybrid wave for low neutral-gas pressure (P approx. = (1--2) x 10/sup -4/ Torr) and the ion acoustic wave for high neutral-gas pressure (P approx. = (7--8) x 10/sup -4/ Torr). The observed phase velocities for three dimensions were found to be in accord with the theoretical ray velocities. The wave fronts of the lower hybrid wave and the ion acoustic wave are parallel and circular to the magnetic field, respectively.

Oblique propagation of electron plasma waves in a magnetoplasma

The electron plasma waves which are radiated from a point source and propagate in an oblique direction with respect to an applied static magnetic field are investigated both theoretically and experimentally. The observed phase velocities and wave amplitudes in the oblique directions are explained by considering anisotropic characteristics of a magnetoplasma in which the directions of phase and group velocities are different. The observed phase velocity in the radial direction was found to agree with the theoretical ray velocity.

Magnetospheric ducting of low-latitude whistlers as deduced from the rocket measurement of their wave normal directions

The measurement of wave normal directions of low-latitude sunset whistlers was carried out by means of crossed loop aerials on board a rocket. It is found that the features of whistler wave normal directions are quite consistent with the concept of a trapping cone for ducted propagation. In addition, the wave normal directions of ionospherically transmitted whistlers lend further strong support to the ducted propagation of sunset whistlers. We deduce the enhancement factor, scale and structure of sunset ducts by making use of the observed results and theoretical ray tracing studies.