Vertical seismic profiling on the sea shelf

Results of a numerical experiment on vertical seismic profiling of the sea bottom on the shelf are presented. The results are obtained by analyzing the acoustic fields in the shelf area with the use of both hydroacoustic and seismic bottom sources of radiation. The influence of both transmission depth and source type on the efficiency of seismic wave excitation in the bottom is investigated. The formation of the vertical wave hodographs and its dependence on the acoustic parameters and structure of the bottom in the oceanic shelf region is analyzed. A high sensitivity of the vertical wave hodographs to variations in the parameters of the bottom medium is revealed. For the layered bottom model, the possibility of estimating the positions of layer boundaries in depth and the velocities of waves within the layers is demonstrated.     

Simulation of wave transmission and reception with phased arrays near the sea shelf bottom

The wave field excited by both single sources and a vertically directed phased transmitting array in a shallow-water waveguide with a layered elastic bottom is numerically simulated. The receivers are equispaced vertical arrays with different apertures, which are positioned at different depths in the water layer. The type of vertical distribution and the transformation of the vertical angular spectrum of pulsed signals arriving at a receiving array are demonstrated for different parameters of the medium, different carrier frequencies, and different distances from the source. It is shown that a selective angular reception ensures a better resolution of the travel-time curves of waves used for sounding the bottom. As an example, the possibility of singling out the shear wave refracted by a deep-seated bottom layer from the tone pattern of the travel-time curves is demonstrated. This possibility can be realized by means of angular scanning in the course of transmission and reception with the use of vertically oriented arrays.     

Dispersion properties of cyclotron waves in periodic semiconductor-insulator structures

The band spectrum of cyclotron waves propagating in a periodic layered semiconductor-insulator structure at an angle to an external magnetic field that is applied perpendicularly to the layers is calculated for two relationships between the characteristic frequencies of the semiconductor: ω_H>ω_P and ω_H<ω_P. The wave field distributions across the layers and over the period of the structure are analyzed. In both spectra, transmission bands arise when the conditions for dimensional resonance across the semiconductor layer are fulfilled. The graphic solution of the dispersion relation demonstrates that the cyclotron wave spectrum can be subdivided into two spectra of normal waves according to the Bloch wavenumbers of the periodic structure. The cases where the band spectra complement each other or overlap are considered.     

Effect of the anisotropy of a conducting layer on the dispersion law of electromagnetic waves in layered metal-dielectric structures

The dispersion laws of electromagnetic waves in layered periodic metal-dielectric structures with anisotropic metal layers have been theoretically analyzed. It has been found that the anisotropy of metal layers is responsible for the appearance of additional allowed energy bands for photons. It has been shown that these bands correspond to plasma (Langmuir) waves propagating in anisotropic metal layers of the structure. Conditions under which the directions of group and phase velocities of Langmuir waves coincide or are opposite have been determined. It has been shown that the penetration of the electromagnetic field of Langmuir waves into dielectric layers is exponentially weak and this field is primarily concentrated in metal layers, where it oscillates in the direction perpendicular to the plane of the layers.     

Numerical modeling of the reflection coefficients for the plane sound wave reflection from a layered elastic bottom

The matrix method and its numerical realization are considered in calculating the complex reflection coefficients and refraction indices of plane sound waves for geoacoustic models of the ocean bottom in the form of homogeneous elastic (liquid) absorbing layers overlying an elastic halfspace. In calculating the reflection coefficients at high frequencies or in the presence of a large numbers of sedimentary layers, a passage from the Thomson-Haskell matrix approach to the Dunkin-Thrower computational scheme is performed. The results of test calculations are presented. With the aim of developing resonance methods for the reconstruction of the parameters of layered elastic media, the behavior of the frequency-angular dependences of the reflection coefficient are studied for various geoacoustic bottom models. The structure of the angular and frequency resonances of the reflection coefficients is revealed. The dependence of the structure (the position, width, and amplitude) of two types of resonances on the parameters of the layered bottom is considered.     

Radiation properties of slot and dipole elements on layered substrates

This paper presents calculations of the radiation properties of slot and dipole antennas on electrically thick, grounded, dielectric substrates. These structures offer the possibility of simplifying the fabrication of imaging array antenna structures which operate at millimeter wave and far infrared freqeuncies. They also offer the possiblility of good beam patterns which can be tailored to suit a specific need. We present an analysis of practical layered structures which have beam patterns that are suitable for millimeter wave and far-infrared imaging array applications. We discuss considerations of the choice of dielectric layers with regard to beam patterns, surface wave losses, and the type of element used. The effects of dielectric and ground plane losses in high-gain structures are also considered. Efficiencies and beam patterns for three and five layer structures are presented, although the analysis techniques are extendable to an arbitrary number of layers. It is found that in combination with the use of a twin element configuration, both slot and dipole antennas can overcome the problems of losses to surface waves in the substrate. Consequently, they can be made to efficiently radiate to air on these layered dielectric structures.This work was supported by the Joint Services Electronics Program, program no. AFOSR-F49620-86-C0045, and the National Science Foundation under grant number ECS-8552868.     

The effect of metallic screens on the spectrum of exchange dipole hybrid electromagnetic-spin waves

A theory of exchange dipole hybrid electromagnetic-spin waves is elaborated. The waves propagate in arbitrary magnetized five-layer screened structures where a ferromagnetic layer is separated from the metallic screens by two insulating layers on both sides, the layers having different permittivities. Within this theory, the effect of the screens on the hybrid surface wave spectrum in layered structures is analyzed. The structures consist of a ferromagnetic film that is applied on an insulating substrate and is in contact with a ferroelectric plate.     

Theory of magnetostatic waves in a ferrite film with transition layers

A dispersion relation is derived for magnetostatic waves in a ferrite film with inhomogeneous layers near the interfaces. It is shown that the transition layers can be taken into account using a single integrated parameter, namely, the effective film thickness. It is established that the layered structure of the film causes an extra wave damping. The relations obtained can be used to analyze the dispersion and damping of the main modes of magnetostatic waves.     

Simplified triangular ground plane cloak by oblique multilayer dielectrics

Based on the effective medium theory, the triangular ground plane cloak can be realized by thin layered systems. Two solutions of parameter setting of the layered cloak are suggested to demonstrate the invisibility performance of a hybrid incoming wave. The hybrid parameters are derived from the equivalent of both anisotropies of permittivity and permeability to the alternating layers. The performance of the designed layered cloak is validated by both TM and TE wave simulations with near-field distributions and average scattering power outflows on an observation semicircle. From the simulation results, the layered cloak with both hybrid parameters and improved hybrid parameters can reflect the incoming TM/TE waves in a specular direction, and the latter behaves with a better overall invisibility performance.     

Excitation of surface electrostatic waves in semibounded layered superconductors by a nonrelativistic electron beam

Excitation of potential surface waves by a nonrelativistic electron beam traveling in a vacuum space near the boundary of a layered superconductor is studied theoretically. Dispersion relations for surface waves at an arbitrary angle between superconductor layers and interface are obtained. Allowance is made for an arbitrary direction of wave propagation in the interfacial plane. Increments of kinetic and hydrodynamic instabilities are found. It is shown that absolute instability may occur.     

Rayleigh wave and detection of low-velocity layers in a stratified half-space

The excitation and propagation of the guided waves in a stratified half-space and a Rayleigh wave exploration method in shallow engineering seismic exploration are studied in this paper. All the modes of the guided waves are calculated by the bisection method in the case where the low velocity layers are contained in a stratified half-space. Cases when the formation shear wave velocity gradually decreases from the top to the bottom layers are also studied. The dispersion curves obtained in actual Rayleigh wave exploration are usually noncontinual zigzag curves, but the dispersion curves given by the elastic theory for given modes of the guided waves are smooth and continual curves. In this paper, the mechanism of zigzag dispersion curves in Rayleigh wave exploration is investigated and analyzed thoroughly. The zigzag dispersion curves can give not only the possible positions of the low-velocity layers but also the other information on the formation structure (fractures, oil, gas, etc.). It is found that the zigzag dispersion curves of the Rayleigh wave are the result of the leap of the modes and the existence of low velocity layers in a stratified half-space. The effects of the compressional wave velocity, shear wave velocity, and density of each layer on zigzag dispersion curves and the relationship of the low velocity layers to zigzag dispersion curves are also investigated in detail. Finally, the exploration depth of the Rayleigh wave is discussed. The exploration depth of the Rayleigh wave is equal to the wavelength multiplied by a coefficient that is variable and usually given by the work experience and the formation properties of the local work area.     

Sound wave propagation in a vertically inhomogeneous ocean

Propagation of sound waves generated by a time dependent acoustic source in a vertically inhomogeneous ocean is considered. The effect of the solid bottom is included so that both the longitudinal and shear waves can be excited inside the bottom. The possibility of exciting a lateral shear wave by the acoustic source is also discussed. Although the results presented here are formal and general, physical interpretations have been offered whenever possible.     

Second-mode nonlinear internal waves over a sloping bottom

The characteristic features of second-mode internal wave propagation over a sloping bottom are investigated by numerical simulation based on the Korteweg-de Vries equation. A comparison of the transformations that occur for first- and second-mode internal solitons in the course of their propagation over a sloping bottom under the hydrological conditions of the South China Sea is carried out. Convex and concave second-mode waves are considered, and the possibility of their transition from the first to the second state in the course of their propagation from the deep ocean to the shelf is demonstrated. This is an analog of the effect of a change in the internal wave polarity, which earlier was known to occur for only first-mode internal waves.     

Shear-horizontal transverse-electric seismoelectric waves in cylindrical double layer porous media

The shear-horizontal(SH) waves excited by the shear source in a borehole are easy to analyze due to the simple waveform. The borehole-side structures make the formation properties discontinuous. We consider a cylindrical double layer structure and study the borehole shear-horizontal and transverse-electric(SH-TE) seismoelectric waves. We first derive the expressions of the basic field quantities, and simulate the acoustic field and electric field using the real axis integral method. Compared with the wave fields of an infinitely homogeneous porous medium outside the borehole, the cylindrical layered structure makes the multi-mode cylindrical Love waves and their accompanying electric fields excited.Next, in order to study the interface response law of the inducing electric fields, we use the secant integral method to calculate the interface converted electromagnetic waves and analyze the causes of each component. It is found that an interface response occurs each time the SH wave impinges the interface in the layered porous medium. The results show that the SH-TE mode has a potential application for borehole-side interface detection in geophysical logs.     

Neutron standing waves in layered systems: Formation,detection, and application in neutron physics and for investigation of nanostructures

Specific features of different regimes of the neutron’s wave field are theoretically considered. The results of experimental studies of the regime of a neutron’s standing waves both in the primary channel of neutron specular reflection and in different channels of registration of secondary emission are considered. Some studies of layered structures performed with the help of a neutron’s standing waves are considered. The prospects of application of a neutron’s standing waves in neutron physics and for investigation of layered nanostructures are considered.     

Reflection of electromagnetic waves from the layered structure of a high-temperature superconductor-multiferroic with cycloidal antiferromagnetic structure

The reflection of electromagnetic waves from the layered structure of a high-temperature superconductor-multiferroic with cycloidal antiferromagnetic structure in an external magnetic field is studied. The frequency dependence of the reflection coefficient at different values of an external magnetic field is calculated. The possibility of effectively controlling the reflective properties of the structure is established.     

On the localization of 2D nonlinear internal waves in a two-layer fluid

A 2D generalized Gardner equation is used to describe 2D nonlinear internal waves in a two-layer fluid. Unlike the previous model based on the Kadomtsev-Petviashvili equation, the model considered here allows for the instability of a plane internal solitary wave. Such a possibility causes the wave to be localized in any direction. Relationships between the thicknesses and densities of the layers under the instability conditions are obtained.     

Constructive methods for synthesizing inhomogeneous layered structures under the effect of elastic waves

A variational statement of the problem of optimal synthesis aimed at obtaining inhomogeneous layered structures that exhibit a required set of properties under the effect of elastic waves is studied. The possibility of targeted control of mode conversion at the boundaries between elastic layers is investigated with a view to extending the limits in designing structurally inhomogeneous systems with preset properties. The optimum conditions are presented for problems of optimal synthesis of inhomogeneous layered structures under the effect of elastic waves in terms of the aforementioned variational statement. Analytical relations are derived for an effective a priori contraction of the allowable set of materials, which makes it possible to increase the efficiency of the search for optimal solutions and to extend the limits of applicability of different approaches.     

Wave based analysis of the Green's function for a layered cylindrical shell

Cylindrical shells composed of concentric layers may be designed to affect the way that elastic waves are generated and propagated, particularly when some layers are anisotropic. To aid the design process, the present work develops a wave based analysis of the Green's function for a layered cylindrical shell in which the response is given as a sum of waves propagating in the axial coordinate. The analysis assumes linear Hookean materials for each layer. It uses finite element discretizations in the radial coordinate and Fourier series expansions in the circumferential coordinate, leading to linear equations in the axial wavenumber domain that relate shell displacements and forces. Inversion to the axial domain is accomplished via a state-space formulation that is evaluated using residue integration. The resulting expression for the Green's function for each circumferential harmonic is a summation over the natural waves of the shell. The finite element discretization in the radial direction allows the approach to be used for arbitrarily thick shells. The approach is benchmarked to results from an isotropic shell and numerical examples are given for a shell composed of a fiber-reinforced material. The numerical examples illustrate the effect of fiber orientation on the Green's function.     

Magnonics: Experiment to prove the concept

An experimental scheme for studying spin wave propagation across thin magnetic film samples is proposed. The scheme is based upon the creation of picosecond pulses of strongly localized effective magnetic field via ultrafast optical irradiation of a specially deposited exchange bias or exchange spring layer. The spin waves are excited near the irradiated surface before propagating across the thickness of the sample. They are then detected near the other surface either within the finite optical skin depth using the linear magneto-optical Kerr effect in metallic samples or by the magnetic second harmonic generation. The experiment can facilitate investigations of propagating spin waves with wavelengths down to several nanometers and frequencies in excess of hundreds of Gigahertz. An experiment upon a periodically layered nanowire (a finite cross-section magnonic crystal) is numerically simulated, although the sample might equally well be a continuous film or an array of elements (e.g. nanowires) that either have uniform composition or are periodically layered as in a magnonic crystal. The experiments could be extended to study domain wall-induced spin wave phase shifts and can be used for the creation of spin wave magnetic logic devices.