Surface Wave Field Research Articles

Multiple diffraction of obliquely incident plane waves by a three‐part impedance plane

Uniform asymptotic high‐frequency solution is developed for the diffraction of obliquely incident plane waves by a three‐part impedance plane. The multiple interaction up to third order between the edges of the three‐part impedance plane is obtained to yield a uniform scattered field. The employed formulations are based on a spectral iteration techique consisting of taking the Fourier integral representation of the n.tuply diffracted field by one junction as the incident field for the two‐part plane where the (n+1)th diffraction will occur. Upon using the field components normal to the three‐part surface, the (n+1)th diffracted field is obtained through the solution of a pair of uncoupled Wiener‐Hopf equations. The contribution of the surface wave fields to the secondary as well as triply diffraction is included in the analysis.

Radiation efficiency and the gain of a source inside a two‐layered grounded dielectric structure

AbstractRadiation characteristics of a source inside a two‐layered grounded dielectric structure are presented. The space and surface wave fields generated by the source are obtained by employing the integral transformation technique. The directivity and the radiation efficiency are found to be maximum for certain thicknesses of the dielectric layers. The overall gain differs considerably from the directive gain, due to the surface wave loss. The bandwidth is found to decrease with the increase in the gain of the structure. © 1992 John Wiley & Sons, Inc.

Partitioning Sequences and Arrays of Directional Ocean Wave Spectra into Component Wave Systems

A method for characterizing wave systems from temporal sequences and spatial arrays of directional ocean surface gravity-wave spectral estimates is presented. The method is structured in two major stages. In the first data-intensive stage, individual spectra are parameterized in terms of the directions, periods, and significant wave heights of wave systems represented in that spectrum. In the second stage, wave systems from spectra are associated with similar wave systems in neighboring spectra, and “large-scale wave systems” representing activity occurring continuously over groups of contiguous spectra are determined. This high-level description of the ocean surface wave field represents a considerable data reduction that preserves directional information in a more meaningful way than either average wave directions computed over the entire spectrum or wind sea-swell decompositions. Each large-scale wave system is normally caused by a distinct meteorological event. Data from the Labrador Sea Extreme Waves Experiment (LEWEX) are used to illustrate applications to numerical wave model and buoy spectra, and these methods should be useful in other spectral data comparison experiments. Other potential applications include the interpretation and archival of global wave-model analyses and the compression of wave spectra in a manner sufficient to preserve information necessary for numerical wave model data-assimilation schemes.

High energy electron generation in surface-wave-produced plasmas

A theoretical model predicting production of hot electrons in the high-frequency field of surface-wave-produced plasmas is presented. The fast-particle generation is caused by the radial component of the surface-wave electric field near the boundary of the discharge by a mechanism so far not considered. As a result of acceleration, due to the interaction with the surface-wave field and due to energy loss by excitation and ionization of atoms in the main volume of the plasma, hot-electron tail of the energy distribution function is formed. The proposed mechanism of hot-electron generation is described for cases when the wave frequency is larger than the electron collision frequency.

SLOPE DATA WATER‐BOTTOM MULTIPLE ATTENUATION1

AbstractMethods for predicting and attenuating water‐bottom multiples by wavefield extrapolation have been discussed by several investigators. Because these prediction methods operate on shot records, boundary conditions must be specified for every shot record.The approach presented operates in the common‐offset plane; a model of expected water‐bottom multiples is generated from the observed surface wavefield using a finite‐difference wave‐equation migration algorithm with an offset term. An accurate water‐depth profile is required, but there is no restriction on the shape of the water bottom other than a dip limit of approximately 18–20°. In generating a multiple model, the water‐bottom primary and each water‐bottom multiple reflection of the observed surface wavefield are extrapolated to a higher order. Thus, the extrapolated water‐bottom primary of the model is lined up with a water‐bottom multiple in the data and each multiple in the model is lined up with a higher‐order (or later) multiple in the data.Prestack multiple attenuation is achieved, for one offset at a time, by first adapting the model of expected multiples to the observed data and then subtracting the predicted multiple energy. An error‐constrained adaptation algorithm is proposed in order to control instabilities. No assumptions are made about primary reflections and no subwater‐bottom velocities are required.Computational efficiency of modelling and adaptation can be improved by applying this method only to near and intermediate offsets as the stacking process usually provides sufficient multiple attenuation at far offsets. A field data example demonstrates the potential of the proposed method for improving the primary‐to‐multiple ratio in prestack and post‐stack data.

The dynamics and acoustics of breaking waves.

In this presentation, recent progress is reviewed in the understanding of the relationship between the dynamics of breaking and the sound generated by breaking waves. Gently breaking waves entrain a small number of bubbles that may break up, resonate, and relax toward their equilibrium shape. Medwin and Daniel’s (1990) measurements of the sound spectrum under gently spilling waves are shown to be consistent with a simple model of the acoustics based on the resonant relaxation of individual bubbles. More energetic breaking may lead to significant air entrainment with recent laboratory and field measurements showing void fractions of O(10−1 ) or more. At these large void fractions the entrained air becomes dynamically significant and up to 50% of the energy lost from the surface wave field by breaking may be expected by entraining air against buoyancy forces. Bubble clouds of large void fraction may oscillate collectively and radiate sound at frequencies much lower that the resonant frequencies of the individual bubbles. Elsewhere at this meeting, measurements of low-frequency sound generated by 2-D and 3-D braking waves are presented, which are consistent with collective oscillation frequencies predicted from independent measurements of the void fraction. Methods are reviewed by which acoustic measurements of breaking may prove useful in indirect measurements of the dynamics of breaking and its role in processes of air–sea interaction. [Work supported by ONR (Ocean Acoustics) and NSF (Physical Oceanography).]

General Dispersion Relation for Surface Waves on a Plasma?Vacuum Interface: Application to Magnetised Plasmas

The general dispersion relation for electromagnetic surface waves on a plasma-vacuum interface, recently derived by Rowe (1991), is applied to the case of a cold magnetised plasma bounded by a vacuum. It is illustrated how the dispersion relation and the surface wave fields may be determined in practice, and some general results are given. It is remarked that a plasma of this type satisfies the consistency conditions which were derived for the general theory by Rowe. These general results are then used to reproduce the dispersion relation of Cramer and Donnelly (1983) for low frequency surface waves in an electron-ion plasma. This example illustrates the general principles of the theory. A major difference between the derivation in their paper and the calculation of this paper is that in the former the plasma was assumed to be infinitely conducting whereas here the plasma is strictly assumed to have finite conductivity.The transition to infinite conductivity, which involves a slight extension of the general theory to include surface currents, is thus also discussed.

Electromagnetic plane wave diffraction by a three‐part thin, planar dielectric/magnetic slab

A uniform asymptotic high‐frequency solution is developed for the plane wave diffraction by a three‐part thin, planar dielectric/magnetic slab. This work is an extension of the previously developed solution for the problem of diffraction by the planar junction of two thin dielectric/magnetic half planes. The multiple interactions up to third order between the edges of the three‐part material slab, which include edge‐diffracted fields, surface wave fields, and two new terms not considered in the majority of papers in the literature, are obtained to yield a uniform far‐zone‐scattered field. The employed formulations are based on the spectrally extended ray method; and the solutions obtained can be specialized to limiting cases for which any part of the material slab can become free space, perfectly conducting, or purely resistive, respectively. Numerical results corresponding to various multiple‐edge planar structures are presented. Some of these numerial examples are compared with the corresponding data based on an available moment method solution; the agreement between these approaches is very good.

Simulation of wave propagation in irregular soil domains by BEM and associated small scale experiments

Refined approaches are developed for the calculation and measurement of waves propagating towards and from rigid or flexible, embedded or surface foundations and between adjacent foundations. The field equation of layered subsoil, foundation slabs and embedded structures are formulated by boundary integral equations and solved by boundary element discretization, after implementing boundary conditions and continuity requirements at the interfaces of substructure domains. Viscoelastic constitutive equations of differential operator or hereditary integral type are generalized by means of fractional order time derivatives in the boundary integral equations. Soil-structure-interaction experiments are performed on a lab scale. Conventional displacement measurement of surface waves by transducers is improved by applying interferometry and holography. Measured surface wave fields prove the necessity of layered boundary element models even for soil with homogeneous compactness.

Diffraction of plane waves by a resistive strip residing between two impedance half-planes

A uniform asymptotic solution is presented for the diffraction of Ez polarized plane waves by a resistive strip residing between two impedance half-planes. The analysis proceeds from triple integral equations approach which leads to a system of uncoupled modified Wiener-Hopf equations (MWHE). This system is then reduced to two pairs of Fredholm integral equations of the second kind which are solved by successive approximations. Diffracted field expressions are derived up to the third order terms which include the surface wave field effects in a uniform manner.

The surface manifestation of internal waves: Doppler sonar observations from SWAPP

During the Surface Wave Processes Program (SWAPP, February–March 1990), a ten‐beam dual frequency Doppler sonar system was used to observe the sea surface velocity field. The sonars were mounted on the research platform FLIP and oriented at various angles in azimuth, forming a directional array. Measurements were obtained out to a range of 1100m. Here, a preliminary effort to use the array to observe that component of the sea surface velocity field associated with internal waves is presented. The internal wave signal exists beneath the far stronger signatures of the surface wave and Langmuir fields. The utility of this approach depends on the ability to isolate these competing motions from the total velocity field. The directional properties of the open ocean internal wave field remain largely unknown. The large horizontal scales of the waves, coupled with the great range of scales, render the measurement problem difficult. The desire for adequate internal wave directional measurements prompted the original development of Doppler sonar at S.I.O. in 1974. Sections of the 20‐day Patchex time series are examined, through a variety of weather and surface wave conditions. Emphasis is placed on the problem of distinguishing small‐scale waves from nonpropagating Langmuir cells. In addition, an attempt to use the full 2‐km aperture of the array to estimate the directional properties of the baroclinic tide will be discussed.

Monte-Carlo simulation studies of the nonlinear imaging of a two dimensional surface wave field by a synthetic aperture radar

Abstract The imaging of ocean surface waves by synthetic aperture radar (SAR) is investigated using two-dimensional Monte-Carlo simulations. The properties of the SAR imaging mechanism for windseas and swell in the Bragg scattering regime are discussed as a function of a few governing non-dimensional parameters formed from a combination of SAR and ocean wave parameters. The parameter ranges may be classified into three regimes corresponding to linear and weakly nonlinear, medium nonlinear and strongly nonlinear imaging. The nonlinearities are induced by motion effects (velocity bunching, velocity spread and acceleration smearing), while the real aperture radar (RAR) tilt and hydrodynamic modulation processes are regarded as linear. In the strongly nonlinear imaging regime, the velocity bunching mechanism causes a rotation of the spectral peak towards the range direction and a stretching of the peak wavelength. In addition, the azimuthal resolution is degraded through the Doppler spreading arising from the...

Surface wave evolution in a waveguide with an axial plasma column upon changing the external magnetic field

The surface wave evolution in a waveguide with a plasma column upon changing the external longitudinal magnetic field from 0 to infinity is investigated. It is shown that a family of anisotropic plasma modes EHl1ap exists when the magnetic field has intermediate values. When Omega < omega p (where Omega is the cyclotron frequency and omega p the plasma frequency), the EHl1ap modes have a surface wave field distribution. When Omega > omega p, they exist only for some system parameters and their field distribution has a quasi-surface character and when kz to infinity (where kz is the longitudinal wave number), the fields have a volume wave distribution.

The non-linear suppression of ionisation waves by the high-frequency field of surface waves. III. Application to He-Ne lasers

For pt.II see ibid., vol.21, p.1605 (1988). Non-linear effects of the field of high-frequency surface waves on the ionisation instability in gas discharge plasma presented previously are applied to gaseous lasers. It is shown that suppression of the ionisation instability by the high-frequency field can be considered as an effective method for improving the output characteristics of He-Ne lasers.

The Coupling of Surface and Internal Gravity Waves: Revisited

A new investigation is made of internal wave generation by surface waves. Previous theories are put into a unified form that includes a model of surface wave damping. Calculations using the complete theory, which do not seem to have been made previously, indicate that for wind speeds between 7 and 20 m s−1 the internal wave field loses about 10−4 W m−2 to the surface wave field. This would lead to a decay time of about 10 days for the high frequency portion of the internal wave field if an energy source were not available to maintain it. Possible sources for this energy are discussed. In contrast to this result for wind waves, a strong highly collimated ocean swell can lead to rapid growth of high frequency internal waves. The effects of nonlinear surface wave modulation and wave blocking are also discussed.

An Evaluation of the WOTAN Technique of Inferring Oceanic Winds from Underwater Ambient Sound

The potential of the WOTAN technique to estimate oceanic winds from underwater ambient sound is thoroughly evaluated. Anemometer winds and sound spectrum levels at 11 frequencies in the range 3–25 kHz from the FASINEX Experiment are used to establish both the frequency and wind speed dependencies of ambient sound. These relationships are then tested using independent data from four other deployments, and found to hold in the deep ocean in the OCEAN STORMS but not in shallow coastal waters. The OCEAN STORMS ambient-sound wind speed estimates are within ±0.5 m s−1 of anemometer values for wind speeds between 4 and 15 m s−1. Causes of differences, including disequilibrium of the surface wave field, are discussed and it is argued that they are no larger than expected. The procedure for processing ambient-sound data is developed. It includes temperature dependent calibration detection of shipping and precipitation contamination, and standardization of measurements to 1 m depth. The latter procedure allows data from different depths and sound speed profiles to be compared. The potential for using the technique on remote platforms is assessed. On-board processing and subsequent despiking and interpolation would result in a continuous wind record. For time scales of 12 hours or longer the results would be very similar to those obtained with an anemometer. Over shorter time scales there may be some important differences.

Ray perturbation theory and the Born approximation

SUMMARY Ray perturbation theory and the Born approximation have both been used extensively in seismological studies to describe the effects of a slowness perturbation on body and surface wavefields. The relationship between the expressions for the perturbed wavefield calculated using the two methods is investigated here. Using the symplectic symmetry of the ray equations we demonstrate the agreement, in the far field, of the two methods to first order in the slowness perturbation and to leading order in the asymptotic ray series. Thus it is shown that geometrical ray effects, like the traveltime perturbation, ray bending and focusing, are contained within the Born scattering formalism, provided these effects are small. The propagator formalism used to present the results is sufficiently general to include body and surface waves with a smoothly varying inhomogeneous elastic reference medium.

Characteristics of space and surface waves in a multilayered structure (microstrip antennas)

Characteristics of space- and surface-wave fields produced by an electromagnetic source in a multilayered structure are explored. Using the integral transformation technique it is shown that the space and surface-wave modes are orthogonal along the longitudinal direction with respect to an appropriate weighting function. It is demonstrated that these properties, together with reciprocity, can be utilized to determine the amplitudes of various surface-wave modes produced by an arbitrarily shaped source. Numerical results for the space- and surface-wave power for a circular patch antenna are presented. The study may find application for millimeter-wave printed antennas where the surface waves will play an important role in determining the radiation and impedance characteristics.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Peculiarities of surface wave fields in laterally inhomogeneous media in the framework of ray theory

Summary The paper presents the results of numerical modelling of Love waves in a layer with varying thickness over a homogeneous half-space. Rays, amplitudes of a transient wave, and synthetic seismograms were calculated by the use of the space-time ray method. It is shown that the lateral heterogeneity strongly affects the surface wave field: the seismograms at two near-by points at the same epicentral distance differ markedly. Anomalies in the Rayleigh and Love wave polarization are analysed by treating the second term in the series, representing the surface wave field in a laterally heterogeneous medium. The effects of lateral heterogeneity on the polarization are similar to those for body waves.

Reflections, diffractions, and surface waves for an interior impedance wedge of arbitrary angle

The asymptotic-impedance wedge solution for plane-wave illumination at normal incidence is examined for interior wedge diffraction. An efficient method for calculating the diffraction coefficient for arbitrary wedge angle is presented, as previous calculations were very difficult except for three specific wedge angles for the uniform geometrical theory of diffraction (UTD) expansion. The asymptotic solution isolates the incident, singly reflected, multiply reflected, diffracted, surface wave, and associated surface wave transition fields. Multiply reflected fields of any order are considered. The multiply reflected fields from the exact solution arise as ratios of auxiliary Maliuzhinets functions; however, by using properties of the Maliuzhinets functions, this representation can be reduced to products of reflection coefficients which are much more efficient for calculation. A surface-wave transition field is added to the surface wave boundaries. Computations are presented for interior wedge diffractions although the formulation is equally valid for both exterior and interior wedges with uniform but different impedances on each face for both soft and hard polarizations. In addition, the accuracy of the high-frequency asymptotic expansion is examined for small diffraction distances by direct comparison of the exact and asymptotic solutions. >