Low-frequency pulse propagation over irregular terrain

Foreword. I: Theoretical problems concerning propagation of surface wave in elastic anisotropic media. On the role of anisotropy in crystalloacoustics V.I. Alshits. Surface waves of non-Rayleigh type S.V. Kuznetsov. Nonlinearity in elastic surface waves acts nonlocally D.F. Parker. Explicit secular equations for surface waves in an anisotropic elastic half space from Rayleigh to today T.C.T. Ting. II: Bending and edge waves in plates and shells. 'Nongeometrical phenomena' in propagation of elastic surface waves V.M. Babich, A.P. Kiselev. Complex rays and internal diffraction at the cusp edge M. Deschamps, O. Poncelet. Edge waves in the fluid beneath an elastic sheet with linear nonhomogeneity R.V. Goldstein, A.V. Marchenko. On continuum modelling of wave propagation in layered medium: bending waves K.B. Ustinov. Edge-localised bending waves in anisotropic media: energy and dispersion III: Experimental, numerical and semi-analytical methods for analysis of surface waves. Surface electromagnetic perturbations induced by unsteady-state subsurface flow P.M. Adler, V.M. Entov. Resonant waves in a structured elastic halfspace M.V. Ayzenberg-Stepanenko. Mumerical analysis of Rayleigh waves in anisotropic media A.V. Kaptsov. Guided waves in anisotropic media: applications R.A. Kline. A general purpose computer model for calculating elastic waveguide properties, with application to Non-Destructive Testing M.J.S. Lowe, P. Cawley, B.N. Pavlakovic. The influence of the initial stresses on the dynamic instability of an anisotropic cone Y.R. Rossikhin, M.V. Shitikova. Embedding theorem and mutual relation for the interface and shear wavespeeds I.V. Simonov. IV: Applications of surface waves to analysis of fracture and damage. The non-uniqueness of constant velocity crack propagation K.B. Broberg. Embedding formulae for planar cracks R.V. Craster, A.V. Shanin. Wave propagation and crack detection in layered structures E.V. Glushkov, N.V. Glushkova, A.V. Ekhlakov. Author Index.

Sound propagation of wind farms is typically simulated by the use of engineering tools that are neglecting some atmospheric conditions and terrain effects. Wind and temperature profiles, however, can affect the propagation of sound and thus the perceived sound in the far field. A better understanding and application of those effects would allow a more optimized farm operation towards meeting noise regulations and optimizing energy yield. This paper presents the parabolic equation (PE) model development for accurate wind turbine noise propagation. The model is validated against analytic solutions for a uniform sound speed profile, benchmark problems for nonuniform sound speed profiles, and field sound test data for real environmental acoustics. It is shown that PE provides good agreement with the measured data, except upwind propagation cases in which turbulence scattering is important. Finally, the PE model uses computational fluid dynamics results as input to accurately predict sound propagation for complex flows such as wake flows. It is demonstrated that wake flows significantly modify the sound propagation characteristics.

In this study, the influence of axial stress on surface wave propagation along cylindrical surfaces is investigated, with particular emphasis on quantifying curvature effects on acoustoelastic coefficients. The classical planar surface wave acoustoelastic formulation is first adopted as a reference. Three-dimensional transient finite element simulations are then performed to model surface wave excitation, propagation, and reception on aluminum cylinders with different radii and excitation frequencies. Stress-free simulations are used to extract surface wave velocities and reference time signals, while prestressed simulations provide stress-induced time delays, from which effective acoustoelastic coefficients are determined. The results indicate that both the surface wave velocity and the acoustoelastic coefficient exhibit clear dependencies on cylinder radius and excitation frequency. Curvature effects are especially pronounced at low frequencies, whereas at higher frequencies the coefficients corresponding to different radii tend to converge. These findings demonstrate that planar surface wave theory may lead to non-negligible errors when applied to cylindrical geometries and provide quantitative guidance for curvature-aware stress evaluation.

On wave diffraction of two-dimensional moonpools in a two-layer fluid with finite depth

The propagation of surface waves on free and metallized surfaces of cubic and hexagonal crystals is investigated numerically. Surface wave velocity curves and relative differential velocity curves are presented for propagation on typical surfaces of four selected crystals (gallium arsenide, bismuth germanium oxide, zinc oxide, and cadmium sulfide). Special attention is paid to degenerate modes wherein decoupling among the various components of mechanical displacement and the electric field occurs.

On a base of developed hydrodynamic models the reflecting capacities of edges, cracks and ice field breaks are studied when the surface waves pass through them. It was investigated the ice flexural rigidity and compression effects on the spatial distribution of wave disturbance amplitudes and bend stresses of the ice cover on both sides of the examined horizontal non-uniformity. Assessment of the ice breaking possibility within the edge-side zones was made. The surface wave propagation from the basin deep-water area through the bottom's step into the area of the finite depth is investigated. The spatial and temporal features of the formed dynamic frontal zones on both sides of the step are revealed. The dependence of the basin surface displacement amplitudes and wave current velocities on the ice thickness, incident wave periods, and depth of the step occurrence, is studied. The three-dimensional bending vibrations of floating ice cover induced by a moving vortex formation are studied. Dependence of the amplitude-phase vibration characteristics on the ice rigidity in bending, angular velocity and forward speed of the vortex is investigated. Spatial distribution of the flexural-gravity disturbances both in front of the moving pressure region and in the back wake is analyzed. A mathematical model of the non-linear dynamics of surface waves of finite amplitude in the ice floes seas was developed involving the method of multiscale expansions. Using the model, it was studiedthe dependence of the amplitude-phase structure of disturbances, formed by the propagating non-linear periodic waves, on the basin depth, ice thickness and non-linearity of its vertical acceleration, frequency and steepness of initial wave harmonics. The ice Stokes drift velocity was estimated as well as the ice induced non-linear mass transport analyzed.&nbsp

The scattering problem of acoustic plane waves from comb-like impedance gratings on a rigid surface has been investigated in this paper. A rigorous analytic approach for homogeneous plane-wave incidence is presented based on the periodicity of the grating structure, in which the problem was solved as a mixed boundary value problem and the scattered field was represented by the tangent velocity difference across a partition wall of the grating. A singular integral equation has been derived for the tangent velocity difference, which can directly be solved with the Gauss-Chebyshev procedure. The resulting solution consists of a series of Bloch-Floquet waves (plane bulk wave and surface wave modes) with explicit expressions for the expansion coefficients. When the grating period is much less than the incident wavelength (ka << 1), the grating structure is equivalent to a plane impedance surface and no surface waves can be excited with homogeneous plane-wave incidence. When the grating period is comparable to the incident wavelength, resonance phenomena are predicted under certain conditions and surface waves can form, even with homogeneous plane-wave incidence. The dispersion relation for surface waves has also been examined. The impedance effects of the grating on the reflection and diffraction waves as well as on the dispersion and formation of surface waves have been studied, with the acoustically hard grating being the special case of the general impedance grating.

SUMMARY The generation of long period (20–50s) surface waves by an explosion on an island, inside a mountain, or near a material discontinuity is studied using 2-D axisymmetrical finite difference calculations. The generation of surface waves can be reduced substantially if the explosion is close to a vertical boundary such as a material velocity reduction, mountain slope, or island/ocean boundary. This occurs because surface waves from a shallow explosion are generated primarily by the horizontal stress components which are reduced at the boundary. Three sets of finite difference calculations were performed for an ‘island’ surrounded by an ‘ocean’ of air, water, and a low-velocity solid. The surface waves were measured on ‘land’ on the other side of the ‘ocean’ . A ‘solid’ calculation with a uniform structure was also performed for comparison. Calculations include ‘ocean’ depths of 3 and 6 km, and the ‘island’ varies in diameter from 6 to 48 km. For a 48 km island, the material boundary has little effect on the surface waves. For an island width equal to the ocean depth, however, the surface wave generation is reduced by an order of magnitude for air and water, and reduced by more than a factor of 2 for the low-velocity solid. Comparisons with the geography of the Amchitka and Mururoa island test sites show that this effect is small for explosions at those sites, however, the effect appears to be important for Novaya Zemlya and other sites where explosions are detonated inside mountains, and at sites with strong material variations. To estimate the effect of a high-velocity boundary surrounding a low-velocity source region, we performed a calculation for an explosion in a sedimentary basin modelled after Yucca Valley at the Nevada Test Site. In order to include finer details of the source region, a finely gridded finite difference calculation was performed in the source region, and surface waves exterior to the calculation were calculated using the representation theorem. We find that surface waves are amplified by about a factor of 2 by the high-velocity interface surrounding the low-velocity basin.

The 3-D surface wave Boundary Element Method is applied to the same set of strong motion records as those of Part 1 (Kato et al. , 1993), namely, the long-period strong motion with periods around 8 sec recorded at the JMA (Japan Meteorological Agency) stations in Yokohama and Tokyo during the 1980 Izu-hanto-toho-oki earthquake. This method simulates exact Love and Rayleigh waves in a laterally heterogeneous medium including 3-D basin structures. The needed computer memory and CPU time are made manageable by assuming the inside and outside of the basin to consist of horizontal flat layers. In addition, vertical interfaces are assumed between the basin and the outside bedrock for simplicity in this study. After checking the range of validity of the flat-layer approximation for the Kanto sedimentary basin, the generation and propagation of surface waves in the basin are numerically investigated. In particular, our investigation places special emphasis on the cause of the observed long duration, which could not be explained in Part 1 using the surface wave Gaussian Beam method. When the bedrock outside the western margin of the Kanto basin is incorporated into the model, the observed long duration at the Tokyo station is successfully reproduced. This can be explained by the strong surface waves generated by the surface wave to surface wave conversion at the margin and appearing in Tokyo after the direct surface waves from the epicenter. This result agrees with Kinoshita et al. (1992), who studied records from an array of seismographs in Tokyo. Moreover, when a sedimentary basin of Sagami Bay is included as was done in Part 1, the simulations are greatly improved, particularly at the Yokohama station, which corresponds to the node in the radiation pattern of the source. This is due to the fact that the radiation pattern is strongly affected by the near-source heterogeneity of the Sagami basin. All these results, together with direct comparisons to 2-D modelings, clearly show the strong 3-D effects of the basin structures on the generation and propagation of surface waves for this earthquake.

Surface wave propagation along multi-mixed-paths with irregular terrain over spherical Earth in two-dimension (2D) is discussed. For the first time in the literature, sea-land-sea (island) transition problem including non-flat (hilly) islands is investigated systematically. A finite element method based multi-mixed path surface wave virtual propagation predictor tool FEMIX is developed for this purpose. FEMIX, tested and calibrated against analytical ray-mode reference models accommodated with the Millington curve fitting approaches, is shown to be capable of modeling propagation along sea surface having multiple hilly islands over MF (300 kHz - 3 MHz) and HF (3 - 30 MHz) bands.

Electromagnetic (EM) biomedical sensors in the mm-wave frequency range must detect small changes in signals in the presence of tissues, which are correlated to a pathological condition. These signals, however, can suffer from artifacts due to complex EM wave interactions such as diffraction and surface wave propagation, which are often overlooked in the design phase of these sensors. This paper studies the impact of these wave phenomena on the signals transmitted and received from a pair of antennas designed to sense glucose changes via changes in transmission through a sample. Numerical simulations and controlled experiments with glucose solutions demonstrate for the first time that unwanted signal contributions from mm surface waves along the tissue can dominate the received signals but can be reduced with the use of appropriately placed absorbers around the antenna sensors. As a result, the sensitivity of such a sensing system to glucose changes is increased. This finding can be very useful in the design and development of the glucose sensor under study, as well as for other EM-based diagnostic medical applications.

Surface wave propagation effects on buried segmented pipelines

This paper focuses on the study of linear and non-linear surface and internal waves, in a complete setting, using a two-layer model of a stratified fluid. The respective Korteweg-de Vries evolutionary equations have been obtained, analysed, and compared with the ‘rigid lid’ model data. Boussinesq-type equations have been derived for the interacting modes pertaining to one type and to different types. It is shown that in addition to the known mechanisms of interaction between internal and surface waves, interaction between long non-linear baroclinic modes and barotropic modes, propagating in the same direction, is likely in such a system.

Summary Hamilton’s wave theory is used to study the effect on seismic surface wave propagation of lateral variations in the composition of the crust and mantle. For cases with small lateral variations the distortion of the propagation paths from the great circle paths is determined by a perturbation method. Moreover, surface wave paths from the Mid-Atlantic Ridge toward the NORSAR and LASA arrays are computed by numerical integration of the ray equations. The computed wave paths agree well with existing observations. The model of the surface wave velocity distribution used in this study cannot, however, explain all the observed multipathing effects. Studies by several authors have shown that the lateral variations in the structure of the Earth’s crust and upper mantle may lead to large distortions of the propagation paths for seismic surface waves. Notably among these works are the works by Evernden (1953, 1954) and the works by Capon (1970, 1971). These works show that surface waves are refracted or reflected at the continental margins and at major geological structures. The oceanic ridge systems are an example to the latter. Estimates of the propagation path for surface waves are presented in the work cited above. Apart from this and a study by Julian (1972) little has been done in order to compute the propagation paths for surface waves. In various branches of Physics, Hamilton’s wave theory (or the eikonal approximation) has successfully been used to study wave propagation in heterogeneous media. Within the theory of seismic surface waves the method has received less attention although the applicability of it has been demonstrated in an elegant study by Backus (1962). The application of this theory to surface wave propagation in cases with lateral variation in the structure of the crust and upper mantle deserves more exploration. A fundamental assumption in Hamilton’s wave theory is that the physical parameters which are important for the wave propagation vary only by a small fraction of their values over a distance comparable to a wavelength. The theory has therefore very misleadingly been referred to as a short wavelength approximation. The propagation of seismic surface waves are for example fundamentally effected by the value of the physical parameters in a surface layer with thickness comparable to the wavelength. Therefore the lateral variations of the parameters describing the wave motion may be smaller for long waves than for short waves. This is confirmed by the observations

SUMMARY This paper presents a numerical simulation of the effects of an underground ridge (UGR) on seismically-induced ground motion characteristics using 2.5-D modelling with variable grid size. The responses of models with and without an UGR, estimated group velocity of later arrivals, horizontal differential ground motion, snapshots and spectral analysis of surface waves have confirmed high frequency surface wave generation near the ridge. Exponential decay of surface wave amplitude with depth was obtained in the vertical component but not in the radial and transverse components. Changes of focal mechanism parameters mainly affected the amplitude but not the spectral shape or bandwidth of UGR-induced surface waves. We predict that anomalous macroseismic intensity due to UGR-induced surface waves might arise in a zone of width of about 2.2 km parallel to a ridge and at a distance of about 0.5 km from the ridge edge, for the model geometry and the soil parameters considered. The UGR-induced surface waves were observed even when a UGR was overlain by a soil deposit. Increased depth or changes in the slope of the flanks of the UGR causes decrease of spectral amplitudes but does not affect the spectral shape, bandwidth or dominant frequency of induced surface waves. The dominant frequency of Rayleigh waves appeared to be twice that the fundamental frequency ( f 0) of first soft soil layer. For Love waves, the dominant frequency was twice that of f 0 for f 0 ≤ 0.63 Hz and was more than twice f 0, when f 0 > 0.63 Hz. The lower cut-off frequency for surface wave generation was approximately f 0. Only spectral amplitudes were affected by a change of S-wave velocity in first soil layer but not the spectral bandwidth, spectral shape or dominant frequency when f 0 was maintained constant. An increase of spectral bandwidth with decrease of f 0 of the first soil layer was found. We obtained a decrease of surface wave amplitude in accordance with the decrease of amplitude of body waves with angle of incidence. The amplitude of surface waves on the side of incidence of body waves was more than the opposite side. Further, it was inferred that dominant frequency and spectral bandwidth were unaffected but spectral shape was highly dependent on the angle of incidence of body waves. The simulations revealed that UGR effects deserve particular attention for earthquake-resistant design and seismic microzonation.