Showing papers on "Phase velocity published in 1997"

Measurements and global models of surface wave propagation

Abstract: A new technique for making single-station phase velocity measurements is developed and applied to a large number of globally recorded Rayleigh and Love waves in the period range 35–150 s. The method is based on phase-matched filter theory and iteratively suppresses the effect of interfering overtones by minimizing residual dispersion. The model surface wave signal is described by its amplitude and apparent phase velocity, both of which are parameterized in terms of smooth B-spline functions of frequency. A misfit function is constructed which represents the difference between the model and observed waveforms, and the optimal spline coefficients are estimated in an iterative misfit minimization algorithm. In order to eliminate cycle skips in the measurements of phase at short periods, the waveforms are first matched at long periods, and the frequency range is gradually extended to include higher frequencies. The application of the algorithm to records from the Global Seismographic Network, using earthquakes in the Harvard centroid-moment tensor catalog, results in the determination of more than 50,000 high-quality dispersion curves. The observed variations in measured dispersion for pairwise similar paths are used to estimate realistic uncertainties in the data. Phase delays at discrete periods are inverted for global maps of variations in phase velocity expanded in spherical harmonics up to degree 40. A realistic resolution test indicates that structures are well recovered up to at least degree 20. The new phase velocity maps explain 70–96% of the observed variance in phase residuals, reflecting the high internal consistency of the dispersion measurements.

A Simple Model of the Decadal Response of the Ocean to Stochastic Wind Forcing

Abstract: A simple linear model is used to estimate the decadal response of the extratropical ocean to wind stress forcing, assuming a flat bottom, a mean state at rest, and no dissipation. The barotropic fields are governed by a time-dependent Sverdrup balance, the baroclinic ones by the long Rossby wave equation. The ocean is bounded by a coast in the east and a radiation condition is used in the west. At each frequency, the baroclinic response consists of a forced response plus a Rossby wave generated at the eastern boundary. For zonally independent forcing, the response propagates westward at twice the Rossby phase speed. The wind stress is assumed to be stochastic with a white frequency spectrum, so the model represents the continuous excitation of the ocean interior by the weather fluctuations. The model predicts the shape and level of the frequency spectra of the oceanic pressure field and their variation with longitude and latitude. The baroclinic response is spread over a continuum of frequencies,...

Reflection moveout and parameter estimation for horizontal transverse isotropy

Abstract: Transverse isotropy with a horizontal axis of symmetry (HTI) is the simplest azimuthally anisotropic model used to describe fractured reservoirs that contain parallel vertical cracks. Here, I present an exact equation for normal-moveout (NMO) velocities from horizontal reflectors valid for pure modes in HTI media with any strength of anisotropy. The azimuthally dependent P -wave NMO velocity, which can be obtained from 3-D surveys, is controlled by the principal direction of the anisotropy (crack orientation), the P -wave vertical velocity, and an effective anisotropic parameter equivalent to Thomsen9s coefficient δ. An important parameter of fracture systems that can be constrained by seismic data is the crack density, which is usually estimated through the shear-wave splitting coefficient γ. The formalism developed here makes it possible to obtain the shear-wave splitting parameter using the NMO velocities of P and shear waves from horizontal reflectors. Furthermore, γ can be estimated just from the P -wave NMO velocity in the special case of the vanishing parameter e, corresponding to thin cracks and negligible equant porosity. Also, P -wave moveout alone is sufficient to constrain γ if either dipping events are available or the velocity in the symmetry direction is known. Determination of the splitting parameter from P -wave data requires, however, an estimate of the ratio of the P -to- S vertical velocities (either of the split shear waves can be used). Velocities and polarizations in the vertical symmetry plane of HTI media, that contains the symmetry axis, are described by the known equations for vertical transverse isotropy (VTI). Time-related 2-D P -wave processing (NMO, DMO, time migration) in this plane is governed by the same two parameters (the NMO velocity from a horizontal reflector and coefficient η) as in media with a vertical symmetry axis. The analogy between vertical and horizontal transverse isotropy makes it possible to introduce Thomsen parameters of the “equivalent” VTI model, which not only control the azimuthally dependent NMO velocity, but also can be used to reconstruct phase velocity and carry out seismic processing in off-symmetry planes.

Experiments on unsteady cavitation

Abstract: The unsteady behaviour of cloud cavitation is obviously influenced by its internal flow pattern. The main purpose of this work is to investigate such a two phase flow during a cavitation cycle. The tests are carried out with a convergent divergent nozzle. Observations are made by using a classical video set in combination with a stroboscopic light sheet. The use of a double optical probe enables void fraction and velocity to be measured inside the two phase flow structure. Data acquisition is governed by a pressure signal measured near the cavity closures to follow their evolution during the shedding process. Special care has been taken in validating the experimental techniques because they have not been used in such flows. The measurements show an extended reversed flow occurring along the solid surface. It plays a significant function in the vapour cloud shedding process.

The simulation of front keyhole wall dynamics during laser welding

Abstract: A physical model of keyhole support and propagation during high-translation-speed laser welding is described. A numerical code for the simulation of the front keyhole wall behaviour is developed on the basis of a `hydrodynamic' physical model assuming that: (i) only the front part of the keyhole wall is exposed to the high-intensity laser beam; and (ii) recoil pressure exceeds surface tension and propagation of the keyhole wall inside the sample is due to melt expulsion similar to that in laser drilling. The front keyhole wall profile, distribution of absorbed laser intensity and phase velocity of the solid/liquid (liquid/vapour) boundary are calculated for various processing parameters. The calculations show that, depending on the processing conditions, the absolute value of the keyhole wall velocity component parallel to the translation velocity vector can be higher than, smaller than or equal to the beam translation speed. When the component of the keyhole velocity vector parallel to the sample surface was higher than the beam translation speed, the formation of the humps on the keyhole wall was observed numerically.

Estimates of momentum flux associated with equatorial Kelvin and gravity waves

Abstract: A new indirect method is proposed to estimate momentum flux based on the theory of slowly varying gravity waves and equatorial waves in vertical shear by Dunkerton [this issue] which explains the discovery by Sato et al. [1994] that the cospectra of temperature and zonal wind fluctuations at Singapore (1.4°N, 104.0°E) are synchronized with the quasi-biennial oscillation (QBO) of mean zonal wind in the stratosphere. The indirect estimates obtained from cospectra correspond to the summation of absolute values of momentum flux associated with each wave, whereas direct estimates from quadrature spectra give the summation of momentum flux. An analysis was made for twice daily rawinsonde data at Singapore. The direct estimate for Kelvin waves (5–20 day components) is 2–9×10−3 m s−2 and accords with the indirect estimate to within the estimation error. This result supports the validity of the indirect method. Although the indirect estimate depends on an assumed wave structure, large values of momentum flux are obtained for all possible equatorial modes having short periods (1–3 days). The indirect estimate for westerly shear is 20–60×10−3 m2 s−2 based on the theory of two-dimensional gravity waves, while the direct estimate is only 0–4×10−3 m2 s−2. The reduction of indirect estimate under the assumption of equatorial waves is about 30–70%. The discrepancy between direct and indirect estimates indicates a large cancelation of positive and negative momentum fluxes. This is the case also for easterly shear. The indirect estimate for westerly shear is almost twice as large as that for easterly shear. The characteristics of waves near the source in the troposphere are thought to be independent of the QBO in the stratosphere, so that the difference in wave activity should be attributed to the differing characteristics of wave propagation under the strong QBO shear. Several possible explanations are discussed. Parameters such as phase velocity and zonal wavelength are estimated from the ratio of potential to kinetic energies assuming that the 1–3 day components are due to equatorial waves. The estimates in this paper were made assuming that the observed frequencies are actual ground-based wave frequencies. If there is aliasing from higher frequencies than 1 day, the actual momentum fluxes can be significantly larger than the estimated values.

On the motion of gas bubbles in homogeneous isotropic turbulence

Abstract: This paper is concerned with the motion of small gas bubbles, equivalent diameter about 1.0 mm, in isotropic turbulent flows. Data on the mean velocity of rise and the dispersion of the bubbles have been obtained numerically by simulating the turbulence as a sum of Fourier modes with random phases and amplitudes determined by the Kraichnan and the von Karman–Pao energy-spectrum functions, and by calculating the bubble trajectories from a reasonably well-established equation of motion. The data cover the range β[les ]1, where β is the ratio between the turbulence intensity and the velocity of rise of the bubbles in still fluid. An approximate analysis based on the assumption that β is small yields results that compare favourably with the numerical data, and clarifies the important role played by the lift forces exerted by the fluid.

Propagation and Decay of Forced and Free Baroclinic Rossby Waves in Off-Equatorial Oceans*

Abstract: Baroclinic Rossby wave motions in the off-equatorial oceans are investigated with emphasis on how eddy dissipation can influence the propagation of the height anomalies when both the forced wave response to wind in the interior ocean and the free wave response originating along the ocean’s coastal and topographic boundaries are present. By explicitly estimating the decay scale for the long baroclinic Rossby wave, the authors show that the forced wave patterns at all off-equatorial regions appear to propagate westward at 2 cr, where cr is the phase speed of the long baroclinic wave. The presence of the boundary-generated, free Rossby waves in the low latitudes, however, reinforces the 1cr phase propagation in the combined height anomaly fields. Toward higher latitudes, this reinforcement weakens as the boundary-generated, free waves become highly dissipative; as a result, the forced wave motion becomes more dominant, which works to increase the apparent phase speed up to 2cr. In the subpolar regions where the annual baroclinic Rossby waves become evanescent, an apparent phase speed higher than 2cr is observed when an annual, standing wave response and a propagating wave response with an interannual frequency coexist. Stronger annual and interannual wind fluctuations over the Southern Hemisphere subpolar regions than over the Northern Hemisphere subpolar regions suggest that this coupling, and the phase speed higher than 2cr, are more likely to be detected in the Southern Hemisphere subpolar oceans.

Crustal anisotropy in the Ural Mountains Foredeep from teleseismic receiver functions

Abstract: Radial and transverse teleseismic receiver functions (RFs) at GSN station ARU, in central Eurasia, display variation in back-azimuth ψ consistent with a 1-D anisotropic crustal structure. In a broad ψ range, the transverse RFs possess a strong phase at ∼5-sec delay relative to direct P, with a polarity reversal at ψ ∼ 50°. The radial RFs peak at the transyerse-RF polarity reversal for this converted phase. The first motion of the transverse RFs varies with ψ also, reversing polarity at ψ ∼ 345°. The azimuthal variation can be modeled by a 5-layer velocity profile with substantial (15%) seismic anisotropy in both the lowermost crust and a low-velocity surface layer. Assuming hexagonal symmetry, the lowermost crust has a tilted “slow” symmetry axis i.e. an oblate phase velocity surface. The strike of the axis is oblique to the north-south Urals trend, but deviates <20° from the mantle fast-axis inferred from SKS splitting. The magnitude and tilt of the model's anisotropy suggests that fine layering and/or aligned cracks augment mineral-orientation anisotropy near the top and bottom of the crust.

Electron trapping in self-modulated laser wakefields by raman backscatter

Abstract: : Simultaneous measurements of high energy electrons and plasma wave characteristics have been conducted in a self-modulated laser wakefield accelerator Approximately 10(exp 8) electrons were accelerated from the background plasma to energies greater than 1 MeV with a peak energy of approximately 30 MeV A strong correlation between the plasma wave amplitude and electron production was measured with no evidence of wave breaking Simulations indicate plasma electrons are trapped by the low phase velocity beat waves produced by backward Raman scattering

A photoacoustic method for characterising thin films

Abstract: A photoacoustic method is presented for characterising thin films. The method uses acoustic surface waves with frequencies up to 200 MHz induced by short laser pulses and detected with a piezoelectric transducer. The surface wave signals are processed by cross-correlation and Fourier transformation to determine a dispersion spectrum (phase velocity depending on frequency) with optimal signal-to-noise ratio. The theoretical curve is fitted to the measured dispersion spectrum to derive film parameters as Young's modulus, density and/or film thickness. The conditions are discussed which enable one, two, or three of these parameters to be obtained. The dispersion spectrum may show normal and anomalous dispersion, which depends on the combination of film and substrate material. Diamond-like carbon and polyamide films on (100) silicon are used to demonstrate this phenomenon. The effects of firm thickness, film and substrate material, and the error of the input parameters on the results are discussed. The peculiarity of surface wave propagation in cubic single crystals is described. The photoacoustic method is particularly suitable to determine the Young's modulus of the film. This material parameter is sensitively related to important microstructural properties and bonding conditions. The large modulus variation of some important covalent and ionic film materials recommends to use Young's modulus for quality control.

On the behaviour of solid particles in a horizontal boundary layer with turbulence and saltation effects

Abstract: A horizontal turbulent boundary layer of air carrying heavy solid particles is investigated experimentally. Mean and r.m.s. velocities of air and particles are measured by LDA, and particle mass flux distributions are obtained by means of a sampling method. The influence of the saltation mechanism is revealed by the large particle r.m.s. velocity in the near-wall region, and by the velocity lag of the particles in the outer region of the boundary layer, which is shown to be closely related to their free fall velocity. The present original results are discussed and compared with available experimental data concerning other kinds of horizontal flows.

Hypersonic Boundary-Layer Stability with Chemical Reactions using PSE

Abstract: Linear stability of reacting flows in hypersonic boundary layers is studied via the parabolized stability equations (PSE) method. Three gas models are considered in the basic flow as well as stability calculations: perfect gas, chemical equilibrium and non-equilibrium (finite-rate chemistry). The finite-rate chemistry model for air contains five species, eight reactants and six reactions. The equilibrium calculation is performed by a table look-up procedure. Amplifying supersonic modes characterized by an oscillating disturbance structure outside the boundary-layer and a relative phase velocity faster than the free-stream sonic speed were found to appear in a Mach-20 flow past a 6° wedge when either equilibrium or finite-rate chemistry model is incorporated. These supersonic modes emerge just downstream of the unstable subsonic second-mode region and they propagate into the free-stream with a phase speed different from the corresponding acoustic wave and decay at a finite distance outside the boundary layer. The Rankine-Hugoniot (shock) conditions applied at the shock have very little effect on the supersonic modes studied here since the mode structure decays before the shock is reached. Due to the presence of supersonic modes, which are enhanced by the chemistry effect, the transition onset (based on N = 10) for the Mach-20 wedge flow is estimated to be at 14 ft, 24ft and 39 ft if one uses equilibrium, non-equilibrium and perfect gas models, respectively. It is therefore very important to account for the chemistry effect in future transition prediction for hypersonic vehicles.

On asymmetric gravity–capillary solitary waves

Abstract: Symme tric gravity–capillary solitary waves with decaying oscillatory tails are known to bifurcate from infinitesimal periodic waves at the minimum value of the phase speed where the group velocity is equal to the phase speed. In the small-amplitude limit, these solitary waves may be interpreted as envelope solitons with stationary crests and are described by the nonlinear Schrodinger (NLS) equation to leading order. In line with this interpretation, it would appear that one may also co nstruct asymmetric solitary waves by shifting the carrier oscillations relative to the envelope of a symmetric solitary wave. This possibility is examined here on the basis of the fifth-order Korteweg–de Vries (KdV) equation, a model for g ravity–capillary waves on water of finite depth when the Bond number is close to 1/3. Using techniques of exponential asymptotics beyond all orders of the NLS theory, it is shown that asymmetric solitary waves of the form suggested by the NLS theory in fact are not possible. On the other hand, an infinity of symmetric and asymmetric solitary-wave solution families comprising two or more NLS solitary wavepackets bifurcate at finite values of the amplitude parameter. The asymptotic results are consistent with numerical solutions of the fifth-order KdV equation. Moreover, the asymptotic theory suggests that such multi-packet gravity–capillary solitary waves also exist in the full water-wave problem near the minimum of t he phase speed.

A Manifestation of Negative Energy Waves in the Solar Atmosphere

Abstract: Magnetosonic modes of magnetic structures of the solar atmosphere in the presence of inhomogeneous steady flows are considered. It is shown that, when the speed of the steady flow exceeds the phase speed of one of the modes, the mode has negative energy, and can be subject to an over-stability due to the negative energy wave instabilities. It is shown that registered steady flows in the solar atmosphere, with speeds below the threshold of the Kelvin–Helmholtz instability, can provide the existence of the magnetosonic negative energy wave phenomena. In particular, in isolated photospheric magnetic flux tubes, there are kink surface modes with negative energy, produced by the external granulation downflows. Dissipative instability of these modes due to finite thermal conductivity and explosive instability due to nonlinear coupling of these modes with Alfven waves are discussed. For coronal loops, it is found that only very high-speed flows (>300 km s-1) can produce negative energy slow body modes. In solar wind flow structures, both slow and fast body modes have negative energy and are unstable.

Internal Generation of Waves for Time-Dependent Mild-Slope Equations

Abstract: The technique of generating waves internally is studied for two time-dependent mild-slope equations developed by Copeland (1985) and by Radder and Dingemans (1985) the velocity of disturbances caused by the incident wave is the phase velocity from the viewpoint of mass transport and it is the energy velocity from the viewpoint of energy transport. For Radder and Dingemans' equations. the desired energy of incident wave cannot be obtained from the viewpoint of mass transport from which the desired energy was successfully obtained by Larsen and Dancy (1983) in the Boussinesq equations and by Madsen and Larsen (1987) in Copeland's equations. However for all the two equations developed by Copeland and Radder and Dingemans. the desired energy of incident wave can be obtained from the viewpoint of energy transport. For horizontally two-dimensional case. an efficient technique of internal generation of multi-directional mono-chromatic waves is suggested.

CHRISTINE: A Multifrequency Parametric Simulation Code for Traveling Wave Tube Amplifiers.

Abstract: : A model and computer code are presented that simulate the operation of traveling wave tube amplifiers (TWTs). The model is based on the well known parametric theory in which the relevant properties of the interaction circuit are the phase velocity and coupling impedance of the waves supported by the slow wave structure. The model includes a multifrequency description of both the fields of the structure and the space charge fields. This allows for the study of harmonic and intermodulation distortion. The beam is treated as an ensemble of disks with an effective axial velocity spread. Several options are available for specifying the parameters of the interaction circuit: using a sheath helix description, importing data from another model, or using data from experimental measurement. The advantages of the code are that it can relatively quickly simulate situations in which the amplifier is driven by multiple input frequencies, it is readily portable to different platforms, and it facilitates tube design by enabling users to vary parameters relatively easily.

Numerical prediction of breaking waves and currents with a boussinesq model

Abstract: This paper describes the extension of a comprehensive numerical model for simulating the propagation and transformation of ocean waves in coastal regions and harbours to include wave breaking, runup and breaking-induced currents. The numerical model is based on a time-domain solution of a fully nonlinear set of Boussinesq-type equations for wave propagation in intermediate and shallow water depths. The equations are able to describe most of the phenomena of interest in the nearshore zone including shoaling, refraction, diffraction, reflection, wave directionality and nonlinear wave-wave interactions. The Boussinesq model is extended to the surf and swash zones by coupling the mass and momentum equations with a one-equation model for the temporal and spatial evolution of the turbulent kinetic energy produced by wave breaking. The waves are assumed to start breaking when the horizontal component of the orbital velocity at the wave crest exceeds the phase velocity of the waves. Numerical and experimental results are presented for the shoaling and runup of regular and irregular waves on a constant slope beach and wave-induced currents behind a detached breakwater.

The transversely isotropic poroelastic wave equation including the Biot and the squirt mechanisms: Theory and application

Abstract: The transversely isotropic poroelastic wave equation can be formulated to include the Biot and the squirt-flow mechanisms to yield a new analytical solution in terms of the elements of the squirt-flow tensor The new model gives estimates of the vertical and the horizontal per- meabilities, as well as other measurable rock and fluid properties In particular, the model estimates phase ve- locity and attenuation of waves traveling at different an- gles of incidence with respect to the principal axis of anisotropy The attenuation and dispersion of the fast quasi P-wave and the quasi SV-wave are related to the vertical and the horizontal permeabilities Modeling sug- gests that the attenuation of both the quasi P-wave and quasi SV-wave depend on the direction of permeability For frequencies from 500 to 4500 Hz, the quasi P-wave attenuation will be of maximum permeability To test the theory, interwell seismic waveforms, well logs, and hydraulic conductivity measurements (recorded in the fluvial Gypsy sandstone reservoir, Oklahoma) provide the material and fluid property parameters For exam- ple, the analysis of petrophysical data suggests that the vertical permeability (1 md ) is affected by the presence of mudstone and siltstone bodies, which are barriers to vertical fluid movement, and the horizontal permeabil- ity (1640 md) is controlled by cross-bedded and planar- laminated sandstones The theoretical dispersion curves based on measurable rock and fluid properties, and the phase velocity curve obtained from seismic signatures, give the ingredients to evaluate the model Theoreti- cal predictions show the influence of the permeability anisotropy on the dispersion of seismic waves These dispersion values derived from interwell seismic signa- tures are consistent with the theoretical model and with the direction of propagation of the seismic waves that travel parallel to the maximum permeability This analy- sis with the new analytical solution is the first step toward a quantitative evaluation of the preferential directions of fluid flow in reservoir formation containing hydrocar- bons The results of the present work may lead to the development of algorithms to extract the permeability anisotropy from attenuation and dispersion data ( de- rived from sonic logs and crosswell seismics) to map the fluid flow distribution in a reservoir

Low‐frequency inertia‐gravity waves in the stratosphere revealed by three‐week continuous observation with the MU radar

Abstract: Continuous observations over three weeks were performed with the MU radar at Shigaraki, Japan (35N, 136E) in April of 1995. By integrating radar echo spectra over 30 min after removing severe interferences, winds were successfully estimated offline up to 24 km in the lower stratosphere. It was found that inertia-gravity waves with a period of about 20 h and a vertical wavelength of 3.5 km are dominant near 22 km in a region of weak background winds. Four wave packets were fitted to plane waves by a least-squares method and wave parameters were estimated theoretically. The inertia-gravity waves propagate energy upward, and their horizontal phase velocity is westward in the range −10 to −20 m s−1. The waves have large momentum fluxes of about −0.002 Nm−2. This result suggests that inertia-gravity wave drag may contribute to the formation of easterlies in the summer stratosphere.

Frequency dependent effects in helicon plasmas

Abstract: Variations in the plasma parameters of a large volume, helicon source as a function of applied rf power (0–2 kW), driving frequency (8–18 MHz), magnetic field (0–1.4 kG) and fill pressure (2–10 mTorr) have been studied. The transitions between the capacitive, inductive, and resonant helicon mode are consistent with previous experiments. Our data indicate that the transition to the helicon mode occurs at a unique magnetic field, independent of the driving frequency. Based on the helicon wave dispersion relation, from which helicon wavelengths can be calculated, the observed variations in plasma density as a function of driving frequency suggest that the wavelength of the helicon wave is a weak function of driving frequency. Calculation of the electron energies which correspond to the phase velocity of the driving wave (i.e., Landau damping) suggest that either Landau damping cannot be responsible for the efficient ionization of helicon sources, or that the helicon portion of the discharge does not extend o...

Dynamic-equivalent medium approach for thinly layered saturated sediments

Abstract: The phase velocity and the attenuation coefficient of compressional seismic waves, propagating in poroelastic, fluid-saturated, laminated sediments, are computed analytically from first principles. The wavefield is found to be strongly affected by the medium heterogeneity. Impedance fluctuations lead to poroelastic scattering; variations of the layer compressibilities cause inter-layer flow (a 1-D macroscopic local flow). These effects result in significant attenuation and dispersion of the seismic wavefield, even in the surface seismic frequency range, 10–100 Hz. The various attenuation mechanisms are found to be approximately additive, dominated by inter-layer flow at very low frequencies. Elastic scattering is important over a broad frequency range from seismic to sonic frequencies. Biot's global flow (the relative displacement of solid frame and fluid) contributes mainly in the range of ultrasonic frequencies. From the seismic frequency range up to ultrasonic frequencies, attenuation due to heterogeneity is strongly enhanced compared to homogeneous Biot models. Simple analytical expressions for the P-wave phase velocity and attenuation coefficient are presented as functions of frequency and of statistical medium parameters (correlation lengths, variances). These results automatically include different asymptotic approximations, such as poroelastic Backus averaging in the quasi-static and the no-flow limits, geometrical optics, and intermediate frequency ranges.

Crust and upper mantle shear velocity structure beneath the Tibetan plateau and surrounding regions from interevent surface wave phase velocity inversion

Abstract: Average a priori shear velocity models are constructed for the Tien Shan, Tarim basin, Pamir-Hindu Kush, Himalaya and NE India. These models are shown to account for most of the lateral velocity heterogeneity at wavelengths >1000 km. Interevent fundamental mode Rayleigh and Love wave phase velocities were measured in the period range 32–200 s and were inverted for path-averaged shear velocity structures. These, in turn, were regionalized to estimate average, isotropic structures beneath the six regions listed above. Low upper mantle velocities are observed beneath the western and eastern Tien Shan. The Tarim basin structure is poorly constrained but exhibits very low upper crustal velocities (probably due to deep sedimentary accumulation), and either very high upper mantle velocities exist (>4.8 km/s) in a layer 70 km thick or the lithosphere is very thick (∼180 km). Low upper mantle velocities are observed beneath the Chang Thang region, the central, and the northeast plateau but not beneath the southern or southeastern plateau. The crustal velocity gradient with depth is zero in the east central plateau consistent with lower crustal basaltic intrusions there but is positive to the west and southeast of this zone. Upper mantle structures show that the Indian lithosphere could have subducted beneath the entire plateau only if either 1 mantle in the depth range 115–185 km has been altered (or removed) to reduce its average velocity or 2 the Indian lithosphere is only ∼85 km thick.

Observation and analysis of lower hybrid solitary structures as rotating eigenmodes

Abstract: Lower hybrid solitary structures (LHSS) are observed to be composed of wave modes rotating in the right-handed sense about the geomagnetic field. The data analyzed were measured at altitudes near 800 km in the auroral ionosphere by the plasma wave interferometer aboard the AMICIST rocket. Clear evidence of these modes is obtained by an estimation of the local frequency-wavenumber spectrum derived from wavelet analysis of the electric field. This evidence demonstrates that the phase velocity direction reverses as the payload traverses the structure, implying that the structure is composed of rotating electric fields. These observations are consistent with the result of three-dimensional numerical simulations investigating lower hybrid wave behavior in the presence of a density cavity. This suggests that the observed characteristics of LHSS may be explained by the excitation of localized lower hybrid eigenmodes and the scattering of background VLH hiss from plasma density depletions.

Hot-Electron Production and Wave Structure in a Helicon Plasma Source

Abstract: Energetic wave-trapped electrons of {approximately}20 eV, moving at the 13.56MHz helicon wave phase velocity, are directly measured with a 3ns response-time retarding-potential analyzer. The rf axial wavelength is measured with B-dot probes scanned axially. Changes in the axial magnetic field or the rf power cause order of magnitude changes in the energetic electron current. The current correlates with either a transition from partially propagating to nearly pure standing waves or quantized jumps in the number of half wavelengths between the two azimuthal antenna straps. {copyright} {ital 1997} {ital The American Physical Society}

Lamb wave excitation by Hertzian contacts with applications in NDE

Abstract: Excitation of Lamb waves in solid plates by point-like Hertzian contacts for material characterization and nondestructive testing is investigated. A 2 dimensional model using normal mode theory is used to predict the relative excitation efficiency of the lowest order Lamb waves in anisotropic solid plates. Hertzian contact transducers with PZT-5H piezoelectric material and quartz buffer rods are realized to operate in the 200 to 500 kHz range for experimental verification. Single mode operation with the lowest order antisymmetric Lamb wave (A/sub 0/) mode is achieved in various plates at in agreement with theoretical predictions. The technique is applied for material characterization on single crystal silicon samples and defect detection in composite plates. The phase velocity anisotropy of the A/sub 0/ mode is measured with signal-to-noise levels exceeding 65 dB. In (111) cut silicon plates the absolute phase velocity is measured with /spl plusmn/0.05% accuracy. The phase velocity anisotropy and effects of delamination in layered composite plates are calculated using the surface impedance approach. The experiments on graphite/epoxy composite plates agree with these calculations and show the potential of the method for defect detection with high resolution.

Modeling frequency-dependent GPR

Abstract: This article addresses the frequency dependence of ground penetrating radar (GPR) propagation due to dispersion caused by frequency dependent material properties. Dispersion is often used loosely to mean any frequency dependent phenomena. In this paper, dispersion only means frequency dependent phase velocity according to the strict, traditional definition.

Dynamics of viscous amphiphilic films supported by elastic solid substrates

Abstract: The dynamics of amphiphilic films deposited on a solid surface is analysed for the case in which shear oscillations of the solid surface are excited. The two cases of surface and bulk shear waves are studied with the film exposed to a gas or to a liquid. By solving the corresponding dispersion equation and the wave equation while maintaining the energy balance, we are able to connect the surface density and the shear viscosity of a fluid amphiphilic overlayer with the experimentally accessible damping coefficient, phase velocity, dissipation factor, and resonant frequency shifts of shear waves.