Variation Of Group Velocity Research Articles

Ambient noise surface wave tomography of the Iranian Plateau

Ambient noise tomography is used to retrieve Rayleigh wave group and phase velocity variations in the period range of 8–40 s based on the vertical component of cross-correlation functions from permanent broad-band and mid-band seismometers across the Iranian Plateau. The iterative, non-linear inversion method of fast marching surface tomography (FMST) is employed to produce 2-D group and phase velocity maps. Shear wave velocities are also estimated using a linear least-square method. Unlike most previous largescale tomographic results, our group, phase and shear wave velocity estimations, emphasize low velocity crustal structure (up to 50 km depth) beneath Zagros Fold and Thrust Belt (ZFTB) and Sanandaj-Sirjan metaphormic Zone (SSZ). The suture zone resulting from the subduction of the Arabian plate under the Central Iran is inferred along the boundary of SSZ and Urmieh-Dokhtar Magmatic Arc (UDMA). The velocity patterns show the main sedimentary basins, and reveal lateral velocity changes indicating the crustal thickening beneath ZFTB, SSZ and Lut Desert (LD), and the crustal thinning beneath Kavir Desert (KD) and UDMA are well inferred. A prominent low velocity is persistent in the whole crust beneath the central part of Alborz mountain range with high topography, and we suggest that it is likely due to elevated crustal temperatures within thin lithosphere.

The character of seafloor ambient noise recorded offshore New Zealand: Results from the MOANA ocean bottom seismic experiment

We analyze the characteristics of ambient noise recorded on ocean‐bottom seismographs using data from the 2009–2010 MOANA (Marine Observations of Anisotropy Near Aotearoa) seismic experiment deployed west and east of South Island, New Zealand. Microseism and infragravity noise peaks are clear on data recorded on the vertical channel of the seismometer and on the pressure sensor. The noise levels in the infragravity band (<0.03 Hz) on the horizontal seismometer channels are too high to show the infragravity peak. There is a small difference (∼0.25 Hz versus ∼0.2 Hz) in microseism peak frequencies between the two sides of the South Island on all three seismic channels. Our results show clear depth dependence between the peak frequency of infragravity waves and the water depth. We find that the product of water depth and wave number at the peak frequency is a constant,koH = 1.5. This relationship can be used to determine the variation of phase and group velocity of infragravity waves with water depth, and the location of the infragravity peak and corresponding noise notch at any water depth. These estimates of spectral characteristics, particularly low noise bands, are useful for future OBS deployments.

Wind-generated water waves in a wind tunnel: Free surface statistics, wind friction and mean air flow properties

This paper describes the systematic measurements of wind and water waves in a wind tunnel with a water tank inside. The velocity fields are measured using a 2-D Laser Doppler velocimetry (LDV) in air and the instantaneous water levels are measured using resistive twin-wire probes. The paper gives details on the free surface statistics and the mean airflow structures, including the statistics of the wind-generated water waves, phase velocity and group velocity of the waves, waves grouping, and interface friction. Comparisons are made between these results with those for wind flows over a fixed solid surface. The wind-generated waves show the typical growing trend with fetch and wind speed and are also asymmetric, with crests more pronounced than troughs. A model is developed to account for the relative variation of phase velocity and group velocity, which includes a dependence of the drift velocity from the wave steepness. The statistics of the wave groups suggest that a separate treatment of the envelopes of the crests and of the troughs should be necessary. The air flow boundary layer over the water waves shows a logarithmic profile with a wake near the free stream and the apparent roughness is related to the wave amplitude. The transition to turbulence occurs at an earlier stage with respect to the transition of a boundary layer over a smooth, flat rigid wall. The thickness of the boundary layer over the water waves grows much faster than for the boundary layer over a plain, smooth solid wall.

The Canadian Rockies and Alberta Network (CRANE): New Constraints on the Rockies and Western Canada Sedimentary Basin

Other| July 01, 2011 The Canadian Rockies and Alberta Network (CRANE): New Constraints on the Rockies and Western Canada Sedimentary Basin Yu Jeffrey Gu; Yu Jeffrey Gu Department of Physics University of Alberta Edmonton, Alberta T6G2G7 Canada ygu@ualberta.ca (Y. J. G.) Search for other works by this author on: GSW Google Scholar Ahmet Okeler; Ahmet Okeler Department of Physics University of Alberta Edmonton, Alberta T6G2G7 Canada ygu@ualberta.ca (Y. J. G.) Search for other works by this author on: GSW Google Scholar Luyi Shen; Luyi Shen Department of Physics University of Alberta Edmonton, Alberta T6G2G7 Canada ygu@ualberta.ca (Y. J. G.) Search for other works by this author on: GSW Google Scholar Sean Contenti Sean Contenti Department of Physics University of Alberta Edmonton, Alberta T6G2G7 Canada ygu@ualberta.ca (Y. J. G.) Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2011) 82 (4): 575–588. https://doi.org/10.1785/gssrl.82.4.575 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Yu Jeffrey Gu, Ahmet Okeler, Luyi Shen, Sean Contenti; The Canadian Rockies and Alberta Network (CRANE): New Constraints on the Rockies and Western Canada Sedimentary Basin. Seismological Research Letters 2011;; 82 (4): 575–588. doi: https://doi.org/10.1785/gssrl.82.4.575 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search Abstract The Canadian Rockies and the neighboring Alberta Basin mark the transition from the old North American continental lithosphere (east) to young accreted “terranes” (west). Geologic, seismic, and magnetic data in this region have suggested complex crustal domains, conductive anomalies, and major seismic velocity gradients in the mantle. However, the nature of the boundaries between the basement domains and their vertical extents remains controversial due to the lack of exposed geology and limited seismic and electromagnetic receivers. Since 2006, the seismic data coverage and depth sensitivity have received a major boost from the establishment of the Canadian Rockies and Alberta Network (nicknamed CRANE), the first semi-permanent broadband seismic array in Alberta. The availability of the array data provides vital constraints on the regional micro-earthquakes and crust/mantle seismic structures. Among the broad range of ongoing efforts, this study highlights promising results from the analyses of P-to-S wave receiver functions, shear wave splitting amplitudes/directions, and ambient seismic noise. Our preliminary receiver-function stacks show that the base of the crust gradually shallows from approximately 60 km beneath the Rockies near the Canadian-U.S. border to 37–40 km beneath central Alberta; the latter range is consistent with earlier findings from active-source experiments. Converted waves from “littered” crust and/or lithosphere have also been detected at a number of stations in the depth range of 80–130 km. Complexities in the lithosphere are further evidenced by our regional shear wave splitting measurements. We infer a strong east-west change of mantle flow pattern, consistent with present-day plate motion. The spatial distribution of the SKS fast orientations highlights the contrasting crust/mantle structures and histories between the Rockies and adjacent domains. Dynamic effects associated with a migrating continental root east of the province may be important. Finally, our preliminary inversions using ambient seismic noise indicate more than 0.8 km/s in peak-to-peak group velocity variations throughout the crust. The upper crust beneath the Alberta Basin is dominated by low Rayleigh-wave group velocities. A lower-than-expected correlation between seismic velocities and tectonic domain boundaries suggests significant tectonic overprinting in the southern Western Canada Sedimentary Basin. Overall, the broadband seismic data from CRANE could play a key role in uncovering the mysteries of the crust and mantle beneath the transition region between cratons and terranes. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

G-Type Seismic Wave in Magnetoelastic Monoclinic Layer

This paper deals with the study of propagation of G type waves along the plane surface at the interface of two different types of media. The upper medium is taken as monoclinic magnetoelastic layer whereas the lower half-space is inhomogeneous isotropic. Dispersion equation and condition for maximum energy flow near the surface are obtained in compact form. The dispersion equation is in assertion with the classical Love-type wave equation for the isotropic case. Effect of magnetic field and inhomogeneity on phase velocity and variation of group velocity with scaled wave number has been depicted by means of graphs. It is observed that inhomogenetity decreases phase velocity and the magnetic field has the favouring effect. A comparative study for the case of isotropic layer and monoclinic layer over the same isotropic inhomogeneous half space has been made through graphs.

Tight focusing of femtosecond elliptically polarised vortex light pulses

This paper studies the tight focusing properties of femtosecond elliptically polarised vortex light pulses. Based on Richards—Wolf vectorial diffraction integral, the expressions for the electric field, the velocity of the femtosecond light pulse and the total angular momentum of focused pluses are derived. The numerical calculations are also given to illustrate the intensity distribution, phase contour, the group velocity variation and the total angular momentum near the focus. It finds that near the focus the femtosecond elliptically polarised vortex light pulse can travel at various group speeds, that is, slower or faster than light speed in vacuum, depending on the numerical aperture of the focusing objective system. Moreover, it also studies the influence of the numerical aperture of the focusing objective and the time duration of the elliptically polarised vortex light pulse on the total angular momentum distribution in the focused field.

Complete Temporal Characterization of Asymmetric Pulse Compression in a Laser Wakefield

We present complete experimental characterization of the temporal shape of an intense ultrashort 200-TW laser pulse driving a laser wakefield. The phase of the pulse was uniquely measured by using (second-order) frequency-resolved optical gating. The pulses are asymmetrically compressed and exhibit a positive chirp consistent with the expected asymmetric self-phase-modulation due to photon acceleration or deceleration in a relativistic plasma wave. The measured pulse duration decreases linearly with increasing length and density of the plasma, in quantitative agreement with the intensity-dependent group velocity variation in the plasma wave.

Wideband slow light and dispersion control in oblique lattice photonic crystal waveguides

We find that the angle between elementary lattice vectors obviously affects the bandwidth and dispersion of slow light in photonic crystal line-defect waveguides. When the fluctuation of group index is strictly limited in a +/-1% range, the oblique lattice structures with the angle between elementary lattice vectors slightly larger than 60 degrees have broader available bandwidth of flat band slow light than triangular lattice structures. For example, for the angle 66 degrees , there are increases of the available bandwidth from 20% to 68% for several different structures. For the same angle and a +/-10% variation in group velocity, when group indices are nearly constants of 30, 48.5, 80 and 130, their corresponding bandwidths of flat band reach 20 nm, 11.8 nm, 7.3 nm and 3.9 nm around 1550 nm, respectively. The increasing of bandwidth is related to the shift of the anticrossing point towards smaller wave numbers.

The dispersion of shear wave in multilayered magnetoelastic self-reinforced media

In this paper, we study the propagation of shear waves in a magnetoelastic self-reinforced medium using finite difference technique. Dispersion equation has been deduced for the case when ( n − 1) layers lie over a half space. It is observed that the obtained dispersion equation is in assertion with the classical Love wave equation for both the cases when a single and double layer lies over a half space. The stability condition for the used finite difference scheme and the expression for the phase and group velocity have been derived. The dispersion curve for different values of magnetoelastic coupling parameter, phase and group velocity variation for different values of stability ratio has been depicted by means of graphs.

Model of the optical Stark effect in semiconductor quantum wells: Evidence for asymmetric dressed exciton bands

The influence of intense coherent $\ensuremath{\omega}$ and $2\ensuremath{\omega}$ laser beams on the electric properties of quantum wells is investigated. In the optical quantum-interference process, an asymmetric dressed band structure can be achieved in $k$ space. By adjusting the relative phase and the polarization direction of the $\ensuremath{\omega}$ and $2\ensuremath{\omega}$ laser beams, the light-induced shift and group velocity variation in electrons and spins can be tuned. The transport effects of asymmetric dressed carriers are studied. We find that if the pseudospin Hall conductance (p-SHC) is dominated by the optically induced band mixing, the p-SHC is nearly invariable with the relative phase of the laser beams. But if the p-SHC is caused by the disorder scattering effect, it is sensitive to the relative phase of the laser beams.

An upper-mantleS-wave velocity model for Northern Europe from Love and Rayleigh group velocities

SUMMARY A model of upper-mantle S-wave velocity and transverse anisotropy beneath northwestern Europe is presented, based on regional surface wave observations. Group velocities for both Love and Rayleigh surface waves are measured on waveform data from international and regional data archives (including temporary deployments) and then inverted for group velocity maps, using a method accounting for Fresnel zone sensitivity. The group velocity variations are larger than in global reference maps, and we are able to resolve unprecedented details. We then apply a linear inversion scheme to invert for local 1-D shear wave velocity profiles which are consequently assembled to a 3-D model. By choosing conservative regularization parameters in the 2-D inversion, we ensure the smoothness of the group velocity maps and hence of the resulting 3-D shear wave speed model. To account for the different tectonic regimes in the study region and investigate the sensitivity of the 1-D inversions to inaccuracies in crustal parameters, we analyse inversions with different reference models of increasing complexity (pure 1-D, 3-D crust/1-D mantle and pure 3-D). We find that all inverted models are very consistent at depths below 70 km. At shallower depths, the constraints put by the reference models, primarily Moho depth which we do not invert for, remain the main cause for uncertainty in our inversion. The final 3-D model shows large variations in S-wave velocity of up to ±12 per cent. We image an intriguing low-velocity anomaly in the depth range 70–150 km that extends from the Iceland plume beneath the North Atlantic and in a more than 400 km wide channel under Southern Scandinavia. Beneath Southern Norway, the negative perturbations are around 10 per cent with respect to ak135, and a shallowing of the anomaly is indicated which could be related to the sustained uplift of Southern Scandinavia in Neogene times. Furthermore, our upper-mantle model reveals good alignment to ancient plate boundaries and first-order crustal fronts around the triple junction of the Baltica-Avalonia–Laurentia collision in the North Sea.

Modified Gain and Mode Characteristics in Two-Dimensional Photonic Crystal Waveguide With Microcavity Structure

Optical gain spectra of InGaAsP MQW for photonic crystal waveguide (PCWG) were simulated by the method with considering the variation of group velocity and the natural broadening synthetically. The dependence of the first mode's gain maximum on the width of PCWG was discussed. To improve the mode characteristics and the gain performances in the 2-D PCWG with relative large width, we proposed a new structure by combining a microcavity inside the 2-D PCWG. Mode characteristics in proposed structure were analyzed and the transmission performance was simulated by the FDTD method. The simulation results show that improved longitudinal mode characteristics can be obtained even in the W3 PCWG with relative wide waveguide width because of the additional frequency selecting mechanism provided by the microcavity.

The group velocity variation of Lamb wave in fiber reinforced composite plate

Experimentally measured Lamb wave group velocities in composite materials with anisotropic characteristics are not the same as the theoretical group velocities which is calculated with the Lamb wave dispersion equation. This discrepancy arises from the fact that the angle between the group velocity direction and the phase velocity direction in anisotropic materials exists. Wave propagation in a composite material with anisotropic characteristics should be considered with respect to magnitude correction in addition to direction correction. In this study, S0 mode phase velocity dispersion curves are depicted with the variation of degree with respect to the fiber direction using a Lamb wave dispersion relation in the unidirectional, bidirectional, and quasi-isotropic composite plates. Slowness surface is sketched by the reciprocal value of the phase velocity curves. The magnitude and direction of the group velocity could be calculated from the slowness surface. The recalculated group velocities with consideration of the magnitude and direction from the slowness surface are compared with experimentally measured group velocities. The proposed method shows good agreements with theoretical and experimental results.

Superluminal propagation in plasma photonic band gap materials

Electromagnetic wave propagation in one-dimensional plasma photonic crystal (PPC) made of alternate thin layers of two materials namely plasma and dielectric materials is theoretically studied. Dispersion relation is obtained. Group velocity, effective group index, reflectivity and superluminal behavior have been studied. The study of the reflectivity plot shows that the designed structure works as a perfect reflector/mirror in a certain range of frequency. Left-right and up-down symmetries (and flip-flop) are observed when we study the variation of group velocity with frequency. Abnormal behavior of group velocity (negative) inside the PPC structure creates the superluminal propagation of electromagnetic waves. The marked existence of symmetries may be exploited for directional switching in optical circuits.

Group velocity measurement using spectral interference in near-field scanning optical microscopy

Near-field scanning optical microscopy provides a tool for studying the behavior of optical fields inside waveguides. In this experiment the authors measure directly the variation of group velocity between different modes of a planar slab waveguide as the modes propagate along the guide. The measurement is made using the spectral interference between pulses propagating inside the waveguide with different group velocities, collected using a near-field scanning optical microscope at different points down the guide and spectrally resolved. The results are compared to models of group velocities in simple guides.

Surface-wave group-delay and attenuation kernels

SUMMARY We derive both 3-D and 2-D Frechet sensitivity kernels for surface-wave group-delay and anelastic attenuation measurements. A finite-frequency group-delay exhibits 2-D off-ray sen- sitivity either to the local phase-velocity perturbation δc/c or to its dispersion ω(∂/∂ω)(δc/c) as well as to the local group-velocity perturbation δC/C. This dual dependence makes the ray- theoretical inversion of measured group delays for 2-D maps of δC/C a dubious procedure, unless the lateral variations in group velocity are extremely smooth.

Dispersion analysis of hollow-core modes in ultralarge-bandwidth all-silica Bragg fibers with nanosupports

Dispersion of the fundamental confined modes in hollow-core all-silica Bragg fibers with nanosupports is analyzed. The transfer-matrix formalism is applied. Anomalies in the group-velocity dispersion are evidenced at long wavelengths, toward the upper limit of the bandgap. The results confirm that, as in microstructured photonic crystal fibers, this anomalous dispersion is due to prevention of the confined hollow-core modes from crossing the surface modes, the avoided crossings are more apparent in the variation of group velocity with wavelength. The dependence of these avoided crossings on the hollow-core radius and the layer thicknesses is briefly analyzed.

Variation of group velocity and complete bandgaps in two-dimensional photonic crystals with drilling holes into the dielectric rods

Two-dimensional square lattices of square cross-section dielectric rods in air, designed with an air hole drilled into each square rod, are studied theoretically. By adjusting the shift of the hole position in the square rod in each unit cell, the dielectric distribution of the square rod will be modified. A sizable complete band gap occurs for certain structural parameters and exhibits very flat photonic bands near such gap edge, which resulting in a sharp peak of density of states. In addition, the zero or small group velocities are observed in a broad region of k-space. This structure can be fabricated with materials widely used today and opens a fascinating area for applications in optoelectric devices.

Lg Group Velocity: A Depth Discriminant Revisited

Lg group velocities are slower for very shallow events than for deeper crustal events in both southern California and Israel. An Lg group velocity depth discriminant applied to data below 1 Hz was proposed and then abandoned two decades ago, but a more precise measure of the energy distribution within Lg leads to the conclusion that higher frequency Lg group velocity may identify event depth. Because of its practical importance to nuclear monitoring and its implications for the generation and propagation of Lg , we quantify and model the delay of shallow-event Lg . We consider two mechanisms. First, shallower, slower modes are more strongly excited by shallow events, while deeper events excite faster modes. Second, Rg -to- Lg scattering for shallow events could introduce a time delay into Lg relative to Lg generated directly by the source. Further, since Rg propagates close to the surface, Rg scattering would also preferentially scatter into the shallower, slower modes. The depth dependence of modal excitation is therefore important for either mechanism, but Rg scattering could increase the time delays. Mode and finite-difference simulations demonstrate the feasibility of both hypotheses. We consider the implications of related observations for the source of the Lg delay, including predictions and observations of the frequency dependence of Lg delays. We conclude that Lg group velocity variation with event depth can be explained by preferential modal excitation and may have the potential to identify very shallow events and events too deep to be man-made. Practical application of the procedure will require regionalization to account for variations in Earth structure.

3D S-wave velocity pattern in the upper mantle beneath the continent of Asia from Rayleigh wave data

Group velocities of Rayleigh waves in the 10–150 s period range along about 1200 paths across Central Asia and Siberia are used to obtain group velocity maps of the region. To image the lateral variations of group velocities we used the tomography method developed earlier for spherical surface [PEPI 122 (2000) 19]. The method is supplemented by the opportunity to estimate the resolving power of the data. The locally averaged dispersion curves have been then inverted to vertical S-wave velocity profiles up to depth of 500 km at different sites in the region, which thus image a 3D velocity distribution. In order to reduce the number of unknown parameters we assumed the relationships between S-wave and P-wave velocities and densities to be those of the PREM model. The 3D S-wave velocity pattern is displayed in the form of maps at depths from 50 to 350 km with a step of 50 km, and of 2D vertical velocity sections along some profiles crossing different tectonic units. The most remarkable feature of the 3D velocity distribution is the presence of a high-velocity rise at the depths of 250–350 km beneath southern Siberia and western China, which borders upon the low-velocity asthenosphere along the longitude about 105°E. Collision of the high- and low-velocity substances results in pressing-out the viscous asthenosphere material in the weakened zone of the Baikal rift, where the lithosphere is thinned. The rise of the asthenosphere in this zone probably produces mantle upwelling, and leads to uplift of the Khangay Plateau.