Wave Phase Velocity Measurements Research Articles

Estimation of the Rod Velocity in Wood using Multi-frequency Guided Wave Measurements

This study presents a new approach for measuring the acoustic “rod velocity” in wood using guided wave measurements. The approach fits the acoustic guided wave longitudinal L(0,1) wave mode dispersion curve, through experimental guided wave phase velocity measurements taken over a range of frequencies. The rod velocity is obtained by measuring the phase velocity of the fitted L(0,1) wave mode dispersion curve at zero frequency. This technique is used to obtain rod velocity measurements for cylindrical wood and aluminium samples. The same approach was also performed on resonance measurements at a wide range of harmonics. These rod velocities are then compared to acoustic velocities obtained using the traditional time of flight and resonance methods.

Compressional wave phase velocity measurements during hydrate growth in partially and fully water saturated sandstone

The compressional wave phase velocity (cP) has been measured as a function of hydrate saturation (SH) during hydrate growth in Bentheim sandstone. The Fourier spectrum signal processing technique was used to specifically obtain the phase velocity at frequency 500 kHz. Eight experiments were conducted on eight Bentheim sandstone samples, having initial water saturation (Swi) in the range 0.51–1. Based on the measurements, it is discussed how Swi might affect the hydrate formation pattern during hydrate growth. For the sample having Swi = 0.51, a clear increase is observed in the measured cP from the beginning of the hydrate formation process. In the literature, micropore models ascribe such an increase to hydrates partly forming in soft, low aspect-ratio pores. For the samples having Swi ≥ 0.68, there is an initial stage in the hydrate formation process with little or no change in the measured cP. The SH interval defining this first stage seems to increase for increasing Swi. As SH further increase, a second stage follows where cP increases. This is explained with hydrates first forming as a hydrate-water slurry before eventually solidifying and taking part of the solid frame. The results aid in understanding how the elastic properties of hydrate-bearing porous rocks change with SH and Swi.

Crustal and Upper Mantle Shear Wave Velocity Structure of Botswana: The 3 April 2017 Central Botswana Earthquake Linked to the East African Rift System

AbstractRayleigh wave group and phase velocity measurements obtained from ambient noise and earthquake data at 51 broadband stations were used to construct the first 3‐D crustal and upper mantle shear wave velocity model of Botswana. The model shows low crustal velocities associated with the Passarge and Nosop sedimentary basins, whereas the Kaapvaal, Zimbabwe, Maltahohe, and Congo Cratons are recognized by high mantle velocities. The lowest upper mantle shear wave velocity, beneath northeastern Botswana, is associated with the southwestern branch of the East African Rift System. This low‐velocity mantle anomaly appears to be linked to the crust of the Okavango Rift Zone and the location of the 3 April 2017 6.5 earthquake in central Botswana. We suggest that fluids or melt at the base of the crust from the southward continuation of the East African Rift Zone triggered the intraplate earthquake in an extensional tectonic setting.

Surface Wave Tomography of the Alps Using Ambient‐Noise and Earthquake Phase Velocity Measurements

AbstractA large data set of surface wave phase velocity measurements is compiled to study the structures of the crust and upper mantle underneath the Alpine continental collision zone. Records from both ambient‐noise and earthquake‐based methods are combined to obtain a high‐resolution 3‐D model of seismic shear velocity. The applied techniques allow us to image the shallow crust and sedimentary basins with a lateral resolution of about 25 km. We find that complex lateral variations in Moho depth as mapped in our model are highly compatible with those obtained from receiver function studies; this agreement with entirely independent data is a strong indication of the reliability of our results, and we infer that our model has the potential to serve as reference crustal map of shear velocity in the Alpine region. Mantle structures show nearly vertical subducting lithospheric slabs of the European and Adriatic plates. Pronounced differences between the western, central, and eastern Alps provide indications of the respective geodynamic evolution: we propose that in the southwestern and northeastern Alps, the European slab has broken off. The complex anomaly pattern in the upper mantle may be explained by combination of remnant European slab and Adriatic subduction. Along‐strike changes in the upper mantle structure are observed beneath the Apennines with an attached Adriatic slab in the northern Apennines and a slab window in the central Apennines. There is also evidence for subduction of Adriatic lithosphere to the east beneath the Pannonian Basin and the Dinarides down to a maximum depth of about 150 km.

Solving large tomographic linear systems: size reduction and error estimation

We present a new approach to reduce a sparse, linear system of equations associated with tomographic inverse problems. We begin by making a modification to the commonly used compressed sparse-row format, whereby our format is tailored to the sparse structure of finite-frequency (volume) sensitivity kernels in seismic tomography. Next, we cluster the sparse matrix rows to divide a large matrix into smaller subsets representing ray paths that are geographically close. Singular value decomposition of each subset allows us to project the data onto a subspace associated with the largest eigenvalues of the subset. After projection we reject those data that have a signal-to-noise ratio (SNR) below a chosen threshold. Clustering in this way assures that the sparse nature of the system is minimally affected by the projection. Moreover, our approach allows for a precise estimation of the noise affecting the data while also giving us the ability to identify outliers. We illustrate the method by reducing large matrices computed for global tomographic systems with cross-correlation body wave delays, as well as with surface wave phase velocity anomalies. For a massive matrix computed for 3.7 million Rayleigh wave phase velocity measurements, imposing a threshold of 1 for the SNR, we condensed the matrix size from 1103 to 63 Gbyte. For a global data set of multiple-frequency P wave delays from 60 well-distributed deep earthquakes we obtain a reduction to 5.9 per cent. This type of reduction allows one to avoid loss of information due to underparametrizing models. Alternatively, if data have to be rejected to fit the system into computer memory, it assures that the most important data are preserved.

An upper-mantle S-wave velocity model for East Asia from Rayleigh wave tomography

We present a new shear velocity model of the upper mantle beneath the East Asia region derived by inverting Rayleigh wave group velocity measurements between 10 and 145 s combined with previously published Rayleigh wave phase velocity measurements between 150 and 250 s. Rayleigh wave group velocity dispersion curves along more than 9500 paths were measured and combined to produce 2D dispersion maps for 10–145 s periods. The group velocity maps benefit from the inclusion of new data recorded by the China National Seismic Network and surrounding global stations. The increase in available data has resulted in enhanced resolution compared with previously published group velocity maps; the horizontal resolution across the region is about 3° for the periods used in this study. The new shear-wave velocity models indicate varying velocity structure beneath eastern China, which yields estimates of a lithosphere–asthenosphere boundary depth from around 160 km beneath the Yangtze block to approximately 140 km beneath the western part of the North China Craton (NCC), up to depths of 70–100 km beneath the eastern NCC, Northeast China, and the Cathaysia block. The models reveal the subduction of two opposite-facing continental plates under the southern and northern margin of Tibet. An obvious low-velocity anomaly appears in the top 200 km of the upper mantle beneath northern Tibet, which is inconsistent with the presence of subducted Asian or Indian mantle lithosphere beneath northern Tibet. The Cenozoic volcanism fields in the Mongolian plateau are characterized by an obvious upper mantle negative anomaly, but no signature of deep-seated plume was observed.

S wave velocity structure of the Arabian Shield upper mantle from Rayleigh wave tomography

The shear wave velocity structure of the shallow upper mantle beneath the Arabian Shield was modeled by inverting Rayleigh wave phase velocity measurements between 45 and 140 s together with previously published Rayleigh wave group velocity measurements between 10 and 45 s. For measuring phase velocities, we applied a modified array method to data from several regional networks that minimizes the distortion of raypaths caused by lateral heterogeneity. The new shear wave velocity model shows a broad low‐velocity region to depths of ∼150 km in the mantle across the Shield and a narrower low‐velocity region at depths ≥150 km localized along the Red Sea coast and Makkah‐Madinah‐Nafud (MMN) volcanic line. The velocity reduction in the upper mantle corresponds to a temperature anomaly of ∼250–330 K. These findings, in particular the region of continuous low velocities along the Red Sea and MMN volcanic line, do not support interpretations for the origin of the Cenozoic plateau uplift and volcanism on the Shield invoking two separate plumes. When combined with images of the 410 and 660 km discontinuities, body wave tomographic models, a S wave polarization analysis, and SKS splitting results for the Arabian Peninsula, the anomalous upper mantle structure in our new velocity model can be attributed to an upwelling of warm mantle rock originating in the lower mantle under Africa that crosses through the mantle transition zone beneath Ethiopia and moves to the north and northwest under the eastern margin of the Red Sea and the Arabian Shield. In this interpretation, the difference in mean elevation between the Arabian Platform and Shield can be attributed to isostatic uplift caused by heating of the lithospheric mantle under the Shield, with the significantly higher elevations along the Red Sea coast possibly resulting also from lithospheric thinning and dynamic uplift.

A model of turbulence in magnetized plasmas: Implications for the dissipation range in the solar wind

This paper studies the turbulent cascade of magnetic energy in weakly collisional magnetized plasmas. A cascade model is presented, based on the assumptions of local nonlinear energy transfer in wave number space, critical balance between linear propagation and nonlinear interaction times, and the applicability of linear dissipation rates for the nonlinearly turbulent plasma. The model follows the nonlinear cascade of energy from the driving scale in the MHD regime, through the transition at the ion Larmor radius into the kinetic Alfvén wave regime, in which the turbulence is dissipated by kinetic processes. The turbulent fluctuations remain at frequencies below the ion cyclotron frequency due to the strong anisotropy of the turbulent fluctuations, k∥ ≪ k⊥ (implied by critical balance). In this limit, the turbulence is optimally described by gyrokinetics; it is shown that the gyrokinetic approximation is well satisfied for typical slow solar wind parameters. Wave phase velocity measurements are consistent with a kinetic Alfvén wave cascade and not the onset of ion cyclotron damping. The conditions under which the gyrokinetic cascade reaches the ion cyclotron frequency are established. Cascade model solutions imply that collisionless damping provides a natural explanation for the observed range of spectral indices in the dissipation range of the solar wind. The dissipation range spectrum is predicted to be an exponential fall off; the power‐law behavior apparent in observations may be an artifact of limited instrumental sensitivity. The cascade model is motivated by a program of gyrokinetic simulations of turbulence and particle heating in the solar wind.

Resolution potential of surface wave phase velocity measurements at small arrays

The deployment of temporary arrays of broadband seismological stations over dedicated targets is common practice. Measurement of surface wave phase velocity across a small array and its depth-inversion gives us information about the structure below the array which is complementary to the information obtained from body-wave analysis. The question is however: what do we actually measure when the array is much smaller than the wave length, and how does the measured phase velocity relates to the real structure below the array? We quantify this relationship by performing a series of numerical simulations of surface wave propagation in 3-D structures and by measuring the apparent phase velocity across the array on the synthetics. A principal conclusion is that heterogeneities located outside the array can map in a complex way onto the phase velocities measured by the array. In order to minimize this effect, it is necessary to have a large number of events and to average measurements from events well-distributed in backazimuth. A second observation is that the period of the wave has a remarkably small influence on the lateral resolution of the measurement, which is dominantly controlled by the size of the array. We analyse if the artefacts created by heterogeneities can be mistaken for azimuthal variations caused by anisotropy. We also show that if the amplitude of the surface waves can be measured precisely enough, phase velocities can be corrected and the artefacts which occur due to reflections and diffractions in 3-D structures greatly reduced.

Seabed property estimation from ambient-noise recordings: Part I — Compliance and Scholte wave phase-velocity measurements

The ability to derive a near-surface shear-velocity profile from ambient-noise records is useful for seismic applications such as shear-wave statics estimation and geohazard prediction. Measurements of seafloor compliance and Scholte wave velocity and amplitude are all related to the near-surface shear-velocity profile. I analyzed a data set of [Formula: see text] of continuous noise records recorded by an ocean bottom cable deployed in [Formula: see text] deep water for seafloor compliance and Scholte waves. I failed to observe seafloor compliance because of limitations in the record length. I have detected Scholte waves on the inline and vertical component geophones and Love waves on the crossline component using [Formula: see text] spectra. Both the Scholte and Love wave phase-velocities can be explained by a simple 1D isotropic near-surface model. The Scholte waves may have been excited by acoustic energy from the recording vessel, while no satisfactory excitation mechanism has been found for the Love waves.

Shear-Wave Structure of the South Indian Lithosphere from Rayleigh Wave Phase-Velocity Measurements

We investigate the upper mantle shear-wave speed structure beneath the south Indian shield by measuring and modeling fundamental mode Rayleigh wave phase-velocity dispersion. Observed phase velocities for the south Indian shield closely match those observed for the Canadian shield. We constrain the south Indian crust using published receiver function results and invert the dispersion data for upper mantle shear-wave structure. The ~155-km-thick seismic lithosphere of the south Indian shield is composed of a 35 km-thick, two-layer crust and a ~120-km-thick, high-velocity upper mantle lid. Beneath the Moho the average Sn wave speed is ~4.7 km sec–1. Both Sn travel times data and the dispersion data suggest a positive sub-Moho shear-wave speed gradient. Beneath the seismic lithosphere there is a low- velocity layer where the shear-wave speed drops to ~4.4 km sec–1.

Teleseismic surface wave study forS-wave velocity structure under an array: Southern California

SUMMARY Teleseismic surface wave data from the dense seismographic network in Southern California (CISN, California Integrated Seismic Network) are used to construct S-wave velocity maps for the region. Surface wave phase velocity measurements are achieved by way of a waveform-matching spectral-domain technique that was developed for use within the CISN array. A two-station phase velocity measurement technique was developed, which takes advantage of the fact that, for surface wave arrivals from any azimuth, there are multiple pairs of CISN stations, which are almost exactly on the same great-circle paths. To ensure the validity of this assumption, particle motions were analysed for measured pairs within the network, showing that the deviation between expected backazimuth from ray theory and observed backazimuths are small enough to be analysed by a simple distance correction. This method was applied to 114 large earthquakes for time periods between 1999 and 2002 and led to the retrieval of phase velocity maps for Rayleigh waves between 20 and 55 mHz and for Love waves between 25 and 45 mHz. Finite frequency effect for propagating surface waves was approximately taken into consideration using a geometrical technique. An ellipse, determined by wavelength and great-circle distance between the source and receiver can describe an influence zone for the finite frequency effect. Phase velocity map reconstructions, using a method which incorporates this effect, show minimal changes in phase velocity maps for our region, but do extend solvable velocity regions. Inversion for S-wave velocity distribution with depth shows that deep structural contrast across the San Andreas extends into the upper mantle. A slow velocity anomaly is evident under the Southern Sierras and the Salton Sea, the latter region indicating the northward extension of tectonic activity in the Gulf of California. Fast velocity roots that reach 60 km are found under the western part of Transverse Ranges and the Northern Peninsular Ranges. Reconstruction of past location of the Western Transverse Range microplate implies that these two ranges may have been adjacent to one another historically. Both the Western Transverse range fast velocity root and the Peninsular range fast velocity root may be a remnant of old oceanic plate.

Computation of Large Anisotropic Seismic Heterogeneities (CLASH)

SUMMARY A general tomographic technique is designed in order (i) to operate in anisotropic media; (ii) to account for the uneven seismic sampling and (iii) to handle massive data sets in a reasonable computing time. One modus operandi to compute a 3-D body wave velocity model relies on surface wave phase velocity measurements. An intermediate step, shared by other approaches, consists in translating, for each period of a given mode branch, the phase velocities integrated along ray paths into local velocity perturbations. To this end, we develop a method, which accounts for the azimuthal anisotropy in its comprehensive form. The weakly non- linear forward problem allows to use a conjugate gradient optimization. The Earth's surface is regularly discretized and the partial derivatives are assigned to the individual grid points. Possible lack of lateral resolution, due to the inescapable uneven ray path coverage, is taken into account through the a priori covariances on parameters with laterally variable correlation lengths. This method allows to efficiently separate the 2ψ and the 4ψ anisotropic effects from the isotropic perturbations. Fundamental mode and overtone phase velocity maps, derived with real Rayleigh wave data sets, are presented and compared with previous maps. The isotropic models concur well with the results of Trampert & Woodhouse. Large 4ψ heterogeneities are located in the tectonically active regions and over the continental lithospheres such as North America, Antarctica or Australia. At various periods, a significant 4ψ signature is correlated with the Hawaii hotspot track. Finally, concurring with the conclusions of Trampert & Woodhouse, our phase velocity maps show that Rayleigh wave data sets do need both 2ψ and 4ψ anisotropic terms.

Ionospheric erosion by Alfvén waves

Using observations from the FAST small explorer spacecraft, we present fields and plasma observations above the dayside auroral oval showing the erosion of ionospheric plasmas from the topside ionosphere by the action of Alfvén waves. Using interferometric techniques, the waves are shown to approximately obey the expected dispersion for Alfvén waves with transverse scales extending from greater than electron inertial lengths down to ion gyroradii. Measurements of the plasma density where these waves are observed show that over latitudinal widths exceeding 100 km total depletion of the cold ionospheric plasma can occur. These depleted regions or cavities are populated by magnetosheath plasmas, upgoing transversely accelerated ionospheric ions, and downgoing field‐aligned electrons. The ionospheric ions and field‐aligned electrons are distributed as conics and beams, respectively. Poynting flux observations on the density gradients comprising the cavity walls show that these waves are directed downward and focused inward toward regions of lower density. Wave phase velocity measurements, while subject to significant uncertainty, show that the wave vector is directed transversely outward from the cavity. These observations suggest a feedback model for Alfvén wave focusing and ion heating on density gradients that can lead to intense ion outflow from the ionosphere and subsequent depletion of ionospheric plasmas.

Rayleigh wave phase velocity measurements across the Philippine sea from a broad-band OBS array

SUMMARY We obtained phase velocities of fundamental Rayleigh waves in the northwestern Pacific ocean and the northern Philippine sea from seismograms retrieved from a long-term broad-band ocean bottom seismometer (LT-OBS) array deployed in 1999‐2000 and from those recorded in Japan, Guam and Ponape islands. This array observation was one of the first attempts to record continuous seismograms on the seafloor using broad-band seismographs on a long-term basis. The geographical distribution of the LT-OBS stations and the surrounding land stations enabled us to determine pure-path phase velocities in a frequency range from 0.01 to 0.05 Hz across the Mariana trough, the Parece Vela basin, the Shikoku basin and the Minami Daito basin, which were found to be systematically lower than those of the Pacific seafloor at comparable ages. This result, along with other geophysical and geological observations in the Philippine sea, may be explained by an uppermost mantle that is relatively Fe rich.

Anisotropic tomography of the Atlantic Ocean

Abstract We present the first regional three-dimensional model of the Atlantic Ocean with anisotropy. The model, derived from Rayleigh and Love wave phase velocity measurements, is defined from the Moho down to 300 km depth with a lateral resolution of about 500 km and is presented in terms of average isotropic S-wave velocity, azimuthal anisotropy and transverse isotropy. The cratons beneath North America, Brazil and Africa are clearly associated with fast S-wave velocity anomalies. The mid-Atlantic ridge (MAR) is a shallow structure in the north Atlantic corresponding to a negative velocity anomaly down to about 150 km depth. In contrast, the ridge negative signature is visible in the south Atlantic down to the deepest depth inverted, that is 300 km depth. This difference is probably related to the presence of hot-spots along or close to the ridge axis in the south Atlantic and may indicate a different mechanism for the ridge between the north and south Atlantic. Negative velocity anomalies are clearly associated with hot-spots from the surface down to at least 300 km depth, they are much broader than the supposed size of the hot-spots and seem to be connected along a north–south direction. Down to 100 km depth, a fast S-wave velocity anomaly is extenting from Africa into the Atlantic Ocean within the zone defined as the Africa superswell area. This result indicates that the hot material rising from below does not reach the surface in this area but may be pushing the lithosphere upward. In most parts of the Atlantic, the azimuthal anisotropy directions remain stable with increasing depth. Close to the ridge, the fast S-wave velocity direction is roughly parallel to the sea floor spreading direction. The hot-spot anisotropy signature is striking beneath Bermuda, Cape Verde and Fernando Noronha islands where the fast S-wave velocity direction seems to diverge radially from the hot-spots. The Atlantic average radial anisotropy is similar to that of the PREM model, that is positive down to about 220 km, but with slightly smaller amplitude and null deeper. Cratons have a lower than average radial anisotropy. As for the velocities, there is a difference between north and south Atlantic. Most hot-spots and the south-Atlantic ridge are associated with positive radial anisotropy perturbation whereas the north-Atlantic ridge corresponds to negative radial anisotropy perturbation.

New images of the Earth's upper mantle from measurements of surface wave phase velocity anomalies

The sensitivity of a surface wave to the lateral structure of the Earth's mantle depends on the local structure and thickness of the crust; particularly at short periods, the strong lateral heterogeneity of the crust has a significant nonlinear effect on the surface wave “sensitivity functions.” Thus far, this effect has not been quantified, and tomographic studies of the upper mantle based on surface wave observations have not taken it into account. On the basis of a Jeffreys‐Wentzel‐Kramers‐Brillouin (JWKB) description of surface wave propagation we find and discuss the lateral variations of the sensitivity of Love and Rayleigh waves to mantle seismic anomalies, associated with a realistic model of the crust [Mooney et al., 1998]. We then use the resulting laterally varying sensitivity kernels to determine a new tomographic image of horizontally and vertically polarized shear velocity in the upper mantle from Love and Rayleigh wave phase velocity measurements (periods from 35 to 300 s). In terms of large‐scale patterns of high and low velocity our model is similar to previously published ones, confirming, among other features, the anomalous anisotropy underlying the central Pacific [Ekström and Dziewonski, 1998]; we find, however, that the lateral dependence of the surface wave sensitivity kernels has a significant effect on the absolute amplitude of the anomalies, which should not be neglected as we attempt to obtain higher‐resolution images of the upper mantle.

Shear‐wave structure of the lithosphere above the Hawaiian Hot Spot from two‐station Rayleigh wave phase velocity measurements

We have measured fundamental mode Rayleigh wave phase velocity dispersion from seismograms of five earthquakes recorded at stations on the islands of Hawaii and Oahu and inverted these data for upper mantle velocity structure. The seismic lithosphere of the velocity model is 88±7 km thick, which is similar to that of 80–90 Myr oceanic lithosphere, indicating that no significant lithospheric thinning takes place above the mantle plume. Below the lithosphere the shear wave velocity decreases to ∼4.0 km s−1. The seismic model is consistent with the structure of the Hawaiian plume model of Watson and McKenzie [1991] which has a 72‐km thick mechanical boundary layer with the shallowest melting occurring at 82 km depth. The difference between the dispersion measured on the Hawaii‐Oahu path and that observed by Woods and Okal [1996] on the Oahu‐Midway path can be explained by the cooling of the injected hot plume material.

Measurement and modeling of dispersive pulse propagation in drawn wire waveguides

An analytical model of dispersive pulse propagation in semi-infinite cylinders due to transient axially symmetric end conditions has been experimentally investigated. Specifically, the dispersive propagation of the first axially symmetric longitudinal mode in thin wire waveguides, which have ends in butt contact with longitudinal piezoelectric ultrasonic transducers, is examined. The method allows for prediction of a propagated waveform given a measured source waveform, together with the material properties of the cylinder. Alternatively, the source waveform can be extracted from measurement of the propagated waveform. The material properties required for implementation of the pulse propagation model are determined using guided wave phase velocity measurements. Hard tempered aluminum 1100 and 304 stainless steel wires, with 127, 305, and 406 μm diam, were examined. In general, the drawn wires were found to behave as transversely isotropic media.

Comments on ‘The correction of great circular surface wave phase velocity measurements from the rotation and ellipticity of the Earth’ by F. A. Dahlen

Comments on ‘The correction of great circular surface wave phase velocity measurements from the rotation and ellipticity of the Earth’ by F. A. Dahlen