Wave Group Velocity Research Articles

Deep Velocity Structure of Southeast Asia from Rayleigh Wave Group Velocities: 3D Isotropic Model of the S-Wave Velocity Distribution in the Upper Mantle

Deep Velocity Structure of Southeast Asia from Rayleigh Wave Group Velocities: 3D Isotropic Model of the S-Wave Velocity Distribution in the Upper Mantle

Mapping lithospheric seismic structure beneath the Shillong plateau (India) and adjoining regions by jointly fitting receiver functions and surface wave dispersion

SUMMARYThe northeastern Indian region is characterized by complex lithospheric structure that developed due to collision between the Indian and Eurasian tectonic plates, in the north, and to subduction beneath the Burmese arc, in the east. We report results from joint modelling of Ps and Sp receiver functions and Rayleigh wave group velocity dispersion curves in which a broad search for acceptable models is performed via simulated annealing. We identify three tectonic domains, the Shillong plateau, Brahmaputra valley and Indo-Burma convergence zone (IBCZ), sampled by teleseismic earthquake data recorded by nine broad-band seismic stations. Our results reveal that the region's thinnest crust lies beneath the Shillong plateau, where it increases slightly from the plateau's eastern edge to its centre and reaches a maximum at the western edge of the plateau. Crustal Vp/Vs ratios range between 1.69 and 1.75 for the Shillong plateau, which is consistent with a felsic composition. Deeper Moho depths beneath the Brahmaputra valley, adjacent to the northern front of the Shillong plateau, may be due to the flexure of Indian lithosphere subducting beneath Asia. Low velocity zones are indicated at ∼5–10 km depth beneath the Brahmaputra valley, which may have been developed by NE–SW trending compressional stresses from the collision at the Himalayan arc and subduction at the Burmese arc. The crust is thickest in Kohima, beneath the Naga thrust in the IBCZ, where a high velocity zone is observed for both Vp and Vs at a depth of 25–40 km. This anomaly may be associated with a high velocity slab, trending N–NE to S–SW, that comprises the subducting Indian lithosphere in the IBCZ.

Direct Calculation of the Group Velocity for Two-Dimensional Complex, Composite and Periodic Structures Using a Wave and Finite Element Scheme

Guided waves have immense potential for structural health monitoring applications in numerous industries including aerospace. It is necessary to evaluate guided wave dispersion characteristics, i.e., group velocity and phase velocity profiles, for using them effectively. For complex structures, the profiles can have highly irregular shapes. In this work, a direct method for calculating the group velocity profiles for complex, composite, and periodic structures using a wave and finite element scheme is presented. The group velocity calculation technique is easy to implement, highly computationally efficient, and works with the standard finite element formulation. The major contribution is summarised in the form of a comprehensive algorithm for calculating the group velocity profiles. The method is compared with the existing analytical and numerical methods for calculation of dispersion curves. Finally, an experimental study in a multilayered composite plate is conducted and the results are found to be in good agreement. The technique is suitable to be used in all guided wave application areas such as material characterisation, non-destructive testing, and structural health monitoring.

Transversely isotropic lower crust of Variscan central Europe imaged by ambient noise tomography of the Bohemian Massif

Abstract. The recent development of ambient noise tomography, in combination with the increasing number of permanent seismic stations and dense networks of temporary stations operated during passive seismic experiments, provides a unique opportunity to build the first high-resolution 3-D shear wave velocity (vS) model of the entire crust of the Bohemian Massif (BM). This paper provides a regional-scale model of velocity distribution in the BM crust. The velocity model with a cell size of 22 km is built using a conventional two-step inversion approach from Rayleigh wave group velocity dispersion curves measured at more than 400 stations. The shear velocities within the upper crust of the BM are ∼0.2 km s−1 higher than those in its surroundings. The highest crustal velocities appear in its southern part, the Moldanubian unit. The Cadomian part of the region has a thinner crust, whereas the crust assembled, or tectonically transformed in the Variscan period, is thicker. The sharp Moho discontinuity preserves traces of its dynamic development expressed in remnants of Variscan subductions imprinted in bands of crustal thickening. A significant feature of the presented model is the velocity-drop interface (VDI) modelled in the lower part of the crust. We explain this feature by the anisotropic fabric of the lower crust, which is characterised as vertical transverse isotropy with the low velocity being the symmetry axis. The VDI is often interrupted around the boundaries of the crustal units, usually above locally increased velocities in the lowermost crust. Due to the north-west–south-east shortening of the crust and the late-Variscan strike-slip movements along the north-east–south-west oriented sutures preserved in the BM lithosphere, the anisotropic fabric of the lower crust was partly or fully erased along the boundaries of original microplates. These weakened zones accompanied by a velocity increase above the Moho (which indicate an emplacement of mantle rocks into the lower crust) can represent channels through which portions of subducted and later molten rocks have percolated upwards providing magma to subsequently form granitoid plutons.

Stress measurement for steel slender waveguides based on the nonlinear relation between guided wave group velocity and stress

The relation between guided wave group velocities and stresses is the key to stress evaluation for slender waveguides. In this study, an investigation of the nonlinear relation between guided wave group velocity and stress was carried out numerically and experimentally, and a stress measurement method based on the nonlinear relation was developed. The group velocity variation-frequency curve was numerically determined. The group velocity and stress are positively correlated in a particular frequency range and the fitted curves show high fitting rates with the quadratic curves. Besides, a numerical method for selecting the most sensitive excitation frequency was given. Loading tests and ultrasonic tests were conducted on four groups of steel slender specimens to obtain the acoustoelastic calibration curve of each specimen. The reliability of the calibration curves was verified and the results show the proposed method has high accuracy in the larger stress levels.

Specular Andreev reflection and its detection

We propose a universal method to detect the specular Andreev reflection taking the simple two dimensional Weyl nodal-line semimetal-superconductor double-junction structure as an example. The quasiclassical quantization conditions are established for the energy levels of bound states formed in the middle semimetal along a closed path. The establishment of the conditions is completely based on the intrinsic character of the specularly reflected hole which has the same sign relation of its wave vector and group velocity with the incident electron. This brings about the periodic oscillation of conductance with the length of the middle semimetal, which is lack for the retro-Andreev reflected hole having the opposite sign relation with the incident electron. The positions of the conductance peaks and the oscillation period can be precisely predicted by the quantization conditions. Our detection method is irrespective of the details of the materials, which may promote the experimental detection of and further researches on the specular Andreev reflection as well as its applications in superconducting electronics.

Delineation of Upper Crustal Structure Beneath the Island of Lombok, Indonesia, Using Ambient Seismic Noise Tomography

We have successfully conducted the first ambient noise tomography on the island of Lombok, Indonesia using local waveform data observed at 20 temporary stations. Ambient noise tomography was used to delineate the seismic velocity structure in the upper crust. The waveform data were recorded from August 3rd to September 9th, 2018, using short-period and broadband sensors. There are 185 Rayleigh waves retrieved from cross-correlating the vertical components of the seismograms. We used frequency-time analysis (FTAN) to acquire the interstation group velocity from the dispersion curves. Group velocity was obtained for the period range of 1 s to 6 s. The group velocity maps were generated using the subspace inversion method and Fast Marching Method (FMM) to trace ray-paths of the surface waves through a heterogeneous medium. To extract the shear wave velocity (Vs) from the Rayleigh wave group velocity maps, we utilize the Neighborhood Algorithm (NA) method. The 2-D tomographic maps provide good resolution in the center and eastern parts of Lombok. The tomograms show prominent features with a low shear velocity that appears up to 4 km depth beneath Rinjani Volcano, Northern Lombok, and Eastern Lombok. We suggest these low velocity anomalies are associated with Quaternary volcanic products, including the Holocene pyroclastic deposits of Samalas Volcano (the ancient Rinjani Volcano) which erupted in 1257. The northeast of Rinjani Volcano is characterized by higher Vs, and we suggest this may be due to the presence of igneous intrusive rock at depth.

Seismic ambient noise tomography to retrieve near-surface properties in soils with significant 3D lateral heterogeneity: the case of Quinta Colorada building in Chapultepec, Mexico

Structural failure due to local soil subsidence is a severe issue in the urban environment, particularly in cities located on thick sediments. We present such a case from Mexico City, where the near-surface soil characteristics beneath a subsided building were examined using 3D ambient noise tomography. We acquired data using dense arrays of 4.5 Hz vertical geophones placed around the building in two geometries. We performed cross-correlation analyses for 12 central frequencies in the range of 4–12 Hz to built tomographic images of surface wave group velocity. The results indicated a low-velocity layer (~ 120 m/s) for the first 10 m depth which we interpret as the typical clay layer of the Lake Zone of the city. Below, a more complex stiff layer (~ 500 m/s) appeared with significant lateral variations. In another independent analysis of HVSR, we observed that the site period varied between 0.4 and 0.55 s, which further signify the complexity of the subsurface. Thus, the conspicuous structural failure was due to the lack of stiffness of the shallow foundation, which subjected to differential motions between consistent materials from the foot of Chapultepec hill, and the soft lacustrine clays of the Lake Zone. As shown in this case, the seismic response variations in short distances, and short periods, for near-source events could be of great importance as demonstrated in Mexico City during the recent Puebla-Morelos Earthquake (M7.1) of September 19th, 2017.

Joint inversion of surface wave and gravity data reveals subbasin architecture of the Congo Basin

AbstractWe investigated the architecture of the greater Congo Basin, one of the largest and least-well-studied sedimentary basins on any continent. Seismograms from a large number of M > 4.5 earthquakes within and surrounding the African plate were used to make event-to-station Rayleigh wave group velocity measurements between periods of 5 and 100 s. Group velocities for discrete periods across the basin, obtained by inverting the event-station measurements, were jointly modeled with gravity data to obtain a three-dimensional S-wave and density model of the basin. The model corroborates the existence of two previously suggested subbasins, one to the north and one to the south, each ~8 km deep and separated by an east-west structural high. Our results favor a salt tectonics origin for the structural high but cannot rule out uplifted basement rock. The northern subbasin is offset to the west from the southern subbasin, consistent with previous studies suggesting sinistral motion along basement faults during periods of transpressional tectonics in late Neoproterozoic-early Paleozoic times.

Non-invasive assessment of urinary bladder compliance using ultrasound: first validation study based on clinical urodynamic study.

This study aims to validate a recently introduced non-invasive method, ultrasound bladder vibrometry (UBV), for the assessment of detrusor compliance in patients with neurogenic bladders. The study was carried out on 79 adult patients with neurogenic bladders (60 male and 19 female). The UBV test was performed on each patient to measure the Lamb wave group velocity (cg ) in the anterior bladder wall at every 50mL volume increment throughout the filling phase. Bladder compliance was assessed based on the trend of Lamb wave group velocity squared (cg 2 ) versus volume. A compliance index was defined to differentiate between the compliant and non-compliant bladders. Results of the UBV compliance assessment were validated using the readings of the corresponding urodynamic studies as the clinical gold standard. The Patients' bladders were divided into non-compliant and compliant groups by an experienced urologist using the information in the urodynamic study (UDS) recordings. The compliance index defined on the basis of cg 2 showed a significant difference (P<0.008) between the compliant and non-compliant groups. The areas under the receiver operating characteristic curve were 0.813, with 95% CI ranging from 0.709 to 0.892. Under the optimal criterion, the bladder was considered as non-compliant if the compliance index was less than 100 mL∙s2/m2, resulting in a sensitivity and specificity of 86.4% and 71.9%, respectively. The results of this study demonstrate that UBV can be used as a non-invasive method for the determination of bladder compliance; thus, it can potentially serve as an alternative method to UDS for the appropriate patient groups.

Mass–action equivalence with respect to Explicit Symmetry Breaking

The mass–action equivalence emerges naturally as the consequence of Relativistic Heisenberg Uncertainty (RHU). It changes the structure of relativistic energy–momentum equation that has been underpinning the chosen Lagrangian of the Standard Model, i.e., the Lagrangian which the Higgs mechanism works. The transition from the mass–energy to the mass–action equivalence zone occurs at a certain value of the particle’s wave group velocity $$v_{{\text{g}}}$$ indicating the crossing curve between relativistic energy and action. Due to this transition, particles in the Standard Model are not going to gain inertial mass coming from the Higgs mechanism and related Higgs field, but from another mechanism which can break the Lagrangian symmetry of the Standard Model. This mechanism corresponds to Explicit Symmetry Breaking by generating the new type of field, i.e., the X-Higgs-like field and works based on the mass–action equivalence. This field provides the mass for the X Matter as well as Higgs field for Ordinary Matter. The gaining mass of particles due to this mechanism is not related to the form of ordinary relativistic energy, but to the form of relativistic action. It leads us to the new dynamical equation instead of the ordinary Klein–Gordon and Dirac equation. There are two phenomena due to the increasing and decreasing of particle’s velocity in this mechanism. From the lower to the higher velocity, particle gains additional inertial mass from the X Matter, while conversely particle loses its inertial mass. It is reasonable for us to view that the missing inertial mass of particles in the Standard Model is compensated as the Dark Matter. The X-Higgs-like field breaking to be the Dark-Higgs-like and recognized Higgs field can be considered to explain the origin of the Dark and Ordinary Matter from the X Matter.

Distinct lithospheres in the Bay of Bengal inferred from ambient noise and earthquake tomography

We present an improved 3-D shear wave velocity image of the uppermost mantle beneath the Bay of Bengal (BoB), the Bengal basin and the adjoining Indian shield to their west. Shear wave velocities were inverted from fundamental mode Rayleigh wave group velocities calculated along 21,600 crisscrossing paths from cross correlation of ambient noise as well earthquake seismograms. This study shows the hitherto unsuspected existence of distinct lithospheres in the eastern and western Bay of Bengal, on either side of 86° E longitude, but understandable in terms of their different origins and ages. The western Bay of Bengal, with greater than 120 km thick layered lithosphere has a shear wave velocity of 4.7 km/s beyond the depth of 90 km. This velocity structure is in lateral continuation with the high velocity in the adjacent cratonic India. The lithosphere thickening can be explained to be the result of conductive cooling of an oceanic plate with a temperature of 1300 ± 50 °C at its base. The remarkable similarity between the velocity structure of the western BoB and the adjoining Indian craton, which could be fortuitous, however, suggests the untested possibility that the fracture plane mediating the initial break-up of India from Antarctica might have been inclined towards the former creating asymmetrically spreading oceanic crust over a cratonic upper mantle. The eastern Bay of Bengal (BoB), in contrast, has thinner lithosphere (60–75 km) with minimum velocity of ~4.2 km/s which is anomalously low for an old ocean. This significant thinning could have been caused by a number of factors such as reheating of the original lithosphere arising from a thermal boundary layer instability or westward flow of mantle due to slab rollback of subducting Indian lithosphere in the Andaman arc.

Imaging of a magma system beneath the Merapi Volcano complex, Indonesia, using ambient seismic noise tomography

SUMMARYMt Merapi, which lies just north of the city of Yogyakarta in Java, Indonesia, is one of the most active and dangerous volcanoes in the world. Thanks to its subduction zone setting, Mt Merapi is a stratovolcano, and rises to an elevation of 2968 m above sea level. It stands at the intersection of two volcanic lineaments, Ungaran–Telomoyo–Merbabu–Merapi (UTMM) and Lawu–Merapi–Sumbing–Sindoro–Slamet, which are oriented north–south and west–east, respectively. Although it has been the subject of many geophysical studies, Mt Merapi's underlying magmatic plumbing system is still not well understood. Here, we present the results of an ambient seismic noise tomography study, which comprise of a series of Rayleigh wave group velocity maps and a 3-D shear wave velocity model of the Merapi–Merbabu complex. A total of 10 months of continuous data (October 2013–July 2014) recorded by a network of 46 broad-band seismometers were used. We computed and stacked daily cross-correlations from every pair of simultaneously recording stations to obtain the corresponding inter-station empirical Green's functions. Surface wave dispersion information was extracted from the cross-correlations using the multiple filtering technique, which provided us with an estimate of Rayleigh wave group velocity as a function of period. The group velocity maps for periods 3–12 s were then inverted to obtain shear wave velocity structure using the neighbourhood algorithm. From these results, we observe a dominant high velocity anomaly underlying Mt Merapi and Mt Merbabu with a strike of 152°N, which we suggest is evidence of old lava dating from the UTMM double-chain volcanic arc which formed Merbabu and Old Merapi. We also identify a low velocity anomaly on the southwest flank of Merapi which we interpret to be an active magmatic intrusion.

Seismic interferometry ambient noise tomography—a study in the north-central part of Iran

Seismic interferometry is one of the noteworthy new subsurface imaging methods that have been used in recent years in many tomographic studies. In this study, the interstation estimated Green functions extracted from the Rayleigh and Love wave ambient noise cross-correlations are used to obtain the Love and Rayleigh wave group velocity maps in different periods across the study area in order to determine the velocity anomalies and their corresponding geological structures within the crust. In the ambient noise method, the quality of the cross-correlations increases when longer time series of data are used, so we used 22 months of continuous data from the 17 broadband seismic stations as the primary database. After performing the initial corrections of the ambient noise data and obtaining the 3-month cross-correlation time series, the longer time series were obtained by stacking the 3-month time series with acceptable quality. In the next step, by using the time-frequency analysis and the estimated Green functions, the Love and Rayleigh wave dispersion curves were determined. Then, using the 2D tomography method, the Love and Rayleigh wave group velocity maps for the periods 10–25 s were produced. In the short periods, a low-velocity zone is observed in the Love and Rayleigh wave group velocity maps from the south Caspian basin toward central Iran, which is likely due to the effect of the thick alluvial deposits and the intermountain sediments in Alborz. At the longer periods, the Rayleigh and Love wave group velocity maps show a low-velocity zone in central Alborz, while in the eastern and western parts of Alborz, they show higher velocities, which is consistent with the prior studies. The dense path coverage permits to produce images that have substantially higher lateral resolution than is currently available from global and regional group velocity studies. Tomographic maps at high frequencies are well correlated with the upper crust structure and especially with sediment layer thickness.

Upper Mantle Anisotropic Shear Velocity Structure at the Equatorial Mid‐Atlantic Ridge Constrained by Rayleigh Wave Group Velocity Analysis From the PI‐LAB Experiment

AbstractThe evolution of ocean lithosphere and asthenosphere are fundamental to plate tectonics, yet high resolution imaging is rare. We present shear wave velocity and azimuthal anisotropy models for the upper mantle from Rayleigh wave group velocities from local earthquake and ambient noise at 15–40‐s period recorded by the Passive Imaging of the Lithosphere Asthenosphere Boundary experiment at the equatorial Mid‐Atlantic Ridge covering 0–80‐Myr‐old seafloor. We find slow velocities along the ridge, with faster velocities beneath older seafloor. We image a fast lid (25–30‐km thick) beneath the ridge that increases to 50–60 km beneath older seafloor. Punctuated, ∼100 km wide low velocity anomalies exist off‐axis. There are multiple layers of azimuthal anisotropy, including (i) a lithosphere (20–40 km depth) characterized by strong anisotropy (4.0%–7.0 %) with fast‐axes that rotate from ridge subparallel toward the absolute plate motion/spreading direction at distances &gt;60 km from the ridge, and (ii) weak anisotropy (1.0%–2.0%) at &gt;40 km depth. Our results are consistent with conductive cooling of lithosphere, although with some complexities. Thickened lithosphere beneath the ridge supports lateral conductive cooling beneath slow‐spreading centers. Undulations in lithospheric thickness and slow asthenospheric velocities are consistent with small scale convection and/or partial melt. Lithospheric anisotropy can be explained by vertical flow and a contribution from either fluid or mineral filled cracks organized melt beneath the ridge and plate motion induced strain off axis. Deep azimuthal anisotropy is suggestive of upwelling beneath the ridge and three‐dimensional flow possibly caused by small scale convection off‐axis.

The Spiral Galaxies Flat Rotational Velocity Curve Explained by the Constant Group Velocity of a Nonlinear Density Wave

Abstract The rotation velocity curves of stars in galaxies, the motions of pairs of galaxies, and the behavior of galaxies in clusters and super-clusters all indicate that there is a lack of mass on different scales in the universe. In this paper, we derive the expression for rotational velocity using the nonlinear density wave theory considering only stellar components and we show that such theory can support the observed flat rotational velocity curve due to the main property of the soliton wave, which is a constant group velocity of the wave. The surface mass density (SMD) function, used to derive gravitational potential gradient and rotational velocity, is not assumed but rather derived as a solution of the nonlinear Srödinger equation, on the contrary to the widely used, in the literature, exponential disk approximation. Three parameters relevant to the curve shape are the intensities of equilibrium SMD, the amplitude of the wave, and total angular velocity or differential rotation, equivalently. Since the shape of the rotational velocity is highly sensitive to the mentioned parameters, this theory eventually provides a method for a very accurate estimation of galaxy mass and angular velocity as well.

The theoretical study on intermittency and propagation of geodesic acoustic mode in L-mode discharge near tokamak edge

Through a systematically developed theory, we demonstrate that the motion of Instanton identified in Zhang et al (2017 Phys. Plasmas 24 122304) is highly correlated to the intermittent excitation and propagation of geodesic acoustic mode (GAM) that is observed in tokamaks. While many numerical simulations have observed the phenomena, it is the first theory that reveals the physical mechanism behind GAM intermittent excitation and propagation. The preceding work is based on the micro-turbulence associated with toroidal ion temperature gradient mode, and slab-based phenomenological model of zonal flow. When full toroidal effect is introduced into the system, two branches of zonal flow emerge: the torus-modified low frequency zonal flow (TLFZF), and GAM, necessitating a unified exploration of GAM and TLFZF. Indeed, we observe that the transition from the Caviton to Instanton is triggered by a rapid zero-crossing of radial group velocity of drift wave and is found to be strongly correlated with the GAM onset. Many features peculiar to intermittent GAMs, observed in real machines, are thus identified in the numerical experiment. The results will be displayed in figures and in a movie; first for single central rational surface, and then with coupled multiple central rational surfaces. The periodic bursting first shown disappears as being replaced by irregular one, more similar to the intermittent characteristics observed in GAM experiments.

Seasonal Variation of Vertical Heat and Energy Fluxes due to Dissipating Gravity Waves in the Mesopause Region Over the Andes

AbstractVertical energy transports due to dissipating gravity waves in the mesopause region (85–100 km) are analyzed using over 400 h of observational data obtained from a narrow‐band sodium wind‐temperature lidar located at Andes Lidar Observatory (ALO), Cerro Pachón (30.25°S, 70.73°W), Chile. Sensible heat flux is directly estimated using measured temperature and vertical wind; energy flux is estimated from the vertical wavenumber and frequency spectra of temperature perturbations; and enthalpy flux is derived based on its relationship with sensible heat and energy fluxes. Sensible heat flux is mostly downward throughout the region. Enthalpy flux exhibits an annual oscillation with maximum downward transport in July above 90 km. The dominant feature of energy flux is the exponential decrease from 10−2 to 10−4 W m−2 with the altitude increases from 85 to 100 km and is larger during austral winter. The annual mean thermal diffusivity inferred from enthalpy flux decreases from 303 m2 s−1 at 85 km to minimum 221 m2 s−1 at 90 km then increases to 350 m2 s−1 at 99 km. Results also show that shorter period gravity waves tend to dissipate at higher altitudes and generate more heat transport. The averaged vertical group velocities for high, medium, and low frequency waves are 4.15 m s−1, 1.15 m s−1, and 0.70 m s−1, respectively. Gravity wave heat transport brings significant cooling in the mesopause region at an average cooling rate of 6.7 ± 1.1 K per day.

Low‐Velocity Zones and Negative Radial Anisotropy Beneath the Plume Perturbed Northwestern Deccan Volcanic Province

AbstractThe Deccan volcanic province (DVP) witnessed a massive outpouring of flood basalts of ∼2 million km3 volume, at ∼65 Ma, in less than a Myr. The volcanic eruption is concomitant with crustal extension, lithospheric thinning and magma influx beneath the major rift systems namely Cambay, Narmada, and Kutch. In this study, we investigate the anisotropic and isotropic variations within the crust and upper mantle beneath the northwestern DVP by estimating the shear wave velocity (VSV, VSH, and VSoigt) and radial anisotropy (ξoigt) models using the Surface Wave Tomography technique. A joint inversion of the regionalized Rayleigh and Love wave group velocities is performed, using the genetic algorithm approach. Our results reveal different intracrustal layers, lid, and a low‐velocity zone (LVZ). This LVZ comprises of a uniform asthenospheric low‐velocity layer (LVL) of average VSV 4.44 km/s and VSH 4.47 km/s, and another LVL below, of average VSV 4.45 km/s and VSH 4.41 km/s. Furthermore, the LVZ represents a negative anomaly with reference to different global models (AK135, STW105, PREM, and S2.9EA). A negative ξoigt is observed in the LVZ, indicating dominance of vertical flow. This could be related to presence of partials melts, volatile materials and/or a thermal anomaly. We also identified the Moho (∼34–40 km) and lithosphere‐asthenosphere boundary (∼84–123 km). The low VS values, negative ξoigt and a thin lithosphere (∼84 km) in the vicinity of Gulf of Cambay affirm the presence of a plume head beneath it, in concurrence with the hypothesis of Indian Plate‐Reunion plume interaction.

Characterizing blood clots using acoustic radiation force optical coherence elastography and ultrasound shear wave elastography

Thromboembolism in a cerebral blood vessel is associated with high morbidity and mortality. Mechanical thrombectomy (MT) is one of the emergenc proceduresperformed to remove emboli. However, the interventional approaches such as aspiration catheters or stent retriever are empirically selected. An inappropriate selection of surgical devices can influence the success rate during embolectomy, which can lead to an increase in brain damage. There has been growing interest in the study of clot composition and using a priori knowledge of clot composition to provide guidance for an appropriate treatment strategy for interventional physicians. Developing imaging tools which can allow interventionalists to understand clot composition could affect management and device strategy. In this study, we investigated how clots of different compositions can be characterized by using acoustic radiation force optical coherence elastography (ARF–OCE) and compared with ultrasound shear wave elastography (SWE). Five different clots compositions using human blood were fabricated into cylindrical forms from fibrin-rich (21% red blood cells, RBCs) to RBC-rich (95% RBCs). Using the ARF–OCE and SWE, we characterized the wave velocities measured in the time-domain. In addition, the semi-analytical finite element model was used to explore the relationship between the phase velocities with various frequency ranges and diameters of the clots. The study demonstrated that the wave group velocities generally decrease as RBC content increases in ARF–OCE and SWE. The correlation of the group velocities from the OCE and SWE methods represented a good agreement as RBC composition is larger than 39%. Using the phase velocity dispersion analysis applied to ARF–OCE data, we estimated the shear wave velocities decoupling the effects of the geometry and material properties of the clots. The study demonstrated that the composition of the clots can be characterized by elastographic methods using ARF–OCE and SWE, and OCE demonstrated better ability to discriminate between clots of different RBC compositions, compared to the ultrasound-based approach, especially in clots with low RBC compositions.