Velocity Anisotropy Research Articles

Detect Multiple-Set Fractures by Crosshole Seismic Tomography for use in Environmental and Engineering Geophysics

Crosshole seismic tomography (CST) could be used to reconstruct the velocity section of media between two holes. One commonly used attribute of fractures reflected in seismic waves is the anisotropy of velocity. Therefore, in this study, the multiple symmetry axes of anisotropic media are utilized to simulate the multiple-set fractures in strata. The anisotropic algebraic reconstruction technique (AART) and anisotropic simultaneous iterative reconstruction technique (ASIRT), extensions of isotropic ART and SIRT, are developed to detect the medium with multiple symmetry axes and strong anisotropy. Full coverage of ray paths in azimuth is needed for these techniques. The performances of these techniques are tested by numerical and physical modeling. Testing results show that the proposed AART and ASIRT methods can successfully detect the multiple-set fractures when the observed data exceeds the estimated parameters. However, in complexly anisotropic and heterogeneous media, the estimation error will significantly increase when using these techniques if the number of estimated parameters is equal to or greater than the number of observed data.

On the Origin of the Variety of Velocity Dispersion Profiles of Galaxies

Observed and simulated galaxies exhibit a significant variation in their velocity dispersion profiles. We examine the inner and outer slopes of stellar velocity dispersion profiles using integral field spectroscopy data from two surveys, SAMI (for z < 0.115) and CALIFA (for z < 0.03), comparing them with results from two cosmological hydrodynamic simulations: Horizon-AGN (for z = 0.017) and NewHorizon (for z ≲ 1). The simulated galaxies closely reproduce the variety of velocity dispersion slopes and stellar mass dependence of both inner and outer radii (0.5 r 50 and 3 r 50) as observed, where r 50 stands for half-light radius. The inner slopes are mainly influenced by the relative radial distribution of the young and old stars formed in situ: a younger center shows a flatter inner profile. The presence of accreted (ex situ) stars has two effects on the velocity dispersion profiles. First, because they are more dispersed in spatial and velocity distributions compared to in situ formed stars, it increases the outer slope of the velocity dispersion profile. It also causes the velocity anisotropy to be more radial. More massive galaxies have a higher fraction of stars formed ex situ and hence show a higher slope in outer velocity dispersion profile and a higher degree of radial anisotropy. The diversity in the outer velocity dispersion profiles reflects the diverse assembly histories among galaxies.

Electron cyclotron maser instability by evolving fast electron beams in the flare loops

The electron cyclotron maser instability (ECMI) stands as a pivotal coherent radio emission mechanism widely implicated in various astrophysical phenomena. In the context of solar activity, ECMI is primarily instigated by energetic electrons generated during solar eruptions, notably flares. These electrons, upon leaving the acceleration region, traverse the solar atmosphere, forming fast electron beams (FEBs) along magnetic field lines. It is widely accepted that as these FEBs interact with the ambient plasma and magnetic fields, they give rise to radio and hard X-ray emission. Throughout their journey in the plasma, FEBs undergo modifications in their energy spectrum and velocity spatial distribution due to diverse energy loss mechanisms and changes in ambient plasma parameters. In this study, we delve into the impact of the evolving energy spectrum and velocity anisotropic distribution of FEBs on ECMI during their propagation in flare loops. Our findings indicate that if we solely consider the progressively flattened lower energy cutoff behavior as FEBs descend along flare loops, the growth rates of ECMI decrease accordingly. However, when accounting for the evolution of ambient magnetic plasma parameters, the growth rates of ECMI increase as FEBs delve into denser atmosphere. This underscores the significant influence of the energy spectrum and velocity anisotropy distribution evolution of FEBs on ECMI. Our study sheds light on a more comprehensive understanding of the dynamic spectra of solar radio emissions.

The Rotation of Classical Bulges in Barred Galaxies in the Presence of Gas

Barred galaxies often develop a box/peanut pseudobulge, but they can also host a nearly spherical classical bulge, which is known to gain rotation due to the bar. We aim to explore how the presence of gas impacts the rotation of classical bulges. We carried out a comprehensive set of hydrodynamical N-body simulations with different combinations of bulge masses and gas fractions. In these models, both massive bulges and high gas content tend to inhibit the formation of strong bars. For low-mass bulges, the resulting bar is stronger in cases of low gas content. In the stronger bar models, bulges acquire more angular momentum and thus display considerable rotational velocity. Such bulges also develop anisotropic velocity dispersions and become triaxial in shape. We found that the rotation of the bulge becomes less pronounced as the gas fraction is increased from 0 to 30%. These results indicate that the gas content has a significant effect on the dynamics of the classical bulge, because it influences bar strength. Particularly in the case of the low-mass bulges (10% bulge mass fraction), all of the measured rotational and structural properties of the classical bulge depend strongly and systematically on the gas content of the galaxy.

Shear-wave attenuation anisotropy: a new constraint on mantle melt near the Main Ethiopian Rift

The behaviour of fluids in preferentially aligned fractures plays an important role in a range of dynamic processes within the Earth. In the near-surface, understanding systems of fluid-filled fractures is crucial for applications such as geothermal energy production, monitoring CO2 storage sites, and exploration for metalliferous sub-volcanic brines. Mantle melting is a key geodynamic process, exerting control over its composition and dynamic processes. Upper mantle melting weakens the lithosphere, facilitating rifting and other surface expressions of tectonic processes.Aligned fluid-filled fractures are an efficient mechanism for seismic velocity anisotropy, requiring very low volume fractions, but such rock physics models also predict significant shear-wave attenuation anisotropy. In comparison, the attenuation anisotropy expected for crystal preferred orietation mechanisms is negligible or would only operate outside of the seismic frequency band.Here we demonstrate a new method for measuring shear-wave attenuation anisotropy, apply it to synthetic examples, and make the first measurements of SKS attenuation anisotropy using data recorded at the station FURI, in Ethiopia. At FURI we measure attenuation anisotropy where the fast shear-wave has been more attenuated than the slow shear-wave. This can be explained by the presence of aligned fluids, most probably melts, in the upper mantle using a poroelastic squirt flow model. Modelling of this result suggests that a 1% melt fraction, hosted in aligned fractures dipping ca. 40° that strike perpendicular to the Main Ethiopian Rift, is required to explain the observed attenuation anisotropy. This agrees with previous SKS shear-wave splitting analysis which suggested a 1% melt fraction beneath FURI. The interpreted fracture strike and dip, however, disagrees with previous work in the region which interprets sub-vertical melt inclusions aligned parallel to the Main Ethiopian Rift which only produce attenuation anisotropy where the slow shear-wave is more attenuated. These results show that attenuation anisotropy could be a useful tool for detecting mantle melt, and may offer strong constraints on the extent and orientation of melt inclusions which cannot be achieved from seismic velocity anisotropy alone.

Shear wave anisotropy response characteristics of laminated shale and its correction methods

Laminated shale exhibits strong anisotropic characteristics in horizontal wells, significantly affecting the application of shear wave travel time data. Using equivalent medium theory and the elastic wave equation, we constructed a physical model for laminated shale, simulating the impact of lamination angle and lamination density on shear wave velocity and shear wave anisotropy coefficient. We established the relationship between the shear wave anisotropy coefficient and the cosine of the lamination angle. The simulation results indicate that shear wave velocity decreases with an increase in lamination angle and density, while the shear wave anisotropy coefficient gradually decreases with an increase in lamination angle and initially increases and then decreases with an increase in lamination density. Moreover, it exhibits a power exponential relationship with the cosine of the lamination angle. After correction, the minimum horizontal principal stress and fracture pressure calculated from shear wave data show an average relative error reduction of 6.72% and 7.77%, respectively, compared to measured data. This demonstrates that the correction method effectively eliminates the impact of laminated shale anisotropy on shear waves.

A new highly accurate and efficient pure visco‐acoustic wave equation for tilted transversely isotropic attenuating media

AbstractThe propagation of seismic waves in attenuating anisotropic media exhibits amplitude dissipation and phase dispersion. To describe its effects, the fractional Laplacian pure visco‐acoustic wave equations capable of producing stable and noise‐free wavefields have been derived. However, except for acoustic approximation, previous wave equations utilize the approximations with lower accuracy in simplifying the denominator of the approximate complex‐valued dispersion relation, resulting in reduced accuracy. To address this concern, we use a combination of complex stiffness coefficients to replace the denominator term of the approximate complex‐valued dispersion relation. This approximation effectively reduces the loss of accuracy caused by ignoring the influence of the velocity anisotropy parameter ε and the attenuation anisotropy parameter εQ in the denominator term, leading to a wave equation with high accuracy in media with large anisotropic parameters ε and δ. In addition, the new wave equation only contains two high‐order spatial partial derivatives and has high computational efficiency. Theoretical analysis and numerical examples demonstrate that the proposed pure visco‐acoustic tilted transversely isotropic wave equation outperforms the previous pure visco‐acoustic wave equation in terms of simulation accuracy. The newly developed wave equation is well suited for the application of Q‐compensated reverse time migration and full waveform inversion in attenuating anisotropic media.

Evolution from Topological Nodal Points to Nodal Line: Realized in Fused Carbon Allotrope

Herein, via first‐principle calculations and theoretical analysis, a new Dirac semimetal carbon system called C32, which is composed of pentagonal, hexagonal, heptagonal, and octagonal carbon rings is systematically investigated. The stability of C32 is verified by calculating the phonon dispersion, elastic constants, etc., and an appropriate pathway for experimental synthesis is proposed. Besides, it is discovered that the system holds the quadruple rotation and inversion symmetry, resulting in the emergence of eight twisted Dirac cones (D1 and D2) with a highly anisotropic Fermi velocity from 3.83 × 105 to 8.96 × 105 m s−1 along different k directions. To substantiate the semimetallic nature of C32, its nontrivial topological properties are confirmed through the presence of topologically protected edge states, and the nonzero ℤ2 topological invariant. More importantly, by introducing biaxial strain, it is uncovered that Dirac cones can gradually evolve into a nodal line, and the perfect nodal line can be obtained when the biaxial strain is 13.3%. Furthermore, by constructing the tight‐binding model, the appearance of the Dirac cone is perfectly repeated and its evolution into the nodal line under biaxial strain is explained.

TriOS Schwarzschild Orbit Modeling: Robustness of Parameter Inference for Masses and Shapes of Triaxial Galaxies with Supermassive Black Holes

Evidence for the majority of the supermassive black holes in the local Universe has been obtained dynamically from stellar motions with the Schwarzschild orbit superposition method. However, there have been only a handful of studies using simulated data to examine the ability of this method to reliably recover known input black hole masses M BH and other galaxy parameters. Here, we conduct a comprehensive assessment of the reliability of the triaxial Schwarzschild method at simultaneously determining M BH, stellar mass-to-light ratio M*/L, dark matter mass, and three intrinsic triaxial shape parameters of simulated galaxies. For each of 25 rounds of mock observations using simulated stellar kinematics and the TriOS code, we derive best-fitting parameters and confidence intervals after a full search in the 6D parameter space with our likelihood-based model inference scheme. The two key mass parameters, M BH and M*/L, are recovered within the 68% confidence interval, and other parameters are recovered between the 68% and 95% confidence intervals. The spatially varying velocity anisotropy of the stellar orbits is also well recovered. We explore whether the goodness-of-fit measure used for galaxy model selection in our pipeline is biased by variable complexity across the 6D parameter space. In our tests, adding a penalty term to the likelihood measure either makes little difference, or worsens the recovery in some cases.

An anisotropic rock physics modeling for the coalbed methane reservoirs and its applications in anisotropy parameter prediction

The elastic properties and anisotropy of coalbed methane reservoirs are still poorly understood due to the complicated pore structure and the existing state of the coalbed methane. Therefore, an anisotropic rock physics model for the coalbed methane reservoirs is proposed based on the related effective medium theories, which focuses on the equivalent calculation of the elastic properties for adsorbed coalbed methane and the characterization of the multiple pore structure. Many factors related to the elastic properties of coal are considered, including the composition, pore structure, adsorbed gas content and pore fluid. The measured ultrasonic and well-logging data demonstrate that the proposed physics model is effective in predicting the elastic constants and velocity anisotropy parameters. The results suggest that the elastic constants increase with the increase of adsorbed gas content to some extent, and the effect of porosity on the elastic constants is much more obvious than that of adsorbed gas content. The velocity anisotropy parameters increase with the aligned fracture porosity and the fracture aspect ratio, and the P-wave anisotropy is more sensitive to these two factors. With increasing organic matter and clay content and porosity, Young's modulus and brittleness index decrease while the Poisson's ratio increases, and the impact of the porosity on the brittleness index is more significant than that of organic matter and clay content. The results are helpful in establishing the relationship between reservoir physics parameters and seismic response and can provide a basis for coalbed methane reservoir prediction and hydraulic fracturing.

Three-dimensional acoustic emission source localization method for layered rock considering anisotropic P-wave velocity

Three-dimensional acoustic emission source localization method for layered rock considering anisotropic P-wave velocity

Shear-wave splitting associated with fluid processes beneath Styra, South Euboea: First results

The intense and persistent seismic activity on the Island of Euboea (or Evia) was a unique opportunity to study anisotropy in a region without any notable prior seismicity. The study area is located in the central-western Aegean Sea and constitutes the endpoint where the North Aegean Trough and four parallel dextral strike-slip fault branches terminate against the Greek mainland, along with their sinistral counterparts. Even though seismicity has been recorded around the island, the affected area close to the town of Styra has no known reported earthquakes. In 2022, surprising activity was initiated by two moderate earthquakes (ML 4.8 and ML 5.0) that occurred on 29 November and were felt as far as Athens. The activity continued well into the second quarter of 2023, with over 1000 microearthquakes being recorded. A pre-planned nearby temporary installation of a broadband instrument (GR27), in the context of the AdriaArray project, offered recordings of the activity. Here, we investigated the occurrence and cause of shear-wave splitting beneath Styra. Based on focal mechanisms of local earthquakes we inferred the stress axes. A fully automated process was used to analyze data for shear-wave splitting and determine the polarization direction of the Sfast (φ), the time-delay (td) and the normalized time-delay (tn). A total of 272 acceptable results showcased a unimodal distribution of φ with an average of N68°E and a mean td of 80 ms. We observed temporal variations of splitting parameters, associated with outbursts of seismicity, not with individual events. The determination of the shear-wave velocity anisotropy, jointly with observations of other splitting parameters, led us to hypothesize that anisotropy is the result of along-fault fluid processes. However, this would require a convincing selection of the NE-SW nodal plane as the preferred fault orientation. Further enrichment of the catalogue and an increase in splitting results is required to draw more robust conclusions.

Heterogeneous mantle effects on the behaviour of SmKS waves and outermost core imaging

SUMMARY Seismic traveltime anomalies of waves that traverse the uppermost 100–200 km of the outer core have been interpreted as evidence of reduced seismic velocities (relative to radial reference models) just below the core–mantle boundary (CMB). These studies typically investigate differential traveltimes of SmKS waves, which propagate as P waves through the shallowest outer core and reflect from the underside of the CMB m times. The use of SmKS and S(m-1)KS differential traveltimes for core imaging are often assumed to suppress contributions from earthquake location errors and unknown and unmodelled seismic velocity heterogeneity in the mantle. The goal of this study is to understand the extent to which differential SmKS traveltimes are, in fact, affected by anomalous mantle structure, potentially including both velocity heterogeneity and anisotropy. Velocity variations affect not only a wave's traveltime, but also the path of a wave, which can be observed in deviations of the wave's incoming direction. Since radial velocity variations in the outer core will only minimally affect the wave path, in contrast to other potential effects, measuring the incoming direction of SmKS waves provides an additional diagnostic as to the origin of traveltime anomalies. Here we use arrays of seismometers to measure traveltime and direction anomalies of SmKS waves that sample the uppermost outer core. We form subarrays of EarthScope's regional Transportable Array stations, thus measuring local variations in traveltime and direction. We observe systematic lateral variations in both traveltime and incoming wave direction, which cannot be explained by changes to the radial seismic velocity profile of the outer core. Moreover, we find a correlation between incoming wave direction and traveltime anomaly, suggesting that observed traveltime anomalies may be caused, at least in part, by changes to the wave path and not solely by perturbations in outer core velocity. Modelling of 1-D ray and 3-D wave propagation in global 3-D tomographic models of mantle velocity anomalies match the trend of the observed traveltime anomalies. Overall, we demonstrate that observed SmKS traveltime anomalies may have a significant contribution from 3-D mantle structure, and not solely from outer core structure.

Crustal Deformation in Eastern Himalayan Syntaxis Constrained by Ambient Noise Tomography

ABSTRACT As the leading edge of the Indian–Asian collision, the eastern Himalayan syntaxis region has experienced extensive tectonic activities, resulting in complex crustal uplift and deformation in the corner area of the southeastern pathway for the extrusion of Tibetan plateau materials. Despite considerable efforts, the corresponding deformation mechanisms remain uncertain. This study presents a new 3D high-resolution azimuthal anisotropic shear-wave velocity model in the crust and uppermost mantle derived from ambient noise dispersion data. Results show that the upper crustal anisotropy aligns with the geological boundaries and major faults nearby, suggesting shape-preferred orientations. The upper crustal low velocity and weak anisotropy beneath the core of the eastern Himalayan syntaxis (EHS) are closely associated with the high fragmentation of shallow rocks and the upwelling of hot materials during the ongoing subduction of the Indian plate. Our model also reveals relatively complex anisotropic patterns in the midlower crust. The eastern Lhasa terrane, in particular, exhibits low velocity and strong anisotropy with a northwest–southeast-oriented fast axis, supporting the local scale midlower crustal “channel flow” model. In addition, a conspicuous, elongated low-velocity zone along the northwest–southeast direction is observed in the midlower crust and uppermost mantle beneath the Bangong–Nujiang suture. The anisotropy in this region increases with depth, and the fast directions are consistently parallel to the northeast subduction of the Indian plate. We infer that this low-velocity zone may result from partial melting under local compression driven by the Indian–Asian collision. On the basis of newly revealed anisotropic model and previous studies, we construct a new dynamic model, which reveals that the migration of mechanically weak material in the midlower crust and the significant contribution of the northeast subduction of the Indian plate jointly control the crustal deformation of the EHS region.

Anisotropic attenuation of GHz-frequency acoustic phonons and the Grüneisen tensor in MgO crystal

We report on measurements of the anisotropy of velocities and attenuation of GHz-frequency acoustic phonons in a cubic MgO crystal at room temperature. They are used to quantify the strong anisotropy of the Grüneisen parameter and calculate the attenuation anisotropy for Rayleigh surface acoustic waves. These observations constitute important building blocks for better understanding of ultrafast laser-based magneto-acoustic and nonlinear acoustic experiments at ultrahigh frequencies.

Phase diagram and physical properties anisotropy of strontianite

The physical properties and anisotropy of carbonates at high pressure are fundamental to understanding the deep carbon cycle and storage. Here, we redraw the phase diagram of strontianite (SrCO3) by first-principles calculations combined with lattice dynamics and study its physical properties and anisotropy at high pressure. The results show that there are only Pnma and Pmmn phases in SrCO3 in the entire Earth's mantle, and there is no R3m phase. The SrCO3 polymorphs have a large elastic anisotropy. SrCO3 polymorphs are a kind of carbonate with high density, low wave velocity, and low wave velocity anisotropy. The linear thermal expansion coefficient of SrCO3-Pmmn was obtained for the first time by using the linear elastic compression coefficient combined with the volume thermal expansion coefficient. The results are of great significance for understanding the high-pressure behavior of Sr-carbonates.

Data merging methods for S-wave velocity and azimuthal anisotropy from different regions

When inverting the S-wave velocity and azimuthal anisotropy from ambient noise data, it is always to obtain the partial overlapped inversion results in contiguous different regions. Merging different data to achieve a consistent model becomes an essential requirement. Based on the S-wave velocity and azimuthal anisotropy obtained from different contiguous regions, this paper introduces three kinds of methods for merging data. For data from different regions with partial overlapping areas, the merged results could be calculated by direct average weighting (DAW), linear dynamic weighting (LDW), and Gaussian function weighting (GFW), respectively. Data tests demonstrate that the LDW and GFW methods can effectively merge data by reasonably allocating data weights to capitalize on the data quality advantages in each zone. In particular, they can resolve the data smoothness at the boundaries of data areas, resulting in a consistent data model in larger regions. This paper presents the effective methods and valuable experiences that can be referred to as advancing data merging technology.

The Radial Orbits of Ram-pressure-stripped Galaxies in Clusters from the GASP Survey

We analyze a sample of 244 ram-pressure-stripped candidate galaxy members within the virial radius of 62 nearby clusters to determine their velocity anisotropy profile β(r). We use previously determined mass profiles for the 62 clusters to build an ensemble cluster by stacking the 62 cluster samples in projected phase space. We solve the Jeans equation for dynamical equilibrium by two methods, MAMPOSSt and the Jeans inversion technique, and determine β(r) both in parametric form and nonparametrically. The two methods consistently indicate that the orbits of the ram-pressure-stripped candidates are increasingly radial with distance from the cluster center, from almost isotropic (β ≃ 0) at the center, to very radial at the virial radius (β ≃ 0.7). The orbits of cluster galaxies undergoing ram pressure stripping are similar to those of spiral cluster galaxies but more radially elongated at large radii.

Decoupled anisotropy in the lower crust and upper mantle extracted by the Wave Gradiometry method resolves the differential deformation mechanisms beneath the northeastern Tibetan Plateau

The Tibet plateau is the ideal place to study how the lithospheres are deformed and how the materials move vertically and horizontally under the tectonic stress applied in an intracontinental collision region. To resolve the deformation and growth mechanism of the northeastern Tibetan Plateau, we constructed a 3-D azimuthally anisotropic shear-wave velocity (Vs) model by applying the recently developed Wave Gradiometry method based on the data recorded at 676 broadband seismic stations. Across the northeastern Tibetan Plateau, we found relatively weak low-velocity zones in the Qilian block and Bayan Har block at depths of 25 to 40 km, as well as decoupled anisotropies in the crust and mantle. In the northeastern Tibetan Plateau, the fast propagation directions (FPDs) in the mid-lower crust (20–60 km) are mostly NW-SE. Within the uppermost mantle (60–100 km), the FPDs beneath the Qilian block and the Qaidam block are nearly NE-SW, and the FPDs under the Bayan Har block are mostly in SE-NW. This result contradicts the lower crustal flow model, which expects the FPDs in the mid-lower crust to align with the assumed flow directions induced by topographic gradient (i.e. NWW-SEE beneath Bayan Har block and NE-SW beneath the Qilian block). On the contrary, our inverted 3D S-wave velocity and azimuthal anisotropy model could be well explained by pure shear deformation in the crust. That is, continuous pure shearing and shortening occur in the crust. Under the influence of asthenosphere material, the lithospheric mantle may move northeast. Crustal anisotropy is related to the strong shear deformation, whereas lithospheric mantle anisotropy is mainly due to the lattice-preferred orientation (LPO) of olivine associated with the mantle movement under the NE-SW compressional stress.

Deformation Behavior and Seismic Characteristics of Sandy Facies Opalinus Clay During Triaxial Deformation Under Dry and Wet Conditions

Unconsolidated, undrained triaxial deformation tests were performed on sandy facies Opalinus Clay at 50 MPa confining pressure to characterize the effect of water and microfabric orientation on the deformation behavior, mechanical properties, and P-wave velocity evolution. Dry and wet (≈ 8 and > 95% initial water saturation, respectively) samples with 12.6 ± 0.4 vol% porosity were deformed parallel and perpendicular to the bedding direction at a constant strain rate of 5 × 10–6 s−1. Dry samples revealed semi-brittle behavior and exhibited strain localization at failure, while deformation was more ductile at saturated conditions, promoting stable, slow faulting. Peak strength, Young’s modulus, and number of cumulative acoustic emissions decreased significantly for wet samples compared to dry samples; the opposite was observed for Poisson’s ratio. P-wave velocity anisotropy was significantly altered by differential stress, primarily due to the interplay between pore and fracture closure and stress-induced microcrack formation. For samples that were deformed perpendicular to bedding, we observed a reduction and reversal of P-wave velocity anisotropy with increasing differential stress, whereas anisotropy of parallel samples increased. The results suggest that water saturation reduces the pressure at the brittle-ductile transition and that the elastic properties and anisotropy of sandy facies Opalinus Clay can be significantly altered in an anisotropic stress field, e.g., adjacent to fault zones or tunnel excavations. Changes in elastic anisotropy are primarily controlled by the orientation between the pre-existing microfabric and the maximum principal stress direction, stress magnitude, and the degree of water saturation.