S-wave Velocity Variations Research Articles

Upper crustal structure across the Xiaojiang fault system revealed by ambient noise tomography from a dense short-period array

The Central Yunnan block (CYB) plays a key role in accommodating the crustal tectonic deformation of the southeastern Tibet, but its exact mechanism is poorly understood. Here we obtained a fine S-wave velocity structure of the upper crust across the Xiaojiang fault system in CYB, using the ambient noise tomography with a linear dense short-period array. The result shows a strong S-wave velocity variation along the profile, which is consistent with geomorphology and hypocenters occurred in this area. The velocity variations along the profile correspond well to the major faults seen on the surface and are consistent with the transition from surface erosion, to ductile basement and to rigid basement in the greater depth. Combining the upper crustal structure from our image with the location of hypocenters, we propose that most faults of Xiaojiang fault system cut through the upper crust steeply and control the tectonic deformation of the upper crust in the CYB.

Dispersion of Scholte wave under horizontally layered viscoelastic seabed

SUMMARY Dispersion inversion of Scholte wave is an effective method for constructing the shear wave velocity models of seabed sediments, but it is usually conducted based on the elastic layered medium theory, which ignores the viscoelasticity of sediments. In this work, we use the transitive matrix method to establish the dispersion equation for Scholte wave under horizontally layered viscoelastic seabed. This equation integrates the kinematic property of seismic wave in a viscoelastic media and the fluid–solid coupling mechanism. The phase velocity and attenuation coefficient dispersion curves of Scholte wave are presented by the real and imaginary parts of the complex-valued roots of the dispersion equation at different frequencies solved by Muller iteration algorithm, respectively. We perform numerical comparisons and analyses on the dispersion curves of Scholte waves for three typical seabed models under both elastic and viscoelastic conditions. Results demonstrate that the seabed viscoelasticity could greatly affect the propagation and dispersion characteristics of Scholte wave. Moreover, the dispersion curves of Scholte wave are sensitive to the variations in S-wave velocity and quality factor of seabed sediments.

Tomographic filtering of shear and compressional wave models reveals uncorrelated variations in the lowermost mantle

SUMMARY Seismic tomography models reveal differences in the geographic distribution and magnitude of P- and S-wave velocity variations (VP and VS, respectively) below ∼2200 km depth in the Earth’s mantle. In particular, large low shear velocity provinces (LLSVPs) beneath the Pacific and Africa exhibit a distinct low velocity population in the distribution of VS that does not stand out in VP models, carrying important implications for the origin of these features. However, it is possible that the absence of a distinct low velocity feature in VP models is an artefact of VP models having lower resolution compared to VS models owing to differences in coverage. Here, we use ‘tomographic filters’ computed from the singular value decomposition of the sensitivity matrices for a pair of VP and VS models in order to test whether such low velocity features are suppressed in VP models. Our ‘cross-filtered’ results show that resolution alone cannot explain the absence of a corresponding low VP population. We additionally apply the joint VP and VS tomographic filter technique to thermochemical mantle convection models to show that cases with distinct phase and/or composition may be differentiated from cases where only temperature varies. We then develop a new proxy for exploring uncorrelated VP and VS more broadly using the difference between the observed VP model and the filtered VS model input. Our results show that ‘large uncorrelated modulus provinces’ (LUMPs) extend beyond the boundaries of LLSVPs, and exhibit anomalies in both fast and slow regions.

Shear wave velocity structure and crustal lithology beneath the ultraslow spreading Southwest Indian Ridge at 50° E

SUMMARY Shear wave velocities provide an important constraint on crustal lithology. Limited crustal shear wave data are available from the ultraslow spreading mid-ocean ridges. We combine observations of both compressional (P) and shear (S) waves in ocean–bottom seismometer data from the Southwest Indian Ridge to determine crustal P-wave velocity (Vp), S-wave velocity (Vs), Vp/Vs and Poisson's ratio variations along the ridge at 49°17′E–50°49′E. Similar layered crustal structures were revealed beneath both the magmatically robust segment centres (Vp/Vs of 1.76–1.94, Poisson's ratio of 0.26–0.32) and the non-transform discontinuity (NTD) between them (Vp/Vs of 1.76–2.03 and Poisson's ratio of 0.26–0.34). Because laboratory measurements show an overlap in Poisson's ratio between mafic igneous and ultramafic rocks, particularly at Vp values typical of oceanic Layer 3, it can be difficult to distinguish crustal composition using this parameter only. However, our observed Vp gradients of 0.1 ± 0.1 s−1 suggest that in this area, oceanic Layer 3 consists primarily of mafic igneous rocks both at segment centres and at the NTD. Oceanic crustal Layers 2A and 2B above are likely also to consist of mafic igneous rocks, with some evidence for increased fracturing at the NTD.

Temperature-Dependent Wave Velocities of Heavy Oil-Saturated Rocks

Abstract Understanding the dependence of the rock properties on temperature is essential when dealing with heavy oil reservoirs. Reported rock physics models can hardly capture the effect of temperature on wave velocities. We propose a dual-porosity temperature-dependent model based on the coherent potential approximation, combining temperature- and frequency-dependent empirical equations for pore fluids with the David-Zimmerman model. The Maxwell model is adopted to obtain the complex shear modulus as a function of temperature and frequency. To verify the validity of the model, a glycerol-saturated tight sandstone and three heavy oil sand samples are considered. The comparison between the predicted and measured wave velocities shows that the model can quantitatively describe the behavior as a function of temperature. We find that there is a viscosity threshold (liquid point), where the P- and S-wave velocity variation trends change, while the porosity has no effect.

Spatial and temporal influence of rainfall on crustal pore pressure based on seismic velocity monitoring

Crustal pore pressure, which controls the activities of earthquakes and volcanoes, varies in response to rainfall. The status of pore pressure can be inferred from observed changes in seismic velocity. In this study, we investigate the response of crustal pore pressure to rainfall in southwestern Japan based on time series of seismic velocity derived from ambient noise seismic interferometry. To consider the heterogeneity of the area, rainfall and seismic velocity obtained at each location were directly compared. We used a band-pass filter to distinguish the rainfall variability from sea level and atmospheric pressure, and then calculated the cross-correlation between rainfall and variations in S-wave velocity (Vs). A mostly negative correlation between rainfall and Vs changes indicates groundwater recharge by rainfall, which increases pore pressure. The correlations differ between locations, where most of the observation stations with clear negative cross-correlations were located in areas of granite. On the other hand, we could not observe clear correlations in steep mountain areas, possibly because water flows through river without percolation. This finding suggests that geographical features contribute to the imprint of rainfall on deep formation pore pressure. We further modelled pore pressure change due to rainfall based on diffusion mechanism. A strong negative correlation between pore pressure estimated from rainfall and Vs indicates that the Vs variations are triggered by pore pressure diffusion in the deep formation. Our modelling results show a spatial variation of diffusion parameter which controls the pore pressure in deep formation. By linking the variations in seismic velocity and crustal pore pressure spatially, this study shows that seismic monitoring may be useful in evaluating earthquake triggering processes or volcanic activity.

Nodal Seismograph Recordings of the 2019 Ridgecrest Earthquake Sequence

Abstract The 2019 Ridgecrest, California, earthquake sequence included Mw 6.4 and 7.1 earthquakes that occurred on successive days beginning on 4 July 2019. These two largest earthquakes of the sequence occurred on orthogonal faults that ruptured the Earth’s surface. To better evaluate the 3D subsurface fault structure, (P- and S-wave) velocity, 3D and temporal variations in seismicity, and other important aspects of the earthquake sequence, we recorded aftershocks and ambient noise using up to 461 three-component nodal seismographs for about two months, beginning about one day after the Mw 7.1 mainshock. The ∼30,000Mw≥1 earthquakes that were recorded on the dense arrays provide an unusually large volume of data with which to evaluate the earthquake sequence. This report describes the recording arrays and is intended to provide metadata for researchers interested in evaluating various aspects of the 2019 Ridgecrest earthquake sequence using the nodal data set.

Joint Inversion of High-Frequency Receiver Functions and Surface-Wave Dispersion: Case Study in the Parnaíba Basin of Northeast Brazil

ABSTRACTWe assess the performance of the joint inversion of receiver functions (RF) and surface-wave dispersion in the characterization of the sedimentary package comprising the Parnaíba basin. This procedure is routinely utilized in passive-source crustal studies to retrieve S-wave velocity variations with depth, and has seldom been used with higher-frequency datasets to investigate fine sedimentary structure. The Parnaíba basin is a Paleozoic cratonic basin composed of five supersequences, accumulating ∼3.5 km of sedimentary rocks interbedded by Late Cretaceous diabase sills. The dataset used for this research was acquired between 2015 and 2017 through deployment of 10 short-period and one broadband seismic stations distributed along an approximately 100-kilometer-long linear array in the center of the basin. The deployment was carried out under the Parnaíba Basin Analysis Project, a multi-institutional and multidisciplinary effort funded by BP Energy do Brasil. High-frequency RFs (f<4.8 Hz) were calculated from deconvolution of teleseismic P waveforms (30°<Δ<90°) after rotation into the great-circle path, whereas high-frequency dispersion curves (0.25–2 Hz) were obtained through multiple filter analysis of empirical Green’s functions developed from cross-correlation (ZZ component) and stacking (six months) of time–frequency-normalized ambient seismic noise recordings. S-wave velocity–depth profiles down to ∼5 km depth were developed through an iterative, linearized joint inversion approach. Comparison to independent active-source seismic profiles overlapping with our passive-source seismic line reveals the inverted velocity models successfully retrieve sedimentary thickness (top of the Cambrian), sedimentary velocity structure, and depth to the Cenozoic sedimentary sequence. In addition, high-velocity zones at depths ranging from 1.5 to 2.5 km are observed in the inverted velocity–depth profiles, which are interpreted as due to the Late Cretaceous sills interbedding the basin’s sedimentary rocks. The relative low cost of our approach makes it ideal for basic characterization of relatively unknown sedimentary basins.

Crustal structure in the middle-southern segments of the Tanlu Fault Zone and adjacent regions constrained by multifrequency receiver function and surface wave data

We successively adopt H-k stacking of receiver functions (RFs) and joint inversion of RF and surface wave dispersion with Gaussian factors of 1.0, 2.0 and 3.0 to invert the crustal thickness (H), average Vp/Vs ratio (k) and S-wave velocity beneath 146 broadband seismic stations in the middle-southern segments of the Tanlu Fault Zone (TLFZ) and adjacent regions. These observations, together with previous geologic and geophysical studies, are further used to investigate Phanerozoic tectonic evolution in the study region and the seismogenic environment of the Tancheng Ms 8.5 earthquake. Our results show that the Moho depth and k generally range from 25 to 38 km and 1.65–1.95, respectively. The study region is roughly divided into three parts with different crustal structures from north to south, based on the Cretaceous Tiefogou fault and Triassic Lu'an fault and their extensional profiles, which may reflect different responses to the westward subduction of the Paleo-Pacific and Pacific Plates in the Mesozoic-Cenozoic. In addition to the observed uplifted Moho, high k and vertically alternating high-/low-velocity layers, lateral variations in S-wave velocity and junctions of multiple faults are imaged beneath the Tancheng earthquake zone, which may also contribute to the occurrence of strong earthquakes.

Elastic least-squares reverse-time migration with density variation for flaw imaging in heterogeneous structures

Ultrasonic techniques are able to accurately detect and characterize flaws in homogeneous structures. Elastic reverse time migration (ERTM) is a powerful tool to reconstruct high-resolution images of flaws. To achieve images with better quality, the solution can be obtained by iteratively finding an image generating the modeled data which can best match the measured data in a least-squares sense, i.e. least-squares migration (LSM). Combing ERTM and LSM, conventional elastic least-squares reverse time migration (ELSRTM) methods are based on the assumption of a constant density, which can lead to inaccurate amplitudes and parameter crosstalk artifacts in the reconstructed images. In this paper, an ultrasonic imaging technique based on the ELSRTM which considers density as well as longitudinal-(L-) and shear-wave (S-wave) velocity variations is explored for imaging flaws in heterogeneous structures. The ELSRTM with density variations can simultaneously reconstruct density and L- and S-wave velocity images, which can provide amplitude-preserving images and mitigate crosstalk artifacts. This method is applied to numerical as well as physical laboratory experiments and the results appear promising for flaw identification in heterogeneous structures.

Ultrasonic wave propagation characteristics for typical anisotropic failure modes of shale under uniaxial compression and real-time ultrasonic experiments

AbstractUniaxial loading and real-time ultrasonic experiments on shales were conducted to study the progressive failure of shale and the ultrasonic propagation characteristics during loading. The results show that the variations in P-wave and S-wave velocities with stress correspond to the crack damage process for the typical anisotropic failure of shale, and that the S-wave amplitude is sensitive to the damage that arises in shale during loading. If the bedding participates relatively simply in the failure (0° and 60° inclinations), the spectrum and the main frequency shape are simple and remain basically unchanged during loading. In the cases where the failure mode is complicated by the participation of the bedding and the matrix (30° and 90° inclinations), the spectrum and the main frequency shape are complex and change dynamically with loading. These results indicate that ultrasonic dynamic parameters reflect the differences in the failure of shale.

S-wave velocity structure of the upper mantle beneath the Arctic region from Rayleigh wave dispersion data

A new model of the upper mantle structure of the Arctic region based on a representative dataset of Rayleigh wave group velocity dispersion curves from 10 to 250 s period is presented. A surface wave tomography procedure is used to image the group velocity distributions. Local dispersion curves are constructed and inverted to vertical S-wave velocity sections up to the depth of 500 km. Results are presented in two forms: as horizontal slices of S-wave velocity variations relative to the average velocities at several mantle depths and as 2D velocity sections along three profiles crossing different tectonic units. The model shows lateral and vertical velocity variations over the whole depth range within the study area. The most prominent velocity heterogeneities are localized in the uppermost mantle up to the 200 km depth. In this depth range, the velocity anomalies are strongly correlated with the main tectonic structures. The calculated S-wave velocity distribution demonstrates that the upper mantle of the continental shields is characterized by high S-wave velocities and contains regional velocity anomalies associated with small-scale blocks. Low mantle velocities are attributed to the areas with high level of tectonic and seismic activity. Significant difference in deep velocity structure of the marginal seas and deep-water oceanic basins of various ages is revealed. The obtained model suggests some insights into the geodynamical evolution of the lithosphere of the Arctic region.

Prestack Stochastic Frequency-Dependent Velocity Inversion With Rock-Physics Constraints and Statistical Associated Hydrocarbon Attributes

Previous studies have demonstrated that P-wave velocity dispersion at seismic frequencies is often related to hydrocarbons, which results in frequency-dependent P-wave reflection coefficients. This effect is neglected in the conventional amplitude-versus-angle (AVA) inversion, or reduced in most AVA inversion involving seismic dispersion due to the linearization of either the forward modeling or the inversion objective function. As a consequence, there are times when nonnegligible error exists in the inverted dispersion-associated result, which is probably nonnegligible in some cases. In this letter, we adopt the propagator matrix forward modeling derived from the wave equation and the particle swarm optimization (PSO)-based inversion to solve the uncollapsed objective function to avoid any linearization operator at the cost of probably more computational complexity and inversion ill-posedness. To address the ill-posedness, in both the forward modeling and inversion, we introduce rock-physics constraints of frequency-independent S-wave velocity and limited variations of P-wave velocity with frequency. Furthermore, we statistically derive the average frequency-dependent P-wave velocity attribute, the P-wave velocity dispersion intensity attribute, and the characteristic frequency attribute corresponding to the maximum velocity dispersion gradient from multiple experimental inverted dispersive P-wave velocity results. These attributes can be directly applied to detect hydrocarbons. A synthetic data example and a real data example through a drilling well are used to demonstrate that PSO-based prestack stochastic inversion method with rock-physics constraints is effective, and that the statistical attributes derived from the multiple inverted dispersive P-wave velocities can be utilized to favorably indicate gas reservoirs.

Elastic least-squares reverse time migration with density variations

Elastic least-squares reverse time migration (ELSRTM) is a powerful tool to retrieve high-resolution subsurface images of the earth’s interior. By minimizing the differences between synthetic and observed data, ELSRTM can improve spatial resolution and reduce migration artifacts. However, conventional ELSRTM methods usually assume constant density models, which cause inaccurate amplitude performance in resulting images. To partially remedy this problem, we have developed a new ELSRTM method that considers P- and S-wave velocity and density variations. Our method can simultaneously obtain P- and S-wave velocity and density images with enhanced amplitude fidelity and suppressed parameter crosstalk. In addition, it can provide subsurface elastic impedance images by summing the inverted velocity images with the density image. Compared with the conventional ELSRTM method, our method can improve the quality of final images and provide more accurate reflectivity estimates. Numerical experiments on a horizontal reflector model and a Marmousi-II model demonstrate the effectiveness of this method.

Application of a complete workflow for 2D elastic full-waveform inversion to recorded shallow-seismic Rayleigh waves

The S-wave velocity of the shallow subsurface can be inferred from shallow-seismic Rayleigh waves. Traditionally, the dispersion curves of the Rayleigh waves are inverted to obtain the (local) S-wave velocity as a function of depth. Two-dimensional elastic full-waveform inversion (FWI) has the potential to also infer lateral variations. We have developed a novel workflow for the application of 2D elastic FWI to recorded surface waves. During the preprocessing, we apply a line-source simulation (spreading correction) and perform an a priori estimation of the attenuation of waves. The iterative multiscale 2D elastic FWI workflow consists of the preconditioning of the gradients in the vicinity of the sources and a source-wavelet correction. The misfit is defined by the least-squares norm of normalized wavefields. We apply our workflow to a field data set that has been acquired on a predominantly depth-dependent velocity structure, and we compare the reconstructed S-wave velocity model with the result obtained by a 1D inversion based on wavefield spectra (Fourier-Bessel expansion coefficients). The 2D S-wave velocity model obtained by FWI shows an overall depth dependency that agrees well with the 1D inversion result. Both models can explain the main characteristics of the recorded seismograms. The small lateral variations in S-wave velocity introduced by FWI additionally explain the lateral changes of the recorded Rayleigh waves. The comparison thus verifies the applicability of our 2D FWI workflow and confirms the potential of FWI to reconstruct shallow small-scale lateral changes of S-wave velocity.

Opportunities and pitfalls in surface-wave interpretation

Many explorationists think of surface waves as the most damaging noise in land seismic data. Thus, much effort is spent in designing geophone arrays and filtering methods that attenuate these noisy events. It is now becoming apparent that surface waves can be a valuable ally in characterizing the near-surface geology. This review aims to find out how the interpreter can exploit some of the many opportunities available in surface waves recorded in land seismic data. For example, the dispersion curves associated with surface waves can be inverted to give the S-wave velocity tomogram, the common-offset gathers can reveal the presence of near-surface faults or velocity anomalies, and back-scattered surface waves can be migrated to detect the location of near-surface faults. However, the main limitation of surface waves is that they are typically sensitive to S-wave velocity variations no deeper than approximately half to one-third the dominant wavelength. For many exploration surveys, this limits the depth of investigation to be no deeper than approximately 0.5–1.0 km.

Sensitivities of phase-velocity dispersion curves of surface waves due to high-velocity-layer and low-velocity-layer models

High-velocity-layer (HVL) and low-velocity-layer (LVL) models are two kinds of the most common irregular layered models in near-surface geophysical applications. When calculating dispersion curves of some extreme irregular models, current algorithms (e.g., Knopoff transfer matrix algorithm) should be modified. We computed the correct dispersion curves and analyzed their sensitivities due to several synthetic HVL and LVL models. The results show that phase-velocity dispersion curves of both Rayleigh and Love waves are sensitive to variations in S-wave velocity of an LVL, but insensitive to that of an HVL. In addition, they are both insensitive to those of layers beneath the HVL or LVL. With an increase in velocity contrast between the irregular layer and its neighboring layers, the sensitivity effects (high sensitivity for the LVL and low sensitivity for the HVL) will amplify. These characteristics may significantly influence the inversion stability, leading to an inverted result with a low level of confidence. To invert surface-wave phase velocities for a more accurate S-wave model with an HVL or LVL, priori knowledge may be required and an inversion algorithm should be treated with extra caution.

The time-lapse AVO difference inversion for changes in reservoir parameters

The result of conventional time-lapse seismic processing is the difference between the amplitude and the post-stack seismic data. Although stack processing can improve the signal-to-noise ratio (SNR) of seismic data, it also causes a considerable loss of important information about the amplitude changes and only gives the qualitative interpretation. To predict the changes in reservoir fluid more precisely and accurately, we also need the quantitative information of the reservoir. To achieve this aim, we develop the method of time-lapse AVO (amplitude versus offset) difference inversion. For the inversion of reservoir changes in elastic parameters, we apply the Gardner equation as the constraint and convert the three-parameter inversion of elastic parameter changes into a two-parameter inversion to make the inversion more stable. For the inversion of variations in the reservoir parameters, we infer the relation between the difference of the reflection coefficient and variations in the reservoir parameters, and then invert reservoir parameter changes directly. The results of the theoretical modeling computation and practical application show that our method can estimate the relative variations in reservoir density, P-wave and S-wave velocity, calculate reservoir changes in water saturation and effective pressure accurately, and then provide reference for the rational exploitation of the reservoir.

Utility of the horizontal-to-vertical spectral ratio passive seismic method for estimating thickness of Quaternary sediments in Minnesota and adjacent parts of Wisconsin

Quaternary glacial and fluvial sediments in Minnesota and adjacent areas are a major source for groundwater, and they are important to many geologic investigations. Unfortunately, the thicknesses of these widespread sediments are often poorly known. The horizontal-to-vertical-spectral ratio (HVSR) passive seismic method is being increasingly used in the region to assist in mapping the thickness of these sediments and the topography of the underlying bedrock surface. HVSR results when control sites with known sediment thickness are used to derive calibration curves that in turn are used to estimate sediment thicknesses in areas lacking control. The HVSR method must be used with appropriate caution. Extreme variations in S-wave velocities of near-surface materials can complicate or even negate the use of calibration curves, and very irregular bedrock surfaces can produce weak, multipeak HVSR spectra that are difficult to interpret. Over soft bedrock, HVSR peaks may reflect intrabedrock features, and velocity contrasts within the glacial sequence can overwhelm HVSR bedrock signatures. Nonetheless, the HVSR method has proven a useful tool to investigate Quaternary geology and depth to bedrock in areas of the state where subsurface data are limited. Thickness estimate errors are usually less than 25%, which is adequate for many geologic applications. The addition of multilocation spectral profiles and cross sections anchored with bedrock control points has allowed us to use data with poor signal quality and to achieve good continuity away from control stations. HVSR methods provide a suitable and cheaper alternative to conventional seismic studies, and they help to target sites for Quaternary drilling programs, bedrock topography evaluation, and determining glacial sediment thickness for 3D mapping. In areas where conventional seismic profiling may be necessary, the HVSR method is useful in selecting and prioritizing targets. The HVSR method should have widespread application in the glaciated areas of the north-central midcontinent.

CLUSTER ANALYSIS OF GEOLOGICAL AND GEOPHYSICAL PARAMETERS OF THE ARCTIC REGION AS THE BASE FOR GEODYNAMIC INTERPRETATION

Cluster analysis is applied for computing stable combinations of geological and geophysical parameters, and areas with such combinations are interpreted as regions that differ in structural and geodynamic features. The shelf areas are distinguished by specific sets and patterns of parameters, including sedimentary cover thickness, tectonic heterogeneity of the basement, heat flow, anomalous magnetic field, and gravity anomalies that reflect the topography of the crust–upper mantle boundary. In the deep oceanic areas, S-wave velocity variations show abnormally ‘cold’ blocks, while the average heat flow values are high. This combination of parameters is typical of transform zones at the junction of the Atlantic and Arctic segments. Superimposed thermal domes are located symmetrically with respect to the axis of the mid-oceanic ridges (MOR). Such domes may occur on the continents located close to MOR. Similar indicators can be revealed along the transition zone to the north of the East Siberian Sea.