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Considering the anisotropy of the earth media is helpful in reducing the depth error between seismic and drilling and providing reliable imaging data for seismic interpretation and inversion. Transversely isotropic media with a vertical axis of symmetry are the most common type of anisotropic media and have been under constant study. The separation of the P‐ and S‐wavefields, which affects the accuracy of elastic wave imaging and the inversion in transversely isotropic media with a vertical axis of symmetry, is a hot research topic. Among the commonly used wavefield decoupling methods, the S‐wave is typically obtained by subtracting the P‐wave from the total wavefield. However, this kind of wavefield decoupling method often leads to severe P‐wave crosstalk in the separated S‐wavefield; thus, it needs further development. In this paper, the principle that the divergence of the S‐wave is zero is employed to solve the unresolved S‐wave elastic parameters of the S‐wave stiffness matrix by utilizing the modified zero‐order pseudo‐Helmholtz decomposition operator. The obtained S‐wave elastic parameters are employed to construct the qS‐wave stress and facilitate the derivation of the relationship between the qS‐wave particle velocity and the qS‐wave stress. Furthermore, a qS‐wave decoupling first‐order velocity–stress equation, which matches the qP‐wave decoupling first‐order velocity–stress equation, is derived. By jointly using the decoupling equations of qS‐ and qP‐wave, the separation of the P‐ and S‐wavefields in transversely isotropic media with a vertical axis of symmetry is realized. The efficiency of the proposed method is demonstrated via numerical tests conducted to assess the wavefield separation. Moreover, a complex model is employed to perform elastic reverse‐time migration, resulting in the acquisition of accurate imaging results for PP, PS, SP, and SS waves, without the presence of significant artefacts. The correctness of the qS‐wave decoupling equation in transversely isotropic media with a vertical axis of symmetry is confirmed by the comparative tests using decoupling methods in isotropic media. [ABSTRACT FROM AUTHOR]

As one of wavefield separation methods in elastic reverse time migration, the decoupled wave equation succeeds to separate the particle velocity of P‐ and S‐wave without mode crosstalk. However, it fails to decouple the stress of P‐ and S‐wave which encounters with the problem of crosstalk. To overcome the crosstalk, the paper has proposed the quasi‐stress equations to decouple the stress tensor wavefields of P‐ and S‐wave based on the decoupled stress–strain relationship. Because P‐ and S‐wave stress wavefields are second‐order tensors, different from the imaging method of vector decoupled particle velocity wavefields, a double dot product imaging condition is proposed to obtain scalar images. Based on the total source‐stress tensor wavefields and separated receiver‐stress tensor wavefields, we obtain the scalar images of quasi‐PP and quasi‐PS from the stress information by the double dot product imaging condition. Several numerical examples are evaluated to demonstrate the effectiveness of the separation of P‐ and S‐wave stress and advantage of the proposed imaging condition from the second‐order stress tensor wavefields. [ABSTRACT FROM AUTHOR]

Two adjacent groundwater wells on the North China Platform are used to study how earthquakes impacted aquifers. We use the response of water level to solid Earth tides to document changes after earthquakes and how aquifer and fracture properties recovered to pre‐earthquake properties. We consider two models for the phase and amplitude of water level response to the lunar diurnal (O1) and semidiurnal (M2) tides: a leaky aquifer model, and a model in which fracture orientation determines the response. In the leaky aquifer model, changes arise from changes in permeability and storage; in the fracture model, changes are due to changes in apparent orientation of transmissive fractures. Responses in one well are best explained by the leaky aquifer model, and can explain the large amplitude coseismic water level and permeability changes and the non‐recoverable changes after the largest earthquake. Responses in the other well are consistent with the fracture model and show little coseismic change in water level but changes in apparent fracture orientation. Larger ground motions lead to larger coseismic water level changes and longer recovery times. We propose that the well in the more permeable and shallow aquifer has less variable pore‐pressures around the well. Larger coseismic strains from water level changes may enable longer‐lasting changes in aquifer properties. We conclude that relatively high permeability aquifers are less susceptible to impacts from seismic waves, and thus have small changes in water levels and hydrogeological properties. Plain Language Summary: After large earthquakes, the properties of aquifers can change, leading to changes in water levels in wells. We study changes in water levels that we attribute to changes in the properties of fractures or the permeabilities of aquifers that connect to the wells. We do so by monitoring how deformation caused by tides leads to changes in water levels in two wells. The deeper well in a less permeable formation has larger changes. The larger the ground motion from earthquakes and the larger the amplitude of the coseismic water level changes, the more likely and larger the changes in permeabilities. The larger the changes, the slower the recovery to pre‐earthquake properties. The shallower well in a more permeable formation has smaller changes and no long‐term changes in water level and hydrogeological properties, which we attribute to more uniform pore‐pressures around the well. Key Points: high permeability aquifers are less susceptible to disruption by seismic waveshydraulic property changes depend on the amplitude and frequency of ground motionlarger water level changes accompany larger changes in fracture orientation and irreversible changes in aquifer properties [ABSTRACT FROM AUTHOR]

Tidal and barometric water‐level responses in wells have been widely used to calculate the hydraulic properties of aquifer systems. The effect of anisotropy induced by shale content on such responses has not received significant attention. In this study, we examine how the presence of shale (anisotropy, extremely low porosity/permeability; approximately 10−4 to 10−3 mD), which occurs as interlayers in aquifers, affects the tidal responses of the leaky model. Our findings show that the number of wells with shale in the screened section of the study aquifer is limited. Here, we focus on the study of the limited number of wells in the North China Platform to ensure a similar geological background for each well. Calculations indicate that wells, even with a small amount of shale (≥∼5%) in the observation aquifer, may exhibit strong anisotropy and heterogeneity, deviating from the theoretical analytical solutions obtained using isotropic and homogeneous assumption models. Generally, the higher the shale content in aquifer lithology, the greater the phase shifts deviated. Thus, theoretical ideal models with isotropic and homogeneous assumptions may only obtain a rough estimation for wells with shale in aquifer lithology, suggesting that we avoid setting observation aquifers onto shale layers. Plain Language Summary: As special lithology, shale is fragile, compact, and anisotropic, and because of its extremely low porosity/permeability (10−4 to 10−3 mD), it will also induce the heterogeneity of the whole layer containing both shale and other lithology rocks. Therefore, aquifers containing shale might exhibit unique characteristics. However, there is mostly no previous research on this topic in the hydro‐geophysics region, perhaps because the number of wells with shale in aquifer lithology is very limited. Key Points: Well aquifers, even with little amount of shale (∼5%), might have strong anisotropy and heterogeneity and deviate from the ideal modelsThe more shale content in the aquifer lithology the larger the deviation occursFurther call for us to avoid setting the observation aquifers onto the shale (low permeability, friability, and anisotropy) layers [ABSTRACT FROM AUTHOR]

Cracks widely exist in rocks and are important in various geophysical applications. However, although electrical methods are conventionally employed for the detection and characterization of cracks, the fundamental question whether cracks improve the conductive ability of porous rocks (a quantification of their electrical conductivity if their porosity and saturant are the same) remains largely unaddressed. We address this knowledge gap through theoretical models with confirmed validity. We show that the conductive ability of a rock containing non‐interacting penny‐shaped cracks with random orientation will be improved only in the case when the aspect ratio of the cracks is below a certain value, which is referred to as the critical crack aspect ratio. We also show that the critical crack aspect ratio is uniquely determined by the porosity and electrical conductivity (or cementation exponent) of the porous rock where the cracks reside. We further demonstrate that the critical crack aspect ratio is some representation of the pore structure, and using two times the critical crack aspect ratio as the pore geometry can give rise to a reasonable agreement between modelled and measured P‐ and S‐wave velocities. The critical crack aspect ratio offers a consistent microstructure for the joint elastic–electrical modelling for the improved characterization of cracks through integrated seismic and electrical surveys. [ABSTRACT FROM AUTHOR]

Water level monitoring data from 5 deep wells on the North China Platform are used to study how two large earthquakes impact deep aquifers. After the passage of seismic waves from earthquakes, a subset of these wells shows changes in the phase shifts of water level responses to lunar diurnal (O1) and semidiurnal (M2) tides. We fit a model for the tidal responses of hydraulically conductive fractures that intersect the wells to explain the phase and amplitude of responses to both O1 and M2 tides. To explain changes after the earthquakes with this model, the apparent orientation of fractures must change. Because the stresses are small, we propose that changes in tidal response occur when the passage of seismic waves modifies the hydraulic connectivity of a network of fractures by unclogging or clogging flow paths, thus changing the apparent orientation of the fractures controlling tidal responses. The seismic energy density that results in changes appears to decrease as the dominant frequency of seismic waves increases. Plain Language Summary: Permeability of Earth's crust is not a static property. Geochemical, biological, and tectonic processes can change permeability and hence fluid and heat transport through the crust. We show that diurnal and semidiurnal solid Earth tides can be used together to monitor changes in deep fractured aquifers over time and after earthquakes. We find that seismic waves from large, distant earthquakes can change the apparent orientation of fractures. Because stresses from the seismic waves are small, we suggest that oscillatory fluid flows driven by seismic waves unclog blocked fracture connections thus changing the apparent orientation of fractures. Key Points: Water level changes produced by solid Earth tides record the orientation of fractures intersected in deep wellsApparent fracture orientations change after large, teleseismic earthquakesDynamic strains from seismic waves may unclog or clog fractures and change fracture connectivity and the mean orientation sensed by tides [ABSTRACT FROM AUTHOR]

Classical third‐order thermoelastic constants are generally derived from the theory of small‐amplitude acoustic waves in isotropic materials during heat treatments. Investigating higher‐order thermoelastic constants for higher temperatures is challenging owing to the involvement of the number of unknown parameters. These Taylor‐type thermoelastic constants from the classical thermoelasticity theory are formulated based on the Taylor series of the Helmholtz free energy density for preheated crystals. However, these Taylor‐type thermoelastic models are limited even at low temperatures in characterizing the temperature‐dependent velocities of elastic waves in solid rocks as a polycrystal compound of different mineral lithologies. Thus, we propose using the Padé rational function to the total thermal strain energy function. The resulting Padé thermoelastic model gives a reasonable theoretical prediction for acoustic velocities of solid rocks at a higher temperature. We formulate the relationship between the third‐order Padé thermoelastic constants and the corresponding higher‐order Taylor thermoelastic constants with the same accuracy. Two additional Padé coefficients α1 ${\mathit{\alpha }}_{1}$ and α2 ${\mathit{\alpha }}_{2}$ can be calculated using the second‐, third‐, and fourth‐order Taylor thermoelastic constants associated with the Brugger's constants, which are consistent with those obtained by fitting the experimental data of polycrystalline material. The third‐order Padé thermoelastic model (with four constants) is validated by the fourth‐order Taylor thermoelastic prediction (with six constants) with ultrasonic measurements for polycrystals (olivine samples) and solid rocks (sandstone, granite, and shale). The results demonstrate that the third‐order Padé thermoelastic model can characterize thermally induced velocity changes more accurately than the conventional third‐order Taylor thermoelastic prediction (with four constants), especially for solid rocks at high temperatures. The Padé approximation could be considered a more accurate and universal model in describing thermally induced velocity changes for polycrystals and solid rocks. Plain Language Summary: Using the Taylor series for thermoelastic constants at higher temperatures is challenging owing to the involvement of the number of unknown parameters. Thus, we propose the third‐order Padé approximation to replace the Taylor series. Applications to laboratory measurements show that, with much less complexity, the Padé approximation presents a tendency of higher accuracy than the third‐order Taylor series and has similar precision with the higher‐order Taylor series at higher temperatures (up to 1500 K for polycrystals and 1000°C for solid rocks). Then we use such a model to predict the temperature‐dependent velocities of elastic waves. Finally, we analyze the physics of two additional Padé coefficients by associating them with the thermal expansion mismatch due to rock heterogeneities in lithology and with the thermally induced deformation of microcracks. Key Points: We apply the Padé rational function to the total thermal strain energy functionWe correlate the Padé and higher‐order Taylor thermoelastic constants through the equivalency of approximation accuraciesThe physics of the Padé coefficients is relevant to the thermal expansion mismatch and thermally induced deformation of microcracks [ABSTRACT FROM AUTHOR]

Joint seismic and controlled‐source electromagnetic surveys can reduce uncertainty and improve hydrocarbon quantification. However, the successful interpretation of joint seismic and controlled‐source electromagnetic survey data requires a valid rock physics model to link the elastic and electrical rock properties. A cross‐property differential effective medium is recently proposed with pores embedded in the mineral matrix. We have presented a technique to test the applicability of the cross‐property differential effective medium model to the joint elastic‐electrical properties of reservoir sandstones. The method is based on the common employment of measured sample porosity and inverted equivalent pore aspect ratio from cementation exponent for the modelling. Application of the modelling approach to a laboratory joint elastic‐electrical dataset shows that there is a systematic discrepancy in the modelled and measured joint elastic–electrical properties. It is also found that an equivalent pore aspect ratio is existing to reasonably predict the trend of the joint elastic–electrical data; however, the porosity used in the modelling is far from their measured values. Conversely, if the measured sample porosity is employed, the mentioned equivalent pore aspect ratio fails to fit either of the measured elastic and electrical rock properties. The results indicate the incapability of the cross‐property differential effective medium model for the joint elastic–electrical modelling of reservoir rocks and suggest the requirement of a new model to link the elastic and electrical properties. [ABSTRACT FROM AUTHOR]

The strength of radial anisotropy can effectively reveal vertical or horizontal material flow, which may provide significant dynamic implications. The South China Block (SCB) is a classic laboratory to study strong deformation under compression from the surroundings. We construct a radially anisotropic 3‐D shear‐wave model of the entire SCB using multimode surface wave tomography with a lateral resolution of 2.0°. Distinctive structural contrasts are revealed in the S‐wave velocity anomalies in the Cathaysia Block and the Yangtze Craton. In the Cathaysia Block, a low‐velocity anomaly becomes evident with increasing depth, particularly in the VSH model. The Yangtze Craton is, however, characterized by a high‐velocity anomaly, especially in the Sichuan Basin, where the high‐velocity feature can extend down to a depth of 300 km. Interestingly, an abrupt change in radial anisotropy is observed beneath the Sichuan Basin, with positive radial anisotropy (faster VSH) in the uppermost mantle and negative radial anisotropy (faster VSV) at depths from 80 to 250 km. The lower negative anisotropic layer may reflect the directional arrangement of constituent materials caused by eastward compression and erosion of mantle flow under the Tibetan Plateau. In addition, remarkable negative radial anisotropy is found in the asthenosphere beneath Hainan Island and its surroundings. The features beneath Hainan Island and its surroundings may be related to the Hainan mantle plume in the Cenozoic. Plain Language Summary: Seismic radial anisotropy represents that the vertically (VSV) and horizontally (VSH) polarized shear waves propagate at different wave velocities in the Earth's medium, which is useful to reflect the deformation characteristics and the deformation pattern of the medium, so as to constrain the mantle dynamics and tectonic processes of the crust and mantle. In this study, we measure multimode Rayleigh wave and Love wave phase velocity dispersion to construct a 3‐D shear‐wave model with radial anisotropy in the entire South China Block. Our results show that the shear‐wave velocity under the Yangtze Craton is anomalously fast and beneath the Cathaysia Block is anomalously slow. Moreover, we find different manifestations of radial anisotropy in the lithosphere of the Sichuan Basin, in which the negative radial anisotropy of 80–250 km is inferred to be affected by eastward compression and erosion of mantle flow under the Tibetan Plateau. A remarkable negative radial anisotropy beneath the Hainan Island and its surroundings in the asthenosphere is interpreted to reflect the Hainan mantle plume in the Cenozoic. Key Points: An anisotropic shear wave model beneath South China is constructed by multimode surface wave tomography with improved vertical resolutionSichuan basin shows negative radial anisotropy, reflecting the influence of eastward compression and mantle flow under the Tibetan PlateauThe low‐velocity anomaly and strong negative radial anisotropy of the Hainan Island may be related to the Hainan mantle plume [ABSTRACT FROM AUTHOR]

Understanding the electrical properties of reservoir rocks has important applications in oil and gas exploration. Small‐scale fractures that are widely existing in reservoir rocks are usually aligned along a certain preferential direction due to reasons such as tectonic movement, making the electrical conductivity of the rocks anisotropic. Since all rocks are experiencing geological pressure, it is theoretically and practically important to study the influence of pressure on the fractures and its control on the anisotropic conductivity of reservoir rocks. We first prepared artificial porous sandstone samples with and without penny‐shaped fractures and experimentally measured the anisotropic conductivity of the samples under differential pressure (the difference between confining pressure and pore fluid pressure). Then, we inverted from the experimental data for the fracture porosity and fracture aspect ratio at each differential pressure. The variation of the fracture parameters caused by the differential pressure and their influence on the anisotropic electrical conductivity of the rocks were further analysed theoretically. The results show that as the differential pressure increases, the measured anisotropic conductivity of the artificial samples with and without fractures all decreases exponentially. We also show that both the inverted fracture porosity and fracture aspect ratio decrease exponentially with the increase of differential pressure, and the fracture porosity and fracture aspect ratio are linearly correlated as an implicit function of differential pressure. The modelling results show that the differential pressure‐induced decrease in the fracture porosity reduces the electrical conductivity in all directions, and the conductivity reduction in the rock parallel to the direction of the fractures is most significant. Comparing with the influence of the fracture porosity resulting from the applied differential pressure, the pressure‐caused variation in the fracture aspect ratio was shown to play a secondary role on affecting the pressure‐dependent anisotropic electrical properties. [ABSTRACT FROM AUTHOR]

Many deep aquifers overlain by barrier formations in the continental US are used as geological repositories for wastewaters coproduced from hydrocarbon exploration. This practice is to protect shallow groundwater following the US Environmental Protection Agency's regulation. Implicit in such practice is the assumption that deep aquifers overlain by mudstones or shales are confined so that the injected fluids will not migrate upward to contaminate shallow groundwater. However, no systematic test of this hypothesis has been made. Here we invert the groundwater response to both the M2 and the O1 tides and to the barometric pressure across a large (2.046 × 106 km2) geologic regime, the North China Platform, to systematically evaluate the hydraulic parameters as functions of depth and time without a priori assumption. Our result, the first of such inversion, shows no depth dependence of aquifer confinement to a depth of 3,400 m and that deep confined aquifers overlain by barrier formations may become leaky after distant earthquakes. It suggests that monitoring of aquifer confinement may be needed to ensure if the targeted deep aquifer for wastewater injection is really confined. The results may be timely and of global significance because the practice of hydrofracking for natural gas and the deep injection for the disposal of the coproduced wastewaters in the US may soon be adopted by other countries, such as China. Plain Language Summary: A great amount of wastewaters coproduced from hydrocarbon exploration in the mid‐continental US has been injected into deep aquifers. Such practice has been based upon an implicit assumption that deep aquifers confined by aquitards are fully confined so that the injected toxic water will not leak back to contaminate shallow groundwater or the environment. This assumption, however, has never been carefully tested. Here we test this hypothesis using groundwater data from wells over a large geological regime, the North China Platform, as a function of depth and time. Our results show no clear dependence of aquifer confinement on depth, that significant leakage occurs even in the deepest studied aquifer (>3400 m) confined by thick clay/mud aquitards, and that an initially confined aquifer became may leaky after a distant earthquake. Because the practice of hydrofracking for natural gas and deep injection for the disposal of the coproduced wastewaters may soon be adopted by other countries, such as China, our study is timely and may be of global significance. Key Points: Systematic evaluation of the confinement of aquifers in the North China Platform at different depths down to 3.4 kmNovel revealing most of the studied aquifers leak, and has no dependence on aquifer depth or thickness of the overlain geologic barrierBoth the M2 and O1 tidal waves are used in the inversion of aquifer parameters without a prior assumption [ABSTRACT FROM AUTHOR]

Seismic waves propagating in a fractured porous rock with fluids can be greatly attenuated due to the wave‐induced fluid flow, including the macroscopic Biot flow, the mesoscopic interlayer fluid flow and the microscopic squirt flow. However, the comprehensive effects of the above wave‐induced fluid flows on the frequency‐dependent seismic properties in layered and fractured rocks with partial saturation are still poorly understood. To obtain such knowledge, we present a superposition approach accounting for the dispersion and attenuation induced by the multi‐scale wave‐induced fluid flows in the above‐mentioned target rocks, and study the influences of the water saturation, layer thickness ratio and fracture properties (i.e. fracture density and fracture aspect ratio) on the frequency‐dependent vertical P‐wave velocity (VP0). The results demonstrate that the increasing water saturation enhances the dispersion and attenuation of VP0 in the full frequency band, and the varying layer thickness ratio not only affects the dispersion and attenuation induced by the interlayer fluid flow, but also influences those caused by the Biot flow and squirt flow. The fracture properties are also found to significantly impact the seismic properties of the target rock. In addition to its effects on the squirt flow, the increasing fracture density is shown to weaken the dispersion and attenuation induced by the Biot flow, and enhance those caused by the interlayer fluid flow. The obtained results provide new insights into the frequency‐dependent seismic properties in the partially saturated layered rocks with fractures, and the knowledge gained from these results has important practical applications for the successful interpretation of fluid and fracture properties applying the seismic exploration data acquired in fractured reservoirs. [ABSTRACT FROM AUTHOR]

Fractures can widely exist in hydrate‐bearing reservoirs and can give rise to anisotropic acoustic properties that play an important role in the accurate assessment of fractured reservoirs. However, the knowledge of the responses of acoustic anisotropy in fractured hydrate‐bearing reservoirs is still poorly understood. To obtain such understanding, we designed and implemented dedicated laboratory experiments to measure the anisotropic acoustic velocities in an artificial sandstone with aligned penny‐shaped fractures parallel to the layers during hydrate formation and dissociation at the confining pressures of 20 and 50 MPa, respectively. We showed that although the velocities at higher confining pressure were systematically greater, all the velocities at both confining pressures increased and decreased during hydrate formation and dissociation, respectively. We also showed that the velocities during hydrate formation were significantly higher than those during dissociation at hydrate saturation greater than 10%. It was found that all the velocities smoothly reduced with decreasing saturation during dissociation, while the varying velocities exhibited distinct increasing gradients during formation, where the compressional wave anisotropy reduced dramatically at saturation between 10% and 25%. Analyses of the experimental results suggested that the floating hydrate evolved to bridge the surfaces of fractures when hydrate saturation exceeded 10% during hydrate formation, and the bridging hydrate gradually transformed to floating in the pore system during dissociation. The results provide new insights into hydrate distribution and its effects on acoustic anisotropy in fractured reservoirs during hydrate formation and dissociation and will contribute to improving the accuracy of hydrate assessment in such reservoirs. Key Points: We study the evolution of hydrate formation and dissociation on the acoustic anisotropy in an artificial sandstone with aligned fracturesThe dependences of anisotropic velocities on hydrate saturation are smooth for hydrate dissociation but are distinct for hydrate formationThe anisotropic results suggest that hydrate floats in the fractures for saturation up to 10% and then bridges fractures during formation [ABSTRACT FROM AUTHOR]

Wave‐induced fluid flow plays an important role in affecting the seismic dispersion and attenuation of fractured porous rocks. While numerous theoretical models have been proposed for the seismic dispersion and attenuation in fractured porous rocks, most of them neglect the wave‐induced fluid flow resulting from the background anisotropy (e.g. the interlayer fluid flow between different layers) that can be normal in real reservoirs. Here, according to the theories of poroelasticity, we present an approach to study the frequency‐dependent seismic properties of more realistic and complicated rocks, i.e. horizontally and periodically layered porous rock with horizontal and randomly orienting fractures, respectively, distributed in one of the two periodical layers. The approach accounts for the dual effects of the wave‐induced fluid flow between the fractures and the background pores and between different layers (the interlayer fluid flow). Because C33 (i.e., the modulus of the normally incident P‐wave) is directly related to the P‐wave velocity widely measured in the seismic exploration, and its comprehensive dispersion and attenuation are found to be most significant, we study mainly the effects of fracture properties and the stiffness contrast between the different layers on the seismic dispersion and attenuation of C33. The results show that the increasing stiffness contrast enhances the interlayer fluid flow of the layered porous rocks with both horizontal and randomly orienting fractures and weakens the wave‐induced fluid flow between the fractures and the background pores, especially for the layered porous rock with horizontal fractures. The modelling results also demonstrate that for the considered rock construction, the increasing fracture density reduces the interlayer fluid flow while improves the dispersion and attenuation in the fracture‐relevant frequency band. Increasing fracture aspect ratio is found to reduce the dispersion and attenuation in the fracture‐relevant frequency band only, especially for the layered porous rock with horizontal fractures. [ABSTRACT FROM AUTHOR]

Generally, local stress induced by individual crack hardly disturbs their neighbours for small crack densities, which, however, could not be neglected as the crack density increases. The disturbance becomes rather complex in saturated porous rocks due to the wave‐induced diffusion of fluid pressures. The problem is addressed in this study by the comparison of two solutions: the analytical solution without stress interactions and the numerical method with stress interactions. The resultant difference of effective properties can be used to estimate the effect of stress interactions quantitatively. Numerical experiments demonstrate that the spatial distribution pattern of cracks strongly affects stress interactions. For regularly distributed cracks, the resulting stress interaction (shielding or amplification) shows strong anisotropy, depending on the arrangement and density of cracks. It has an important role in the estimation of effective anisotropic parameters as well as the incident‐angle‐dependency of P‐ and SV‐wave velocities. Contrarily, randomly distributed cracks with a relative small crack density generally lead to a strong cancellation of stress interactions across cracks, where both the numerical and analytical solutions show a good agreement for the estimation of effective parameters. However, for a higher crack density, the incomplete cancellation of stress interactions is expected, exhibiting an incidence‐angle dependency, slightly affecting effective parameters, and differentiating the numerical and analytical solutions. [ABSTRACT FROM AUTHOR]

The existence of cracks can significantly influence the electrical properties of rocks, and electrical exploration has been applied for the detection and characterization of cracks. However, the premise is the knowledge of the quantitative effects of cracks on the electrical properties of fractured rocks, and theoretical models have provided means to obtain such knowledge. In this work, we derive explicit theoretical models for the anisotropic electrical conductivity of transversely isotropic (TI) rocks with inclined and rotated cracks, as well as cracks with random orientation distribution. Based on the developed models, we study systematically and quantitatively the effects of the crack properties and distributions on the anisotropic electrical conductivity of cracked rocks. We show that for TI rocks with aligned cracks, the inclination angle of the cracks affects both the vertical and the horizontal electrical conductivities, whereas the rotation angle only influences the horizontal electrical conductivity, although both the two angles can lead to the global rotations of the principal axes of the electrical conductivity tensor. On the other hand, the conductivity of TI rocks containing randomly distributed cracks remains transversely isotropic. For both aligned and randomly distributed cracks, the increasing crack volume fraction can linearly increase the electrical conductivity of the effective rocks, and cracks with lower aspect ratio show more significant effects on the electrical conductivity. The knowledge of the effects of the cracks on the TI rocks obtained in this study can be applied for more accurate interpretation of the electrical exploration data to better characterize cracked formations. Key Points: We develop theoretical models for the electrical conductivity of transversely isotropic rocks with inclined cracks of varying orientationsThe effects of crack parameters and crack orientation distribution on the anisotropic electrical conductivity of cracked rocks are studiedThe models can be applied for more accurate interpretation of electrical exploration data to better characterize cracked formations [ABSTRACT FROM AUTHOR]

P and SV wave scattering by thermally constrained saturated cracks couples elastic deformation with temperature. We estimate the scattering attenuation and velocity dispersion by aligned fluid‐saturated cracks with finite thickness, which are sparsely and randomly embedded in an isothermal and isotropic elastic medium. By incorporating thermoelastic effects into the representation theorem with the non‐interaction Foldy approximation, we formulate the dispersion and attenuation of P and SV waves induced by displacement discontinuities across aligned cracks constrained by a thermoelastic boundary condition. The frequency‐dependent response as a function of temperature is calculated for various P and SV wave incidence angles. The resulting scattering attenuation and dispersion are compared with those of the conventional aligned‐crack model. The examples show that the dissipation increases with increasing temperature, but the scattering is inhibited at a given temperature. The P wave shows a strong sensitivity to temperature along the direction normal to the cracks, whereas the SV wave has an opposite behavior. This theory has the potential to assess the distribution of temperature from seismic attributes. Key Points: We study the thermoelastic dispersion and attenuation of P and SV wave scattering by aligned‐crack in an isothermal elastic mediumThe dissipation increases with increasing temperature, but the scattering is inhibited at a given temperatureThe P wave shows a strong sensitivity to temperature along the crack normal, whereas the SV wave behaves in opposite manner [ABSTRACT FROM AUTHOR]

The lithospheric layering and evolution of Archean cratons can be effectively constrained by seismic properties. We present fine‐scale lithospheric discontinuity images of the North China Craton (NCC) using the seismic daylight imaging (SDI) technique, which is also known as the autocorrelogram method. With the aid of large‐scale surface‐wave velocity models, the characters of autocorrelograms related to lithospheric discontinuities in the NCC are investigated. Spatial variation in discontinuity characteristics is revealed across the craton. In the modified/destroyed eastern NCC, both the midlithospheric discontinuity (MLD) and the lithosphere‐asthenosphere boundary (LAB) are rather sharp (~2 km). Various dynamic processes, including juvenile underplating, asthenospheric upwelling, and corner flow induced by paleo‐Pacific Plate subduction, may contribute to such sharp interfaces. In the stable western NCC, P wave reflections cannot easily reveal the LAB, suggesting a diffuse (thicker than 30 km) lithosphere‐asthenosphere transition (LAT). Multiple MLDs are also found that may have arisen during craton formation and multiple phases of rejuvenation of the cratonic lithosphere as evidenced by the petrological results. The use of higher‐frequency data enables us to distinguish among the lithospheric discontinuity styles and favor multiple origins of the MLDs and LAB in the cratonic lithosphere. Key Points: High‐frequency autocorrelograms are used to investigate the fine‐scale lithosphere structures beneath North ChinaA sharp MLD/LAB is observed (~2 km thick) in the destroyed eastern North China CratonGradual MLDs/LAT are present in the stable craton region exceeding the 30 km transition [ABSTRACT FROM AUTHOR]

We develop a numerical algorithm for simulation of wave propagation in linear nonisothermal poroelastic media, based on Biot theory and a generalized Fourier law of heat transport in analogy with Maxwell model of viscoelasticity. A plane wave analysis indicates the presence of the classical P and S waves and two slow waves, namely, the Biot and the thermal slow modes of propagation, which present diffusive behavior under certain conditions, depending on viscosity, frequency, and the thermoelastic constants. The wavefield is computed with a direct meshing method using the Fourier differential operator to calculate the spatial derivatives. We propose two alternative time‐stepping algorithms, namely, a first‐order explicit Crank‐Nicolson method and a second‐order splitting method. The Fourier differential operator provides spectral accuracy in the calculation of the spatial derivatives. Modeling the thermal diffusive mode is relevant for high‐temperature high‐pressure fields and since it leads to mesoscopic attenuation by mode conversion of the fast waves to the thermal waves. Key Points: The differential equations of dynamic thermoporoelasticity are solved with a direct grid methodThe solver is based on the Fourier pseudospectral method to compute the spatial derivativesThe simulation reveals the behavior of four wave modes, namely, the fast P, slow P, S, and the thermal wave-diffusion field [ABSTRACT FROM AUTHOR]

Seismic characterization of fractures is of great importance in many disciplines, for which rock physics models provide the basis to link fracture properties to seismic attributes. However, to date, most rock physics models neglect the background anisotropy that may exist in many fractured formations. Hence, in this work, we developed a theoretical model for rock effective elastic properties with penny‐shaped cracks embedded in the transversely isotropic (TI) background medium. We first derived analytical solutions for the case with dry or fluid‐filled penny‐shaped cracks parallel to the isotropic plane of TI background medium. Further, the results were extended to the case with cracks inclined to the isotropic plane with any angles. We then studied, based on the developed theoretical model, two tight sand samples with TI and isotropic background, respectively, to illustrate effects of background anisotropy on effective elastic properties of fractured rocks. The results show that the background anisotropy has significant influence on P and S wave velocity anisotropy, as well as on shear wave splitting. The background anisotropy can either increase or decrease P and SH wave velocity anisotropy depending on crack inclination angles, whereas it has relatively small influence on SV wave velocity anisotropy. To further illustrate the important effects of background anisotropy, we compared theoretical predictions to ultrasonic measurements on a synthetic fractured sandstone sample with TI background medium. The results show notable improvement of the agreement between theoretical predictions and experimental results after considering background anisotropy. Key Points: A theoretical model is developed to study effects of background anisotropy on elastic properties of fractured rocksBackground anisotropy has significant influence on the P and S wave velocity anisotropy, as well as on the shear wave splittingEffects of background anisotropy are also illustrated by comparing theoretical predictions to experimental data [ABSTRACT FROM AUTHOR]

Abstract: We estimate the seismic attenuation and velocity dispersion induced by fluid pressure diffusion in a saturated porous medium containing a sparse distribution of aligned slit fractures. This is done by solving the scattering problem for a single crack and using Waterman‐Truell scattering approximation for a distribution of cracks. This approach gives a numerical solution for the entire frequency range and analytical solutions for the low‐ and high‐frequency limits. Analysis of the results shows that the characteristics of the seismic dispersion and attenuation for the slit cracks are similar to known results for penny‐shaped crack. At high frequencies, the attenuation and dispersion are controlled by the crack density regardless of the crack geometry. At low frequencies, due to the different geometries of the slit and penny‐shaped cracks, the characteristics of seismic dispersion and attenuation are different. For both the slit and penny‐shaped crack case, the slope of the frequency dependence attenuation of factor (inverse quality factor) Q−1 tends to be 1. Yet the low‐frequency asymptotic solutions for these two cases are somewhat different. For penny‐shaped cracks, the Q−1(ω) on the bilogarithmic scale has a straight line asymptote with slope 1, so that the attenuation factor at low frequencies is proportional to the frequency ω. However, for slit cracks, the asymptotic low‐frequency solution contains a logarithmic function of ω, and no such asymptote exists. At the same time, similarly to the penny‐shaped crack case, the solution for compressional velocity in the low‐frequency limit is consistent with the anisotropic Gassmann theory. [ABSTRACT FROM AUTHOR]

Abstract: The characteristics of internal lithospheric discontinuities carry crucial information regarding the origin and evolution of the lithosphere. However, the formation and mechanisms of the midlithosphere discontinuity (MLD) are still enigmatic and controversial. We investigate the midlithospheric discontinuities beneath the Archean Western Australian Craton, which represents one of the oldest continents on the globe, using a novel receiver‐based reflectivity approach combined with other geophysical information comprising tomographic P and S wave velocity, radial anisotropy, electrical resistivity, and heat flow data. The MLD is rather shallow with a depth of 68–82 km. Multiple prominent discontinuities are observed in the lithospheric mantle using constructed high‐frequency (0.5–4 Hz) P wave reflectivities. These multiple discontinuities coincide well with the broad‐scale reduction of relative P and SV wave velocities at the top of the graded transition zone from the lithosphere to the asthenosphere. Strong radial anisotropy in the upper lithosphere mantle tends to be weak across the MLD, which might reflect quasi‐laminar lithospheric heterogeneity behavior with a horizontal correlation length that is greater than its vertical correlation length. Broad‐scale electrical resistivity variations show little coherence with the MLD. Given these various geophysical observations, the upper lithosphere exhibits rigid and elastic properties above the MLD, while the lower lithosphere tends to be ductile and rheological or viscous. A model comprising quasi‐laminar lithospheric heterogeneity could effectively represent the MLD characteristics beneath the Archean continent. [ABSTRACT FROM AUTHOR]

Fractures are commonly existing in the subsurface rocks, and understanding the seismic anisotropy in fractured rocks is of great importance in a range of geophysical applications. However, although all the fractured rocks are suffering from geological stresses and most of them are experiencing partial water saturation, the combined effects of confining pressure and water saturation on the seismic anisotropy in fractured rocks remain poorly understood. To obtain such knowledge, we measured and compared the anisotropic velocities and corresponding seismic anisotropy in two artificial porous sandstone samples with and without aligned fractures, respectively, as functions of varying confining pressure and degree of water saturation. We showed that the existence of aligned fractures significantly reduced the five measured velocities but dramatically enhanced the seismic anisotropy in the fractured sample. We also showed that confining pressure and water saturation had an interdependent complex effect on the seismic anisotropy of the fractured sample. The seismic anisotropy of the dry sample showed the highest‐pressure sensitivity, and the saturation effect was most significant at low confining pressure. Integrated interpretation of the experimental data indicated that wave‐induced fluid flow due to pore‐scale and mesoscale fluid distribution in the rocks is responsible for most of the observed pressure and saturation‐dependent anisotropic behaviors in the rocks. The results have helped to gain new knowledge on the seismic anisotropy in partially saturated fractured rocks subject to varying pressures and suggest the requirement for advanced theoretical models to fully understand the acquired data. Key Points: Combined pressure and water saturation effects on the seismic anisotropy in artificial porous sandstone with aligned fractures are studiedExistence of aligned fractures reduces the measured velocities but enhances the seismic anisotropy in the fractured sampleSeismic anisotropy of dry sample shows the highest pressure sensitivity and saturation effect is most significant at low confining pressure [ABSTRACT FROM AUTHOR]