P-wave Velocity Anisotropy Research Articles

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.

Anisotropy of Thermal Conductivity and P-wave Velocity in Sedimentary Soft Rocks and their Relationships with Porosity

Anisotropy of Thermal Conductivity and P-wave Velocity in Sedimentary Soft Rocks and their Relationships with Porosity

The 3-D P-wave velocity and radial anisotropy model of southern Africa (South of 5°S).

The 3-D P-wave velocity and radial anisotropy model of southern Africa (South of 5°S).

The 3-D P-wave velocity and radial anisotropy model of southern Africa (South of 5°S).

The 3-D P-wave velocity and radial anisotropy model of southern Africa (South of 5°S).

Deformation of the NE Tibetan Plateau revealed by velocity and azimuthal anisotropy structures

Because of the complex interaction among the Tibetan Plateau, the Alxa block and the Ordos block, the deformation mechanisms of the northeastern (NE) Tibetan Plateau have been controversial for decades. To shed light on these mechanisms, we obtain high-resolution 3-D images of the P-wave velocity and azimuthal anisotropy structures beneath the NE Tibetan Plateau and adjacent area. We invert high-quality manually picked first arrivals using an eikonal equation-based seismic traveltime tomography method. Tomographic results mainly present two features. First, low-velocity anomalies are evident in the lower crust of the Qilian block. In addition, weak azimuthal anisotropy with NW–SE-oriented fast wave directions (FVDs) is detected in the lower crust. These characteristics indicate compressional deformation of the Qilian block. Second, the lower crust and uppermost mantle below the eastern Qilian orogenic belt display high-velocity anomalies. These high-velocity anomalies are collected with the high-velocity body beneath the Alxa block. This feature may imply that the Alxa block is underthrusting beneath the eastern Qilian orogenic belt. Our tomographic results show that the 1920 Ms 8.5 Haiyuan earthquake occurred near the boundary of the high-velocity anomaly, where the FVDs change dramatically. In addition, we observe that the 1927 Ms 7.9 Gulang earthquake occurred in the normal upper crust and above the strong low-velocity anomalies. From our tomographic results, we propose that two different seismogenic structures were associated with these two great earthquakes.

Application of Laser-Ultrasonics for Evaluating Textures and Anisotropy

Various approaches are reviewed for determining elastic anisotropy and its coupling to crystallographic texture, with special reference to ultrasonic measurements. Two new methods are described for measuring the anisotropy of P-wave velocity using laser-ultrasonics. Making measurements across the diameter of a cylindrical specimen as it is rotated makes it possible to maintain a very constant known path length. This permits extremely accurate measurements with a precision of better than 0.01%. Results on 316 stainless steel in different conditions are compared with calculated values obtained from EBSD textures together with measured densities and crystalline coefficients from the literature. Excellent agreement is obtained when applying the Hill geometrical average procedure. A similar approach is adopted to measure the variation of wave velocity in a martensitic steel, after tempering at a range of temperatures. Changes in the anisotropy associated with thermal softening are discussed. The second method uses Galvano mirrors to steer the generating laser to different positions over a sheet surface, allowing wave velocities to be determined along different directions in the anisotropic material.

Experimental study of the anisotropic behaviour of the Naparima Hill argillite

Argillite rocks are often anisotropic due to the presence of microfractures , clay minerals, laminations , and bedding planes . Understanding the anisotropic behaviour of organic-rich argillite rocks is critical for their successful exploitation as unconventional reservoirs. In this study, the anisotropic rock properties of the argillite in the Naparima Hill Formation, Trinidad, an unconventional reservoir, was studied in laboratory experiments. A series of laboratory measurements including permeability, P- and S-wave velocities, and compressive strength were conducted at effective pressures of up to 130 MPa. The rock samples tested were collected from the Naparima Hill outcrop described by three lithofacies : siliceous calcareous mudstones , calcareous mudstone, and siliceous mudstones. The anisotropy of the argillite was determined by the ratio of the rock property measured perpendicular to the outcrop bedding to the rock property measured parallel to the outcrop bedding. For all the studied argillites, the permeability anisotropy ratios is about 2, while the P- and S-wave velocities anisotropy ratios, and compressive strength anisotropy ratios are close to 1, indicating that the rocks are fairly anisotropic. With increasing effective pressure, the permeability, velocities, and strength anisotropy ratios all decreased marginally. A comparison analysis reveals that the anisotropies of permeability and strength, anisotropies of P-velocity and permeability and anisotropies of S-velocity and permeability are all unrelated. However, anisotropies of P-wave velocity and strength, anisotropies of S-wave velocity and strength, and anisotropies of P-wave velocity and S-wave velocity anisotropy all have a 1:1 relationship. Overall, the results of the study suggest that the Naparima Hill argillite is weak to fairly anisotropic, which can be attributed to the low clay content, poor laminations, lack of foliation and few microfractures in the rocks.

P-wave velocity anisotropy in Hamra Quartzites reservoir, Hassi Messaoud oil field in Algeria

P-wave velocity anisotropy in Hamra Quartzites reservoir, Hassi Messaoud oil field in Algeria

The crustal and uppermost mantle dynamics of the Tengchong–Baoshan region revealed by P-wave velocity and azimuthal anisotropic tomography

SUMMARY The Tengchong–Baoshan region (TBR) is located at the margin of the southeastern Tibetan Plateau (SETP) and is subject to the complex interaction among multiple tectonic blocks. Therefore, the TBR is approximate for investigating the mechanism responsible for the interaction among the SETP and its surrounding blocks. Based on eikonal equation-based seismic tomography, we obtain high-resolution P-wave velocity and azimuthal anisotropy at depths of 0–40 km beneath the TBR. We discover a large magma reservoir (with horizontal scale of ∼50 km and vertical scale of ∼15 km) in the middle-lower crust beneath the Tengchong volcanic zone, which is directly fed by hot materials from the uppermost mantle. We also detect a shallow magma chamber in the uppermost crust above the large reservoir. These magma reservoirs at different depths constitute a multiscale magma system. Furthermore, we observe a low-velocity belt beneath the Weixi–Qiaohou–Weishan and Nantinghe faults and postulate that it corresponds to lower crustal flow and hot mantle upwelling. Additionally, we infer that two M > 7.0 earthquakes that have occurred in Baoshan block nucleated in the transition zone between low- and high-velocity anomalies. Based on the azimuthal anisotropy tomography results, we consider that incoherent deformation in the crust causes the uneven accumulation of stress, which facilitated the occurrence of these two events. The 2021 Ms 6.4 Yangbi earthquake also occurred in the transition zone between low- and high-velocity anomalies. Based on the P-wave velocity tomography results, we suggest that hot material decreases the fault strength and leads to the instability of the fault.

Development of a New Empirical Relation to Assess P-wave Velocity Anisotropy of Rocks

Several physical parameters and anisotropy related to rock textural arrangements, schistosity and weakness planes such as cracks and joints affect the P-wave velocity (VP). First, VP anisotropy of quartz-mica schist as a common type of widespread metamorphic rock was compared with VP anisotropy of jointed homogeneous limestone specimens to clarify effect of these two different types of anisotropies. The results showed that the VP anisotropy of quartz-mica schist texture is stronger than the VP anisotropy of jointed limestone, because all body of quartz-mica schist specimens have VP anisotropy behavior. Many rocks are anisotropic and degree of anisotropy varies from one to another. Various investigations have been carried out on VP anisotropy but there is not a unique comprehensive relation to represent the influence of different degrees of anisotropy on the VP for different rocks. The relation between VP and angle (θ) between the axis of symmetry (perpendicular to weakness planes) with the wave propagation direction was analyzed for a wide range of anisotropy degrees using the results of nine different types of rocks including: Angouran quartz-mica schist, Golgohar mica schist, amphibole schist, mica-quart schist, Marcellus shale, Withby shale WUK47B, WUK70 and WUK2, and Veroia-Polymylos gneiss. A new simple empirical relation fitted to all groups of results was obtained to assess VP for different degrees of anisotropies with a good correlation of determination (R2 = 0.937), low RMSE (RMSE = 320 m/s) and low CV (CV = 7.0%). P wave velocity anisotropy can simply be predicted by the developed relation using only two parameters of VP0 and VP90 or ε that is the percentage change of VP0 with respect to VP90. A VP anisotropy classification diagram was also developed based on the different values of ε.

Influence of the microstructure on stress-dependent P-wave anisotropy in sandstone

SUMMARY To understand the factors that affect the stress-dependent P-wave velocity anisotropy, a method is proposed to simulate anisotropic microcracks and minerals based on the discrete element method (DEM). Laboratory triaxial tests and numerical simulations were performed on sandstone samples with bedding orientations parallel and perpendicular to the maximum principal stress. The ellipse fitting method was applied to analyse the variation in P-wave anisotropy. The micromechanism of stress-dependent P-wave anisotropy was revealed. The evolution of microcracks is the main reason for the change in P-wave anisotropy under compression. As the confining pressure increases, the magnitude of the P-wave anisotropy is reduced. The weakening of the P-wave anisotropy results from the decrease in the number of open microcracks. Under deviatoric stress loading, the P-wave anisotropy of the bedding-parallel sample in the axial direction is strengthened. Anisotropy reversal occurs in the bedding-normal sample. The microcrack behaviour depends on the direction of maximum principal stress. The variation in microcrack anisotropy induced by stress controls the evolution of P-wave velocity anisotropy. The stress at which anisotropic reversal occurs depends on the preferred orientation mineral. The DEM model offers the unique ability to directly examine the variation in microstructure anisotropy that causes the change in P-wave anisotropy.

Prediction of sweet spots in tight sandstone reservoirs based on anisotropic frequency-dependent AVO inversion

Abstract The frequency-dependent amplitude-versus-offset (FAVO) method has become a practical method for fluid detection in sand reservoirs. At present, most FAVO inversions are based on the assumption that reservoirs are isotropy, but the application effect is not satisfactory for fractured reservoirs. Hence, we analyse the frequency variation characteristics of anisotropy parameters in tight sandstone reservoirs based on a new petrophysical model, and propose a stepwise anisotropic FAVO inversion method to extract frequency-dependent attributes from prestack seismic field data. First, we combine the improved Brie's law with the fine-fracture model to analyse frequency-dependent characteristics of velocities and Thomsen anisotropy parameters at different gas saturations and fracture densities. Then, we derive an anisotropic FAVO inversion algorithm based on Rüger's approximation formula and propose a stepwise anisotropic FAVO inversion method to obtain the dispersions of anisotropy parameters. Finally, we propose a method that combines the inversion spectral decomposition with the stepwise anisotropy FAVO inversion and apply it to tight sand reservoirs in the Xinchang area. We use P-wave velocity dispersion and anisotropy parameter ε dispersion to optimise favourable areas. Numerical analysis results show that velocity dispersion of the P-wave is sensitive to fracture density, which can be used for fracture prediction in fractured reservoirs. In contrast, anisotropic parameter dispersion is sensitive to gas saturation and can be used for fluid detection. The seismic data inversion results show that velocity dispersion of the P-wave and anisotropic parameter dispersion are sensitive to fractured reservoirs in the second member of Xujiahe Group, which is consistent with logging interpretation results.

Layer parallel stretching? Characterising magnetic and pore-fabric styles at a rifted continental margin: New insights from the Otway Ranges, Australia

This study presents the first analysis of the anisotropy of magnetic susceptibility (AMS) coupled with anisotropy of P-wave velocity (APV) in sedimentary rocks from a passive continental margin. In the Otway Ranges, Australia, the Lower Cretaceous sediments deposited during a period of subsidence and structural quiescence display triaxial-magnetic fabrics typical of extensional deformation. The extensional interpretation from AMS is also supported by the orientation of the pore fabric measured using APV.The inferred extensional azimuths from both AMS and APV analyses are in good agreement with axes for renewed Late Cretaceous rifting and the continental break up of Gondwana, a period of structural evolution characterised by abnormally-high levels of paleostress in the Otway Basin. Predicated on this we introduce the concept of layer parallel stretching (LPSt), a process that describes pore-scale structuring representative of the first phase of extensional deformation during periods of intense tectonic extension, in this case the LPSt was oriented NE-SW. These extensional structural fabrics in the Otway Basin were preserved during NE-SW oriented basin inversion, a process that involved reactivation of the local detachment, the partitioning of strain, and low levels of coupling between the basement and the cover. The results highlight the applicability of AMS and APV for characterising rock anisotropy in sedimentary basins at continental margins and have distinct outcomes with respect to the structural framework of the study area in the Otway Ranges.

Assessment of structural, magnetic, and P-wave velocity anisotropy of two biotite gneisses from X-ray and neutron tomography

Anisotropy of the physical properties of rocks is directly related to their internal structure, which is often exhibited by lineation and foliation fabrics as anisotropy markers. Conventional methods of diffraction measurements only provide a part of the structural information that is related to the crystallographic preferred orientation of minerals. In this work, we show the contribution of imaging methods such as X-ray and neutron tomography in studying rock structure that exhibits anisotropic character through lineation and foliation fabrics. In our tomography studies, spherical samples of two different biotite gneisses were tested. Virtual 3D structure models of samples were obtained by both neutron and X-ray tomography, which depict the spatial distribution of mica minerals (biotite and muscovite) - the major carriers of rock anisotropy in the studied samples. An application of scanning method to the 3D data of mica's spatial distribution allowed us to reveal the presence, orientation and strength of foliation and lineation fabrics. We have also performed grain shape analysis using the approximation of the individual elements of segmented mica phase by the equivalent (Legendre's) ellipsoids. As a result, we determined the shape preferred orientations of mica grains and calculated corresponding shape orientation distribution functions. The comparison of structural properties obtained from tomography studies with experimentally determined P-wave, and magnetic anisotropy demonstrated their mutual correlation, providing a basis for the quantitative interpretation of anisotropic rock fabric in relation to the physical properties of rock. The effective elastic and magnetic properties were calculated based on the shape orientation distribution functions determined from tomography data. The good agreement between calculated and measured effective properties have shown the potential of X-ray and neutron tomography for the prediction of magnetic and seismic anisotropy using a 3D model of a rock sample.

Unexpected Consequences of Transverse Isotropy

ABSTRACTIn a series of articles, Kawakatsu et al. (2015) and Kawakatsu (2016a,b, 2018) introduced and discussed a new parameter, ηκ, that characterizes the incidence angle dependence (relative to the symmetry axis) of seismic body-wave velocities in a transverse isotropy (TI) system. During the course of these exercises, several nontrivial consequences of TI were realized and summarized as follows: (1) P-wave velocity (anisotropy) strongly influences the conversion efficiency of P-to-S and S-to-P, as much as S-wave velocity perturbation does; (2) Rayleigh-wave phase velocity has substantial sensitivity to P-wave anisotropy near the surface; (3) a trade-off exists between ηκ and the VP/VS ratio if the latter is sought under an assumption of isotropy or the elliptic condition. Among these findings, the first two deserve careful attention in interpretation of results of popular seismic analysis methods, such as receiver function analysis and ambient-noise Rayleigh-wave dispersion analysis. We present simple example cases for such problems to delineate the effect in actual situations, as well as scalings among TI parameters of the crust and mantle materials or models that might help understanding to what extent the effect becomes important.

Uniaxial compressive strength estimation based on the primary wave velocity in coal: considering scale effect and anisotropy

ABSTRACT Coal mass strength estimation has attracted broad attentions for decades. Investigating the effect of scale and anisotropy on the Uniaxial Compressive Strength (UCS) estimation via P-wave velocity is crucial in upscaling the lab-obtained correlation to coal mass strength evaluation. Microstructures were considered as the major impact factors for the scale difference and anisotropy of mechanical properties in coal. Thus, the microstructure correlated influence of specimen sizes and anisotropy angles on the UCS estimation based on the primary wave (P-wave) velocity were investigated here. Specimens with four diameters (25 mm to 75 mm) and six anisotropy angles (angles between bedding plane orientation and coring direction, 0° to 90°) were processed. Roles microstructures played for the P-wave propagation were explored by the X-ray computed tomography scanning and P-wave velocity tests. The UCS estimation via P-wave velocity in coal were investigated by regression analysis, significances of scale and anisotropy effect on this correlation were quantitatively evaluated by an introduced one-way analysis of variance method. P-wave velocity for specimens with different diameters exhibits a U-shaped variation against anisotropy angles, it reduces at anisotropy angles 0° ~ 45°, while raises within that of 45° ~ 90°, the minimum, maximum, and second largest value were acquired, respectively, at the anisotropy angle 45°, 90°, and 0°. This was considered caused by the complementary variation of angles between directions of P-wave propagation and orientations of bedding plane and major cleats. P-wave velocity wakens in larger specimens, velocity reductions are significantly at the anisotropy angle of 45°, while less obviously at 0° and 90°. While the anisotropy of P-wave velocity strengthens as diameters gain. Specimen sizes and anisotropy angles were proved to affect the UCS estimation via P-wave velocity by the one-way analysis of variance. Exponential correlation was verified more appropriate in the UCS estimation via P-wave velocity in coal, specimens with diameter 38 mm and anisotropy angle 45° were proved optimal for the UCS estimating by P-wave velocity in coal, since the correlation between them is stronger than other diameters and anisotropy angles. This research facilitates us in quickly and accurately strength estimating in anisotropy coal, meanwhile, it also provides references in extending the lab-obtained mechanical properties of coal/rock specimens to coal/rock mass.

Influence of bedding structure on stress-induced elastic wave anisotropy in tight sandstones

To understand the evolution of stress-induced elastic wave anisotropy, three triaxial experiments were performed on sandstone specimens with bedding orientations parallel, perpendicular, and oblique to the maximum principal stress. P-wave velocities along 64 different directions on each specimen were monitored frequently to understand the anisotropy change at various stress levels by fitting Thomsen's anisotropy equation. The results show that the elastic wave anisotropy is very sensitive to mechanical loading. Under hydrostatic loading, the magnitude of anisotropy is reduced in all three specimens. However, under deviatoric stress loading, the evolution of anisotropic characteristics (magnitude and orientation of the symmetry axis) is bedding orientation dependent. Anisotropy reversal occurs in specimens with bedding normal/oblique to the maximum principal stress. P-wave anisotropy ε ′ is linearly related to volumetric strain S v and dilatancy, indicating that stress-induced redistribution of microcracks has a significant effect on P-wave velocity anisotropy. The closure of initial cracks and pores aligned in the bedding direction contributes to the decrease of the anisotropy. However, opening of new cracks, aligned in the maximum principal direction, accounts for the increase of the anisotropy. The experimental results provide some insights into the microstructural behavior under loading and provide an experimental basis for seismic data interpretation and parameter selection in engineering applications.

EXPERIMENTAL EVALUATION OF ULTRASONIC VELOCITIES AND ANISOTROPY IN THE TUSCALOOSA MARINE SHALE FORMATION

Tuscaloosa Marine Shale (TMS) formation is a clay- and organic-rich emerging shale play with a considerable amount of hydrocarbon resources. Despite the substantial potential, there have been only a few wells drilled and produced in the formation over the recent years. The analyzed TMS samples contain an average of 50 wt% total clay, 27 wt% quartz and 14 wt% calcite and the mineralogy varies considerably over the small intervals. The high amount of clay leads to pronounced anisotropy and the frequent changes in mineralogy result in the heterogeneity of the formation. We studied the compressional (VP) and shear-wave (VS) velocities to evaluate the degree of anisotropy and heterogeneity, which impact hydraulic fracture growth, borehole instabilities, and subsurface imaging. The ultrasonic measurements of P- and S-wave velocities from five TMS wells are the best fit to the linear relationship with R2 = 0.84 in the least-squares criteria. We observed that TMS S-wave velocities are relatively lower when compared to the established velocity relationships. Most of the velocity data in bedding-normal direction lie outside constant VP/VS lines of 1.6–1.8, a region typical of most organic-rich shale plays. For all of the studied TMS samples, the S-wave velocity anisotropy exhibits higher values than P-wave velocity anisotropy. In the samples in which the composition is dominated by either calcite or quartz minerals, mineralogy controls the velocities and VP/VS ratios to a great extent. Additionally, the organic content and maturity account for the velocity behavior in the samples in which the mineralogical composition fails to do so. The results provide further insights into TMS Formation evaluation and contribute to a better understanding of the heterogeneity and anisotropy of the play.

Frequency-dependent P-wave anisotropy due to scattering in rocks with aligned fractures

Frequency-dependent P-wave anisotropy due to scattering often occurs in fractured formations, whereas the corresponding theoretical study is lacking. Hence, based on a newly developed P-wave scattering model, we have studied the frequency-dependent P-wave scattering anisotropy in rocks with aligned fractures. To describe P-wave scattering anisotropy, we develop the corresponding anisotropy parameters similar to those for elastic anisotropy. Our results indicate that the P-wave velocity anisotropy parameters [Formula: see text] and [Formula: see text] do not change with frequency monotonically, which is different from that caused by wave-induced fluid flow. Fluid saturation in fractures can greatly decrease the P-wave velocity anisotropy, whose effects depend on the ratio of the fluid bulk modulus to the fracture aspect ratio. The P-wave exhibits elliptical anisotropy for the dry fracture case at low frequencies, but anelliptical anisotropy for the case with fluid-filled fractures. The P-wave attenuation anisotropy parameters [Formula: see text] and [Formula: see text] vanish in the low- and high-frequency limits but reach their maxima at the characteristic frequency when the P-wavelength is close to the fracture length. The influence of fluid on the P-wave attenuation anisotropy is similar to that on the velocity anisotropy. To further analyze frequency-dependent P-wave scattering anisotropy, theoretical predictions are compared with experimental results, which indicate reasonable agreement between them.

A method to model the effect of pre-existing cracks on P-wave velocity in rocks

Abstract Crack closure is one of the reasons inducing changes of P-wave velocity of rocks under compression. In this context, a method is proposed to investigate the relationships among P-wave velocity, pre-existing cracks, and confining pressure based on the discrete element method (DEM). Pre-existing open cracks inside the rocks are generated by the initial gap of the flat-joint model. The validity of the method is evaluated by comparing the P-wave velocity tested on a sandstone specimen with numerical result. As the crack size is determined by the diameter of particles, the effects of three factors, i.e. number, aspect ratio, and orientation of cracks on the P-wave velocity are discussed. The results show that P-wave velocity is controlled by the (i.e. number) of open micro-cracks, while the closure pressure is determined by the aspect ratio of crack. The reason accounting for the anisotropy of P-wave velocity is the difference in crack number in measurement paths. Both of the number and aspect ratio of cracks can affect the responses of P-wave velocity to the applied confining pressure. Under confining pressure, the number of open cracks inside rocks will dominate the lowest P-wave velocity, and the P-wave velocity of the rock containing narrower cracks is more sensitive to the confining pressure. In this sense, crack density is difficult to be back-calculated merely by P-wave velocity. The proposed method offers a means to analyze the effect of pre-existing cracks on P-wave velocity.