Generalized moveout approximation for P–SV converted waves in vertically inhomogeneous transversely isotropic media with a vertical symmetry axis          下载免费PDF全文

Qi Hao  Alexey Stovas 《Geophysical Prospecting》2016,64(6):1469-1482

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Effect of Decoupling of Lithospheric Plates on the Observed Geoid     

Mikhail K. Kaban  Alexey G. Petrunin  Harro Schmeling  Meysam Shahraki 《Surveys in Geophysics》2014,35(6):1361-1373

A joint effect of weak zones, dividing lithospheric plates, and lateral viscosity variations (LVV) in the whole mantle on the observed geoid is investigated by a new numerical approach. This technique is based on the substantially revised method introduced by Zhang and Christensen (Geophys J Int 114:531–547, 1993) for solving the Navier–Stokes–Poisson equations in the spectral domain with strong LVV. Weak plate boundaries (WPB) are introduced based on an integrated global model of plate boundary deformations GSRM (Kreemer et al. in Geophys J Int 154:8–34, 2003). The effect of WPB on the geoid is significant and reaches ?40 to 70 m with RMS ~20 m. The peaks are observed over large subduction zones in South America and the southwestern Pacific in agreement with previous studies. The positive geoid anomaly in South America could be explained largely by a dynamic effect of decoupling of the Nazca and South American plates. The negative changes of the geoid mostly relate to mid-oceanic ridges. The amplitude of the effect depends on the viscosity contrasts at WPB compared with the plate viscosity until its value reaches the limit of 2.5–3 orders of magnitude. This value might be considered as a level at which the plates are effectively decoupled. The effect of WPB exceeds the effect of LVV in the whole mantle and generally does not correlate with it. However, inclusion of LVV reduces the geoid perturbations due to WPB by about 10 m. Therefore, it is important to consider all factors together. The geoid changes mainly result from changes of the dynamic topography, which are about ?300 to +500 m. The obtained results show that including WPB may significantly improve the reliability of instantaneous global dynamic models.  相似文献   

Sensitivity of the southern annular mode to greenhouse gas emission scenarios     

Graham R. Simpkins  Alexey Yu. Karpechko 《Climate Dynamics》2012,38(3-4):563-572

The leading mode of southern hemisphere (SH) climatic variability, the southern annular mode (SAM), has recently seen a shift towards its positive phase due to stratospheric ozone depletion and increasing greenhouse gas (GHG) concentrations. Here we examine how sensitive the SAM (defined as the leading empirical orthogonal function of SH sea level pressure anomalies) is to future GHG concentrations. We determine its likely evolution for three intergovernmental panel on climate change (IPCC) special report on emission scenarios (SRES) for austral summer and winter, using a multi-model ensemble of IPCC fourth assessment report models which resolve stratospheric ozone recovery. During the period of summer ozone recovery (2000–2050), the SAM index exhibits weakly negative, statistically insignificant trends due to stratospheric ozone recovery which offsets the positive forcing imposed by increasing GHG concentrations. Thereafter, positive SAM index trends occur with magnitudes that show sensitivity to the SRES scenario utilised, and thus future GHG emissions. Trends are determined to be strongest for SRES A2, followed by A1B and B1, respectively. The winter SAM maintains a similar dependency upon GHG as summer, but over the entire twenty-first century and to a greater extent. We also examine the influence of ozone recovery by comparing results to models that exclude stratospheric ozone recovery. Projections are shown to be statistically different from the aforementioned results, highlighting the importance of ozone recovery in governing SAM-evolution. We therefore demonstrate that the future SAM will depend both upon GHG emissions and stratospheric ozone recovery.  相似文献   

Geothermochronology based on noble gases: I. Stability of the U-Xe isotopic system in nonmetamict zircons     

Yu. A. Shukolyukov  M. M. Fugzan  I. P. Paderin  S. A. Sergeev  D. P. Krylov 《Petrology》2009,17(1):1-24

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Effects of Hypomagnetic Conditions and Reversed Geomagnetic Field on Calcium-Dependent Proteases of Invertebrates and Fish     

N. P. Kantserova  V. V. Krylov  L. A. Lysenko  N. V. Ushakova  N. N. Nemova 《Izvestiya Atmospheric and Oceanic Physics》2017,53(7):719-723

The effects of hypomagnetic conditions and the reversal of the geomagnetic field (GMF) on intracellular Ca²⁺-dependent proteases (calpains) of fish and invertebrates have been studied in vivo and in vitro. It is found that the intravital exposure of examined animals to hypomagnetic conditions leads to a significant decrease in its calpain activity. The activity of preparations of calcium-dependent proteases was tested in separate experiments. It is shown that preparations of Ca²⁺-dependent proteases from invertebrates and fish are also inactivated substantially under effect of hypomagnetic conditions. The ambiguous results obtained in the experiments with a reversed GMF do not make it possible to discuss the biological response of calcium-dependent proteases to the reversal of the GMF.  相似文献   

Local distribution of oxygen isotopes and fluid exchange during genesis of the corundum-bearing rocks of Khitostrov Island     

D. P. Krylov  V. A. Glebovitsky 《Doklady Earth Sciences》2017,473(2):441-443

New data are presented on the distribution of oxygen isotopes and conditions of the local isotope equilibrium in high-Al rocks rocks of Khitostrov Island showing abnormally low δ¹⁸O values (below–25‰). The temperatures of isotope equilibrium are within 400–475°C. The minimum δ¹⁸O values have been registered in the in plagioclase, whereas the same phases in kyanite-bearing rocks lacking corundum demonstrate δ¹⁸O values usually 3–5‰ higher. The fluid δ¹⁸O value varies from–22 to–16‰ at 475 ± 15°C, from–18 to–23‰ at 425 ± 25°C, and from–17 to–22‰ at 380 ± 15°C. The results obtained do not require abnormal depletion of δ¹⁸O values owing to the infiltration of an external fluid under the Svecofennian transformations. The association of corundum-bearing rocks with the basic intrusions, the presence of zircon cores of older ages compared to these rocks, and the peculiarities of rock chemistry may be ascribed to the fact that lower crustal layers of ancient rocks depleted in δ¹⁸O before metamorphism were captured by basite melts.  相似文献   

Using the Digital Models of Gravity Anomalies for Zoning of the Earth’s Gravity Field     

V. N. Koneshov  S. A. Krylov  D. S. Loginov  V. B. Nepoklonov 《Izvestiya Physics of the Solid Earth》2018,54(6):964-970

This paper addresses the questions associated with using the models in regioning the Earth’s gravity field depending on its roughness and type of anomalies. A formalized technique for zoning based on the analysis of the intensity and variability of gravity anomalies is suggested. The suggested technique includes preprocessing the initial gravity anomalies aimed at eliminating the noise component; partitioning the studied region into the elementary segments; calculating the primary characteristics of the complexity and roughness (the intensity and variability of gravity anomalies) for each segment; and, based on these characteristics, classifying each segment into a particular category of complexity. The proposed system of classification of segments relies on the use of three classes of intensity and three classes of variability of gravity anomalies and four categories of complexity of the regions, which are derived from these classes. As a result of applying the technique, the mapped territory is subdivided into the regions that are uniform in terms of their geophysical properties. The developed technique is used for comparing the quality of the different digital models of gravity anomalies, including the Russian RGM model (A.P. Karpinsky Russian Geological Research Institute (VSEGEI)) and the WGM model (Bureau Gravimétrique International (BGI)) in the region of the Mongol–Okhotsk orogenic belt and Sikhote–Alin fold system. The results of the study can be used for zoning of the other geophysical fields and for planning the locations of the new survey networks for increasing the accuracy of the initial data used for compiling the maps.  相似文献   

Low-frequency anisotropy in fractured and layered media     

Shuo Pang  Alexey Stovas 《Geophysical Prospecting》2020,68(2):353-370

Computing effective medium properties is very important when upscaling data measured at small scale. In the presence of stratigraphic layering, seismic velocities and anisotropy parameters are scale and frequency dependent. For a porous layer permeated by aligned fractures, wave-induced fluid flow between pores and fractures can also cause significant dispersion in velocities and anisotropy parameters. In this study, we compare the dispersion of anisotropy parameters due to fracturing and layering at low frequencies. We consider a two-layer model consisting of an elastic shale layer and an anelastic sand layer. Using Chapman's theory, we introduce anisotropy parameters dispersion due to fractures (meso-scale) in the sand layer. This intrinsic dispersion is added to anisotropy parameters dispersion induced by layering (macro-scale) at low frequencies. We derive the series coefficients that control the behaviour of anisotropy parameters at low frequencies. We investigate the influences of fracture length and fracture density on fracturing effect, layering effect and combined effect versus frequency and volume fraction of sand layer. Numerical modelling results indicate that the frequency dependence due to layering is not always the dominant effect of the effective properties of the medium. The intrinsic dispersion is not negligible compared with the layering effect while evaluating the frequency-dependent properties of the layered medium.  相似文献   

Frequency-dependent PP and PS reflection coefficients in fractured media     

Shuo Pang  Alexey Stovas 《Geophysical Prospecting》2020,68(3):926-940

When a porous layer is permeated by mesoscale fractures, wave-induced fluid flow between pores and fractures can cause significant attenuation and dispersion of velocities and anisotropy parameters in the seismic frequency band. This intrinsic dispersion due to fracturing can create frequency-dependent reflection coefficients in the layered medium. In this study, we derive the frequency-dependent PP and PS reflection coefficients versus incidence angle in the fractured medium. We consider a two-layer vertical transverse isotropy model constituted by an elastic shale layer and an anelastic sand layer. Using Chapman's theory, we introduce the intrinsic dispersion due to fracturing in the sand layer. Based on the series coefficients that control the behaviour of velocity and anisotropy parameters in the fractured medium at low frequencies, we extend the conventional amplitude-versus-offset equations into frequency domain and derive frequency-dependent amplitude-versus-offset equations at the elastic–anelastic surface. Increase in fracture length or fracture density can enlarge the frequency dependence of amplitude-versus-offset attributes of PP and PS waves. Also, the frequency dependence of magnitude and phase angle of PP and PS reflection coefficients increases as fracture length or fracture density increases. Amplitude-versus-offset type of PP and PS reflection varies with fracture parameters and frequency. What is more, fracture length shows little impact on the frequency-dependent critical phase angle, while the frequency dependence of the critical phase angle increases with fracture density.  相似文献   

The kinematics of S waves in acoustic orthorhombic media     

Shibo Xu  Alexey Stovas 《Geophysical Prospecting》2019,67(8):1983-2002

Orthorhombic models are often used in the seismic industry nowadays to describe azimuthal and polar anisotropy and reasonably realistic in capturing the features of the earth interior. It is challenging to handle so many model parameters in the seismic data processing. In order to reduce the number of the parameters for P wave, the acoustic orthorhombic medium is proposed by setting all on-axis S wave velocities to zero. However, due to the coupled behaviour for P and S waves in the orthorhombic model, the ‘S wave artefacts’ are still remained in the acoustic orthorhombic model, which kinematics needs to be defined and analysed. In this paper, we analyse the behaviour of S wave in acoustic orthorhombic media. By analysis of the slowness surface in acoustic orthorhombic media, we define the S waves (or S wave artefacts) that are more complicated in shape comparing to the one propagating in an acoustic transversely isotropic medium with a vertical symmetry axis. The kinematic properties of these waves are defined and analysed in both phase and group domain. The caustics, amplitude and the multi-layered case for S wave in acoustic orthorhombic model are also discussed. It is shown that there are two waves propagating in this acoustic orthorhombic medium. One of these waves is similar to the one propagating in acoustic vertical symmetry axis media, whereas another one has a very complicated shape consisting of two crossing surfaces.  相似文献