Reflectivity method for geomechanical equilibria     

Boris N. Kuvshinov 《Geophysical Journal International》2007,170(2):567-579

It is shown that the block LU decomposition of the transfer and scattering matrix convert these matrices into each other. This allows to introduce a generalization of the Kennett reflectivity method, which is applicable to arbitrary systems of linear differential equations. The introduced method is convenient to analyse equilibria, where the governing matrix is degenerate. The resulting algorithm is compact and numerically stable. To illustrate the concept, we consider elastic equilibrium of a layered medium. We also derive closed-form expressions for a quasi-stationary poroelastic case taking into account solid–fluid and electrokinetic coupling.  相似文献   

Fluid-dependent shear-wave splitting in a poroelastic medium with conjugate fracture sets     

Robert J. Galvin  Boris Gurevich  Colin M. Sayers 《Geophysical Prospecting》2007,55(3):333-343

The dependence of shear‐wave splitting in fractured reservoirs on the properties of the filling fluid may provide a useful attribute for identifying reservoir fluids. If the direction of wave propagation is not perpendicular or parallel to the plane of fracturing, the wave polarized in the plane perpendicular to the fractures is a quasi‐shear mode, and therefore the shear‐wave splitting will be sensitive to the fluid bulk modulus. The magnitude of this sensitivity depends upon the extent to which fluid pressure can equilibrate between pores and fractures during the period of the deformation. In this paper, we use the anisotropic Gassmann equations and existing formulations for the excess compliance due to fracturing to estimate the splitting of vertically propagating shear waves as a function of the fluid modulus for a porous medium with a single set of dipping fractures and with two conjugate fracture sets, dipping with opposite dips to the vertical. This is achieved using two alternative approaches. In the first approach, it is assumed that the deformation taking place is quasi‐static: that is, the frequency of the elastic disturbance is low enough to allow enough time for fluid to flow between both the fractures and the pore space throughout the medium. In the second approach, we assume that the frequency is low enough to allow fluid flow between a fracture set and the surrounding pore space, but high enough so that there is not enough time during the period of the elastic disturbance for fluid flow between different fracture sets to occur. It is found that the second approach yields a much stronger dependence of shear‐wave splitting on the fluid modulus than the first approach. This is a consequence of the fact that at higher wave frequencies there is not enough time for fluid pressure to equilibrate and therefore the elastic properties of the fluid have a greater effect on the magnitude of the shear‐wave splitting.  相似文献   

Reflection coefficients for weak anisotropic media   总被引：1，自引：0，他引：1  

Matthias Zillmer  Dirk Gajewski  Boris M. Kashtan 《Geophysical Journal International》1997,129(2):389-398

The interaction of plane elastic waves with a plane boundary between two anisotropic elastic half-spaces is investigated. The anisotropy dealt with in this study is of a general type. Explicit expressions for energy-related reflection and transmission coefficients are derived. They represent an approximation which is valid for a small deviation of the elastic parameters from isotropy.
Classical perturbation theory is applied on a 6times6 non-symmetric real eigenvalue problem to calculate first-order corrections for the polarization and stress of the plane waves. The explicit solution of the isotropic problem is used as a reference case. Degenerate perturbation theory is used to consider the splitting of the isotropic S -wave into two anisotropic qS-waves. The boundary conditions for two half-spaces in welded contact lead to a 6times6 system of linear equations. A correction to the isotropic solution is calculated by linearization. The resultant coefficients are functions of horizontal slowness, Lamé parameters and densities of the reference media, and of the perturbation of the elasticity tensors from isotropy.  相似文献   

Detection of the water level in fractured phreatic aquifers using nuclear magnetic resonance (NMR) geophysical measurements   总被引：1，自引：0，他引：1  

Israel Gev  Mark Goldman  Boris Rabinovich  Michael Rabinovich  Arie Issar 《Journal of Applied Geophysics》1996,34(4):277-282

Correlation of geophysical data collected using the NMR method in the Negev Desert, Israel, with hydrogeological data from nearby observation wells is presented. The experiment was conducted near Kibbutz Revivim in the Besor drainage system (Fig. 1). The objective of the survey was to detect groundwater layers in the Quaternary cover filling and Eocene fractured aquifers down to a depth of 100 m. The experiment was performed using a combination of two different geophysical techniques, namely the NMR and time domain electromagnetic (TDEM) methods. The geophysical results were verified by measuring the water level in three observation wells, two of which were drilled several months after the geophysical survey was carried out.The water level measured in these follow-up observation wells shortly after drilling did not coincide with the geophysical data. However, it settled over a period of time and finally stabilized at a depth very similar to that obtained from the NMR measurements. This phenomenon is caused by the fractured nature of the phreatic aquifer. Since the flow of water in such aquifers is confined by the fractures, the appearance of water in the well during or shortly after drilling is determined solely by the intersection of the well and the fracture. Our experiments showed that geophysical measurements in fractured phreatic aquifers may have a distinct advantage over direct borehole measurements, since the former average the depth to the water table over large areas (several thousand square meters) while the latter are limited by the area of the borehole cross-section (several tens of square centimeters).  相似文献   

Climatic and limnological changes at Lake Karakul (Tajikistan) during the last ~29 cal ka     

Liv Heinecke  Steffen Mischke  Karsten Adler  Anja Barth  Boris K. Biskaborn  Birgit Plessen  Ingmar Nitze  Gerhard Kuhn  Ilhomjon Rajabov  Ulrike Herzschuh 《Journal of Paleolimnology》2017,58(3):317-334

We present results of analyses on a sediment core from Lake Karakul, located in the eastern Pamir Mountains, Tajikistan. The core spans the last ~29 cal ka. We investigated and assessed processes internal and external to the lake to infer changes in past moisture availability. Among the variables used to infer lake-external processes, high values of grain-size end-member (EM) 3 (wide grain-size distribution that reflects fluvial input) and high Sr/Rb and Zr/Rb ratios (coinciding with coarse grain sizes), are indicative of moister conditions. High values in EM1, EM2 (peaks of small grain sizes that reflect long-distance dust transport or fine, glacially derived clastic input) and TiO₂ (terrigenous input) are thought to reflect greater influence of dry air masses, most likely of Westerly origin. High input of dust from distant sources, beginning before the Last Glacial Maximum (LGM) and continuing to the late glacial, reflects the influence of dry Westerlies, whereas peaks in fluvial input suggest increased moisture availability. The early to early-middle Holocene is characterised by coarse mean grain sizes, indicating constant, high fluvial input and moister conditions in the region. A steady increase in terrigenous dust and a decrease in fluvial input from 6.6 cal ka BP onwards points to the Westerlies as the predominant atmospheric circulation through to present, and marks a return to drier and even arid conditions in the area. Proxies for productivity (TOC, TOC/TN, TOC _Br ), redox potential (Fe/Mn) and changes in the endogenic carbonate precipitation (TIC, δ¹⁸O _Carb ) indicate changes within the lake. Low productivity characterised the lake from the late Pleistocene until 6.6 cal ka BP, and increased rapidly afterwards. Lake level remained low until the LGM, but water depth increased to a maximum during the late glacial and remained high into the early Holocene. Subsequently, the water level decreased to its present stage. Today the lake system is mainly climatically controlled, but the depositional regime is also driven by internal limnogeological processes.  相似文献   

Spotless flares and type II radio bursts     

Michal Šeršeň  Boris Valníček 《Solar physics》1993,145(2):339-345

Flares accompanied by type II meter radio bursts that occurred in plages with no visible spots are examined in this paper. There have been found 12 such spotless flares observed in the period of January 1981–August, 1990. Six out of all the flares may be said to have not been associated with any filament activation or disruption. A few of these flares have shown features of major events. The study suggests that a filament activation seems not to be the crucial factor for the occurrence of major flares in regions with no visible spots.  相似文献   

Rheology of the Earth and a thermoconvective mechanism for sedimentary basin formation   总被引：1，自引：0，他引：1  

Boris I. Birger 《Geophysical Journal International》1998,134(1):1-12

A power-law non-Newtonian fluid is usually assumed to model slow flows in the mantle and, in particular, convective flows. However, the power-law fluid has no memory, in contrast to a real material. A new non-linear integral (having a memory) model is proposed to describe the rheology of rocks. The model is consistent with the theory of simple fluids with fading memory and with laboratory studies of rock creep. The proposed model reduces to the power-law fluid model for stationary flows and to the Andrade model for flows associated with small strains. Stationary convection beneath continents has been studied by Fleitout & Yuen (1984 ), who used the power-law fluid model and obtained the cold immobile boundary layer (continental lithosphere). In a stability analysis of this layer, the Andrade model must be used. The analysis shows that the lithosphere is overstable (the period of oscillation is about 200  Ma). In the present study, it is suggested that these thermoconvective oscillations of the lithosphere are a mechanism for sedimentary basin formation. The vertical crustal movement in sedimentary basins can be considered as a slow subsidence on which small-amplitude oscillations are superimposed. The longest period of oscillatory crustal movement is of the same order of magnitude as the period of convective oscillation of the lithosphere found in the stability analysis. Taking into account the difference between depositional and erosional transport rates we can explain the permanent subsidence as well as the oscillations.  相似文献   

Contourites associated with pelagic mudrocks and distal delta-fed turbidites in the Lower Proterozoic Timeball Hill Formation epeiric basin (Transvaal Supergroup), South Africa     

Patrick G. Eriksson  Boris F. F. Reczko 《Sedimentary Geology》1998,120(1-4):319-335

Five genetic facies associations/architectural elements are recognised for the epeiric sea deposits preserved in the Early Proterozoic Timeball Hill Formation, South Africa. Basal carbonaceous mudrocks, interpreted as anoxic suspension deposits, grade up into sheet-like, laminated, graded mudrocks and succeeding sheets of laminated and cross-laminated siltstones and fine-grained sandstones. The latter two architectural elements are compatible with the Te, Td and Tc subdivisions of low-density turbidity current systems. Thin interbeds of stromatolitic carbonate within these first three facies associations support photic water depths up to about 100 m. Laterally extensive sheets of mature, cross-bedded sandstone disconformably overlie the turbidite deposits, and are ascribed to lower tidal flat processes. Interbedded lenticular, immature sandstones and mudrocks comprise the fifth architectural element, and are interpreted as medial to upper tidal flat sediments. Small lenses of coarse siltstone–very fine-grained sandstone, analogous to modern continental rise contourite deposits, occur within the suspension and distal turbidite sediments, and also form local wedges of inferred contourites at the transition from suspension to lowermost turbidite deposits. Blanketing and progressive shallowing of the floor of the Timeball Hill basin by basal suspension deposits greatly reduced wave action, thereby promoting preservation of low-density turbidity current deposits across the basin under stillstand or highstand conditions. A lowstand tidal flat facies tract laid down widespread sandy deposits of the medial Klapperkop Member within the formation. Salinity gradients and contemporaneous cold periglacial water masses were probably responsible for formation of the inferred contourites. The combination of the depositional systems interpreted for the Timeball Hill Formation may provide a provisional model for Early Proterozoic epeiric basin settings.  相似文献   

The regularizing function in resonance problems     

Boris Garfinkel 《Celestial Mechanics and Dynamical Astronomy》1980,22(1):13-18

The regularizing function ψ(x) in the theory of resonance removes the singularities discovered by Poincaré (1893), of the form 1/x, at theturning points x ₁ andx ₂ of thecritical argument x, librating in the rangex ₁≤x≤x ₂. This function has been explicitly introduced into the HamiltonianF ₀ of the Ideal Resonance Problem in the author's recent paper (1977) in order to remove the singularities in the second-order perturbations. It is shown here that this procedure can be extended toall orders. ThenF ₀ can be put into the form $$F_0 = B(y) + \Psi (x)$$ where ψ is thecomplete regularizing function, removing theclassical singularity of thesmall divisor, in addition to the singularities of Poincaré.  相似文献   

A three-dimensional hydrodynamic model of estuarine circulation with an application to Southampton Water, UK     

Anne Levasseur  Lei Shi  Neil C. Wells  Duncan A. Purdie  Boris A. Kelly-Gerreyn   《Estuarine, Coastal and Shelf Science》2007,73(3-4):753-767

A three-dimensional hydrodynamic model has been developed to simulate water mass circulation in estuarine systems. This model is based on the primitive equation in Cartesian coordinates with a terrain-following structure, coupled with a Mellor–Yamada 2.5 turbulence scheme. A fractional-step method is applied and the subset of equations is solved with finite volume and finite element methods. A dry–wet process simulates the presence of the tidal flat at low water. River inputs are introduced using a point-source method. The model was applied to a partially mixed, macrotidal, temperate estuary: Southampton Water, UK. The model is validated by comparisons with sea surface elevation, ADCP measurements and salinity data collected in 2001. The mean spring range 2(M₂ + S₂) and the mean neap range 2(M₂ − S₂) are modelled with an error relative to observation of 12 and 16%, respectively. The unique tidal regime of the system with the presence of the ‘young flood stand’ corresponding to the slackening conditions occurring at mid flood and ‘double high water’ corresponding to an extension of the slackening conditions at high tide is accurately reproduced in the model. The dynamics of the modelled mean surface and bottom velocity closely match the ADCP measurements during neap tides (rms of the difference is 0.09 and 0.01 m s⁻¹ at the bottom and at the surface, respectively), whereas at spring the difference is greater (rms of the difference is 0.25 and 0.20 m s⁻¹ at bottom and surface, respectively). The spatial and temporal variation of the degree of stratification as indicated by salinity distributions compares well with observations.  相似文献