共查询到20条相似文献，搜索用时 15 毫秒
1.
《Geophysical Journal International》1969,18(5):441-460
Summary Many accept thermal convection within the mantle of the Earth as the driving mechanism for continental drift. It is also of considerable interest to determine whether thermal convection is occurring within Venus, Mars, and the Moon. In this paper a systematic treatment of the stability of planetary interiors is given. The thermal stability problem for a layer of fluid heated from below is solved when the viscosity of the fluid increases exponentially with depth. For a semi-infinite fluid with exponentially increasing viscosity the critical Rayleigh number based on the surface viscosity and the scale length of the viscosity increase is found to be 30 for a fixed surface boundary condition and 23 for a free surface boundary condition. This stability analysis is also extended to include volume heat release. The thermal stability of the interiors of the Earth, Venus, Mars and the Moon is examined. Using temperature-depth profiles in the literature and a theoretical expression for the viscosity of a crystalline solid based on diffusion creep, viscosity depth profiles for planetary interiors are obtained. Because of the strong pressure effect the viscosity within the Earth and Venus increases greatly from a near surface minimum. For Mars the increase is less pronounced and within the interior of the Moon the viscosity is nearly constant. For all the cases considered the planetary interiors are found to be thermally unstable. Because of the dependence of viscosity on depth the interiors of the Moon and Mars are found to be considerably less stable than Venus and the Earth. It is concluded that thermal convection is occurring within the planetary bodies considered. 相似文献
2.
The vertical wind-velocity profile within the saltation layer is investigated theoretically. New equations for the fluid shear stress distribution in the saltation layer and the velocity profile affected by saltation are derived. The dependence of the velocity profile on the threshold shear velocity, the vertical variations of sand grain velocity, the vertical sand-mass-flux distribution, and the fluid shear stress within the saltation are demonstrated. Velocity profiles with or without Bagnold's focus are predicted. These velocity profiles are not very sensitive to the vertical distribution of sand-mass flux. Predicted velocity profiles are compared with profiles measured in the wind tunnel, and satisfactory agreement has been achieved. 相似文献
3.
Hydrostatic or “normal” pressure can be easily visualized as a water column with pressure given by ρgh and any departures classified as abnormal pressure. This is the basis for commonly used hydrostatic pressure depth trends in sedimentary basins that are constructed on assumptions of constant gradients and are datumed at mean sea level or ground level. But the straightforward water column concept does not upscale in a simple way to sedimentary basins where the zones of interest are several thousands of metres below the land or sea surface. Sedimentary basins are heterogeneous, including stacked, confined reservoirs and variations in pore water composition. It is possible to construct pressure-depth profiles that honour the geology and hydrostratigraphy of a basin and these give different hydrostatic baselines from simple constant gradients hung from familiar local datums such as ground level. Key steps are using a reservoir-specific datums such as the water table or potentiometric surface relevant to that unit, then building a pressure-depth trend that represents the pore fluid salinity variation and density profile throughout the reservoir unit. At a given depth, this version of hydrostatic may predict pressures several hundred psi different from a single density gradient hung from a datum local to the well, and exhibit a notched profile reflecting the geological and hydrological stratigraphy. This construct redefines normal and abnormal pore fluid pressures in sedimentary basins. The impacts of this alternative approach to sedimentary basin hydrostatics, even if data are limited and pressure profiles have to be framed probabilistically, extend to many aspects of studying and interacting with fluid systems in sedimentary basins including basin modelling, petroleum systems analysis, well planning and well operations. 相似文献
4.
A. G. Davies 《Geophysical Journal International》1977,51(2):503-529
5.
风沙研究者非常重视对输沙通量随高度变化特征的研究,并为寻找可靠的测量手段付出了不懈的努力。基于高速摄影技术获得的沙粒平均水平速度与沙粒数的垂直剖面,推导了较低风速下环境风洞内输沙通量的垂直剖面。结果表明：沙粒平均水平速度随高度呈幂函数增加,颗粒浓度随高度的算数平方根呈指数衰减。由颗粒平均水平速度剖面与浓度剖面的乘积可获得输沙通量剖面。所获得的输沙通量随高度变化曲线在距床面1~3 mm处均有一个明显的拐点,拐点上方输沙通量随高度呈指数衰减。在床面与拐点之间输沙通量没有明显的变化趋势,这可能是由于气流中颗粒间的碰撞以及颗粒与床面碰撞的影响。平均跃移高度和相对衰减系数是描述输沙通量随高度变化的两个重要参数,两者有着很好的相关性,表明了随着风速增加和沙粒粒径减小跃移颗粒可以达到更大的高度,随着风速减小与粒径增大,输沙通量迅速衰减。 相似文献
6.
In a blowing sand system,the wind provides the driving forces for the particle movement while the moving particles exert the opposite forces to the wind by extracting its momentum.The wind-sand interaction that can be characterized by shear stress and force exerted on the wind by moving particles results in the modification of wind profiles.Detailed wind pro-files re-adapted to blown sand movement are measured in a wind tunnel for different grain size populations and at differ-ent free-stream wind velocities.The shear stress with a blowing sand cloud and force exerted on the wind by moving par-ticles are calculated from the measured wind velocity profiles.The results suggest that the wind profiles with presence of blowing sand cloud assume convex-upward curves on the u(z)-ln(z) plot compared with the straight lines characterizing the velocity profiles of clean wind,and they can be better fitted by power function than log-linear function.The exponent of the power function ranging from 0.1 to 0.17 tends to increase with an increase in wind velocity but decrease with an increase in particle size.The force per unit volume exerted on the wind by blown sand drift that is calculated based on the empirical power functions for the wind velocity profiles is found to decrease with height.The particle-induced force makes the total shear stress with blowing sand cloud partitioned into air-borne stress that results from the wind velocity gradient and grain-borne stress that results from the upward or downward movement of particles.The air-borne stress in-creases with an increase in height,while the grain-borne stress decreases with an increase in height.The air-borne shear stress at the top of sand cloud layer increases with both wind velocity and grain size,implying that it increases with sand transport rate for a given grain size.The shear stress with a blowing sand cloud is also closely related to the sand transport rate.Both the total shear stress and grain-borne stress on the grain top is directly proportional to the squ 相似文献
7.
Magnus Wangen 《Geophysical Journal International》1997,130(3):757-764
The fluid-pressure build-up due to porosity reduction in sedimentary basins during burial is studied. The model assumes that the void ratio decreases exponentially with depth, and that the permeability is proportional to the void ratio to an arbitrary exponent. Simple analytical solutions are obtained for the Darcy velocity and the fluid excess pressure. The pressure build-up during burial is studied with these solutions, and it is found to be inversely proportional to the gravity number. The importance of the permeability exponents on the fluid pressure is also studied. Gravity numbers much less than 1 are shown to yield high excess pressures during burial. A reasonable approximation for the maximum Darcy velocity is found to be the product of the surface void ratio and the burial rate. Hydrofracturing is discussed in relation to the pressure build-up, and cases characterized by gravity numbers much less than 1 are found to yield hydrofracturing over large depth ranges. It is suggested that the average permeability of hydrofractured sediments during burial corresponds to a gravity number equal to 1. 相似文献
8.
Summary. A simplified model of convection in the mantle is used to investigate the transient effect of cooling a fluid layer from above, The model, representing the mantle overlain by the lithosphere, consists of a two-dimensional fluid layer overlain by a solid conducting lid. The initial temperature of both layers is the same, with the top surface of the lid kept at 0°C throughout. We observe the onset of small-scale flow in the model. In the absence of internal heating the behaviour of the system is controlled by the Rayleigh number, R , and the ratio of the thicknesses of the two layers, a . The onset time of convection as defined by reference to conduction temperature profiles is related simply to a boundary layer critical Rayleigh number. The mean temperature profiles for the convection model are also compared with the observed depth—age relation for oceanic lithosphere and the results are used to estimate the viscosity of the mantle. 相似文献
9.
S. A. Miller 《Geophysical Journal International》2008,173(1):334-338
Recently reported rain-triggered seismicity from three separate storms occurred exclusively in karst geology. In this paper, I discuss how the hydrogeology of karst controls rain-triggered seismicity by channeling of the watershed after intense rainfall directly into the karst network. Such channeling results in very large increases in hydraulic head, and more importantly, substantially increases the vertical stress acting on the underlying pore-elastic media. Rapid loading upon a pore-elastic media induces seismicity by increasing pore pressure at depth in a manner similar to that observed from reservoir impounding. Using a simple 1-D model of a pore-elastic medium, it is shown that the instantaneous fluid pressure increase at depth is a substantial fraction of the pressure step applied at the boundary, followed by time-dependent pore pressure increases associated with the typical linear diffusion problem. These results have implications for the change in fluid pressure necessary to trigger earthquakes, and leads to the following hypothesis to be tested: Unambiguous rain-triggered seismicity will only occur in karst regions. 相似文献
10.
We investigated the effects of rainfall and the number of animals on changes in vegetation and on the output of milk and meat from the communal areas of Namaqualand. Previously published short- and long-term models link processes that range from the levels of tissue (in, for example, the mammary gland), to the milk yields of individual animals, to the growth and survival of their young and to long-term changes in plant species populations at the ecosystem level. These models have been used to study how different factors and management strategies affect livestock productivity and vegetation composition on a 20,000 ha rangeland in Namaqualand. First, the inter-relations between rainfall, stocking rate and productivity were studied using the short-term model. This model shows that in addition to total rainfall and stocking rate, the timing of rainfall within a year also influences doe live weight and survival to the end of the year. When the long-term model is run, using recorded rainfall, predictions of small stock numbers agree closely with livestock data recorded over the same 30-year period. One thousand replicates of 100-year runs of the long-term model were then used to study the probable impact of different upper limits to stock numbers on animal performance. Off take (sales and slaughterings) are maximal when stock numbers are limited to 2000 adults. Animal numbers increase marginally as the limit is increased above this level, but the variability between years in numbers increases. Secondly, the long-term model was used to study the long-term effects of the stocking rate strategies on rangeland condition. The model predicts that although these effects are variable, when moderately degraded range is stocked with an upper limit at the recommended level it is unable to recover to less degraded states over 100 years. Thirdly, the model was used to examine the effects of reduction in stock numbers and slaughtering of kids in a drought year on goat numbers during the subsequent 5 years. Finally, the model predicts that a 10% reduction in mean annual rainfall will lead to a 35% reduction in animal numbers over 200 years. 相似文献
11.
Jinwei Gao Nathan Bangs Shiguo Wu Guanqiang Cai Shuoshuo Han Benjun Ma Jiliang Wang Yangbing Xie Wenkai Huang Dongdong Dong Dawei Wang 《Basin Research》2019,31(4):688-708
Submarine magmatism and associated hydrothermal fluid flows has significant feedback influence on the petroleum geology of sedimentary basins. This study uses new seismic profiles and multibeam bathymetric data to examine the morphology and internal architecture of post‐seafloor spreading magmatic structures, especially volcanoes of the Xisha uplift, in extensive detail. We discover for the first time hydrothermal systems derived from magmatism in the northwestern South China Sea. Numerous solitary volcanoes and volcanic groups occur in the Xisha uplift and produce distinct seismic reflections together with plutons. Sills and other localized amplitude anomalies were fed by extrusions/intrusions and associated fluid flow through fractures and sedimentary layers that may act as conduits for magma and fluid flows transport. Hydrothermal structures such as pipes and pockmarks mainly occur in the proximity of volcanoes or accompany volcanic groups. Pipes, pockmarks and localized amplitude anomalies mainly constitute the magmatic hydrothermal systems, which are probably driven by post‐seafloor spreading volcanoes/plutons. The hydrothermal fluid flows released by magma degassing or/and related boiling of pore fluids/metamorphic dehydration reactions in sediments produced local overpressures, which drove upward flow of fluid or horizontal flow into the sediments or even seafloor. Results show that post‐seafloor spreading magmatic activity is more intense during a 5.5 Ma event than one in 2.6 Ma, whereas the hydrothermal activities are more active during 2.6 Ma than in 5.5 Ma. Our analysis indicates that post‐seafloor spreading magmatism may have a significant effect on hydrocarbon maturation and gas hydrate formation in the Xisha uplift and adjacent petroliferous basins. Consequently the study presented here improves our understanding of hydrocarbon exploration in the northwestern South China Sea. 相似文献
12.
Vertical profiles of aeolian sand mass flux 总被引：5，自引：0，他引：5
Vertical profiles of the horizontal mass flux of blown sand are investigated experimentally using a passive vertical array in a wind tunnel. Considering lower sampling efficiency of the sand trap in the near-bed region, this investigation is complemented by the measurements of the longitudinal profiles of mass flux made using a horizontal sand trap. The experiments were conducted with two test sands and five different stream velocities.In the upper part of the vertical profile, the measured data exhibit an exponential decay distribution with a positive deviation occurring in the near-bed region. The measured longitudinal profiles are similar to the measured vertical profiles. Linking both profiles and the modes of sand transport, it is possible that saltating sand grains give rise to the well-known exponential decay distribution of mass flux, and that creeping and reptating grains force a deviation from it. A simple equation applicable for both the vertical and the longitudinal sand mass flux variations is introduced and the parameters are estimated from experimental data. 相似文献
13.
C. Coruh J.K. Costain R.D. Hatcher Jr. T.L. Pratt R.T. Williams R.A. Phinney 《Geophysical Journal International》1987,89(1):147-156
Summary. In 1985, 180 km of regional vibroseis profiles were acquired in the Carolinas and Georgia, southeastern United States, as part of the Appalachian Ultra-Deep Core Hole (ADCOH) Site Study. The data quality is excellent, with large-amplitude reflections from faults and crystalline rocks, lower Palaeozoic shelf strata and from within autochthonous Grenville basement. The profiles image the subsurface more clearly than other available data and allow the possibility of alternative interpretations of important elements of the tectonic framework of the southern Appalachians.
The major points in the interpretation are: 1) The Blue Ridge master decollement is at a depth of 2-3 km beneath the Blue Ridge. This thrust increases in dip just NW of the Brevard fault zone. 2) The Brevard fault zone appears to splay from the master decollement at 6 km (2.2 s) near Westminster, S.C., and defines the base of the crystalline Inner Piedmont allochthon. 3) Below the Blue Ridge thrust sheet are images of duplex and imbricate structures ("duplex tuning wedges") connected by other thrust faults that duplicate shelf strata to a thickness of 4–5 km. 4) Subhorizontal reflections from depths of 6 to 9 km may be from relatively undisturbed lower Palaeozoic strata as suggested by others. 5) Eocambrian-Cambrian(?) rift basins in the Grenville basement are also imaged.
The ADCOH data were originally recorded with 14–56 Hz bandwidth and 8 s length, but an extended Vibroseis correlation was used to produce 17 s data length revealing reflections from within the upper crust. Below 8 s, reflections from within the Grenville basement become weak, but are observable as late as 13 s; however, these Moho (?) reflections are generally short segments. 相似文献
The major points in the interpretation are: 1) The Blue Ridge master decollement is at a depth of 2-3 km beneath the Blue Ridge. This thrust increases in dip just NW of the Brevard fault zone. 2) The Brevard fault zone appears to splay from the master decollement at 6 km (2.2 s) near Westminster, S.C., and defines the base of the crystalline Inner Piedmont allochthon. 3) Below the Blue Ridge thrust sheet are images of duplex and imbricate structures ("duplex tuning wedges") connected by other thrust faults that duplicate shelf strata to a thickness of 4–5 km. 4) Subhorizontal reflections from depths of 6 to 9 km may be from relatively undisturbed lower Palaeozoic strata as suggested by others. 5) Eocambrian-Cambrian(?) rift basins in the Grenville basement are also imaged.
The ADCOH data were originally recorded with 14–56 Hz bandwidth and 8 s length, but an extended Vibroseis correlation was used to produce 17 s data length revealing reflections from within the upper crust. Below 8 s, reflections from within the Grenville basement become weak, but are observable as late as 13 s; however, these Moho (?) reflections are generally short segments. 相似文献
14.
库布齐沙漠南缘抛物线形沙丘表面风沙流结构变异 总被引：5，自引：0，他引：5
对库布齐沙漠南缘抛物线形沙丘表面气流和输沙率的野外观测和分析结果表明,沙丘表面约90%的风沙输移集中在距沙面0.10 m高度范围内,输沙率随高度递减的形式在沙丘各部位因风速、下垫面状况和坡面形态不同而发生变异。沙丘迎风坡坡脚因出露坚硬、含砾石地表,颗粒跃移高度大,风沙流上层相对输沙率大;迎风坡沙粒沿坡向上运动,颗粒跃移高度减小,风沙流中近地表相对输沙率大;沙丘背风坡沙粒沿坡向下运动,加之来自丘顶变型跃移物质的影响,风沙流上层相对输沙率较大;脊线受迎风坡各个断面地形差异的影响,各观测点间风沙流结构差异显著。风沙流结构在迎风坡和丘顶均遵循指数递减规律(Q=aexp(-z/b)),其中,指数函数拟合中系数a与输沙率具有良好的幂函数关系,随风速增加而增加,但二者关系较弱;b与二者无相关性。背风坡风沙流结构具有明显的分段现象,以0.10 m高度为界,下层符合指数函数,上层符合幂函数。 相似文献
15.
植株减小风速从而抑制风蚀,风速廓线可以反映植株对风速的影响。在风洞中测量了细长状植株和上大下小形状植株在不同覆盖密度、不同来流风速下的风速廓线,并对这两种植株地表上空气动力学粗糙度、零平面位移高度进行了比较。结果表明:两种植株地表的空气动力学粗糙度均随植株密度按幂函数规律增加,其与植株高度之比也随侧影盖度按幂函数规律增加。在相同覆盖密度或侧影盖度条件下,上大下小形状植株地表的空气动力学粗糙度大于细长状植株。细长状植株地表的零平面位移高度随植株密度的增加先增加而后减小,而上大下小形状植株地表的零平面位移高度则基本不受植株密度的影响。 相似文献
16.
Previous studies have suggested the presence of large-scale flow structures in gravel-bed rivers. These structures are pictured as intermittent high-speed wedges separated by regions of lower velocity. However, the characteristics of these structures have not been examined in detail through either visualisation techniques or detailed field measurements. This paper confirms the presence of large-scale flow structures in gravel bed rivers, pictures their sequence and patterns and characterises their mean and individual properties. The analysis relies on a new technique for displaying velocity fluctuations in a space–time matrix that allows one to see the structures as they pass an array of current meters. Streamwise and vertical velocities were measured simultaneously with an array of three electromagnetic current meters. The sampling frequency was 20 Hz. Five velocity profiles of up to 13 1-min series of measurements and one profile of three 20-min measurements were sampled. These data suggest the presence of large wedges of faster fluid joined by regions of slower fluid. Space–time correlation analysis confirmed the presence of vertical coherence of the flow. The average angle of the front of the wedges is 36°. Although individual structures are variable in size and shape, a new detection technique using all three velocity signals simultaneously showed that their average frequency is nine events per minute and their duration is more than 2 s. The high-speed wedges display a complex organisation and do not show a preferred sequence of events as was postulated by previous studies. Because of their duration and size, these high-speed wedges are likely to play a major role in bedload sediment transport. 相似文献
17.
To enhance the quality of oil- and gas-resource assessments and to reduce the risks in oil and gas exploration, a number of
assessment techniques have been developed. Unfortunately, these techniques have not always been effective in the timely transfer
of information. The amount of time that is required for preparing assessments does not always allow for the necessary high-quality
data to be generated. To overcome this problem, a method based on an analysis of the phase state of oil and the dynamics of
fluids in secondary migration of hydrocarbons is proposed. The phase state of the oil and fluid potential for secondary migration
is estimated initially for each prospect together with the extent of the drainage area. On the basis of these estimates, statistical
calculations can be made for the generation and expulsion of hydrocarbons. As a result, more reliable data are available for
prospect assessment. The application of this method has a practical significance in that it brings the role of basin modeling
in prospect assessment into full play, increases the reliability of petroleum-resource assessments, and reduces the risks
in exploration. A case study from the Beitang region in eastern China is presented. 相似文献
18.
An analogy between turbulent fluid systems and landscape drainage systems [Parker, G., Haff, P.K., Murray, A.B., 2001, EOS, Transactions of the American Geophysical Union, 82, pp. F564.] is suggested by the observation that transport in both systems can be approximated by diffusion with size-proportional effective diffusivities, with a cross-over at small scales to Fickian diffusion. The “landscape” Reynolds number of a typical fluvial landscape is estimated to be of order ReL 106 to 109, these large values reflecting the relative efficiency of fluvial transport compared to creep. ReL is the ratio of the large-scale effective diffusivity of rivers to the small-scale diffusivity of creep processes on hillslopes. The spatial dependence of the effective diffusivity produces rivers with logarithmic long-profiles, similar to the profiles of many rivers in nature, and analogous to the logarithmic dependence of mean fluid velocity on distance from a wall in turbulent flow. The landscape example suggests how other generalized “Reynolds numbers” can be constructed as ratios of large-scale to small-scale diffusivities to measure the efficiencies of complex processes that affect the surface. As an example, the global airline transportation network is estimated to have an efficacy relative to that of direct human mechanisms for transport of similar goods and materials of about 108 as measured by a corresponding “technology” Reynolds number. The appearance of such large dimensionless numbers, pertaining to the consequences of human invention and design, reflects the emergence of the technosphere as an increasingly efficient overlay on the historical domain of biology and surficial geology. 相似文献
19.
A model for the distribution of sulfur in the sediments of a lake in the Adirondack Mountains, New York, is developed to gain insight into the timing and magnitude of anthropogenic increases in sulfur loading. Surficial sulfur concentrations are about 3500 µg g–1, increase to 6000–7000 µg g–1 at a depth of about 6 cm, and then decrease downcore to background levels of about 2500 µg g–1. Sulfate concentrations are about 60–80 µM in the overlying water and decrease rapidly below the sediment-water interface. This reflects assimilatory reduction and/or dissimilatory reduction of sulfate and incorporation into the inorganic and/or organic solid-phase fractions of the sediment. A mathematical model is constructed assuming sulfur is incorporated into sediments by burial of detrital organic matter and by diffusion of sulfate from the overlying water with subsequent fixation in the sediment. Several historical scenarios of atmospheric sulfate loading rates were examined as model boundary conditions. Model results are compared with observed sedimentary sulfur profiles. The observed sediment sulfur profiles are best described using model boundary conditions showing increases in lake water sulfate concentrations since about 1940. Assuming as much as a decade of retention of sulfate within the terrestrial portion of the ecosystem, this suggests that significant increases in sulfate deposition rates began sometime after 1930 in this relatively remote area. 相似文献
20.
ELIZABETH CHACKO 《Geographical review》2003,93(4):491-506
ABSTRACT. Ethiopians are a recent immigrant group in the United States, having entered the country in significant numbers during the 1980s and 1990s. This preliminary study examines the ethnic and racial identities of children of first‐generation Ethiopian immigrants living in the Washington, D.C. metropolitan area. The results of twenty in‐depth interviews demonstrate that race is a much more fluid and contested form of identification than is ethnicity to the young immigrants, who equate the latter unilaterally with their Ethiopian heritage. Immigrants also adopt different subject identities in various locales, favoring those that are most in accordance with their needs and sense of self. 相似文献