全文获取类型
收费全文 | 7698篇 |
免费 | 1448篇 |
国内免费 | 1332篇 |
专业分类
测绘学 | 254篇 |
大气科学 | 727篇 |
地球物理 | 2679篇 |
地质学 | 4136篇 |
海洋学 | 1074篇 |
天文学 | 53篇 |
综合类 | 348篇 |
自然地理 | 1207篇 |
出版年
2024年 | 43篇 |
2023年 | 124篇 |
2022年 | 228篇 |
2021年 | 296篇 |
2020年 | 320篇 |
2019年 | 324篇 |
2018年 | 292篇 |
2017年 | 326篇 |
2016年 | 326篇 |
2015年 | 339篇 |
2014年 | 457篇 |
2013年 | 620篇 |
2012年 | 389篇 |
2011年 | 454篇 |
2010年 | 400篇 |
2009年 | 480篇 |
2008年 | 561篇 |
2007年 | 492篇 |
2006年 | 504篇 |
2005年 | 377篇 |
2004年 | 362篇 |
2003年 | 322篇 |
2002年 | 303篇 |
2001年 | 268篇 |
2000年 | 243篇 |
1999年 | 250篇 |
1998年 | 202篇 |
1997年 | 189篇 |
1996年 | 161篇 |
1995年 | 134篇 |
1994年 | 121篇 |
1993年 | 118篇 |
1992年 | 97篇 |
1991年 | 77篇 |
1990年 | 71篇 |
1989年 | 48篇 |
1988年 | 45篇 |
1987年 | 21篇 |
1986年 | 18篇 |
1985年 | 13篇 |
1984年 | 10篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1978年 | 16篇 |
1977年 | 5篇 |
1976年 | 1篇 |
1972年 | 1篇 |
1954年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Daniel Hillel 《水文研究》1987,1(2):143-147
Release of water from the soil in the process of internal drainage, and its continued downward movement through the vadose zone, constitute the main mechanism of groundwater recharge. Water released from the soil generally contains solutes, and these are conveyed to the groundwater via the same pathways as the drained water. Knowledge of those pathways is essential in any attempt to minimize the likelihood of groundwater pollution. Solutes generally interact with the medium in which they reside or travel, and the spatial and temporal pattern of their movement influences the nature and extent of their interactions. For many years, the assumption had prevailed that flow in the vadose zone is a steady-state, uniform process. Hence the vadose zone can serve to filter, attenuate, as well as degrade, potential pollutants. Recently, however, the existence of preferred pathways has come to light. Such pathways might connect the soil's upper zone directly to the water-table, thus bypassing the greater volume of the vadose zone and evading its filtering mechanisms. Groundwater recharge models that ignore the possibility of such spurts of contamination may be highly misleading. Preferred flow path may be cracks, animal burrows, or decayed root channels. Less easily discernible are transient and random paths associated with the phenomenon of ‘unstable flow’, which is most likely to occur in layered soils during infiltration. The wetting front, instead of remaining horizontal and advancing continuously from one layer to the next, may begin (particularly in transition from a fine-textured to a coarse-textured layer) to form bulges, called ‘fingers’, which propagate downwards and may become, in effect, vertical pipes. At present we are aware only of the occasional occurrence and potential importance of such phenomena, but as yet have neither the systematic empirical data, nor a proven comprehensive theoretical framework, by which to assess where, when, and according to what pattern, they are likely to occur. 相似文献
942.
Barry J. Dempsey 《国际地质力学数值与分析法杂志》1978,2(1):19-34
An investigation was conducted to develop a comprehensive moisture model for predicting non-isothermal moisture conditions in soils. An extensive literature review indicated that a model based on the Philip and de Vries equations for non-isothermal moisture movement and heat conduction would give the best results. By using numerical methods, the implicit finite difference approximations to the moisture movement and heat-transfer equations were programmed for computer solution of water content and temperature in the soil with time. Validation studies indicate that the moisture model can be used to predict accurately moisture conditions in the soil. The model was validated by using hydraulic data from laboratory studies conducted on soil columns compacted with AASHO A-3 and AASHO A-4 soil. The application of the moisture model to the study of non-isothermal moisture movement in the field is demonstrated. The influence of parameters such as water table depth, precipitation, and soil hydraulic properties on soil moisture content are shown by use of the moisture model. The model is shown to be applicable to a wide range of boundary conditions and that it predicts the moisture-temperature regime with time in soils utilizing climatic input data. 相似文献
943.
水流冲刷沉积物生成泥石流的条件及运动规律的试验研究 总被引:6,自引:0,他引:6
在铺满卵石的陡槽上释放不同浓度、不同流量的泥浆或清水,观测卵石在泥浆流作用下的运动,研究泥石流的形成条件和运动规律。试验结果表明,当K(=rqJ)大于某临界值K_c时就会发生泥石流,但这个临界值K_c随波相泥浆浓度而变。泥石流龙头由大卵石组成,在不断摩擦和碰撞中翻滚着推进,消耗大量动能,同时控制着后续流不离散。后续流液相流速显著大于龙头,不断把能量输送到龙头,维持龙头运动。大卵石在运动中受到液相流的推动力较大,而受到的碰撞阻力较小,比龙头运动的快。因而大卵石逐渐集中于龙头。 相似文献
944.
A breccia vein sampled from a shear zone in greenschist facies metapelites at Mount Isa, Queensland, Australia, shows a systematic variation in vein geometry that is related to the geometry of folding and faulting within the sample. Calcite vein-fill is coarse grained and equigranular, suggesting precipitation in a fluid-filled space. Partially folded veins suggest that veining occurred during folding and faulting. The breccia vein contains a central zone in which dilation has occurred simultaneously in all directions in the plane of section, implying that this was a zone of high fluid pressure and nearly isostatic differential stress during folding and faulting. From these observations, it can be inferred that the breccia vein was a zone of high permeability and a likely fluid channel during deformation. This hypothesis was tested by stable isotope analysis of veins and host rocks. The calcite veins have δ13 C values of -11.1 ± 0.1% and δ18 O values of 6-10%o , whereas the host metapelite has δ13 C values of -10.62 and -10.11% and δ18 O values of 14-15%o . These values are consistent with an igneous-derived, H2 O-dominated fluid that exchanged little oxygen with the host rocks, but derived much of its carbon from the wall rock. The isotopic disequilibrium between the veins and the wall rock confirms that the fluid was externally derived, and that the breccia vein acted as a channel for large-volume fluid flow within the shear zone. 相似文献
945.
Stabilization of Bottom Hole Temperature With Finite Circulation Time and Fluid Flow 总被引:4,自引:1,他引:3
As an alternative to finite element or finite difference modelling, analytical solutions are derived by the method of Laplace transformation and numerical results obtained for several models of the bottom hole temperature stabilization. Included in the models are the finite circulation time, the thermal property contrast between the borehole mud and the surrounding formation, and the presence of radial or lateral fluid flow in the formation, all of which are found to have significant effects on the dissipation of the thermal disturbance induced by drilling.
The mud circulation is considered to have the effect of either maintaining the borehole mud at a constant temperature or supplying a constant amount of heat per unit length per unit time to the borehole. For small circulation times, the former reduces to the 'zero circulation' model in which the mud circulation creates an instantaneous temperature anomaly at the hole bottom; for small borehole radii, the latter reduces to the line source model and the traditional 'Homer plot'.
For typical drilling operations in which the bottom hole temperatures are measured several hours to several tens of hours after the hole is shut in, the new models generally predict higher equilibrium formation temperatures than does the Horner plot. However, predictions from the various models converge if the BHTs are taken after the hole has been shut in for a period which is greater than about five times the circulation period. 相似文献
The mud circulation is considered to have the effect of either maintaining the borehole mud at a constant temperature or supplying a constant amount of heat per unit length per unit time to the borehole. For small circulation times, the former reduces to the 'zero circulation' model in which the mud circulation creates an instantaneous temperature anomaly at the hole bottom; for small borehole radii, the latter reduces to the line source model and the traditional 'Homer plot'.
For typical drilling operations in which the bottom hole temperatures are measured several hours to several tens of hours after the hole is shut in, the new models generally predict higher equilibrium formation temperatures than does the Horner plot. However, predictions from the various models converge if the BHTs are taken after the hole has been shut in for a period which is greater than about five times the circulation period. 相似文献
946.
Ernesto A. Lacomba Ernesto Pérez-Chavela 《Celestial Mechanics and Dynamical Astronomy》1992,54(4):343-355
We obtain a compact model for the global study of the planar rhomboidal 4-body problem in a level of constant negative energy. This model is a variation of the non compact model obtained through a McGehee blow up transformation. but compactness permits to obtain results which are not clear in the other case. 相似文献
947.
A. J. Desbarats 《Mathematical Geology》1994,26(1):1-21
Steady-state radial flow in three-dimensional heterogeneous media is investigated using a geostatistical approach. The goal of the study is to develop a model of the relationship between corescale hydraulic conductivities measured at the wellbore and the conductivity of the surrounding drainage region as measured by a larger scale flow experiment such as a pump test. Conductivity at the point or core-scale is modeled as a stationary and multivariate lognormal spatial random function. Conductivity of the drainage region is obtained by a weighted nonlinear spatial average over the point-scale values within. This empirical spatial averaging process is shown to yield excellent approximations of true effective drainage region conductivities calculated using a numerical flow model. The geostatistical model for point-scale conductivity and the spatial averaging process are used to determine the first and second order ensemble moments of drainage region conductivity. In particular, an expression is derived for the conditional expectation of drainage region conductivity given point-scale values measured at the wellbore. The results are illustrated in a case study of a well from a sandstone oil reservoir where both core and transient-test conductivity data from the same interval are available for comparison. 相似文献
948.
949.
950.
Xiao-yong Liu Pi-bo Yang Lan-tian Yang Shao-li Chen 《Chinese Astronomy and Astrophysics》1997,21(4):405-414
Considering nonthermal e±-pair production, this paper is focused on the radial structure of hot one-temperature accretion flows with advection under the condition of small viscous parameter ( = 0.1) and low mass-accretion rates. Self-similar relations of hot flow with advection have been adopted. The authors have used a scheme, in which the two regions of accretion flow are calculated separately and then joined together. Some typical features of hot flow with advection have been confirmed, and several new results obtained: there exists a critical radius rcr; the local cooling rate is inversely proportional to the square of the mass of the central object; the e±-pair process affects significantly the radiation of hot flows with advection. 相似文献