首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   558篇
  免费   15篇
  国内免费   3篇
测绘学   29篇
大气科学   36篇
地球物理   142篇
地质学   165篇
海洋学   21篇
天文学   146篇
综合类   10篇
自然地理   27篇
  2022年   9篇
  2021年   12篇
  2020年   7篇
  2019年   5篇
  2018年   25篇
  2017年   30篇
  2016年   20篇
  2015年   26篇
  2014年   30篇
  2013年   58篇
  2012年   22篇
  2011年   23篇
  2010年   16篇
  2009年   22篇
  2008年   22篇
  2007年   12篇
  2006年   13篇
  2005年   13篇
  2004年   13篇
  2003年   9篇
  2002年   8篇
  2001年   9篇
  2000年   4篇
  1999年   5篇
  1998年   7篇
  1997年   13篇
  1996年   7篇
  1995年   6篇
  1994年   2篇
  1993年   4篇
  1992年   6篇
  1991年   8篇
  1990年   8篇
  1989年   6篇
  1988年   6篇
  1987年   5篇
  1986年   3篇
  1985年   9篇
  1984年   15篇
  1983年   12篇
  1982年   6篇
  1981年   10篇
  1979年   3篇
  1978年   3篇
  1977年   4篇
  1976年   5篇
  1975年   4篇
  1972年   4篇
  1971年   3篇
  1957年   1篇
排序方式: 共有576条查询结果,搜索用时 31 毫秒
561.
The Karharbari and Barakar coal measures of Giridih and Saharjuri basins of Bihar, eastern India, comprise an interbedded assemblage of sandstone, shale and coal in variable abundance. The lithofacies composition records a progressive decrease in sandstone and enrichment of shale and coal from Karharbari up to Barakar. Application of first-order embedded Markov-chain statistics to subsurface data of Karharbari (52 borehole logs) and Barakar (10 borehole logs) reveals that deposition in both the coal measures followed a Markovian mechanism with variable probability, to yield a sequence of upward transition from sandstone through shale to coal. The repetitive fining-upward cycles are asymmetrical, i.e. sandstone → shale → coal → sandstone in the case of Karharbari, but symmetrical as sandstone → shale → coal → shale in Barakar.The abundance of sandstone and the asymmetrical nature of Karharbari cycles are attributed to abrupt shifting of channel bars in low-sinuosity anabranching streams. By contrast, the subequal amount of sandstone, shale and coal forming symmetrical cycles in the overlying Barakar Formation is due perhaps to a slow and gradual shift of the stream channels over and across the adjacent subenvironments of the flood plain.  相似文献   
562.
Summary An analysis of the seismicity of Broach region, Gujarat (India) has been made. The activity of the network of faults present in this region, perhaps, has been responsible for the present earthquake. Earthquake energy, magnitude and focal depth are connected by a single equation. An empirical relation for focal depth and magnitude has been derived for the area.  相似文献   
563.
The concentrations of the rare earth elements (REE), K, Rb, Sr, Ba, U, Zr and Cr for the Luna 20 soil and four different Apollo 16 soils are reported. These trace element abundances imply: (1) that the lunar highlands consist of a mixture of rocks rich in large ion lithophile (LIL) elements and LIL-element impoverished anorthosites; or (2) that the bulk of the aluminum-rich crust did not originate by upward segregation of plagioclase in a primitive liquid shell. The Luna 20 soil is distinguished from the Apollo 16 soil by lower aluminum and LIL element abundances.  相似文献   
564.
The concept that the plutonism of the lunar highlands and the mare-type volcanism are two separate problems in both time (> 4.4 AE versus < 3.95 AE) and space is seriously questioned by the discovery of a 4.23-AE low-Ti mare basalt from Fra Mauro Formation.Apollo 14 breccia 14305 contains a clast (,122) which is an olivine gabbronorite that is texturally and mineralogically similar to several Apollo 12 basalts (e.g., 12005, 12035, 12040). It consists of cumulus olivine (40 modal %; Fo 62–70) and Ti-chromite (2.5 modal %); post-cumulus phases include low-Ca pyroxene (29 modal %; Wo 7–13 En 68–75), augite (10 modal %; Wo 31–40 En 47–50), plagioclase (15 modal %, An 82–93), and ilmenite (4 modal %, 5–7 MgO). The TiO2 content of this rock = 4.3%; CaO/Al2O3 ? 1.0, CaO = 5.1%; MgO/FeO ? 1.0, MgO = 21.9%. The REE pattern, normalized to chondritic abundances, is approximately 30 × Ch and “hump-shaped” with a pronounced Eu depletion and a non-KREEPy signature. A four-point Rb-Sr isochron reveals an age of 4.23 ± 0.05 AE. The sample has a low initial 87Sr/86Sr= 0.69911 ± 3.The data presented here show that non-KREEPy, mare-type volcanism commenced at least as early as 4.2 AE in the Fra Mauro region and probably across much of the lunar surface. Massive bombardment during the “terminal cataclysm” and the subsequent veneer of younger mare basalts has obliturated most of the evidence for these ancient volcanic events. These old, mare-type volcanics may be related to basin-forming events such as made Procellarum (i.e., impact-triggered igneous activity).  相似文献   
565.
Ram Fishman 《Climatic change》2018,147(1-2):195-209
Recent studies have found that increasing intra-seasonal precipitation variability will lead to substantial reductions in rice production in India by 2050, independently of the effect of rising temperatures. However, these projections do not account for the possibility of adaptations, of which the expansion of irrigation is the primary candidate. Using historical data on irrigation, rice yields, and precipitation, I show that irrigated locations experience much lower damages from increasing precipitation variability, suggesting that the expansion of irrigation could protect Indian agriculture from this future threat. However, accounting for physical water availability shows that under current irrigation practices, sustainable use of irrigation water can mitigate less than a tenth of the climate change impact. Moreover, if India continues to deplete its groundwater resources, the impacts of increased variability are likely to increase by half.  相似文献   
566.
Anthropogenic greenhouse gas emissions that induce changes in the Earth’s climate affect particular variables and locations differently. A key part of this difference is the timescale at which this change takes place, which will eventually have important consequences for adaptation requirements. This idea of timescale associated with climate change has been used several times in the past to estimate the urgency of adaptation in particular regions. The definition of climate-change timescale is, however, not unique. For example, we can think of it in terms of an expected trend (e.g. in temperature) reaching a given threshold, or think of it in terms of the time it may take this trend to become statistically significant. We may also wonder about the validity of this speculation given that, due to natural variability, the expected trend may in fact not be realized. In this article we explore alternative ways of defining the timescale of climate-change, compare their properties, and illustrate them with an example for the case of projected surface temperature over North America. It is shown that these timescales are analytically related but may differ substantially in magnitude under certain conditions. In particular, it is shown that climate change impact on vulnerable systems may arrive before statistical detection of the variable’s trend takes place. This fact may have implications on how climate change impacts are seen by those with diverging interests.  相似文献   
567.
568.
Photogravitational Restricted Three-Body Problem (PGRTBP) is considered and halo orbits are generated in the vicinity of the Sun–Mars L1 Lagrangian point. Deviation of properties such as time period, size and velocity variation in the halo orbits with Sun as a source of radiation are discussed. With increase in solar radiation pressure, the halo orbits are found to elongate and move towards the Sun and the time period of the halo orbits is found to increase. The variation in the behaviour of invariant manifolds with change in radiation pressure is also computed and it is found that as the radiation pressure increases, the transition from Mars-centric path to heliocentric path is delayed. Certain implications of the velocity profile of the invariant manifolds are also discussed.  相似文献   
569.
In this study, an assessment of seismicity parameters in the northwest Himalaya and adjoining regions using an earthquake catalog from India Meteorological Department covering a period from June 1, 1998 to June 30, 2011 has been carried out. The spatial distributions of seismicity parameters, namely magnitude of completeness, M C, a value, b value, and correlation fractal dimension, D C, are estimated for the studied region. The M C, a, and b values are found to be 2.5, 4.601, and 0.83, respectively. Despite significant gaps, the spatial distributions of a and b values are seen to follow similar trend and are found scattering in between Main Boundary Thrust (MBT) and South Tibet Detachment, adjoining areas of Mahendragarh-Dehradun Fault (MDF), Delhi-Haridwar Ridge (DHR) and Moradabad Fault (MF), and the southern flank of Karakoram Fault and Indus-Tsangpo Suture Zone. The estimated spatial distribution of b and a values is within 90 % of confidence level, thereby indicating non-uniform stress accumulation or higher rock fracturing density in the studied region caused by strong tectonization following several earthquakes. Negative correlation between low b value and high D C is observed predominantly in the region between the MBT and Munsiari Thrust or Main Central Thrust-I of Garhwal and Kumaon Himalaya, adjoining zones of MDF, DHR, and MF of Indo-Gangetic plain, and the eastern flank of the studied region, suggesting the presence of asperities in the zone. At the same time, active creeping process can be inferred in between the MBT and Main Central Thrust of Garhwal Himalaya and the surrounding areas of Shimla region of the Himalayan arc to the northwestern part of the studied region from the positive correlation between b value and D C. The results indicate that the structural heterogeneity caused by different stress accumulation and rock fracturing densities exists due to continuous tectonic adjustments between different geomorphic features of the studied region. An attempt has also been made to classify the studied region into smaller seismic zones by observing the spatial patterns of b value and D C that are fractal properties of the observed seismicity, along with the prevalent fault networks.  相似文献   
570.
The Cameros Basin is a part of the Mesozoic Iberian Rift. It is an extensional basin formed during the late Jurassic and early Cretaceous, in the Mesozoic Iberian Rift context, and it was inverted in the Cenozoic as a result of the Alpine contraction. This work aims to reconstruct the tectono-stratigraphic evolution of the basin during the Mesozoic, using new and revised field, geophysical and subsurface data. The construction of a basin-wide balanced section with partial restorations herein offers new insights into the geometry of the syn-rift deposits. Field data, seismic lines and oil well data were used to identify the main structures of the basin and the basin-forming mechanisms. Mapping and cross-sectional data indicate the marked thickness variation of the depositional sequences across the basin, suggesting that the extension of the depositional area varied during the syn-rift stage and that the depocentres migrated towards the north. From field observation and seismic line interpretation, an onlap of the depositional sequences to the north, over the marine Jurassic substratum, can be deduced. In the last few decades, the structure and geometry of the basin have been strongly debated. The structure and geometry of the basin infill reconstructed herein strongly support the interpretation of the Cameros Basin as an extensional-ramp synclinal basin formed on a blind south-dipping extensional ramp. The gradual hanging-wall displacement to the south shifted the depocentres to the north over time, thus increasing the basin in size northwards, with onlap geometry on the pre-rift substratum. The basin was inverted by means of a main thrust located in a detachment located in the Upper Triassic beds (Keuper), which branched in depth with the Mesozoic extensional fault flat. The reconstruction of the tectono-stratigraphic evolution of the Cameros Basin proposed herein represents a synthesis and an integration of previous studies of the structure and geometry of the basin. This study can be used as the basis for future basin-scale research and for modelling the ancient petroleum system of the basin.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号