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   3479篇
  免费   178篇
  国内免费   681篇
测绘学   256篇
大气科学   295篇
地球物理   1382篇
地质学   1388篇
海洋学   407篇
天文学   165篇
综合类   190篇
自然地理   255篇
  2024年   7篇
  2023年   31篇
  2022年   29篇
  2021年   68篇
  2020年   68篇
  2019年   118篇
  2018年   51篇
  2017年   15篇
  2016年   39篇
  2015年   52篇
  2014年   88篇
  2013年   87篇
  2012年   185篇
  2011年   280篇
  2010年   146篇
  2009年   171篇
  2008年   202篇
  2007年   195篇
  2006年   200篇
  2005年   176篇
  2004年   169篇
  2003年   122篇
  2002年   124篇
  2001年   103篇
  2000年   142篇
  1999年   66篇
  1998年   74篇
  1997年   67篇
  1996年   81篇
  1995年   82篇
  1994年   90篇
  1993年   112篇
  1992年   111篇
  1991年   114篇
  1990年   102篇
  1989年   81篇
  1988年   88篇
  1987年   60篇
  1986年   43篇
  1985年   45篇
  1984年   43篇
  1983年   22篇
  1982年   26篇
  1981年   18篇
  1980年   36篇
  1973年   6篇
  1964年   6篇
  1959年   8篇
  1958年   16篇
  1957年   9篇
排序方式: 共有4338条查询结果,搜索用时 15 毫秒
961.
Allen M  Yung YL  Gladstone GR 《Icarus》1992,100(2):527-533
The observed ratio of C2H6 to C2H2 in the Jovian stratosphere increases from approximately 55 at 2 mbar to approximately 277 at 12 mbar. In current photochemical models this ratio typically increases between 2 and 12 mbar by a factor of < or = 3. Recent laboratory kinetics studies on the reaction between C2H3 and H2 to form C2H4 suggest an efficient chemical mechanism for hydrogenation of C2H2 to C2H6. Inclusion of this scheme as part of a comprehensive updated model for hydrocarbon photochemistry in the atmosphere of Jupiter provides an explanation of the altitude variation of the C2H6/C2H2 ratio. The sensitivity of these results to uncertainties in the key rate constants at low temperatures is illustrated, identifying needs for additional laboratory measurements. Since the key reaction rate constants decrease with decreasing temperature, the hydrogenation of C2H2 as proposed predicts a qualitatively decreasing trend in the C2H6/C2H2 value with decreasing distance from the Sun. The observed variation between Jupiter and Saturn is consistent with this prediction.  相似文献   
962.
Global Martian atmospheric results derived from the infrared imaging spectrometer ISM flown aboard the Phobos 2 Soviet spacecraft are presented. Over low altitude regions the expected CO mixing ratio of (8 +/- 3) x 10(-4) is measured. Variations of the 2.35-micrometers feature are inconsistent with this value over the Great Martian Volcanoes. If the 2.35-micrometers band is entirely attributable to carbon monoxide, the CO mixing ratio is typically depleted by a factor of 3 over these high altitude areas. Orography should play a major role in the existence of this CO "hole." If, however, these spectral variations at 2.35 micrometers are due to the surface composition, the fraction of the surface covered by the responsible mineral must smoothly decrease as the surface elevation decreases. This phenomenon implies a strong interaction between the surface and the atmosphere for the Great Martian Volcanoes. Diurnal behavior and latitudinal variations of water vapor are globally consistent with Viking measurements. During the Phobos observations, the water vapor amounts over the bright equatorial regions range around 11 pr-micrometers during the day. These amounts are slightly larger than those inferred from 1976 to 1979. The lack of global dust storms during 1988-1989 could explain the enhancement of H2O in the atmosphere.  相似文献   
963.
Al-gabbani M 《GeoJournal》1992,24(4):375-385
"This paper examines and assesses the changes in the population density pattern in the city of Riyadh, the capital of Saudi Arabia where the city has undergone dramatic changes in its physical as well as its social structure. Analysis of data revealed a decline in population density at an exponential rate from the city center with variation between city sectors and zones. The city's rapid horizontal expansion and dispersal of population is the result of factors such as increases in city population and in size and number of single family dwellings, widening of streets, rise in income and residential mobility rates, mass use of automobiles, change in family size, zoning regulations, leapfrogging, and huge governmental city development projects."  相似文献   
964.
The effect of radiogenic heating on the thermal evolution of spherical icy bodies with radii 1 km < R < 100 km was investigated. The radioisotopes considered were 26Al, 40K, 232Th, 235U, and 238U. Except for the 26Al abundance, which was varied, the other initial abundances were kept fixed, at values derived from those of chondritic meteorites and corresponding to a gas-to-dust ratio of 1. The initial models were homogeneous and isothermal (To = 10 K) amorphous ice spheres, in a circular orbit at 10(4) AU from the Sun. The main object of this study was to examine the conditions under which the transition temperature from amorphous into cubic ice (Ta = 137 K) would be reached. It was shown that the influence of the short-lived radionuclide 26Al dominates the effect of other radioactive species for bodies of radii up to approximately 50 km. Consequently, if we require comets to retain their ice in amorphous form, as suggested by observations, an upper limit of approximately 4 x 10(-9) is obtained for the initial 26Al abundance in comets, a factor of 100 lower than that of the inclusions in the Allende meteorite. A lower limit for the formation time of comets may thus be derived. The possibility of a coexistence of molten cometary cores and extended amorphous ice mantles is ruled out. Larger icy spheres (R > 100 km) reached Ta even in the absence of 26Al, due to the decay of the other radionuclides. As a result, a crystalline core formed whose relative size depended on the composition assumed. Thus the outermost icy satellites in the solar system, which might have been formed of ice in the amorphous state, have probably undergone crystallization and may have exhibited eruptive activity when the gas trapped in the amorphous ice was released (e.g., Miranda).  相似文献   
965.
If the "11.3 microns" emission feature seen in the spectra of many planetary nebulae, H II regions, and reflection nebulae is due to polycyclic aromatic hydrocarbons (PAHs), then additional features should be present between 11.3 and 13.0 microns (885 and 770 cm-1). Moderate-resolution spectra of NGC 7027, HD 44179, IRAS 21282+5050, and BD + 30 degrees 3639 are presented which show that the "11.3 microns" feature actually peaks at 11.22 microns (891 cm-1). The spectra also show evidence of new emission features near 11.9 and 12.7 microns (840 and 787 cm-1). These are consistent with an origin from PAHs and can be used to constrain the molecular structure of the family of PAHs responsible for the infrared features. The observed asymmetry of the "11.3 microns" band is consistent with the slight anharmonicity expected in the C--H out-of-plane bending mode in PAHs. Laboratory experiments show that the intensity of this mode relative to the higher frequency modes depends on the extent of molecular "clustering." The observed strengths of the "11.3 microns" interstellar bands relative to the higher frequency bands are most consistent with the features originating from free molecular PAHs. The intensity and profile of the underlying broad structure, however, may well arise from PAH clusters and amorphous carbon particles. Analysis of the 11-13 microns (910-770 cm-1) emission suggests that the molecular structures of the most intensity emitting free PAHs vary somewhat between the high-excitation environment in NGC 7027 and the low-excitation but high-flux environment close to HD 44179. Finally, a previously undetected series of regularly spaced features between 10 and 11 microns (1000 and 910 cm-1) in the spectrum of HD 44179 suggests that a simple polyatomic hydride is present in the gas phase in this object.  相似文献   
966.
2020年1月3日,由河南省自然资源厅主办,省测绘地理信息局信息中心承办的2019年河南省测绘地理信息统计工作培训会在郑州召开。省自然资源厅测绘地理信息管理处处长宋新龙参加会议。宋新龙指出,2019年前三季度工作表明,全省测绘地理信息单位对统计工作的重视程度不断提高,统计的信息服务保证能力逐渐凸显,统计队伍的专业素质日益提升,广大统计工作者的责任感和主人翁意识显著增强,但仍存在对统计工作的指导监督力度不够、统计队伍稳定性和专业性有待加强、统计分析报告质量尚待提高等问题。  相似文献   
967.
For more than 20 years, the concept of near-fault pulse-like ground motion has been a topic of great interest due to its distinct characteristics, particularly due to directivity or fling effects, which are hugely influenced by the rupture mechanism. These unexpected characteristics, along with their effective frequency, energy rate, and damage indices, create a near-fault, pulse-like ground motion capable of causing severe damage to structures. One of the most common approaches for identifying ...  相似文献   
968.
A systematic sedimentological and chronological study of typical Paleogene basins in eastcentral Tibet suggests that the depositional characteristics of extensively developed huge-bedded, purplish-red coarse clastic rocks formed in a tectonic setting of regional thrusting and strike-slipping represent a typical dry and hot subaerial alluvial fan environment formed in a proximal and rapidaccumulating sediment body in debris flows and a fan-surface braided river. Combining results from basin-fill sequences, sequences of coarse clastic rocks, fauna and sporo-pollen associations and thermochronological data, it is conduded that the coarse clastic rocks formed in the period of 54.2- 24.1 Ma, nearly coeval with the formation of Paleogene basins in the northern (Nangqen-Yushu thrust belt), middle (Batang-Lijiang fault belt), and disintegration of large basins in the southern (LanpingSimao fold belt) segments of Tibet. The widespread massive-bedded coarse clastic rocks, fold thrusting and strike-slip, thrust shortening, and igneous activities in the Paleogene basins of eastcentral Tibet indicate that an early diachronous tectonic uplift might have occurred in the Tibetan Plateau from Middle Eocene to Oligocene, related to the initial stage of collision of the Indian and Asian plates.  相似文献   
969.
Traditional approaches to develop 3D geological models employ a mix of quantitative and qualitative scientific techniques,which do not fully provide quantification of uncertainty in the constructed models and fail to optimally weight geological field observations against constraints from geophysical data.Here,using the Bayesian Obsidian software package,we develop a methodology to fuse lithostratigraphic field observations with aeromagnetic and gravity data to build a 3D model in a small(13.5 km×13.5 km)region of the Gascoyne Province,Western Australia.Our approach is validated by comparing 3D model results to independently-constrained geological maps and cross-sections produced by the Geological Survey of Western Australia.By fusing geological field data with aeromagnetic and gravity surveys,we show that 89%of the modelled region has>95%certainty for a particular geological unit for the given model and data.The boundaries between geological units are characterized by narrow regions with<95%certainty,which are typically 400-1000 m wide at the Earth's surface and 500-2000 m wide at depth.Beyond~4 km depth,the model requires geophysical survey data with longer wavelengths(e.g.,active seismic)to constrain the deeper subsurface.Although Obsidian was originally built for sedimentary basin problems,there is reasonable applicability to deformed terranes such as the Gascoyne Province.Ultimately,modification of the Bayesian engine to incorporate structural data will aid in developing more robust 3D models.Nevertheless,our results show that surface geological observations fused with geophysical survey data can yield reasonable 3D geological models with narrow uncertainty regions at the surface and shallow subsurface,which will be especially valuable for mineral exploration and the development of 3D geological models under cover.  相似文献   
970.
抽提岩石(沉积物)中吸附气和吸收气的方法可用于预测矿藏,这是因为这些气体会逐渐聚集和保存在岩石中。来自地幔的流体会补充热解吸附流体,运移到地表的流体则产生岩石圈气田。产生于这种方式的背景气田可能会受到物理场和来自不同矿床流体的影响。矿床的物理场可能会改变背景气田的成分。  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