全文获取类型
收费全文 | 2785篇 |
免费 | 431篇 |
国内免费 | 727篇 |
专业分类
测绘学 | 107篇 |
大气科学 | 198篇 |
地球物理 | 740篇 |
地质学 | 1861篇 |
海洋学 | 466篇 |
天文学 | 96篇 |
综合类 | 125篇 |
自然地理 | 350篇 |
出版年
2024年 | 22篇 |
2023年 | 43篇 |
2022年 | 76篇 |
2021年 | 112篇 |
2020年 | 121篇 |
2019年 | 142篇 |
2018年 | 128篇 |
2017年 | 144篇 |
2016年 | 155篇 |
2015年 | 138篇 |
2014年 | 138篇 |
2013年 | 173篇 |
2012年 | 179篇 |
2011年 | 164篇 |
2010年 | 143篇 |
2009年 | 163篇 |
2008年 | 194篇 |
2007年 | 189篇 |
2006年 | 166篇 |
2005年 | 155篇 |
2004年 | 151篇 |
2003年 | 115篇 |
2002年 | 136篇 |
2001年 | 86篇 |
2000年 | 104篇 |
1999年 | 116篇 |
1998年 | 78篇 |
1997年 | 79篇 |
1996年 | 64篇 |
1995年 | 51篇 |
1994年 | 49篇 |
1993年 | 41篇 |
1992年 | 21篇 |
1991年 | 16篇 |
1990年 | 21篇 |
1989年 | 16篇 |
1988年 | 11篇 |
1987年 | 13篇 |
1986年 | 10篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有3943条查询结果,搜索用时 31 毫秒
81.
地裂缝灾害研究现状与展望 总被引:6,自引:0,他引:6
1927年美国首先发现地裂缝以来,各国学者对地裂缝的发生、发展进行了细致的研究。美国学者首先从地震研究入手,认为地裂缝的形成受构造因素影响。后逐渐加强研究地下水超采对地裂缝的影响,形成了构造与地下水开采复合成因观点。我国学者重点对汾渭地堑地裂缝成因问题进行了较系统的研究,提出几种成因机理模型,并用GIS技术建立简单的预测预报系统。文章在详细介绍国内、外对地裂缝成因机理和水文特性的研究以及预测、预报工作的同时,对今后研究趋势进行了展望。目前,地裂缝的研究正从以往的定性静态描述转向以过程为目标的精确定量动力学研究。利用相似材料模拟地裂缝的形成与发展,以非线性理论(如分形)对地裂缝系统进行定量的数学描述,建立准确的预测、预报模型(如人工神经网络模型),并与GIS相结合,建立完善的预测、预报系统,减少地裂缝灾害对环境的影响,进而对它加以利用,是今后地裂缝研究的主要方向。 相似文献
82.
Multi-parameter studies (stable isotopes in carbonate and organic matter, pigment,organic carbon and nitrogen contents) from a 660-yr continuous sediment core from Lake Cheng-hai, a closed, eutropic lake in southern China, provide information on lake historical eutrophi-cation. During the last 660 years, great changes have taken place in productivity and eutrophi-cation of Lake Chenghai in response to human activities. In 1690, the productivity of the lakebegan to increase as Lake Chenghai became closed from agriculture in the lake‘‘ s watershed. In 1942, Lake Chenghai evolved to eutrophic state, marked by an increase in organic carbon, ni-trogen, CaC03, pigment contents and obvious negative values of stable isotopes, which is more or less simultaneous with the large-scale population immigration during the period. In 1984, in-tensive human activities induced modern lacustrine productivity and eutrophic level. Human-in-duced trophic changes during the past few decades have affected the Lake Chenghai ecosystem tosuch an extent that it has never experienced before in the last 660 years. 相似文献
83.
84.
Fluid mixing as the mechanism of formation of the Dajing Cu-Sn-Ag-Pb-Zn ore deposit,Inner Mongolia ——Fluid inclusion and stable isotope evidence 总被引:2,自引:0,他引:2
Since the 1990s, interest in the magmatic fluids and their relation to mineralization has been re-aroused[1—6]. Studies on stable isotopes of low-sulfidation deposits commonly show the predominance of meteoric water[7]. Paradoxically, the evidence for me… 相似文献
85.
A critical state interpretation for the cyclic liquefaction resistance of silty sands 总被引:1,自引:0,他引:1
George D. Bouckovalas Konstantinos I. Andrianopoulos Achilleas G. Papadimitriou 《Soil Dynamics and Earthquake Engineering》2003,23(2):115-125
Contrary to many laboratory investigations, common empirical correlations from in situ tests consider that the increase in the percentage of fines leads to an increase of the cyclic liquefaction resistance of sands. This paper draws upon the integrated Critical State Soil Mechanics framework in order to study this seemingly not univocal effect. Firstly the effect of fines on the Critical State Line (CSL) is studied through a statistical analysis of a large data set of published monotonic triaxial tests. The results show that increasing the content of non-plastic fines practically leads to a clockwise rotation of the CSL in (e–ln p) space. The implication of this effect on cyclic liquefaction resistance is subsequently evaluated with the aid of a properly calibrated critical state elasto-plastic constitutive model, as well as a large number of published experimental results and in situ empirical correlations. Both sets of data show clearly that a fines content, less than about 30% by weight, may prove beneficial at relatively small effective stresses (p0<50–70 kPa), such as the in situ stresses prevailing in most liquefaction case studies, and detrimental at larger confining stresses, i.e. the stresses usually considered in laboratory tests. To the extent of these findings, a correction factor is proposed for the practical evaluation of liquefaction resistance in terms of the fines content and the mean effective confining stress. 相似文献
86.
为了客观地反映各因素对目标问题的影响程度,本文首次提出了最佳因素权重概念。定义最佳因素权重为研究区域内系统质量处于极限状态时的各因素权重之组合。结合深港西部通道工程深圳湾公路大桥桥址比选方案,详细说明了本方法的研究思路及其确定方法。通过最佳因素权重方法,能客观地、定量地确定出多因素复杂系统中的最优场址。 相似文献
87.
Oscillatory zoning in low δ18O skarn garnet from the Willsboro wollastonite deposit, NE Adirondack Mts, NY, USA, preserves a record of the temporal evolution of mixing hydrothermal fluids from different sources. Garnet with oscillatory zoning are large (1–3 cm diameter) euhedral crystals that grew in formerly fluid filled cavities. They contain millimetre‐scale oscillatory zoning of varying grossular–andradite composition (XAdr = 0.13–0.36). The δ18O values of the garnet zones vary from 0.80 to 6.26‰ VSMOW and correlate with XAdr. The shape, pattern and number of garnet zones varies from crystal to crystal, as does the magnitude of the correlated chemistry changes, suggesting fluid system variability, temporal and/or spatial, over the time of garnet growth. The zones of correlated Fe content and δ18O indicate that a high Fe3+/Al, high δ18O fluid mixed with a lower Fe3+/Al and δ18O fluid. The high δ18O, Fe enriched fluids were likely magmatic fluids expelled from crystallizing anorthosite. The low δ18O fluids were meteoric in origin. These are the first skarn garnet with oscillatory zoning reported from granulite facies rocks. Geochronologic, stable isotope, petrologic and field evidence indicates that the Adirondacks are a polymetamorphic terrane, where localized contact metamorphism around shallowly intruded anorthosite was followed by a regional granulite facies overprint. The growth of these garnet in equilibrium with meteoric and magmatic fluids indicates an origin in the shallow contact aureole of the anorthosite prior to regional metamorphism. The zoning was preserved due to the slow diffusion of oxygen and cations in the large garnet and protection from deformation and recrystallization in zones of low strain in thick, rigid, garnetite layers. The garnet provide new information about the hydrothermal system adjacent to the shallowly intruded massif anorthosite that predates regional metamorphism in this geologically complex, polymetamorphic terrane. 相似文献
88.
Zuoheng Zhang Jingwen Mao Jianmin Yang Zhiliang Wang Zhaochong Zhang 《Resource Geology》2003,53(2):101-114
Abstract. The Ta'ergou tungsten deposit in Gansu province, northwestern China, is located in the western part of the North Qilian Caledonian orogen, and consists of scheelite skarn bodies and wolframite quartz veins. The tungsten‐bearing skarn developed by the replacement of carbonate layers intercalated in the Precambrian schist and amphibolite whereas wolframite‐quartz ore veins developed along a group of fractures that cut through horizontal skarns. The Ta'ergou tungsten deposit is genetically related to the Caledonian Yeniutan granodiorite intrusion and occurs ca. 500 m wide in the exo‐contact zone 300 ~ 500 m apart from the intrusion. The granodiorite displays a lower grade of differentiation, low content of SiO2 and high contents of mafic components. There are three types of fluid inclusions in the wolframite‐quartz vein systems, i. e. aqueous, CO2‐H2O and CO2‐rich. The homogenization temperature of aqueous inclusion ranges from 140 to 380d?C and their salinities from 6.4 to 17.4 equivalent wt% NaCl. Laser Raman spectroscopy shows that the inclusions contain a relatively high content of CO2. The δ34S values of skarn type sulfides range from +8.1 to +12.7 per mil and those of quartz vein sulfides from +9.3 to +14.9 per mil, similar to sulfides of the granodiorite with from +6.0 to +11.7 per mil. The δ18O values of quartz are between +10.5 and +13.3 per mil and those of wolframite between +3.4 and +5.1 per mil. The δ18O water values of ore forming fluids range from +0.6 to +6.4 per mil and suggest the mixture of magmatic fluids with meteoric water formed the ore‐forming fluids. It has been proved that Precambrian strata in the west sector of North Qilian region are enriched in tungsten. We propose the strata were remelted to be tungsten‐granitoid during subduction. The polymetallic tungsten was gradually accumulated into the roof pendants of the granite intrusion by fractional crystallization and then was deposited by hydrothermal fluids during metasomatism and infilling along fractures. On the other hand, the granite intrusion also acted as “heating machine” to make hydrothermal fluids leach out the metals from Precambrian strata and these metals joined the ore‐forming hydrothermal system. 相似文献
89.
Prosenjit Ghosh S. K. Bhattacharya Ashok Sahni R. K. Kar D. M. Mohabey K. Ambwani 《Cretaceous Research》2003,24(6):743-750
A single locality of the Late Cretaceous (Maastrichtian) Lameta Formation at Pisdura in Central India has yielded a large number of coprolites attributed to titanosaurian dinosaurs. Internally the coprolites are dark grey and contain abundant plant tissues and other organic materials. The plant tissues are mostly of gymnospermous origin. In addition, remains of bacterial colonies, fungal spores and algae are seen in the macerated fraction under scanning electron microscope. The dark grey appearance is probably attributable to fine-grained organic matter within voids in tracheids or xylem. The average 13C/12C ratio of the organic matter in the coprolites is -24‰ (relative to PDB) suggesting that plants of C3type were the main diet of their producers. A comparison of δ15N value (about 4‰ w.r.t. air) of the coprolites with that of faecal matter of modern herbivores and carnivores suggests that gut fermentation may not have been an active mechanism in the digestion process of titanosaurs. 相似文献
90.