全文获取类型
收费全文 | 57462篇 |
免费 | 9237篇 |
国内免费 | 12078篇 |
专业分类
测绘学 | 6298篇 |
大气科学 | 8038篇 |
地球物理 | 11960篇 |
地质学 | 28319篇 |
海洋学 | 6784篇 |
天文学 | 4388篇 |
综合类 | 3907篇 |
自然地理 | 9083篇 |
出版年
2024年 | 221篇 |
2023年 | 671篇 |
2022年 | 1784篇 |
2021年 | 2140篇 |
2020年 | 2170篇 |
2019年 | 2798篇 |
2018年 | 1920篇 |
2017年 | 2391篇 |
2016年 | 2405篇 |
2015年 | 2643篇 |
2014年 | 3350篇 |
2013年 | 3637篇 |
2012年 | 3491篇 |
2011年 | 3745篇 |
2010年 | 3106篇 |
2009年 | 3956篇 |
2008年 | 3839篇 |
2007年 | 4058篇 |
2006年 | 3927篇 |
2005年 | 3488篇 |
2004年 | 3118篇 |
2003年 | 2840篇 |
2002年 | 2363篇 |
2001年 | 2081篇 |
2000年 | 2021篇 |
1999年 | 1826篇 |
1998年 | 1642篇 |
1997年 | 1273篇 |
1996年 | 1042篇 |
1995年 | 912篇 |
1994年 | 827篇 |
1993年 | 733篇 |
1992年 | 504篇 |
1991年 | 430篇 |
1990年 | 304篇 |
1989年 | 263篇 |
1988年 | 217篇 |
1987年 | 124篇 |
1986年 | 89篇 |
1985年 | 82篇 |
1984年 | 46篇 |
1983年 | 27篇 |
1982年 | 34篇 |
1981年 | 28篇 |
1980年 | 28篇 |
1979年 | 18篇 |
1978年 | 27篇 |
1977年 | 25篇 |
1976年 | 29篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
我国水资源承载力研究现状及展望 总被引:1,自引:0,他引:1
概要说明了我国水资源承载力的研究现状,阐述了水资源承载力的概念、内涵及影响因素,探讨了目前几种常用的水资源承载力评价方法,并对当前研究中存在的问题进行了初步分析,进而提出了我国水资源承载力的研究趋势。 相似文献
992.
993.
Paleoproterozoic accretion in the Northeast Siberian craton: Isotopic dating of the Anabar collision system 总被引:1,自引:0,他引:1
O. M. Rosen L. K. Levskii D. Z. Zhuravlev A. Ya. Rotman Z. V. Spetsius A. F. Makeev N. N. Zinchuk A. V. Manakov V. P. Serenko 《Stratigraphy and Geological Correlation》2006,14(6):581-601
Geochronological database considered in the work and characterizing the Anabar collision system in the Northeast Siberian craton includes coordinated results of Sm-Nd and Rb-Sr dating of samples from crustal xenoliths in kimberlites, deep drill holes, and bedrock outcrops. As is inferred, collision developed in three stages dated at 2200–2100, 1940–1760, and 1710–1630 Ma. The age of 2000–1960 Ma is established for substratum of mafic rocks, which probably originated during the lower crust interaction with asthenosphere due to the local collapse of the collision prism. Comparison of Sm-Nd and Rb-Sr isochron dates shows that the system cooling from ≈700 to ≈300°C lasted approximately 300 m.y. with a substantial lag relative to collision metamorphism and granite formation. It is assumed that accretion of the Siberian craton resulted in formation of a giant collision mountainous structure of the Himalayan type that was eroded by 1.65 Ga ago, when accumulation of gently dipping Meso-to Neoproterozoic (Riphean) platform cover commenced. 相似文献
994.
Skier-triggered avalanches are the main cause of avalanche accidents in backcountry skiing. The risk of accidents during backcountry
skiing was analysed statistically and related to factors such as elevation level, aspect, stability rating and the time of
the year. The analysis is based on a database about terrain usage and avalanche accidents from a large heli-skiing operator
in Canada, which makes it possible to study the conditional probability of accidents given the recorded pattern of terrain
usage. This study shows that the historical risk of accidentally triggering an avalanche greater than size 1 depends highly
on the stability rating, with the highest risk occurring during “poor” stability. The risk is greater at high elevations,
and it is lower during the late season than earlier on. Skier risk does not depend as much on aspect as may be indicated from
avalanche data alone. However, it is relatively high in the N–NE–E sector. These factors are not independent of each other
and therefore analyses of combined factors were also performed. Questionnaires and interviews were used to gain knowledge
about the terrain selection of professional mountain guides. These results indicate that when selecting terrain, guides first
look at the overall shape and size of the terrain, but avalanche history of terrain and inclination are also important factors.
Finally, remarks in avalanche reports were analysed, and common human factors identified. 相似文献
995.
John R. Greenwood Joanne E. Norris Jo Wint 《Geotechnical and Geological Engineering》2006,24(3):467-481
The procedure for geotechnical site investigation is well established but little attention is currently given to investigating
the potential of vegetation to assist with ground stability. This paper describes how routine investigation procedures may
be adapted to consider the effects of the vegetation. It is recommended that the major part of the vegetation investigation
is carried out, at relatively low cost, during the preliminary (desk) study phase of the investigation when there is maximum
flexibility to take account of findings in the proposed design and construction. The techniques available for investigation
of the effects of vegetation are reviewed and references provided for further consideration. As for general geotechnical investigation
work, it is important that a balance of effort is maintained in the vegetation investigation between (a) site characterisation
(defining and identifying the existing and proposed vegetation to suit the site and ground conditions), (b) testing (in-situ and laboratory testing of the vegetation and root systems to provide design parameters) and (c) modelling (to analyse the
vegetation effects). 相似文献
996.
In this study, hydrogeologic and hydrochemical information from the Mersin-Erdemli groundwater system were integrated and
used to determine the main factors and mechanisms controlling the chemistry of groundwaters in the area and anthropogenic
factors presently affecting them. The PHREEQC geochemical modeling demonstrated that relatively few phases are required to
derive water chemistry in the area. In a broad sense, the reactions responsible for the hydrochemical evolution in the area
fall into four categories: (1) silicate weathering reactions; (2) dissolution of salts; (3) precipitation of calcite, amorphous
silica and kaolinite; (4) ion exchange. As determined by multivariate statistical analysis, anthropogenic factors show seasonality
in the area where most contaminated waters related to fertilizer and fungicide applications that occur during early summer
season. 相似文献
997.
硝基苯污染可使地下水环境质量急剧恶化。利用阻截墙控制地下水中的硝基苯污染范围的扩大,是控制地下水环境继续恶化的经济、有效方法之一。以粘土、沸石、铁粉、硫酸钙等与水调配成泥浆,利用压力向井中回灌,将泥浆注入到含水层中,可有效地阻截地下水中硝基苯的扩散。 相似文献
998.
鄂尔多斯盆地北部下二叠系下石盒子组为纵横向变化大,岩性、孔隙结构等复杂的河流相地层,很难用统一的计算公式来完成对孔隙度的计算。采用多元统计分析、散点图和孔隙度对比图来确定其孔隙度。运用上述方法,比较精确地确定鄂尔多斯盆地北部下二叠系下石盒子组的孔隙度,对鄂尔多斯盆地油气储量的估计,提供一个很好的基础资料。 相似文献
999.
Gehad M. Saleh 《中国地球化学学报》2006,25(4):307-317
1 Introduction The association of massive Fe-Ni-Cu sulfides andchromite is a very unusual feature of podiformchromitites occurring in mantle tectonites of ophioliticcomplexes. It has only been described in theSoutheastern Desert, Egypt, where sulfides a… 相似文献
1000.
作为公益性财政拨款事业单位,省级地震系统实现开发富局,应该要从稳定项目收入,拓展自营收入,培育和发展生产点,推进制度创新着手。本文试图从预算和管理的角度对其作一点简单分析。 相似文献