全文获取类型
收费全文 | 57080篇 |
免费 | 9442篇 |
国内免费 | 12313篇 |
专业分类
测绘学 | 5415篇 |
大气科学 | 7409篇 |
地球物理 | 11371篇 |
地质学 | 31031篇 |
海洋学 | 7454篇 |
天文学 | 3424篇 |
综合类 | 3878篇 |
自然地理 | 8853篇 |
出版年
2024年 | 433篇 |
2023年 | 1053篇 |
2022年 | 2015篇 |
2021年 | 2527篇 |
2020年 | 2326篇 |
2019年 | 2751篇 |
2018年 | 2050篇 |
2017年 | 2405篇 |
2016年 | 2439篇 |
2015年 | 2678篇 |
2014年 | 3384篇 |
2013年 | 3558篇 |
2012年 | 3412篇 |
2011年 | 3663篇 |
2010年 | 3162篇 |
2009年 | 3722篇 |
2008年 | 3772篇 |
2007年 | 3889篇 |
2006年 | 3770篇 |
2005年 | 3364篇 |
2004年 | 2991篇 |
2003年 | 2753篇 |
2002年 | 2478篇 |
2001年 | 2171篇 |
2000年 | 2072篇 |
1999年 | 1807篇 |
1998年 | 1596篇 |
1997年 | 1225篇 |
1996年 | 1002篇 |
1995年 | 862篇 |
1994年 | 806篇 |
1993年 | 648篇 |
1992年 | 485篇 |
1991年 | 401篇 |
1990年 | 272篇 |
1989年 | 244篇 |
1988年 | 189篇 |
1987年 | 117篇 |
1986年 | 62篇 |
1985年 | 62篇 |
1984年 | 40篇 |
1983年 | 34篇 |
1982年 | 28篇 |
1981年 | 21篇 |
1980年 | 16篇 |
1979年 | 7篇 |
1978年 | 24篇 |
1977年 | 9篇 |
1971年 | 5篇 |
1954年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
海拉尔盆地贝尔凹陷的油气运聚分析 总被引:22,自引:2,他引:22
海拉尔盆地贝尔凹陷是一个以白垩系沉积为主的陆相断陷区。采用条件分析法,对贝尔凹陷的油气运聚条件及其成藏的基本模式进行了系统研究,认为,贝尔凹陷在剖面上有3套成藏组合,其中中部成藏组合最佳;在平面上可分为5个运聚系统,其中以位于凹陷西北部的3个系统为最佳。该凹陷不同构造部位的运聚条件相差较大,生烃洼陷深部位的油气运移以砂岩储集层和断裂为主要输导通道,构造高部位的油气运移以断裂为主,而两者之间的油气输导则以不整合面和断裂为主。在油气运聚和成藏模式分析的基础上,认为贝尔凹陷油气运聚的有利条件主要包括生油凹陷有充足的油源、生油区内有继承性发育的圈闭构造以及各运聚要素间存在良好的时空配置关系3个方面。 相似文献
103.
104.
以累计频度定量计算方法,分区讨论了河南及邻区1970年以来地震活动非线性度ZL值的时间进程曲线,系统计算了M≥5.0级地震前的ZL值,结果表明:开始出现活跃(平静)异常的时间集中在地震前1—2年,结束异常的时间在地震前一年内,集中于0—4个月。同时提出了异常的定量指标以及发震类型。 相似文献
105.
科技期刊编辑应具备的基本素质 总被引:2,自引:0,他引:2
对科技期刊质量和编辑素质的关系进行了研究,认为编辑是期刊的设计师。一本期刊从总体创意、策划选题、编审稿件到排版设计,无不体现编辑的主体意识。期刊质量的好坏,读者评价的高低都与编辑息息相关。编辑的素质关系到期刊的生存和发展。得出不断提高科技期刊编辑的素质是十分必要的结论。 相似文献
106.
The effect of asymmetry and irregularity of the inputted seismic waves on the earthquake-induced differential settlement of the buildings on natural subsoil is investigated in terms of the earthquake damage phenomena, theoretical analyses, dynamic triaxial tests and shaking table tests. A conclusion can be drawn from the investigation results that, the asymmetrical and irregular character of the inputted seismic waves themselves may have a significant contribution to the differential settlement of subsoil and buildings in some cases, and this is a necessary factor to be considered in reasonable evaluation for the differential settlement and other problems relating to the soil deformation due to earthquakes. 相似文献
107.
The importance of plant root characteristics in controlling concentrated flow erosion rates 总被引:3,自引:0,他引:3
While it has been demonstrated in numerous studies that the aboveground characteristics of the vegetation are of particular importance with respect to soil erosion control, this study argues the importance of separating the influence of vegetation on soil erosion rates into two parts: the impact of leaves and stems (aboveground biomass) and the influence of roots (belowground biomass). Although both plant parameters form inseparable constituents of the total plant organism, most studies attribute the impact of vegetation on soil erosion rates mainly to the characteristics of the aboveground biomass. This triggers the question whether the belowground biomass is of no or negligible importance with respect to soil erosion by concentrated flow. This study tried to answer this question by comparing cross‐sectional areas of concentrated flow channels (rills and ephemeral gullies) in the Belgian Loess Belt for different cereal and grass plant densities. The results of these measurements highlighted the fact that both an increase in shoot density as well as an increase in root density resulted in an exponential decrease of concentrated flow erosion rates. Since protection of the soil surface in the early plant growth stages is crucial with respect to the reduction of water erosion rates, increasing the plant root density in the topsoil could be a viable erosion control strategy. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
108.
Riverbanks along the Arno River have been investigated with the aims of de?ning the main mechanisms of failure and retreat, their spatial distribution, and their causes. Geomorphological aspects were investigated by a reconnaissance of riverbank processes, for a number (26) of representative sites. Laboratory and in situ tests were then performed on a selected number of riverbanks (15). Based on the material characteristics, six main typologies of riverbanks have been de?ned, with homogeneous ?ne‐grained and composite banks representing the most frequent types. Slab‐type failures are the most frequent mechanism observed on ?ne‐grained banks, while cantilever failures prevail on composite banks. The role of river stage and related pore water pressure distributions in triggering the main observed mechanisms of failure has been investigated using two different types of stability analysis. The ?rst was conducted for 15 riverbanks, using the limit equilibrium method and considering simpli?ed hypotheses for pore water pressure distribution (annulment of negative pore pressures in the portion of the bank between low water stage and peak stage). Stability conditions and predicted mechanisms of failure are shown to be in reasonably good agreement with ?eld observations. Three riverbanks, representative of the main alluvial reaches of the river, were then selected for a more detailed bank stability analysis, consisting of: (a) de?nition of characteristic hydrographs of the reach with different return periods; (b) modelling of saturated and unsaturated ?ow using ?nite element seepage analysis; and (c) stability analysis with the limit equilibrium method, by adopting pore water pressure values derived from the seepage analysis. The results are compared to those obtained from the previous simpli?ed analysis, and are used to investigate the different responses, in terms of stability, to different hydrological and riverbank conditions. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
109.
结合水动力学原理在尼尔基水利枢纽·右岸副坝下游浸没研究中的应用 总被引:3,自引:0,他引:3
应用结合水动力学原理,采用野外调查和室内试验研究相结合、宏观 观研究和粘土微观研究相结合的方法,对粘土层的渗流特征进行了专门研究,确定粘土起始比降I0值,以确定测压水位与粘土不带水位、毛细力与毛管水上升高度之间的关系。在三维渗流电网络计算的基础上,进一步进行了粘土层内含水带的埋深计算,并重新选定毛管水上升高度值和浸没临界深度。研究结果表明,副坝下游浸没范围和程度大大减轻,只要进行适当的工程处理,下游浸没影响当可消除,建筑物安全也是有保证的。 相似文献
110.
陕西省地下水水质监测发展探析 总被引:1,自引:0,他引:1
本文针对陕西省地下水水质监测控制范围局限于农灌区,监测项目单一的现状,通过分析研究,提出水质监测的发展思路,剖析实现新思路的相关条件。 相似文献