全文获取类型
收费全文 | 40633篇 |
免费 | 6136篇 |
国内免费 | 7854篇 |
专业分类
测绘学 | 6785篇 |
大气科学 | 4738篇 |
地球物理 | 6846篇 |
地质学 | 19864篇 |
海洋学 | 4275篇 |
天文学 | 3500篇 |
综合类 | 3036篇 |
自然地理 | 5579篇 |
出版年
2024年 | 245篇 |
2023年 | 644篇 |
2022年 | 1300篇 |
2021年 | 1531篇 |
2020年 | 1413篇 |
2019年 | 1742篇 |
2018年 | 1245篇 |
2017年 | 1552篇 |
2016年 | 1570篇 |
2015年 | 1764篇 |
2014年 | 2258篇 |
2013年 | 2253篇 |
2012年 | 2464篇 |
2011年 | 2563篇 |
2010年 | 2254篇 |
2009年 | 2854篇 |
2008年 | 2768篇 |
2007年 | 2828篇 |
2006年 | 2757篇 |
2005年 | 2536篇 |
2004年 | 2201篇 |
2003年 | 2088篇 |
2002年 | 1801篇 |
2001年 | 1579篇 |
2000年 | 1555篇 |
1999年 | 1331篇 |
1998年 | 1155篇 |
1997年 | 804篇 |
1996年 | 667篇 |
1995年 | 564篇 |
1994年 | 534篇 |
1993年 | 456篇 |
1992年 | 316篇 |
1991年 | 280篇 |
1990年 | 194篇 |
1989年 | 160篇 |
1988年 | 127篇 |
1987年 | 73篇 |
1986年 | 39篇 |
1985年 | 31篇 |
1984年 | 19篇 |
1983年 | 15篇 |
1982年 | 12篇 |
1981年 | 9篇 |
1980年 | 9篇 |
1979年 | 8篇 |
1978年 | 14篇 |
1977年 | 5篇 |
1976年 | 4篇 |
1954年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
62.
介绍了TDP—0844型地震前兆数据综合采集器采用的设计方案及工作原理,及各个功能模块的设计准则,并列出了采集器实现的功能。文章还给出了采集器的部分应用图例,证明该采集器能够满足地震前兆观测需要。 相似文献
63.
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
64.
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
65.
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
66.
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
67.
68.
69.
The anisotropy of magnetic susceptibility (AMS) of single crystals of biotite, muscovite and chlorite has been measured in order to provide accurate values of the magnetic anisotropy properties for these common rock-forming minerals. The low-field AMS and the high-field paramagnetic susceptibility are defined. For the high-field values, it is necessary to combine the paramagnetic deviatoric tensor obtained from the high-field torque magnetometer with the paramagnetic bulk susceptibility measured from magnetization curves of the crystals. This leads to the full paramagnetic susceptibility ellipsoid due to the anisotropic distribution of iron cations in the silicate lattice. The ellipsoid of paramagnetic susceptibility, which was obtained for the three phyllosilicates, is highly oblate in shape and the minimum susceptibility direction is subparallel to the crystallographic c-axes. The anisotropy of the susceptibility within the basal plane of the biotite has been evaluated and found to be isotropic within the accuracy of the instrumental measurements. The degree of anisotropy of biotite and chlorite is compatible with previously reported values while for muscovite the smaller than previously published values. The shape of the chlorite AMS ellipsoid for all the samples is near-perfect oblate in contrast with a wide distribution of oblate and prolate values reported in earlier studies. Reliable values are important for deriving models of the magnetic anisotropy where it reflects mineral fabrics and deformation of rocks. 相似文献
70.