全文获取类型
收费全文 | 665篇 |
免费 | 148篇 |
国内免费 | 185篇 |
专业分类
测绘学 | 2篇 |
大气科学 | 3篇 |
地球物理 | 196篇 |
地质学 | 674篇 |
海洋学 | 39篇 |
综合类 | 15篇 |
自然地理 | 69篇 |
出版年
2024年 | 2篇 |
2023年 | 6篇 |
2022年 | 8篇 |
2021年 | 23篇 |
2020年 | 22篇 |
2019年 | 37篇 |
2018年 | 36篇 |
2017年 | 28篇 |
2016年 | 40篇 |
2015年 | 28篇 |
2014年 | 36篇 |
2013年 | 51篇 |
2012年 | 39篇 |
2011年 | 43篇 |
2010年 | 22篇 |
2009年 | 51篇 |
2008年 | 46篇 |
2007年 | 44篇 |
2006年 | 40篇 |
2005年 | 42篇 |
2004年 | 42篇 |
2003年 | 35篇 |
2002年 | 22篇 |
2001年 | 27篇 |
2000年 | 38篇 |
1999年 | 29篇 |
1998年 | 27篇 |
1997年 | 17篇 |
1996年 | 15篇 |
1995年 | 15篇 |
1994年 | 19篇 |
1993年 | 14篇 |
1992年 | 12篇 |
1991年 | 11篇 |
1990年 | 6篇 |
1989年 | 2篇 |
1988年 | 10篇 |
1987年 | 4篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1976年 | 1篇 |
排序方式: 共有998条查询结果,搜索用时 31 毫秒
21.
在中国北部边疆西起天山东端的北山,东至大兴安岭北段的4200km地域内,气候干旱少雨,水资源贫乏,第四系地下水是主要供水源之一。本文首次比较系统地研究了该区第四纪堆积物的分布和地下水的赋存规律。文中依据不同地貌单元、地层岩性及气候条件诸因素将本区第四系地下水划分为枝状沟谷砂砾石层孔隙水、河谷冲积平原砂砾石层孔隙水两种主要类型,并着重对有重要供水意义的河谷冲积平原砂砾石层孔隙水的分布、埋藏条件、富水性及水化学特征进行了论述。 相似文献
22.
南京下蜀土的岩石磁学特征 总被引:5,自引:0,他引:5
对南京附近的下蜀土进行了岩石磁学测定,通过磁化率各向异性测量研究了它的沉积磁性组构特征,下蜀土的各异向性很不明显,各向异性废P小于水成岩的P值(>1.02),且与困面理度F高度相关,与磁线理度L相关不明显,这种特征与中国中部风成黄土极相似。对新生圩剖面进行的磁化率测量发现用化率曲线上有7个旋回的波峰、波谷变化,这与剖面中的古土壤-黄土系列相一致,说明下蜀土的剖面磁化率同样可以作为地层划分与对比,反映古气候冷暧变化的物理参数。对典型下蜀土样品进行的低温磁化率测量揭示出古土壤和黄土的成颗粒的构成不同,古土壤以超顺磁质为主,黄土以顺磁质为主。细小磁颗粒在古土壤中的积聚可能与古土壤的发育及古气候的暧湿程度有关。 相似文献
23.
1∶5万庐山地区区域地质调查和专题研究,在星子县黄岭一带发现一套巨厚层状复成分的变质砾岩。根据该变质砾岩的砾石成分、变形组构及区域分布特点,以及上部(双桥山群)与下部(星子岩群)地层在变质与变形方面存在明显差别,认为变质砾岩应属于双桥山群的底部层位,是中元古界双桥山群与下伏下元古界星子岩群不整合关系的佐证。由于受后期构造叠加改造的影响,现今二者呈断层或隐蔽不整合接触关系。 相似文献
24.
25.
26.
In permafrost regions of Qinghai-Tibetan Plateau, the critical embankment height must be considered in the process of the construction of highway, especially for the global climatic warming. In this paper, the two-dimensional numerical analysis for the critical embankment height (for gravel road surface and coarse-grained soil) has been performed by using thefinite element method. In the calculation, we think that the service life of the construction is at least 50 years. The mean annual air temperatures applied to the calculation model are -6.5 ℃, -6.0 ℃, -5.5 ℃, -5.0 ℃, -4.5 ℃ and -4.0 ℃, respectively, and the value of temperature rise are taken as 1.10℃ in the coming 50 years. The minimum embankment heights derived from the analysis are 0.85 m, 0.92 m, 1.01 m, 1.18 m, 1.60 m and 2.66 m for the different mean annual air temperatures and the maximum embankment heights are 7.68 m,7.55 m, 7.34 m, 7.00 m, 6.45 m and 5.85m, accordingly. On condition that the service life of embankment is 50 years, the critical value of the mean annual air temperature is -3.5 ℃. Namely, in the areas where the mean annual air temperature is higher than -3.5 ℃, the critical embankment height does not exist. 相似文献
27.
This paper introduces and evaluates a novel method for ascertaining the grain‐size distribution of subsurface sediments that involves profoundly less sampling effort than standard methods. It is based on hybrid sampling principles previously applied to the construction of synthetic surface grain‐size distributions. The method is developed from an empirical demonstration of the approximate similarity of surface and subsurface grain‐size distributions when compared over a common range of sizes. Subsurface hybrid models are found to provide good facsimiles of grain‐size distributions de?ned using standard criteria and to yield distribution percentiles with millimetre accuracy. The technique is presented as an expedient alternative to standard methods for large, perennial gravel‐bed rivers. As this is a new technique, prudent application is advised in lieu of further investigation. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
28.
Jiongxin Xu 《地球表面变化过程与地形》2004,29(5):645-657
A comparison has been made between the hydraulic geometry of sand‐ and gravel‐bed rivers, based on data from alluvial rivers around the world. The results indicate a signi?cant difference in hydraulic geometry among sand‐ and gravel‐bed rivers with different channel patterns. On this basis, some diagrams for discrimination of meandering and braided channel patterns have been established. The relationships between channel width and water discharge, between channel depth and water discharge, between width–depth ratio and water discharge and between channel slope and water discharge can all be used for channel pattern discrimination. The relationship between channel width and channel depth can also be used for channel pattern discrimination. However, the accuracy of these relationships for channel pattern discrimination varies, and the depth–discharge relationship is a better discriminator of pattern type than the classic slope–discharge function. The cause for this difference has been explained qualitatively. To predict the development of channel patterns under different natural conditions, the pattern discriminator should be searched on the basis of independent or at least semi‐independent variables. The relationship between stream power and bed material grain size can be used to discriminate channel patterns, which shows a better result than the discriminator using the slope–discharge relationship. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
29.
30.
The evolution, migration and deposits of a gravelly braid bar in the Sagavanirktok River, northern Alaska, are described in unprecedented detail using annual aerial photographs, ground‐penetrating radar (GPR) profiles, trenches and cores. Compound braid bars in the Sagavanirktok River form by chute cut‐off of point bars and by growth of mid‐channel unit bars. Subsequent growth is primarily by accretion of unit bars onto their lateral and downstream margins. The upstream ends of braid bars may be sites of erosion or unit bar deposition. Compound braid bar deposits vary in thickness laterally and are thickest in medial sections and near cut banks. Compound bar deposits are typically composed of three to seven sets of simple large‐scale inclined strata, each simple set formed by a unit bar. The simple large‐scale strata contain medium‐scale cross‐strata (from dune migration) and planar strata (from migration of bedload sheets). The upstream and medial parts of compound braid bar deposits show very little vertical variation in grain size, but downstream and lateral margins tend to fine upwards. The deposits are mostly poorly sorted sands and gravels, although sands tend to be deposited at the top of the braid bar, and open‐framework gravels preferentially occur near the top and base of the braid bar. The patterns of braid bar growth and migration, and the nature of the deposits, described from the Sagavanirktok River are generally similar to other sandy and gravelly braided rivers, and consistent with the theoretical braid bar model of Bridge (1993). 相似文献