全文获取类型
收费全文 | 7550篇 |
免费 | 1299篇 |
国内免费 | 678篇 |
专业分类
测绘学 | 1452篇 |
大气科学 | 112篇 |
地球物理 | 1340篇 |
地质学 | 3410篇 |
海洋学 | 730篇 |
天文学 | 1547篇 |
综合类 | 399篇 |
自然地理 | 537篇 |
出版年
2024年 | 13篇 |
2023年 | 74篇 |
2022年 | 230篇 |
2021年 | 358篇 |
2020年 | 301篇 |
2019年 | 286篇 |
2018年 | 171篇 |
2017年 | 259篇 |
2016年 | 252篇 |
2015年 | 286篇 |
2014年 | 479篇 |
2013年 | 411篇 |
2012年 | 453篇 |
2011年 | 419篇 |
2010年 | 335篇 |
2009年 | 454篇 |
2008年 | 472篇 |
2007年 | 602篇 |
2006年 | 461篇 |
2005年 | 374篇 |
2004年 | 357篇 |
2003年 | 342篇 |
2002年 | 287篇 |
2001年 | 225篇 |
2000年 | 243篇 |
1999年 | 236篇 |
1998年 | 227篇 |
1997年 | 164篇 |
1996年 | 170篇 |
1995年 | 117篇 |
1994年 | 91篇 |
1993年 | 62篇 |
1992年 | 68篇 |
1991年 | 33篇 |
1990年 | 54篇 |
1989年 | 43篇 |
1988年 | 36篇 |
1987年 | 26篇 |
1986年 | 19篇 |
1985年 | 10篇 |
1984年 | 6篇 |
1983年 | 1篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 5篇 |
1978年 | 4篇 |
1954年 | 4篇 |
排序方式: 共有9527条查询结果,搜索用时 15 毫秒
121.
We investigate the global evolution of a turbulent protoplanetary disk incorporating the effects of Maxwell stress due to a large-scale magnetic field permeating the disk. A magnetic field is produced continuously by an dynamo and the resultant Maxwell stress assists the viscous stress in p roviding the means for disk evolution. The most striking feature of magnetized disk evolution is the presence of the surface density bulge located in the magnetic gap, the region of the disk where the degree of ionization is too low to allow for coupli ng between the magnetic field and the gas. The bulge persists for a time of the order of 105–106 yr. The presence and persistence of the surface density bulge may have important implications for the process of planet formation and the overall characteristics of resultant planetary systems.Operated by USRA under contract No. NASW-4574 with NASA. 相似文献
122.
123.
Besides granites of the ilmenite series, in which the anisotropy of magnetic susceptibility (AMS) is mainly controlled by paramagnetic minerals, the AMS of igneous rocks is commonly interpreted as the result of the shape-preferred orientation of unequant ferromagnetic grains. In a few instances, the anisotropy due to the distribution of ferromagnetic grains, irrespective of their shape, has also been proposed as an important AMS source. Former analytical models that consider infinite geometry of identical and uniformly magnetized and coaxial particles confirm that shape fabric may be overcome by dipolar contributions if neighboring grains are close enough to each other to magnetically interact. On these bases we present and experimentally validate a two-grain macroscopic numerical model in which each grain carries its own magnetic anisotropy, volume, orientation and location in space. Compared with analytical predictions and available experiments, our results allow to list and quantify the factors that affect the effects of magnetic interactions. In particular, we discuss the effects of (i) the infinite geometry used in the analytical models, (ii) the intrinsic shape anisotropy of the grains, (iii) the relative orientation in space of the grains, and (iv) the spatial distribution of grains with a particular focus on the inter-grain distance distribution. Using documented case studies, these findings are summarized and discussed in the framework of the generalized total AMS tensor recently introduced by Cañon-Tapia (Cañon-Tapia, E., 2001. Factors affecting the relative importance of shape and distribution anisotropy in rocks: theory and experiments. Tectonophysics, 340, 117–131.). The most important result of our work is that analytical models far overestimate the role of magnetic interaction in rock fabric quantification. Considering natural rocks as an assemblage of interacting and non-interacting grains, and that the effects of interaction are reduced by (i) the finite geometry of the interacting clusters, (ii) the relative orientation between interacting grains, (iii) their heterogeneity in orientation, shape and bulk susceptibility, and (iv) their inter-distance distribution, we reconcile analytical models and experiments with real case studies that minimize the role of magnetic interaction onto the measured AMS. Limitations of our results are discussed and guidelines are provided for the use of AMS in geological interpretation of igneous rock fabrics where magnetic interactions are likely to occur. 相似文献
124.
This paper contributes an empirical test of key themes of the literature on natural resource conflict. Survey and interview data from an ongoing project in Azerbaijan provide insights into an unexpected lack of conflict in Azerbaijan related to the environment, resources and energy despite the predictions of resource conflict literature. We contend that questions about public perceptions about the environment and other daily concerns are critical if we are to understand who is likely (or unlikely) to be involved in conflict and why. The data presented in this paper demonstrate that Azerbaijani citizens rank environmental and resource issues among their immediate concerns and their top concerns for the country. However, compared to other day-to-day concerns such as the Nagorno-Karabakh conflict and economic concerns, environment-related concerns do not appear to be sufficient to motivate widespread violent conflict or citizen dissent related to environmental or natural resource conditions. 相似文献
125.
126.
航空伽马能谱地形改正新方法及应用 总被引:5,自引:2,他引:5
针对航空伽马能谱测量特点,推导了基于二维地形影响系数的航空伽马能谱测量地形逐点改正的理论公式。通过模型试验,总结了不同地形上不同飞行方式时的航空放射性异常特征,给出实测数据处理与野外异常查证对比结果,地形改正效果较好。 相似文献
127.
128.
129.
130.
内蒙古中部重、磁场特征与地壳密度结构 总被引:2,自引:2,他引:2
中亚造山带中的古生代——中生代花岗岩普遍具有正εNd值,在世界上是十分独特的。美国西部加利福尼亚中生代-新生代花岗岩同样具有正εNd值,并且其地壳的速度分布特征与内蒙古中部十分相似。本文通过与美国加利福尼亚的地球物理研究成果进行对比,来研究内蒙古中部的地壳密度结构,特别是下地壳的组成,试图探讨产生正£。值花岗岩的深部地质原因。研究结果表明,在大兴安岭-内蒙古造山带下地壳中可能存在与美国加里福尼亚类似的洋壳物质。此外,还对重、磁异常进行了处理,以确定蛇绿岩带的延展情况。由于在西拉木伦河附近存在切割至莫霍面的深断裂,结合地表出现的蛇绿岩带,故提出温都尔庙-西拉木伦河一线可能是内蒙古中部最重要的地质构造界线。 相似文献