全文获取类型
收费全文 | 3102篇 |
免费 | 663篇 |
国内免费 | 877篇 |
专业分类
测绘学 | 84篇 |
大气科学 | 955篇 |
地球物理 | 769篇 |
地质学 | 766篇 |
海洋学 | 1570篇 |
天文学 | 47篇 |
综合类 | 147篇 |
自然地理 | 304篇 |
出版年
2024年 | 15篇 |
2023年 | 39篇 |
2022年 | 88篇 |
2021年 | 112篇 |
2020年 | 130篇 |
2019年 | 151篇 |
2018年 | 97篇 |
2017年 | 117篇 |
2016年 | 123篇 |
2015年 | 137篇 |
2014年 | 192篇 |
2013年 | 195篇 |
2012年 | 195篇 |
2011年 | 204篇 |
2010年 | 179篇 |
2009年 | 222篇 |
2008年 | 188篇 |
2007年 | 263篇 |
2006年 | 224篇 |
2005年 | 209篇 |
2004年 | 203篇 |
2003年 | 162篇 |
2002年 | 175篇 |
2001年 | 133篇 |
2000年 | 114篇 |
1999年 | 120篇 |
1998年 | 113篇 |
1997年 | 92篇 |
1996年 | 69篇 |
1995年 | 60篇 |
1994年 | 73篇 |
1993年 | 61篇 |
1992年 | 44篇 |
1991年 | 37篇 |
1990年 | 20篇 |
1989年 | 22篇 |
1988年 | 18篇 |
1987年 | 8篇 |
1986年 | 3篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 9篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 3篇 |
排序方式: 共有4642条查询结果,搜索用时 31 毫秒
71.
借用河流水力学顺直河道中有两个次生横向环流的假说来解释潮流沙脊的成因,水槽实验和实际观察也支持了这一观点.正是这一次生的纵轴横向环流(即螺旋流)将泥沙从沟槽带向脊顶,建造了大致与潮流方向平行的潮流沙脊.根据底质粒度成份和分布特点.可分为粗底质沙脊和细底质沙脊两类.前者组成物质偏粗,底质分布为脊细槽粗,后者组成物质偏细,底质分布为脊粗槽细.这是因为影响颗粒起动流速的主导因素不同,前者以重力为主,后者以颗粒间粘结力为主.它们都反映沟槽中流速大于脊顶处流速. 相似文献
72.
GPS observations of the ionospheric F2-layer behavior during the 20th November 2003 geomagnetic storm over South Korea 总被引:4,自引:1,他引:3
The ionospheric F2-layer peak density (NmF2) and its height (hmF2) are of great influence on the shape of the ionospheric
electron density profile Ne (h) and may be indicative of other physical processes within the ionosphere, especially those
due to geomagnetic storms. Such parameters are often estimated using models such as the semiempirical international reference
ionosphere (IRI) models or are measured using moderately priced to expensive instrumentation, such as ionosondes or incoherent
scatter radars. Global positioning system (GPS) observations have become a powerful tool for mapping high-resolution ionospheric
structures, which can be used to study the ionospheric response to geomagnetic storms. In this paper, we describe how 3-D
ionospheric electron density profiles were produced from data of the dense permanent Korean GPS network using the tomography
reconstruction technique. These profiles are verified by independent ionosonde data. The responses of GPS-derived parameters
at the ionospheric F2-layer to the 20th November 2003 geomagnetic storm over South Korea are investigated. A fairly large
increase in the electron density at the F2-layer peak (the NmF2) (positive storm) has been observed during this storm, which
is accompanied by a significant uplift in the height of the F2 layer peak (the hmF2). This is confirmed by independent ionosonde
observations. We suggest that the F2-layer peak height uplift and NmF2 increase are mainly associated with a strong eastward
electric field, and are not associated with the increase of the O/N2 ratio obtained from the GUVI instruments aboard the TIMED satellite. It is also inferred that the increase in NmF2 is not
caused by the changes in neutral composition, but is related to other nonchemical effects, such as dynamical changes of vertical
ion motions induced by winds and E × B drifts, tides and waves in the mesosphere/lower thermosphere region, which can be dynamically
coupled upward to generate ionospheric perturbations and oscillations. 相似文献
73.
74.
本文根据层序地层学的基本原理,立足东部箕状断陷盆地的特点,以冀中拗陷和二连盆地为重点研究对象,提炼出陆相断陷盆地层序地层学模式,在缓坡带,以风暴浪基面作为“陆架边角”的参照物,层序地层特征相似于海相被动大陆边缘背景;在陡坡带则截然不同,并以湖面与边界断裂的交点取代“陆架边角”,各体系域所含沉积体系与海相模式不尽相同,在盆地形成初期的层序,低水位体系域发育,湖泊消亡以后,主要发育高水位体系域。此外,本文还探索了层序地层与油气赋存的关系,指出富砂的“盆底扇”是今后勘探开发的重要方向。 相似文献
75.
Neetu Agnihotri P. Chittibabu Indu Jain P. C. Sinha A. D. Rao S. K. Dube 《Natural Hazards》2006,39(1):83-101
The paper describes a two-dimensional bay–river coupled numerical model for storm surges along the Andhra coast of India.
The effect of the Krishna and Godavari rivers on the surge development is analysed. A comparative study of the surge generated
by a tropical cyclone with and without the inclusion of rivers is done in detail. Three cyclones that struck the Andhra coast
in November 1977, May 1990 and November 1996 were used for the simulation studies. It is found that the idealized model without
a river overestimates the sea-level elevation as compared to a more realistic bay–river coupled model. The temporal variation
of surge values at the mouth of the rivers is also studied for all three cyclone cases. It is found that the effect of the
presence of rivers depends on the strength of the cyclone, its point of landfall and the location of the rivers with respect
to the landfall point. 相似文献
76.
The degree to which dust emissions are controlled by geomorphic conditions, wind environments and land use was investigated using the dust storm frequency (DSF) and data from more than 300 meteorological stations throughout northern China. Our analysis showed that most dust emissions originated in gobi deserts that developed in piedmont alluvial fans of the Kunlun, Qilian and Helan mountains. Dust emissions are low from other gobi desert regions, such as the northern Gurbantunggut and eastern Taklimakan, where high vegetation coverage restrained dust emissions or where dust-size particles are not abundant after a long period of strong wind erosion. Sandy deserts with relatively high vegetation coverage or an extensive cover by mobile sands are not a major dust source. Although the highest dust emissions did not appear in regions with the highest wind energy, DSF trends in each region from 1960 to 2003 were closely related to local wind activity. DSF was low in regions with high levels of human activity, where the mean DSF from 1960 to 2003 did not exceed 4 days/year; even from the 1960s to the early 1970s, the period with the greatest DSF, frequency did not exceed 8 days/year, which indicates that extensive land use did not contribute to DSF. The low DSF in these areas might result from the fact that although land use could produce abundant fine soil fractions, vegetation coverage and soil moisture remained higher than in the gobi deserts of arid China, thereby decreasing dust-storm occurrence. 相似文献
77.
选取四川省汶川至马尔康公路线的磨子沟作为试验对象,采用弯道超高法推算其泥石流流速。进而可以计算出该次泥石流的平均流量,为公路的改建工程提供设计依据。经过实地调查,用此方法计算磨子沟2005年8月17日暴发的泥石流流速为4.4m/s。同其他流速方法计算的结果相比较,前者值偏大,且偏向于工程安全值。弯道超高法可以解决泥石流观测的局限性和泥石流发生的不可重复性。此方法简便易行,对泥石流防治工程具有一定的实用价值。 相似文献
78.
79.
2004年全球重大气候事件概述 总被引:1,自引:3,他引:1
2004年全球气候持续偏暖。年初,暴雪席卷欧洲多国,美国受到罕见大雪袭击,南亚和墨西哥遭遇低温严寒。北半球夏秋季,西太平洋、大西洋热带风暴活动频繁。日本、菲律宾和美国受灾严重。年内,非洲、亚洲部分国家发生严重干旱;同时,南亚、南美洲及非洲多国暴雨致洪。年内,欧洲伊比利亚半岛、日本和澳大利亚遭遇高温热浪。 相似文献
80.