全文获取类型
收费全文 | 8705篇 |
免费 | 2110篇 |
国内免费 | 3057篇 |
专业分类
测绘学 | 1213篇 |
大气科学 | 1360篇 |
地球物理 | 1983篇 |
地质学 | 5702篇 |
海洋学 | 1491篇 |
天文学 | 195篇 |
综合类 | 735篇 |
自然地理 | 1193篇 |
出版年
2024年 | 111篇 |
2023年 | 305篇 |
2022年 | 623篇 |
2021年 | 678篇 |
2020年 | 585篇 |
2019年 | 619篇 |
2018年 | 632篇 |
2017年 | 559篇 |
2016年 | 584篇 |
2015年 | 597篇 |
2014年 | 598篇 |
2013年 | 624篇 |
2012年 | 661篇 |
2011年 | 670篇 |
2010年 | 667篇 |
2009年 | 550篇 |
2008年 | 559篇 |
2007年 | 545篇 |
2006年 | 443篇 |
2005年 | 438篇 |
2004年 | 356篇 |
2003年 | 297篇 |
2002年 | 420篇 |
2001年 | 357篇 |
2000年 | 306篇 |
1999年 | 258篇 |
1998年 | 131篇 |
1997年 | 93篇 |
1996年 | 94篇 |
1995年 | 82篇 |
1994年 | 75篇 |
1993年 | 61篇 |
1992年 | 67篇 |
1991年 | 34篇 |
1990年 | 37篇 |
1989年 | 18篇 |
1988年 | 16篇 |
1987年 | 16篇 |
1986年 | 13篇 |
1985年 | 16篇 |
1984年 | 11篇 |
1982年 | 11篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1958年 | 4篇 |
1957年 | 3篇 |
1954年 | 9篇 |
1928年 | 3篇 |
1925年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
S. Y. Fu Q. G. Zong Z. Y. Pu C. J. Xiao A. Korth P. Daly H. Reme 《Surveys in Geophysics》2005,26(1-3):241-254
In this paper we report energetic ion behavior and its composition variations observed by the Cluster/RAPID instrument when the spacecraft was travelling in the high latitude magnetospheric boundary region on the day of the 31 March, 2001, strongest magnetic storm in the past 50 years. The Dst index reached −360 nT at about 09:00 UT. During its early recovery phase, large amounts of oxygen and helium ions were observed; the ratio of oxygen to hydrogen in the RAPID energy range reached as high as 250%, which suggests that the observed energetic particles might be of magnetospheric origin. The observations further show that enhanced energetic electron fluxes are confined in a very narrow region, while protons have occupied a larger region, and heavy ions have been observed in an even larger region. The flux of energetic electrons show a slight enhancement in a region where the magnetic field magnitude is around zero. These observed energetic ions could be quasi-trapped by the current sheet in the stagnation region of the cusp. 相似文献
73.
Although Eddy Covariance (EC) technique is one of the best methods for estimating the energy and mass exchanges between underlying surface and atmosphere in micrometeorology, errors and uncertainties still exist without necessary corrections. In this paper, we will focus on the effect of coordinate system on the eddy fluxes. Based on the data observed over four sites (one farmland site, one grassland site and two forest sites), the effects of three coordinate system transforming methods (Double Rotation-DR, Triple Rotation-TR and Planar Fit-PF)on the turbulent fluxes are analyzed. It shows that (i) the corrected fluxes are more or less than the uncorrected fluxes, which is related mainly to the sloping degree of surface, wind speed and wind direction; and (ii) pitch angle has a sinusoidal dependence on wind direction, especially in the regular sloping terrain; and (iii) PF method is something like the simplification of TR or DR,and there are not obvious distinctions in correction in sloping grassland and flat farmland, but PF method is not suitable for uneven and irregular forest sites. 相似文献
74.
75.
Wenzhi Zhao Shuichang Zhang Feiyu Wang Jianping Chen Zhongyao Xiao Fuqing Song 《Organic Geochemistry》2005,36(12):1602-1616
Oil and gas exploration in eastern Tarim Basin, NW China has been successful in recent years, with several commercial gas accumulations being discovered in a thermally mature to over-mature region. The Yingnan2 (YN2) gas field, situated in the Yingnan structure of the Yingjisu Depression, produces gases that are relatively enriched in nitrogen and C2+ alkanes. The δ13C1 (−38.6‰ to −36.2‰) and δ13C2 values (−30.9‰ to −34.7‰) of these gases are characteristic of marine sourced gases with relatively high maturity levels. The distributions of biomarkers in the associated condensates suggest close affinities with the Cambrian–Lower Ordovician source rocks which, in the Yingjisu Sag, are currently over-mature (with 3–4%Ro). Burial and thermal maturity modeling results indicate that paleo-temperatures of the Cambrian–Lower Ordovician source rocks had increased from 90 to 210 °C during the late Caledonian orogeny (458–438 Ma), due to rapid subsidence and sediment loading. By the end of Ordovician, hydrocarbon potential in these source rocks had been largely exhausted. The homogenization temperatures of hydrocarbon fluid inclusions identified from the Jurassic reservoirs of the YN2 gas field suggest a hydrocarbon emplacement time as recent as about 10 Ma, when the maturity levels of Middle–Lower Jurassic source rocks in the study area were too low (<0.7%Ro) to form a large quantity of oil and gas. The presence of abundant diamondoid hydrocarbons in the associated condensates and the relatively heavy isotopic values of the oils indicate that the gases were derived from thermal cracking of early-formed oils. Estimation from the stable carbon isotope ratios of gaseous alkanes suggests that the gases may have been formed at temperatures well above 190 °C. Thus, the oil and gas accumulation history in the study area can be reconstructed as follows: (1) during the late Caledonian orogeny, the Cambrian–Lower Ordovician marine source rocks had gone through the peak oil, wet gas and dry gas generation stages, with the generated oil and gas migrating upwards along faults and fractures to form early oil and gas accumulations in the Middle–Upper Ordovician and Silurian sandstone reservoirs; (2) since the late Yanshanian orogeny, the early oil accumulations have been buried deeper and oil has undergone thermal cracking to form gas; (3) during the late Himalayan orogeny, the seals for the deep reservoirs were breached; and the gas and condensates migrated upward and eventually accumulating in the relatively shallow Jurassic reservoirs. 相似文献
76.
新疆阿尔泰造山带构造作用的锆石裂变径迹分析 总被引:1,自引:0,他引:1
在新疆阿尔泰造山带所获得的19个锆石裂变径迹年龄变化于155-243Ma之间,明显地分为2组,分别对应于2个构造活动期,早期为155-189Ma,晚期为189-243Ma。这与磷灰石裂变径迹年龄反映的62-100Ma和100-160Ma两个构造期完全一致。早期和晚期构造活动期持续的时间分别为54-60Ma和34-38Ma,而这两个构造期之间的间隔时间,则从早到晚由83-89Ma变为89-93Ma。同时,锆石裂变径迹年龄与距特斯巴汗断裂和巴寨断裂的距离有关,反映这两条断裂带对区域构造演化的控制作用。 相似文献
77.
78.
79.
80.