全文获取类型
收费全文 | 34495篇 |
免费 | 6222篇 |
国内免费 | 8398篇 |
专业分类
测绘学 | 1715篇 |
大气科学 | 7866篇 |
地球物理 | 8731篇 |
地质学 | 17400篇 |
海洋学 | 4019篇 |
天文学 | 1694篇 |
综合类 | 3744篇 |
自然地理 | 3946篇 |
出版年
2024年 | 151篇 |
2023年 | 594篇 |
2022年 | 1388篇 |
2021年 | 1686篇 |
2020年 | 1440篇 |
2019年 | 1487篇 |
2018年 | 1793篇 |
2017年 | 1669篇 |
2016年 | 1952篇 |
2015年 | 1515篇 |
2014年 | 2059篇 |
2013年 | 1807篇 |
2012年 | 1724篇 |
2011年 | 1826篇 |
2010年 | 1969篇 |
2009年 | 1919篇 |
2008年 | 1650篇 |
2007年 | 1597篇 |
2006年 | 1359篇 |
2005年 | 1247篇 |
2004年 | 999篇 |
2003年 | 1014篇 |
2002年 | 1019篇 |
2001年 | 985篇 |
2000年 | 1203篇 |
1999年 | 1752篇 |
1998年 | 1517篇 |
1997年 | 1576篇 |
1996年 | 1299篇 |
1995年 | 1206篇 |
1994年 | 1092篇 |
1993年 | 954篇 |
1992年 | 760篇 |
1991年 | 579篇 |
1990年 | 389篇 |
1989年 | 415篇 |
1988年 | 359篇 |
1987年 | 237篇 |
1986年 | 206篇 |
1985年 | 149篇 |
1984年 | 130篇 |
1983年 | 97篇 |
1982年 | 97篇 |
1981年 | 63篇 |
1980年 | 62篇 |
1979年 | 41篇 |
1978年 | 18篇 |
1977年 | 9篇 |
1976年 | 6篇 |
1958年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
Fuzzy neural network models for liquefaction prediction 总被引:1,自引:0,他引:1
Integrated fuzzy neural network models are developed for the assessment of liquefaction potential of a site. The models are trained with large databases of liquefaction case histories. A two-stage training algorithm is used to develop a fuzzy neural network model. In the preliminary training stage, the training case histories are used to determine initial network parameters. In the final training stage, the training case histories are processed one by one to develop membership functions for the network parameters. During the testing phase, input variables are described in linguistic terms such as ‘high’ and ‘low’. The prediction is made in terms of a liquefaction index representing the degree of liquefaction described in fuzzy terms such as ‘highly likely’, ‘likely’, or ‘unlikely’. The results from the model are compared with actual field observations and misclassified cases are identified. The models are found to have good predictive ability and are expected to be very useful for a preliminary evaluation of liquefaction potential of a site for which the input parameters are not well defined. 相似文献
42.
Yao Chun-xial Li Bao-sheng Jin He-ling David Dian Zhang Yan Man-cun Zhu Yi-zhi Li Hou-xin Zhang Yu-hong Luo Kai-li 《地理学报(英文版)》2002,12(1):65-71
The paper makes some analyses on 11 trace elements in the Milanggouwan stratigraphical section in the Salawusu River valley,
which is regarded as a prototype geology-palaeoclimate record since 150 ka BP. The results show that the content and variation
of trace elements has experienced remarkably regular changes in the pace with coarse and fine sedimentary cycles of palaeo-aeolian
sands to its overlying fluvio-lacustrine facies or/and palaeosols. The trace elements with chemical properties of relatively
active (V, Sr, Cu, Ni, As) and relatively stable (P, Pb, Rb, Mn, Nb, Zr) are a manifestation of the corresponding 27 changeable
cycles between peak and valley values, appearing a multi-fiuctuational process line of relative gathering and migration since
then. The low numerical value distribution of these two types of trace elements in the aeolian sand facies represents erosion
and accumulation under wind force during the cold-dry climate. Whereas their enrichments in both fluvio-lacustrine facies
and palaeosols are related to the valley’s special low-lying physiognomic position between the Ordos Plateau and the Loess
Plateau under the warm and humid climate conditions. The above relatively migrated and gathered change of the trace elements
is the result of 27 climatic cycles of cold-dry and warm-humid, which is probably caused by repeated alternations of winter
monsoon and summer monsoon in the Mu Us Sandy Land influenced by the climate vicissitudes in northern hemisphere during glacial
and interglacial periods since 150 ka BP. 相似文献
43.
44.
Samples of dune sands, surveys of the morphology and field measurements of wind velocity and direction of a simple linear dune in Taklimakan Sand Sea show that the airflow and sand flux vary with the change of wind direction on the dune surface. Decrease of the airflow stress on the lee flank does not result in much decrease of the sand flux because of the low threshold shear velocities and the airflow conditions. There are no significant relations between the sand flux on the lee flank and the angle of incidence of the airflow. The low threshold shear velocities and the maintenance of the sand flux at the lee flank are the main mechanisms keeping the linear shape of the dunes. Measurements of the sand flux shows that it reaches a maximum on the crest of the dune. The grain size of the transported sands has some differences compared to that of the dune surface. The sands transported are finer than that on the dune surface, but better sorted under the influence of the medium to low wind activity. The field experiment results exhibit that it is possible for the dunes to be shaped as linear dunes during the processes of accumulation and elongation. 相似文献
45.
46.
We use 23298 Pn arrival-time data from Chinese national and provincial earthquake bulletins to invert fine structure of Pn velocity and anisotropy at the top of the mantle beneath the Sichuan-Yunnan and its adjacent region. The results suggest that the Pn velocity in this region shows significant lateral variation; the Pn velocity varies from 7.7 to 8.3 km/s. The Pn-velocity variation correlates well with the tectonic activity and heat flow of the region. Low Pn velocity is observed in southwest Yunnan, Tengchong volcano area, and the Panxi tectonic area. These areas have very active seismicity and tectonic activity with high surface heat flow. On the other hand, high Pn velocity is observed in some stable regions, such as the central region of the Yangtze Platform; the most pronounced high velocity area is located in the Sichuan Basin, south of Chengdu. Pn anisotropy shows a complex pattern of regional deformation. The Pn fast direction shows a prominent clockwise rotation pattern from east of the Tibetan block to the Sichuan-Yunnan diamond block to southwest Yunnan, which may be related to southeastward escape of the Tibetan Plateau material due to the collision of the Indian Plate to the Eurasia Plate. Thus there appears to be strong correlation between the crustal deformation and the upper mantle structure in the region. The delay times of events and stations show that the crust thickness decreases from the Tibetan Plateau to eastern China, which is consistent with the results from deep seismic sounding. 相似文献
47.
48.
49.
50.
首先证明了加权最小范数平差一般解的一个重要性质,由此提出了利用附加的观测值来求解加权最小范数平差问题的方法,并证明了该法与利用一般解法等价性的充要条件。 相似文献