全文获取类型
收费全文 | 6098篇 |
免费 | 1229篇 |
国内免费 | 1767篇 |
专业分类
测绘学 | 452篇 |
大气科学 | 1409篇 |
地球物理 | 1512篇 |
地质学 | 3360篇 |
海洋学 | 643篇 |
天文学 | 283篇 |
综合类 | 729篇 |
自然地理 | 706篇 |
出版年
2024年 | 41篇 |
2023年 | 150篇 |
2022年 | 292篇 |
2021年 | 368篇 |
2020年 | 289篇 |
2019年 | 296篇 |
2018年 | 414篇 |
2017年 | 333篇 |
2016年 | 385篇 |
2015年 | 344篇 |
2014年 | 373篇 |
2013年 | 379篇 |
2012年 | 319篇 |
2011年 | 330篇 |
2010年 | 413篇 |
2009年 | 349篇 |
2008年 | 319篇 |
2007年 | 334篇 |
2006年 | 250篇 |
2005年 | 242篇 |
2004年 | 194篇 |
2003年 | 174篇 |
2002年 | 155篇 |
2001年 | 181篇 |
2000年 | 195篇 |
1999年 | 256篇 |
1998年 | 252篇 |
1997年 | 237篇 |
1996年 | 209篇 |
1995年 | 161篇 |
1994年 | 154篇 |
1993年 | 150篇 |
1992年 | 115篇 |
1991年 | 88篇 |
1990年 | 62篇 |
1989年 | 66篇 |
1988年 | 59篇 |
1987年 | 31篇 |
1986年 | 32篇 |
1985年 | 20篇 |
1984年 | 14篇 |
1983年 | 14篇 |
1982年 | 10篇 |
1981年 | 11篇 |
1980年 | 8篇 |
1979年 | 4篇 |
1975年 | 7篇 |
1960年 | 4篇 |
1959年 | 2篇 |
1958年 | 7篇 |
排序方式: 共有9094条查询结果,搜索用时 10 毫秒
71.
Dong-Mei Qin Ping Guo Zhan-Yi Hu Yong-Heng ZhaoNational Laboratory of Pattern Recognition Laboratory Institute of Automation Chinese Academy of Sciences Beijing dmqin@nlpr.ia.ac.cnDepartment of Computer Sciences Beijing Normal University Beijing National Astronomical Observatories Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2003,3(3)
For LAMOST, the largest sky survey program in China, the solution of the problem of automatic discrimination of stars from galaxies by spectra has shown that the results of the PSF test can be significantly refined. However, the problem is made worse when the redshifts of galaxies are not available. We present a new automatic method of star/(normal) galaxy separation, which is based on Statistical Mixture Modeling with Radial Basis Function Neural Networks (SMM-RBFNN). This work is a continuation of our previous one, where active and non-active celestial objects were successfully segregated. By combining the method in this paper and the previous one, stars can now be effectively separated from galaxies and AGNs by their spectra-a major goal of LAMOST, and an indispensable step in any automatic spectrum classification system. In our work, the training set includes standard stellar spectra from Jacoby's spectrum library and simulated galaxy spectra of EO, SO, Sa, Sb types with redshift ranging from 0 to 1 相似文献
72.
常量元素氧化物含量及其比值揭示的中晚全新世以来玛曲高原的环境演变 总被引:2,自引:0,他引:2
对玛曲高原具有代表性的黄土-古土壤-风成砂剖面(OL剖面)常量元素氧化物含量及其比值的地球化学特征分析表明:(1)常量元素氧化物总含量为91.93%且SiO2(67.93%) >Al2O3(11.2%) >Fe2O3(3.97%) >CaO(3.76%)>K2O(2.16%) >Na2O(1.81%) >MgO(1.1%);(2)玛曲高原处于较弱的脱Ca、Na初级风化阶段,并伴有碳酸盐的淋失;(3)玛曲高原的气候经历了冷干-凉润-温湿-凉干的冷暖旋回。依据常量元素氧化物含量及其比值揭示的环境特征并结合14C测年将玛曲高原中晚全新世以来的环境演变划分为4.6 ka BP以前的风沙活动强烈期、4.6~4.3 ka BP的风沙活动较弱期、4.3~0.3 ka BP的固结成壤期和0.3 ka BP至今的风沙活动增强期4个阶段。 相似文献
73.
74.
TingSheng Xiang XiaoBo Liu Min Zhang Fang Liu BaiGuang Li HuaE Fu Li Zhao WangShui Hu 《中国科学D辑(英文版)》2009,52(1):128-134
The distribution of indigenous microorganisms was surveyed in Block 1 of Daqing Oilfield. Based on this survey, the indigenous microorganisms in the formation water were activated with different activator systems at the simulated stratum ecological environment. The changes of the number of bacteria of various physiological groups were determined during the process of activation. Also changes of pH value and composition of gas productions were analyzed at the end of culturing. The results showed that the selected block formation water contained a great number of saprophytic bacteria, hydrocarbon-oxidizing bacteria, fermentative bacteria, methane-producing bacteria and sulfate-reducing bacteria. Under the conditions that the growth of sulfate-reducing bacteria was controlled the block had the potential to enhance oil recovery by activating beneficial bacteria. The growth of sulfate-reducing bacteria can be inhibited through the activation of nitrate-reducing bacteria. The number of nitrate-reducing bacteria reached 106–107 cells/mL, but sulfate-reducing bacteria reached only 0–45 cells/mL in A system. Methane-producing bacteria can be activated by C, D activators. The relative content of biological methane in the light hydrocarbon gas reached 80% in C, D systems. B activator was conducive to the propagation of acid-producing bacteria, so that the pH value of the culture medium decreased from 7.5 to around 5.0. Hydrocarbon-oxidizing bacteria can be activated by various activator systems. There was low molecular light hydrocarbon in gas production according to the analysis of gas chromatograph. According to the content of methane and the number of methane-producing bacteria, methane only can be generated through activating methane-producing bacteria. By choosing different activator systems, various populations of indigenous microorganisms can be activated accordingly. 相似文献
75.
青藏高原及邻区广泛分布的漂砾层、杂砾岩、含砾板岩是一种特殊的冰海相沉积岩,称为冰海杂砾岩(Ice-seaMix-conglomerates)。它们和一起产出的冷水型动物群、耐冷植物群视为这一地区冈瓦纳相最典型的标志,代表着冈瓦纳大陆上塔尔切尔(Talchir)冰期所影响的时间和空间。除“标准型”冰海杂砾岩外,羌塘腹地发现了蛇绿质和钙质冰海杂砾岩。这两种冰海杂砾岩的发现,不但丰富了冰海杂砾岩的家族成员,同时也为冰海沉积、物质来源问题、冈瓦纳大陆北界及特提斯演化研究提供了新的信息。经研究,冰海杂砾岩中的砾石来自印度半岛──冈瓦纳大陆本体,以冰筏为载体融冰落石成因。 相似文献
76.
77.
78.
随着旅游者对休闲娱乐类旅游需求的不断增加,山地型旅游区已成为最受欢迎的旅游目的地之一,利用新兴且高效的轨迹记录方式挖掘山地型旅游区游客在小尺度范围内动态的时空行为特征和制约因素,对旅游区线路设计和旅游产品优化尤为重要,也将为山地型旅游区的深化发展和科学管理提供建议。本文以湖北省恩施州恩施大峡谷为案例地,基于“六只脚”平台的GNSS轨迹及位置照片数据,结合GIS在线地图绘制的景区路网和从BigMap平台获取的POI数据,选择GNSS轨迹栅格化的概念模式解析出恩施大峡谷游客轨迹的时空行为特征,尝试探讨与其他类别旅游区时空行为特征的差异,从而总结出山地型旅游区的行为特征,并通过网络点评数据和游记文本等网络文本挖掘行为特征形成的影响因素。研究表明:(1)月份和季相层面,游客行为5月游客量处于峰值,节日效应明显,短假期效应较强;淡旺季游客行为轨迹呈非集聚性,旺季游客量主要受气候条件影响呈不稳定态势;(2)游客行为轨迹于11:00—14:00游客量达到顶峰,日内结束时间相对较早,且兴趣点选择偏向知名度高的七星寨景区;(3)空间分布特征层面,受时空因素控制,单一路径景区轨迹集中度较强;(4)不同性别... 相似文献
79.
腾冲火山区地表垂直形变分析 总被引:8,自引:3,他引:8
对腾冲火山区1998~2000年的3期垂直形变资料进行了分析,进而利用相关资料得出与计算有关的参数,再用多元Mogi模型反演了岩浆房的位置和大小,所得出的结果与地震层析反演结果基本相似。分析反演结果,认为火山区可能存在多个岩浆房,并有岩脉相通。 相似文献
80.
Potential promoted productivity and spatial patterns of medium- and low-yield cropland land in China 总被引:1,自引:1,他引:1
With a continuously increasing population and better food consumption levels, improving the efficiency of arable land use and increasing its productivity have become fundamental strategies to meet the growing food security needs in China. A spatial distribution map of medium- and low-yield cropland is necessary to implement plans for cropland improvement. In this study, we developed a new method to identify high-, medium-, and low-yield cropland from Moderate Resolution Imaging Spectroradiometer (MODIS) data at a spatial resolution of 500 m. The method could be used to reflect the regional heterogeneity of cropland productivity because the classification standard was based on the regionalization of cropping systems in China. The results showed that the proportion of high-, medium-, and low-yield cropland in China was 21%, 39%, and 40%, respectively. About 75% of the low-yield cropland was located in hilly and mountainous areas, and about 53% of the high-yield cropland was located in plain areas. The five provinces with the largest area of high-yield cropland were all located in the Huang-Huai-Hai region, and the area amounted to 42% of the national high-yield cropland area. Meanwhile, the proportion of high-yield cropland was lower than 15% in Heilongjiang, Sichuan, and Inner Mongolia, which had the largest area allocated to cropland in China. If all the medium-yield cropland could be improved to the productive level of high-yield cropland and the low-yield cropland could be improved to the level of medium-yield cropland, the total productivity of the land would increase 19% and 24%, respectively. 相似文献