全文获取类型
收费全文 | 5501篇 |
免费 | 1071篇 |
国内免费 | 1276篇 |
专业分类
测绘学 | 413篇 |
大气科学 | 1131篇 |
地球物理 | 1524篇 |
地质学 | 2750篇 |
海洋学 | 630篇 |
天文学 | 160篇 |
综合类 | 607篇 |
自然地理 | 633篇 |
出版年
2024年 | 16篇 |
2023年 | 80篇 |
2022年 | 244篇 |
2021年 | 269篇 |
2020年 | 209篇 |
2019年 | 273篇 |
2018年 | 315篇 |
2017年 | 286篇 |
2016年 | 323篇 |
2015年 | 262篇 |
2014年 | 285篇 |
2013年 | 324篇 |
2012年 | 264篇 |
2011年 | 268篇 |
2010年 | 290篇 |
2009年 | 283篇 |
2008年 | 258篇 |
2007年 | 267篇 |
2006年 | 204篇 |
2005年 | 206篇 |
2004年 | 184篇 |
2003年 | 184篇 |
2002年 | 251篇 |
2001年 | 273篇 |
2000年 | 243篇 |
1999年 | 265篇 |
1998年 | 225篇 |
1997年 | 197篇 |
1996年 | 168篇 |
1995年 | 156篇 |
1994年 | 148篇 |
1993年 | 123篇 |
1992年 | 109篇 |
1991年 | 80篇 |
1990年 | 64篇 |
1989年 | 54篇 |
1988年 | 45篇 |
1987年 | 32篇 |
1986年 | 18篇 |
1985年 | 22篇 |
1984年 | 15篇 |
1983年 | 17篇 |
1982年 | 9篇 |
1981年 | 10篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1958年 | 8篇 |
1954年 | 1篇 |
排序方式: 共有7848条查询结果,搜索用时 31 毫秒
641.
Linking Tarim Basin sea retreat (west China) and Asian aridification in the late Eocene 总被引:1,自引:0,他引:1
R. Bosboom G. Dupont‐Nivet A. Grothe H. Brinkhuis G. Villa O. Mandic M. Stoica W. Huang W. Yang Z. Guo W. Krijgsman 《Basin Research》2014,26(5):621-640
The Tarim Basin in western China formed the easternmost margin of a shallow epicontinental sea that extended across Eurasia and was well connected to the western Tethys during the Paleogene. Climate modelling studies suggest that the westward retreat of this sea from Central Asia may have been as important as the Tibetan Plateau uplift in forcing aridification and monsoon intensification in the Asian continental interior due to the redistribution of the land‐sea thermal contrast. However, testing of this hypothesis is hindered by poor constraints on the timing and precise palaeogeographic dynamics of the retreat. Here, we present an improved integrated bio‐ and magnetostratigraphic chronological framework of the previously studied marine to continental transition in the southwest Tarim Basin along the Pamir and West Kunlun Shan, allowing us to better constrain its timing, cause and palaeoenvironmental impact. The sea retreat is assigned a latest Lutetian–earliest Bartonian age (ca. 41 Ma; correlation of the last marine sediments to calcareous nannofossil Zone CP14 and correlation of the first continental red beds to the base of magnetochron C18r). Higher up in the continental deposits, a major hiatus includes the Eocene–Oligocene transition (ca. 34 Ma). This suggests the Tarim Basin was hydrologically connected to the Tethyan marine Realm until at least the earliest Oligocene and had not yet been closed by uplift of the Pamir–Kunlun orogenic system. The westward sea retreat at ca. 41 Ma and the disconformity at the Eocene–Oligocene transition are both time‐equivalent with reported Asian aridification steps, suggesting that, consistent with climate modelling results, the sea acted as an important moisture source for the Asian continental interior. 相似文献
642.
选择旅游酒店数、旅行社数、旅游酒店从业人员数和旅行社从业人员数为旅游投入因素,旅游总收入为产出变量,运用数据包络分析方法中的 C2 R 模型分析2001-2010年中国区域旅游效率及其空间格局和变化趋势。中国四大区域的平均旅游效率由高至低依次为东部地区、东北地区、中部地区和西部地区。在31个中国大陆省级行政区中,天津在2001-2010年期间每年都为旅游效率最高的省份。从时间的角度,中国区域旅游效率经历了一个波浪式变化过程,2003年和2008年是2个明显的波谷。2001-2005年四大区域旅游效率的变化过程基本一致。2005-2010年四大区域旅游效率变化各不相同,东部地区呈逐步下降趋势,中部地区先增长、2010年开始下降,西部地区为起伏不定,东北地区为持续增长。 相似文献
643.
644.
645.
Pavement snow and icing are worldwide problems, but effective countermeasures are just beginning to be developed in China. The two most common snow and ice removal methods are mechanical clearance and chemical melting, and the advantages and disadvantages of each approach are discussed here, including environmental and structural damage caused by corrosive snow melting agents. New developments in chemical melting agents and mechanical equipment are discussed, and an overview of alternative thermal melting systems is presented, including the use of geothermy and non-geothermal heating systems utilizing solar energy, electricity, conductive pavement materials, and infrared/microwave applications. Strategic recommendations are made for continued enhancement of public safety in snow and ice conditions. 相似文献
646.
总结了自主研制的MWP967KV型地基35通道微波辐射计系统设计原理和主要特点。系统将K、V双频段宽带天线及接收机、宽带调谐本振、温湿压计以及电源模块紧凑集成为一台整机设备。对各传感器的测量输出实时进行一体化综合处理,采用BP神经网络实时反演大气温度、湿度廓线和汽、水总量。廓线的垂直覆盖范围为地表至顶空10km,共划分为58层。辐射计样机于2012年秋冬季节在北京地区开展了为期2个月的观测试验,利用这段时期内的69个探空资料样本对辐射计2个层次的实时观测输出都进行了对比检验,计算了水汽、氧气通道亮温和反演所得大气温湿廓线的平均偏差、均方差以及相关性。结果表明该系统能够满足实时气象监测的需求,达到国际先进水平。 相似文献
647.
A new in-situ seabed acoustic measurement system is developed for direct in-situ measurement of sediment geoacoustic properties (compressional wave velocity and attenuation). The new in-situ system consists of two parts: the deck control unit and the underwater measurement unit. The underwater measurement unit emits sonic waves that propagate through the seafloor sediment, receives the returning signals, and transmits them to the deck control unit for waveform display and analysis. The entire operation is controlled and monitored in real time by the deck control unit on the research vessel and can provide recording of full waveforms to determine the sound velocity and attenuation. This paper outlines the design of the system, the measurement process, and demonstrates its application in tests carded out on seabed sediment off the Qingdao coast, China. The test results show that the system performed well and rapidly provided accurate in-situ acoustic velocity and attenuation estimates of the seafloor sediment. 相似文献
648.
Modeling of spatial distributions of farmland density and its temporal change using geographically weighted regression model 总被引:1,自引:1,他引:0
Haitao Zhang Long Guo Jiaying Chen Peihong Fu Jianli Gu Guangyu Liao 《中国地理科学(英文版)》2014,24(2):191-204
This study used spatial autoregression(SAR) model and geographically weighted regression(GWR) model to model the spatial patterns of farmland density and its temporal change in Gucheng County,Hubei Province,China in 1999 and 2009,and discussed the difference between global and local spatial autocorrelations in terms of spatial heterogeneity and non-stationarity. Results showed that strong spatial positive correlations existed in the spatial distributions of farmland density,its temporal change and the driving factors,and the coefficients of spatial autocorrelations decreased as the spatial lag distance increased. SAR models revealed the global spatial relations between dependent and independent variables,while the GWR model showed the spatially varying fitting degree and local weighting coefficients of driving factors and farmland indices(i.e.,farmland density and temporal change). The GWR model has smooth process when constructing the farmland spatial model. The coefficients of GWR model can show the accurate influence degrees of different driving factors on the farmland at different geographical locations. The performance indices of GWR model showed that GWR model produced more accurate simulation results than other models at different times,and the improvement precision of GWR model was obvious. The global and local farmland models used in this study showed different characteristics in the spatial distributions of farmland indices at different scales,which may provide the theoretical basis for farmland protection from the influence of different driving factors. 相似文献
649.
2013-06~07??????????3?????????????С??????????п???????У???????????????????????????????????????????????????????????????????????????????????????????????????????????(ETAS???)??????????????????????崥???????????????й??????????????????? 相似文献
650.
?????????????????????????ο??????????????????????????????????????????????????????????NGA?????????????????????????飬??????????????????????о?????????????????????????????????????????2012?????4.7??????????е?????????????????????????????????????PGA??????S??a??T???????б????о?????????????????????????????????????????????????????????????????????????????????????????????? 相似文献