首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   103228篇
  免费   2097篇
  国内免费   1660篇
测绘学   2690篇
大气科学   7515篇
地球物理   22058篇
地质学   36943篇
海洋学   8792篇
天文学   22308篇
综合类   468篇
自然地理   6211篇
  2022年   678篇
  2021年   1176篇
  2020年   1228篇
  2019年   1266篇
  2018年   2751篇
  2017年   2566篇
  2016年   3247篇
  2015年   2026篇
  2014年   3146篇
  2013年   5519篇
  2012年   3528篇
  2011年   4588篇
  2010年   3975篇
  2009年   5076篇
  2008年   4598篇
  2007年   4377篇
  2006年   4146篇
  2005年   3253篇
  2004年   3131篇
  2003年   2927篇
  2002年   2666篇
  2001年   2461篇
  2000年   2362篇
  1999年   1897篇
  1998年   2045篇
  1997年   1887篇
  1996年   1567篇
  1995年   1588篇
  1994年   1371篇
  1993年   1248篇
  1992年   1222篇
  1991年   1109篇
  1990年   1233篇
  1989年   1044篇
  1988年   947篇
  1987年   1175篇
  1986年   969篇
  1985年   1252篇
  1984年   1394篇
  1983年   1270篇
  1982年   1249篇
  1981年   1081篇
  1980年   1032篇
  1979年   932篇
  1978年   924篇
  1977年   876篇
  1976年   835篇
  1975年   793篇
  1974年   794篇
  1973年   818篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
In this paper, the self-adaptive artificial fish swarm algorithm (SAAFSA) is used to optimize the coarse graining of segment numbers, which are used in the Lempel-Ziv complexity algorithm. This approach improves the Lempel-Ziv complexity (LZC) algorithm of equal probability coarse graining. As a case study, the complexities of monthly series of groundwater depth were analyzed at seven farms in the Hongxinglong Administration. GIS technology was used to create spatial distributions of monthly groundwater depths. A projection pursuit model based on the SAAFSA was established and used for complexity attribution analysis at selected farms with different degrees of complexity. The three selected farms, Hongqiling, 852, and Youyi, each represent a certain degree of complexity. Further analysis shows that precipitation, evaporation, temperature, and human activities are the primary factors that cause complexity variations in local groundwater depth. The results reveal the evolution of the complexity characteristics of local groundwater depth and provide scientific evidence for the need to effectively allocate regional water resources. Additionally, the proposed method can be applied in complexity analyses of other hydrologic features, as well as in research regarding nonlinear time series in economic, engineering, medical, and signal analyses.  相似文献   
993.
Facies and sequence stratigraphic analysis of the Kometan Formation (Upper Cretaceous) were studied from Kometan village, Kurdistan region of northeastern Iraq. Lithologically, the formation consists of 44 m of white weathering, light grey, thin to medium-bedded highly fractured limestones with chert nodules. Petrographic study of the carbonates shows that both skeletal and non-skeletal grains were present. The skeletal grains include a variety of planktonic foraminifera (including Oligostegina), calcispheres, ostracods, pelecypods, larva ammonite, and echinoderms. Non-skeletal grains include peloids only. Three main microfacies types are distinguished in the studied formation. The results of stable carbon and oxygen isotopes of the studied carbonate samples show negative values of δ18O. These indicate that the seawater was warm with low salinity during precipitation of the carbonates in the Kometan Formation in northeastern Iraq. The positive δ13C values of carbonate samples, in the middle part of the formation, reflect the widespread deposition of organic-rich marine sediments during a transgression and deepening of the basin. Petrographic, facies and stable isotopic analyses revealed that the Kometan Formation was deposited in a warm, basinal, pelagic (open marine) environment with low salinity. The Kometan Formation consists of one complete third-order depositional sequence, separated by a sequence boundary (SB) of type 2. The third-order sequence is subdivided into a transgressive systems tract (TST) and highstand systems tract (HST). This reflects episodes of transgression and still stands of the relative sea level. The TSTs are topped by maximum flooding surface (MFS) characterized by deepening-/fining-upward parasequences implying a retrogradational stacking pattern. The HST is marked by shallowing-/coarsening-upward parasequences implying a progradational stacking pattern.  相似文献   
994.
During the last 30 years, UAE witnessed earthquakes that ranged from minor to moderate, with maximum magnitude of 5.1 that occurred in the Masafi area (eastern UAE, on March 11, 2002). Recent earthquakes that hit Iran such as on May 11, 2013, caused tremors and mild shaking of buildings in some UAE cities. Although the tremors are small in magnitude, their sequences apparently become an important research topic and deserve more assessment from different perspectives such as geographical, geological, engineering, and social. This is because low risk does not equal no risk. This study is concerned with public perceptions of earthquake preparedness (reduction of disaster impact) that can be measured by various variables such as developing an emergency plan, preparation of disaster supply kits, and training. The methodology consists of a survey of 470 people who live around the Masafi area, near Fujairah city, UAE. GIS and GPS were used for site selection in conducting the survey, and remote sensing was used as an aid in identification of buildings’ ages. Results show that around 90% of the people surveyed have water tanks that can support them up to 3 days, and 60% of them normally buy food that can support them up to 2 days. Thirty percent of the respondents were familiar with storing first-aid kits and tools such as flashlights. The findings point to a need for more research regarding public awareness about earthquakes. The findings of this study may be useful for people who are involved in the four cornerstones of disaster risk reduction: community participation, public policy actions, safer construction and urban development, and development of a culture of prevention.  相似文献   
995.
This work presents the application of the ground-penetrating radar (GPR) method and electric resistivity tomography (ERT) technique in outlining a zone of contamination due to the light non-aqueous phase liquid (LNAPL) plume underground in the area of an impacted fuel station, close to Abha City. The GPR has been performed using SIR3000 unit with the 100 and 400 MHz antennas. The main objective of the GPR survey was to evaluate the lateral extension of contamination. The complex GPR signature of the plume was well characterized. Low reflectivity zone corresponds to hydrocarbon vapor phase in the vadose zone. Enhanced reflections are associated with free and residual products in the fractured saturated zone directly above the water table. An electric resistivity tomography (ERT) survey was performed on four profiles within the site to investigate the vertical and horizontal extent of the contamination plume and to define the bottom of the landfill. The 2D electric profiles show the presence of low-resistivity (4O to 37 Ω m) anomalies that refers to the presence of accumulated hydrocarbons. From the interpretation of the GPR and ERT profile, it was possible to locate the top and bottom of the contamination plume of the waste disposal site. The radar signal penetrated deep enough and enabled the identification of a second reflector at approximately 10-m deep, interpreted as the hard basement surface which causes the strong amplitude reflection in the GPR profile. The results of GPR and ERT showed good agreement.  相似文献   
996.
997.
M. S. Barash 《Oceanology》2016,56(6):863-875
The second of the five great mass extinctions of the Phanerozoic occurred in the Late Devonian. The number of species decreased by 70–82%. Major crises occurred at the Frasnian–Famennian and Devonian–Carboniferous boundary. The lithological and geochemical compositions of sediments, volcanic deposits, impactites, carbon and oxygen isotope ratios, evidence of climate variability, and sea level changes reflect the processes that led the critical conditions. Critical intervals are marked by layers of black shales, which were deposited in euxinic or anoxic environments. These conditions were the main direct causes of the extinctions. The Late Devonian mass extinction was determined by a combination of impact events and extensive volcanism. They produced similar effects: emissions of harmful chemical compounds and aerosols to cause greenhouse warming; darkening of the atmosphere, which prevented photosynthesis; and stagnation of oceans and development of anoxia. Food chains collapsed and biological productivity decreased. As a result, all vital processes were disturbed and a large portion of the biota became extinct.  相似文献   
998.
Izvestiya, Atmospheric and Oceanic Physics - International investigations into the correlation between tropical cyclones (TCs) and the ionosphere are associated with great difficulties in proving...  相似文献   
999.
The results of comprehensive research into the area of the upper reach of the Mzymta River (Western Caucasus) are presented. The evidence of strong earthquakes that struck the study area throughout history is studied. The periods when mud flows and rockslides associated with the earthquakes are identified to fall between 3100–4000 and 700–1200 years ago. Paleoseismological results are correlated to the results of archeological excavations. As a result, it is revealed that the periods of tectonic disasters coincided with the onsets of the period when the area was uninhabited (abandoned) by people. The results of the present comprehensive research provide additional information to specify the seismic hazard and the chronology of human habitation in the region.  相似文献   
1000.
A short survey prepared by the Russian Commission on Atmospheric Radiation contains the most significant results of work in the field of atmospheric-radiation studies performed in 2011–2014. It is part of the Russian National Report on Meteorology and Atmospheric Sciences prepared for the International Association on Meteorology and Atmospheric Sciences (IAMAS)1. During this period, the Russian Commission on Atmospheric Radiation, jointly with the concerned departments and organizations, organized two International Symposiums on Radiation and Dynamics (ISARD-2011 and ISARD-2013). At these conferences, the central problems in modern atmospheric physics were discussed: radiative transfer (RT) and atmospheric optics; greenhouse gases, clouds, and aerosols; remote methods of measurements; and new measurement data. This survey presents six directions covering the whole spectrum of investigations performed in the field of atmospheric radiation.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号