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   77388篇
  免费   1131篇
  国内免费   411篇
测绘学   2116篇
大气科学   5804篇
地球物理   15174篇
地质学   28942篇
海洋学   6045篇
天文学   16486篇
综合类   268篇
自然地理   4095篇
  2021年   409篇
  2020年   479篇
  2019年   452篇
  2018年   3921篇
  2017年   3748篇
  2016年   2943篇
  2015年   1085篇
  2014年   1523篇
  2013年   2856篇
  2012年   2502篇
  2011年   4479篇
  2010年   4216篇
  2009年   4736篇
  2008年   3950篇
  2007年   4491篇
  2006年   1957篇
  2005年   2222篇
  2004年   2019篇
  2003年   2005篇
  2002年   1734篇
  2001年   1384篇
  2000年   1322篇
  1999年   1211篇
  1998年   1170篇
  1997年   1142篇
  1996年   832篇
  1995年   899篇
  1994年   868篇
  1993年   781篇
  1992年   755篇
  1991年   714篇
  1990年   825篇
  1989年   710篇
  1988年   675篇
  1987年   764篇
  1986年   639篇
  1985年   867篇
  1984年   971篇
  1983年   950篇
  1982年   900篇
  1981年   858篇
  1980年   784篇
  1979年   737篇
  1978年   741篇
  1977年   662篇
  1976年   620篇
  1975年   558篇
  1974年   620篇
  1973年   628篇
  1972年   390篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The autonomous underwater glider "Spray"   总被引:4,自引:0,他引:4  
A small (50-kg, 2-m long) underwater vehicle with operating speeds of 20-30 cm/s and ranges up to 6000 km has been developed and field tested. The vehicle is essentially an autonomous profiling float that uses a buoyancy engine to cycle vertically and wings to glide horizontally while moving up and down. Operational control and data relay is provided by GPS navigation and two-way communication through ORBCOMM low-Earth-orbit satellites. Missions are envisioned with profile measurements repeated at a station or spaced along a transect. The initial instrument complement of temperature, conductivity, and pressure sensors was used to observe internal waves and tides in the Monterey underwater canyon  相似文献   
992.
Gallo  D. G.  Kidd  W. S. F.  Fox  P. J.  Karson  J. A.  Macdonald  K.  Crane  K.  Choukroune  P.  Seguret  M.  Moody  R.  Kastens  K. 《Marine Geophysical Researches》1984,6(2):159-185
During the Fall of 1979, a manned submersible program, utilizing DSRV ALVIN, was carried out at the intersection of the East Pacific Rise (EPR) with the Tamayo Transform boundary. A total of seven dives were completed in the vicinity of the EPR/Tamayo intersection depression and documented the geologic relationships that characterize the juxtaposition of these types of plate boundaries. The young volcanic terrain of the EPR axis can be traced into and across the Tamayo Transform valley but becomes buried by sedimentary talus that is being shed from sediment scarps along the unstable sediment slope that defines the north side of the intersection depression. Within 4 km of the transform boundary, the dominant trend (000°) of the fissures and faults that disrupt the rise-generated volcanics is markedly oblique to the regional direction of sea floor spreading (120°). Since no evidence was found to suggest that these structures accommodate significant amounts of strike-slip displacement, they are taken to reflect a distortion of the EPR extensional tectonic regime by a transform generated shear couple. The floor of the Tamayo Transform valley in this area is inundated by mass-wasted sediment, and the principal transform displacement zone is characterized at the surface by a narrow (<1.5 km) interval of fault scarps in sediment that trends parallel with the transform valley. Extrapolated to the west, this zone links with zones of transform deformation investigated during earlier submersible studies (CYAMEX and Pastouret, 1981). Evidence of low-level hydrothermal discharge was seen at one locality on the EPR axis and at another 8 km west of the axis at the edge of the zone of transform deformation.  相似文献   
993.
994.
Three methods for the determination of dissolved organic carbon in seawater were compared. Samples were analysed using persulphate oxidation, high-temperature combustion, and ultraviolet photo-oxidation. The dissolved organic carbon content of the seawater samples ranged from 0.6 to 1.6 mg C/I. This study shows that results of high-temperature oxidation and photo-oxidation procedures differ by less than 5%, whereas results with persulphate oxidation are about 15% less than those obtained with the high-temperature oxidation. The relative merits of each of the oxidation techniques for the determination of organic matter in seawater are discussed.  相似文献   
995.
996.
997.
The chemical and mineralogical composition of suspended particle suites has been used to assess off-shelf transport on the West African shelves of Liberia and Sierra Leone. Using the ratios of Si/Al, Fe/Al, Mg/Al and Mn/Al as tracers, it was possible to detect shelf-derived materials in slope waters. In the majority of cases, these inputs could not have been detected using particle mass or light scattering measurements and could only be measured by using the chemical signatures of the particulate matter. At the time of sampling, the suspensate which had been moved seaward over the slope was detected adjacent to submarine canyons and highly turbid areas on the outer shelf. The chemical and mineralogical composition of the suspensates, and sediments in the adjacent eastern Atlantic basin are similar to those found in the water column seaward of the West African continental shelves and yet distinct from the Sahelian dust which is considered the major source of sedimentary material for the tropical and semi-tropical areas of the deep eastern basin. These findings suggest that materials from this shelf area could be a more important input to the deep sea than was previously realised.  相似文献   
998.
Several areas of erosion and accretion were observed along the Burullus-Gamasa beach. The accreted sands are coarser and less sorted than the eroded ones. Differentiation between them could be achieved on the basis of their grain-size fractions, shape of grain-size distribution curves and statistical grain-size parameter relationships.  相似文献   
999.
Abstract. Benthic metabolism and standing stocks were investigated in the deep Red Sea between 21o and 27oN, Activity was assessed by the determination of respiration rates with a shipboard method and by calculating oxygen consumption from the activity in the electron transport system. We attempted to compare results from different latitudes within the warm Red Sea and with data from cold Atlantic environments. Our investigations were part of an environmental risk assessment to evaluate future mining of metalliferous sediments from the Atlantis II Deep.  相似文献   
1000.
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