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   49394篇
  免费   640篇
  国内免费   324篇
测绘学   1334篇
大气科学   3830篇
地球物理   9285篇
地质学   16261篇
海洋学   4172篇
天文学   12164篇
综合类   108篇
自然地理   3204篇
  2020年   339篇
  2019年   337篇
  2018年   828篇
  2017年   810篇
  2016年   1052篇
  2015年   696篇
  2014年   1170篇
  2013年   2441篇
  2012年   1110篇
  2011年   1543篇
  2010年   1436篇
  2009年   1930篇
  2008年   1774篇
  2007年   1776篇
  2006年   1678篇
  2005年   1544篇
  2004年   1508篇
  2003年   1413篇
  2002年   1357篇
  2001年   1230篇
  2000年   1155篇
  1999年   1121篇
  1998年   1056篇
  1997年   1062篇
  1996年   844篇
  1995年   836篇
  1994年   794篇
  1993年   738篇
  1992年   709篇
  1991年   680篇
  1990年   776篇
  1989年   674篇
  1988年   648篇
  1987年   741篇
  1986年   620篇
  1985年   824篇
  1984年   947篇
  1983年   912篇
  1982年   861篇
  1981年   819篇
  1980年   722篇
  1979年   698篇
  1978年   692篇
  1977年   627篇
  1976年   596篇
  1975年   517篇
  1974年   597篇
  1973年   586篇
  1972年   362篇
  1971年   334篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
LANDSAT-1 (ERTS-1) data of the complete scene, 1218-04320, covering an area 185 Km x 185 Km in the Godavari Valley, India was computer processed in the Computer Division of the Oil and Natural Gas Commissíon to produce an imagery of readily recognizable land forms. Comparison with the original maps supplied by NASA showed an amazing agreement in quality. Computer oriented data enhancement techniques were developed and the data was reprocessed with the help of these techniques. The resulting imagery reveal an improvement over the originals. The computer software package developed to-date comprises nine different programmes to process the LANDSAT data on the IBM 360, 370 series.  相似文献   
992.
The Arc Marine data model is a generalized template to guide the implementation of geographic information systems (GIS) projects in the marine environment. It developed out of a collaborative process involving research and industry shareholders in coastal and marine research. This template models and attempts to standardize common ocean and coastal data types to facilitate data sharing and analytical tool development. In this study, Arc Marine is extended from its core model to fit the research goals of the whale satellite-telemetry-tagging program of the Oregon State University Marine Mammal Institute (MMI). The study sought the best customization of the generic Arc Marine data model to enhance the key advantages of satellite telemetry for mapping the distribution and movement of endangered marine mammal species. It was found that three new groups of object classes were needed (Animal, Telemetry, and Operations). Further customization involved the development of a comprehensive framework for animal tracking with Argos satellite telemetry data. A new multidimensional data cube model was also devised, showing how this extension of Arc Marine serves as an appropriate target schema for the application of on-line analytical processing (OLAP) tools and spatial data mining of satellite telemetry tracking datasets.  相似文献   
993.
Eight samples of the beryl variety aquamarine were selected from four pegmatites in the Governador Valadares and Araçuaí regions in northeastern Minas Gerais State, Brazil. These samples were fully characterized by chemical analysis, infrared and UV-visible spectroscopy, thermal analyses, and high-temperature X-ray diffraction (from room temperature up to 800 °C). Several physical and chemical properties of beryl were found to depend on the amount of water and ions residing in the structural channels. The thermal expansion coefficients from room temperature to about 800 °C are temperature-independent, with αa ? ?3.2 × 10?6 ° C?1 and αc ? ?8.7 × 10?6 ° C?1. The contraction of both a and c unit-cell parameters with increasing temperature and the shift of the infrared band centered at about 1200 cm?1 were tentatively ascribed to interactions between channel water and the silicate rings.The color of beryl seems to be dictated by the relative proportions of Fe3+ in the octahedralsites and of fe2+ in the channels. Thus, deep-blue samples have little Fe3+, whereas greener samples have more Fe3+ or less channel Fe2+.  相似文献   
994.
995.
996.
By reviewing the history of fishery exploitation in the coastal waters of west Canada and east Korea, related with contrasting life history strategies of the dominant species, the fishery management challenges that each country would face in the upcoming decades were outlined. In the ecosystem of the Canadian western coastal waters, the dominant oceanographic feature is the coastal upwelling domain off the west coast of Vancouver Island, the northernmost extent of the California Current System in the eastern North Pacific. In the marine ecosystem of the eastern coasts of Korea (the Japan/East Sea), a major oceanographic feature is the Tsushima Warm Current, a branch of the Kuroshio Current in the western North Pacific. Fishes in the Canadian ecosystem are dominated by demersal, long-lived species such as flatfish, rockfish, sablefish, and halibut. During summer, migratory pelagic species such as Pacific hake, Pacific salmon, and recently Pacific sardine, move into this area to feed. In the late 1970s, Canada declared jurisdiction for 200 miles from their coastline, and major fisheries species in Canadian waters have been managed with a quota system. The overall fishing intensity off the west coast of Vancouver Island has been relatively moderate compared to Korean waters. Fishes in the ecosystem of the eastern Korean waters are dominated by short-lived pelagic and demersal fish. Historically, Korea has shared marine resources in this area with neighbouring countries, but stock assessments and quotas have only recently (since the late-1990s) been implemented for some major species. In the Korean ecosystem, fisheries can be described as intensive, and many stocks have been rated as overfished. The two ecosystems responded differently to climate impacts such as regime shifts under different exploitation histories. In the future, both countries will face the challenge of global warming and subsequent impacts on ecosystems, necessitating developing adaptive fisheries management plans. The challenges will be contrasting for the two countries: Canada will need to conserve fish populations, while Korea will need to focus on rebuilding depleted fish populations.  相似文献   
997.
Wind sedimentation in the Jafurah sand sea, Saudi Arabia   总被引:2,自引:0,他引:2  
The Jafurah sand sea of the Eastern Province of Saudi Arabia extends along the Arabian Gulf coastline from Kuwait in the north to the Rub Al Khali in the south, a distance of about 800 km. Sand drifts southward to south-eastward from regions of high wind energy in the north to low wind energy in the south. The aeolian landscape is zoned, with areas of deflation, transport and deposition from north to south. Drift rates in the zone of transport, near Abqaiq, range from 2 m3 m-w-1 yr-1 on sabkhas, to 29 m3 m-w-1 yr-1 on the crests of dunes. Average drift rates of approximately 18 m3 m-w-1 yr-1 observed during the study can cause about 1 m of accumulation per 5500 yr in a 100 km zone of deposition downwind, not including the bulk transport represented by the forward advance of dunes. Dune advance ranged from 23 m (2.9 m high dune) to 3 m (23 m high dune) during April-October 1980. The study area consists of dune, interdune, sand sheet and siliciclastic sabkha terrains, each of which is characterized by differing drift rates, and differing rates of erosion or deposition. Sedimentation occurs by lateral movement of dunes and interdunes, and vertical accretion by sand sheets and sabkhas.  相似文献   
998.
A new spectral factorization method is presented for the estimation of a causal as well as a causally invertible ARMA operator from the correlation sequence of seismic traces. The method has been implemented for multichannel deconvolution of seismic traces with the aim of exploiting the trace-to-trace correlation that exists within seismograms. A layered earth model with a small reflectivity sequence has been considered, and the seismic traces have been considered as the output of a linear system driven by white noise reflection coefficient sequences. The present method is the concatenation of three algorithms, namely Kung's method for state variable ( F , G , H ) realization using a singular value decomposition (SVD) algorithm, Faurre's technique for computation of the strong spectral factor and Leverrier's algorithm for ARMA representation of the spectral factor. The inverted ARMA operator is used as a recursive filter for deconvolution of seismic traces. In the example shown, two traces with a covariance sequence of 160 ms length have been considered for multichannel deconvolution of stacked seismic traces. The results presented, when compared with those obtained from a conventional deconvolution algorithm, have shown encouraging prospects.  相似文献   
999.
Since the advent of CHAMP, the first in a series of low-altitude satellites being almost continuously and precisely tracked by GPS, a new generation of long-wavelength gravitational geopotential models can be derived. The accuracy evaluation of these models depends to a large extent on the comparison with external data of comparable quality. Here, two CHAMP-derived models, EIGEN-1S and EIGEN-2, are tested with independent long-term-averaged single satellite crossover (SSC) sea heights from three altimetric satellites (ERS-1, ERS-2 and Geosat). The analyses show that long-term averages of crossover residuals still are powerful data to test CHAMP gravity field models. The new models are tested in the spatial domain with the aid of ERS-1/-2 and Geosat SSCs, and in the spectral domain with latitude-lumped coefficient (LLC) corrections derived from the SSCs. The LLC corrections allow a representation of the satellite-orbit-specific error spectra per order of the models spherical harmonic coefficients. These observed LLC corrections are compared to the LLC projections from the models variance–covariance matrix. The excessively large LLC errors at order 2 found in the case of EIGEN-2 with the ERS data are discussed. The degree-dependent scaling factors for the variance-covariance matrices of EIGEN-1S and –2, applied to obtain more realistic error estimates of the solved-for coefficients, are compatible with the results found here.  相似文献   
1000.
At 31 sites in Finland the Zn content was determined of the epiphytic lichen Hypogymnia physodes (L.) Nyl. growing on pine and birch, the terricolous lichen Cladina spp. in pine bark, and the inner part of birch bark. The anomalous contents of Zn in H. physodes on pine and Cladina spp. in soil appeared to be strongly related to sulphide ore deposits. Anthropogenic Zn correlated closely with S and Fe in H. physodes on pine and with Fe in Cladina spp. The binding of geogenic Zn was related to iron hydroxides, and was attributed to the different uptake mechanism of S in these lichens. Elevated values of Zn in H. physodes on birch were due to both geogenic and anthropogenic sources. Zinc in pine bark increased with the Zn content in H. physodes and reached a limiting value.  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