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   293篇
  免费   13篇
  国内免费   3篇
测绘学   6篇
大气科学   5篇
地球物理   48篇
地质学   142篇
海洋学   15篇
天文学   79篇
自然地理   14篇
  2022年   4篇
  2021年   3篇
  2020年   2篇
  2019年   4篇
  2018年   21篇
  2017年   7篇
  2016年   18篇
  2015年   11篇
  2014年   20篇
  2013年   13篇
  2012年   12篇
  2011年   8篇
  2010年   9篇
  2009年   18篇
  2008年   8篇
  2007年   9篇
  2006年   15篇
  2005年   6篇
  2004年   9篇
  2003年   4篇
  2002年   3篇
  2001年   5篇
  2000年   5篇
  1999年   6篇
  1998年   6篇
  1997年   5篇
  1996年   3篇
  1995年   2篇
  1993年   3篇
  1991年   7篇
  1990年   5篇
  1988年   3篇
  1987年   7篇
  1986年   4篇
  1985年   5篇
  1984年   5篇
  1982年   5篇
  1981年   1篇
  1980年   6篇
  1979年   3篇
  1978年   2篇
  1977年   2篇
  1976年   2篇
  1975年   3篇
  1974年   1篇
  1973年   1篇
  1971年   3篇
  1970年   2篇
  1969年   1篇
  1965年   1篇
排序方式: 共有309条查询结果,搜索用时 15 毫秒
131.
Possibility of a persistent structure of source free pure magnetic field contained by the curvature of a static cylindrically symmetric space-time associated with its own strength of energy density in five dimensional Kaluza-Klein space-time is investigated. It may be considered as the generalization of Melvin's 4D ‘magnetic universe’ in five dimensional K-K space-time. Time like and null geodesics are studied in order to investigate the nature of gravitation produced by it and are found not to deviate much from those noted in Melvin's spacetime. It is further interesting to note that near the axis of symmetry our 5D space-time exhibits the conical defect similar to that in the case of a string. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
132.
Human has always modified its surroundings for better adaptation which include the building of urban landscapes, dams, and engineering constructions like bridges and other infrastructures. It has been unveiled from the study of River Chel that when the rail bridge was constructed before 1913, the natural width of the river was nevertheless maintained but the problem was initiated and after the road bridge construction in 1970s when the river width was minimized,the morphological changes started in a large scale. Such modifications have affected nature as well as human communities both positively and negatively. This paper will try to portray the evolution of channel shifting and changes of sediment size regime that the River Chel is experiencing due to the construction of bridges across the middle part of its course. So, this study is concerned about the changes in channel pathway since last 100 years and field-based hydrological parameters and sediment size analysis reveals some distinct changes in the channel planform with alteration of sediment size regime. Simulation of the hydraulic modeling in HEC-RAS specifies the probable affected area with bridges and without bridges in both upstream and downstream of the river course. Hence, the study unveils the alteration of the river hydrology and sediment size caused due to anthropogenic effects and impact of such flow analysis has been evaluated through the bridge scour calculation by CUS method. The discussion concludes that the Odlabari road bridge construction (after 1970) has modified the normal hydrological behavior of the river with a higher probability of bank erosion in downstream and floods in upstream resulting river bottleneck condition.  相似文献   
133.
Mylonitic gabbro and altered gabbro were recovered from off-axis high and corner high locations at ridge-transform intersection, adjacent to Vityaz transform fault of the slow spreading (32–35 mm/yr, full spreading) Northern Central Indian Ridge. Both the varieties show signatures of extensive alteration caused due to interaction with sea water. Mylonitic gabbro represents high temperature metamorphism (∼700–800°C) and comprised of hornblende mineral which exhibits well defined foliation/gneissic appearance along with dynamically recrystallised plagioclase grains frequently intercalated with magnetite-ilmenite. Altered gabbro from corner high generally includes low temperature greenschist grade (∼300°C) mineralogical assemblages: chlorite, albite, quartz and locally magnesio hornblende. Crystal plastic deformation resulted in mylonite formation and often porphyroclasts of plagioclase and clinopyroxene grains, while altered gabbro locally exhibits cataclastic texture. Presence of Vityaz transform fault and adjacent megamullion at the weakly magmatic ridge-transform intersection and off-axis high locations prompted the present scenario very much conducive for hydrothermal circulation and further facilitate the exhumation of present suite of gabbro.  相似文献   
134.
This work illustrates the effectiveness of hyperspectal image spectroscopy and lab spectroscopy in identification techniques of minerals in alteration zone of ore body. The adopted procedure involves testing of Hyperion image spectra, their processing for noise, spectral matching and spectral similarity measurement with selected library spectra. Average weighted spectral similarity; visual and statistical matching techniques were used to select end-members from image spectra. Minimum noise fraction and pixel purity index technique were used to retrieve end-member spectra from hyperion image. Hyperspectral image like hyperion has the capability to deliver laboratory standard spectroscopic result. This paper illustrates the capability of hyperion image spectra in copper ore identification and mapping of chalcopyrite outcrop. A systematic approach has been made in this paper. This approach describes how image end member spectra and laboratory spectra can be co-related to fetch accurate spectral form of chalcopyrite ores. Thus, statistical and graphical comparison has been carried out between image derived end member spectra and laboratory spectra of chalcopyrite for better accuracy. The visual measurements is satisfactory, R = 0.973 for fine and 0.976 for medium grained chalcopyrite ore. Excellent statistical significance levels (90–97%) are found while comparing these spectra. There are many success stories of sub-pixel and N-dimensional feature space methods to separate the hydrothermal alteration zones from iron oxide mixed ore bodies. Thus, unmixing methods are very useful for mapping of most dominating mineral parts of a pixel from hyperspectral images which have coarse spatial resolution. Finally, mapping of mineralized zone has been achieved through sub-pixel based classifiers like spectral angle mapper (SAM), constrained energy minimization (CEM) and adaptive coherence estimator (ACE) techniques.  相似文献   
135.
136.
In the hypersaline lagoon at Laguna Figueroa vertically stratified diverse communities of microorganisms thrive. The modern sediments of Baja California at Laguna Figueroa contain cyanobacterial communities and sedimentary structures produced by these blue greens that have already been studied by Horodyski and his colleagues. This paper provides an introduction to the complex microbial communities, primarily those that underlie the laminated Microcoleus mats. They are composed of anaerobic photosynthetic and heterotrophic bacteria.The following genera of cyanobacteria at least are components of these mat communities: Lyngbya, Microcoleus, Entophysalis, Phormidium, Pseudoanabaena, Anabaena and Schizothrix. Among the photosynthetic bacteria several species of Thiocapsa-like microbes formed major surface components of certain mats and scums; rhodospirilli, rhodopseudomonads, chromatis and others were seen.The following nonphotosynthetic bacteria were identified: Nocardia sp., three types of spirilli, two types of Spirochaeta sp., two types of Desulfovibria sp., a new strain of red Beneckea and four distinctive unidentified coccoid and filamentous bacteria. Reasons are given for believing several of the species are new to science and that the microbial diversity is far greater than the approximately twenty species reported here. Eukaryotes are extremely rare. Only one species of animal, a herpachtechoid copepod, was ever seen in the 8-km long microbial communities of the hypersaline basin. Dunaliella salina, a chlorophyte and Aspergillus sydowi, an ascomycetous fungus were the only eukaryotes that were observed to be regular components of mat communities. Ciliates, amoebae (including a chrysarchnion-like microbe) and diatom tests, mostly empty, were the only other eukaryotes observed. Attempts to enrich for eukaryotic microorganisms were not successful whereas attempts to enrich for bacteria, especially anaerobes led to such a profusion of forms that to continue detailed study of them was beyond our means. Unidentified small rods and cocci constituted the largest fraction of individuals in the subsurface community. The microbes isolated from mats are adapted for alternating dry and wet conditions as well as high concentrations of salt and low concentrations of oxygen.  相似文献   
137.
Exact solutions are obtained in (4+1) dimensions for plane symmetric and cylindrically symmetric inhomogeneous spacetimes. In the former case the three space depends on time only while the metric corresponding to the extra dimension is dependent on space as well as time coordinates. The cylindrically symmetric nonstatic solutions for the perfect fluid have no singularity near the axis, but show big bang type of singularity in the finite past. One of the classes of such solutions satisfies the barotropic equation of state of the form =p. Static solutions with cylindrically symmetric solutions are also obtained in 5 dimensions.  相似文献   
138.
We have observed natural remanent magnetizations (NRM), ranging from 10−3 to 10−5 Gauss cm3/g, at least two orders of magnitude greater than 10−7, the noise level of our spinner magnetometer, in the carbonaceous chondrites, Orgueil, Cold Bokkeveld, Nogoya and Mighei. Preliminary alternating field demagnetization of two samples of Orgueil showed a moderate sized stable component of 10−4 Gauss cm3/g. Microscopic study reveals that a grey spinel oxide, Ni-Fe and an Fe sulphide are the principal opaque minerals in both Cold Bokkeveld and Orgueil; the oxide phase predominates in both, while the Ni-Fe is more visible in the former. Coercive force determinations in Orgueil and Cold Bokkeveld indicate single domain particles as the NRM carrier. These meteorites are suitable for the determination of the magnetic field intensity in the primary solar nebula.  相似文献   
139.
140.
Along the coast of East and South India, a slow and minor sea level fall as an immediate response to the Last Glacial Maximum (LGM) followed by strong aridity and fast retreat of the sea to the outer shelf are indicated by coastal ironstones, traces of channels in nearshore shallow seismic reflectors, and submerged, structureless, multimineral placer sands. Nearshore peat beds and inland beach ridges reflect perturbations during Early to Mid-Holocene sea level rise. The Little Ice Age witnessed shifts in littoral sediment budget, possibly as a result of reduced transport of sediments from the Himalaya to the Bay of Bengal.  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