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   2321篇
  免费   319篇
  国内免费   416篇
测绘学   419篇
大气科学   190篇
地球物理   476篇
地质学   1232篇
海洋学   269篇
天文学   23篇
综合类   175篇
自然地理   272篇
  2024年   10篇
  2023年   37篇
  2022年   57篇
  2021年   80篇
  2020年   99篇
  2019年   111篇
  2018年   79篇
  2017年   131篇
  2016年   139篇
  2015年   111篇
  2014年   122篇
  2013年   128篇
  2012年   137篇
  2011年   139篇
  2010年   129篇
  2009年   179篇
  2008年   146篇
  2007年   158篇
  2006年   123篇
  2005年   116篇
  2004年   112篇
  2003年   89篇
  2002年   81篇
  2001年   70篇
  2000年   56篇
  1999年   48篇
  1998年   54篇
  1997年   55篇
  1996年   37篇
  1995年   39篇
  1994年   31篇
  1993年   29篇
  1992年   34篇
  1991年   29篇
  1990年   13篇
  1989年   8篇
  1988年   13篇
  1987年   6篇
  1986年   6篇
  1985年   4篇
  1984年   1篇
  1982年   1篇
  1980年   2篇
  1978年   2篇
  1976年   1篇
  1954年   4篇
排序方式: 共有3056条查询结果,搜索用时 15 毫秒
811.
Six morphometrical characters are analyzed on each of 64-specimens of Gabonella elongata, de Klasz & Meijer (1960), picked out from the same level in the Upper Campanian part of a borehole core from western Nigeria. Multivariate statistical analyzes by means of standard methods, show the existence of trimorphism with one microspheric and two megalospheric generations, in this paper denoted Me1 and Me2. The generations are well separated by differences in size, morphological variability and ontogeny. There is a tendency for a reduction in size of the last chamber in the megalospheric populations, especially in Me1.  相似文献   
812.
Celico  F.  Musilli  I.  Naclerio  G. 《Environmental Geology》2004,46(2):233-236
Hydrogeological and microbiological research is in progress to analyze the interaction between groundwater and microbial pollutants, produced by pasture and/or manure spreading, in the areas of different carbonate aquifers of southern Italy. Several springs and wells were studied, and the precipitation, the discharge, the groundwater level and the classic microbial indicators of pollution were monitored weekly or daily. The experimental results show that the pasture and the manure spreading produced microbial contamination of the groundwater, even if runoff infiltration in swallow holes does not exist. The time dependence of microbial contamination shows a series of peaks irregularly distributed, related to the precipitation that produce effective infiltration.  相似文献   
813.
The concentrations of fluorine in groundwater of North Jordan range from 0.009 to 0.055 mg/l. Other chemical parameters, e.g. pH, EC, TDS, Cl, TH, HCO3, PO4, SO4, NO3, NH4, K, Ca, Mg, and NO3 have been studied and showed higher concentrations in HCO3 and NO3 of 307 and 51 mg/l, respectively. Thermodynamic considerations show that almost all the analyzed samples are undersaturated with respect to calcite and fluorite. This undersaturation is probably due to their low availability in the locations. Fluoride concentration shows a positive relation to pH and HCO3, whereas Cl, Mg, Ca, and Na initially increase and then decrease with increasing fluoride in the water. Saturation indexes of fluorite and calcite are estimated. The chemistry of the groundwater is controlled by the fluorite and calcite solubility. The topography of the area has exerted control on the aerial extent of fluoride concentration.  相似文献   
814.
We present new UB V observations of the symbiotic nova V 1329 Cyg. Based on all our UB V observations of a uniform system, we redetermined the orbital period of the binary and estimated the magnitudes and luminosities of its components. We show that the pre-outburst visual luminosities of the red giant and the hot star were almost equal and that the rapid irregular photographic variability of the star was caused by the nonstationary behavior of the hot component. The outburst amplitude of the hot component (subdwarf) in 1964 was found to be ~2m in the V band, which is typical of ordinary symbiotic stars. We estimated the continuum luminosity of the gaseous component that appeared after the outburst. In the V band, it was almost 1m fainter than the flared hot star. Structurally, the gaseous component is an ionized gaseous disk comparable in size to an M giant.  相似文献   
815.
The sedimentological study of thirteen sediment cores from the periplatform setting surrounding Pedro Bank (Northern Nicaragua Rise, Caribbean Sea) shows that during the last 300 ka turbidite deposition is controlled by at least four factors: (1) late Quaternary sea level fluctuations, (2) prolific fine-grained sediment production and export resulting in oversteepening of the upper slope environment, (3) the proximity to the bank margin, and (4) local slope and seafloor morphology. The most intriguing finding of this study is the paucity of turbidites, with only 101 turbidites in 13 cores in this tectonically active setting near the Caribbean plate boundary. Throughout the last 300 ka, the frequency of turbidite input during interglacial stages is three times higher than during glacial stages. Also it is obvious that changes in sea level influence the timing of turbidite deposition. This is especially evident during the transgressions resulting in rapid renewed bank-top flooding, subsequent neritic sediment overproduction, and offbank export. The flooding event during each transgression is usually recorded by the onset of turbidite deposition at various sites along several platform-to-basin transects in down- and upcurrent slope settings. Overall, however, more turbidites are deposited during the regressive rather than the transgressive phases in sea level, probably as a result of sediment reorganisation on the slope resulting in slope failure. Five cores show "highstand bundling" of calciturbidites, i.e. higher number of turbidites during highstands than during lowstands in sea level.  相似文献   
816.
Calcite crystals were grown from solution with single-crystal dimensions up to 3 mm and doped up to 0.1 at% with Nd3+ ions. Phase purity was verified by powder X-ray diffraction. The concentration of Nd3+ was measured by energy-dispersive spectrometry and Rutherford backscattering spectrometry. Micro X-ray fluorescence mapping of the calcite grains indicates uniform Nd distribution in as-grown crystal grains. X-ray absorption fine structure suggests that Nd3+ is substituted for Ca2+ with local lattice distortion. Temperature-dependent near-infrared spectroscopy of Nd3+ impurities in calcite reveals large inhomogeneous linewidths and smooth line profiles that are characteristic of glassy hosts, though the samples are well crystallized.  相似文献   
817.
基于小波包变换与IHS变换的遥感图像融合   总被引:10,自引:0,他引:10  
曹闻  张勇 《测绘学院学报》2004,21(2):114-117
针对多光谱图像与金色图像的融合,提出了基于IHS变换和小波包变换的遥感图像融合新方法。该方法首先对多光谱图像作IHS变换得到3个分量:亮度I、色度H和饱和度S;其次利用小波包变换融合方法融合多光谱图像的亮度分量与金色图像,并用融合后的图像替代多光谱图像的亮度分量;最后作IHS反变换得到新的多光谱图像。实验分析表明,新方法的性能优于IHS变换融合方法、小波变换融合方法和基于小波变换与IHS变换融合方法,在保留多光谱图像光谱信息的基础上,增强了融合图像的空间细节表现能力。  相似文献   
818.
The microfacies of a Lower Cretaceous carbonate drillcore from Oman are characterized using optimizing matrices of Jaccard's similarity coefficients of community. Other than systems tract boundaries, there is no obvious evidence of individual parasequences in the core. However, diagnostic patterns in microfossil distribution identify environmental gradients recording changes in water depth. These gradients are used to define individual parasequences, parasequence sets, stacking patterns and key surfaces. The patterns suggest that deposition was controlled by successive fourth‐ to fifth‐order (high‐frequency) relative sea‐level cycles superimposed on an underlying third‐order relative sea‐level rise. Although the correlation of these depositional subunits to systematic changes in water depth and the rate of carbonate accumulation alone is not incontrovertible proof of such a sea‐level control, concurrent multiorder relative sea‐level cyclicity provides by far the most likely explanation. A microfacies deposited when the water depth was shallowing is characterized by a relay of microfossils with affinities that shallow upwards. Conversely, a microfacies that records a gradual increase in water depth has a relay of microfossils with affinities that deepen upwards. Microfacies characterized by an assemblage of microfossils with similar affinities record deposition when the benthic environmental conditions remained stable, either because of an equilibrium between shallow water carbonate deposition and rising sea level, or in deeper water where sediment composition was relatively insensitive to changes of water depth. Microfacies characterized by mixed affinity assemblages record syndepositional reworking. During periods of embedded multiorder sea‐level changes, individual parasequences within systems tracts are shown to record more complex environmental gradients than simply the repetition of successive shallowing‐up units as traditionally represented in carbonate sequence stratigraphic models. The microfacies of an individual parasequence may shallow up, or may record both deepening‐up and shallowing‐up depositional phases, as well as periods of sedimentation when benthic environments remained stable. Individual parasequence boundaries may be submarine or subaerial unconformities, or be conformable, as part of a predictable stratigraphic pattern related to the temporal position of an individual parasequence within the underlying third‐order cycle of relative sea‐level change. The hitherto ubiquitous use of assemblages to describe carbonate microfacies, coupled with the widespread use of the metre‐scale shallowing‐up template to identify parasequences, may have led to such complexities previously being overlooked.  相似文献   
819.
城市职能体系研究的思路与方法--以太原市为例   总被引:3,自引:0,他引:3  
全面认识城市职能体系需从其结构属性和空间属性两方面着手,从职能层次、职能规模和职能强度三方面来界定,思路是将城市职能的结构属性与空间属性结合起来,研究城市在不同空间尺度的主要职能。文章以太原市为案例,构造了区位熵、职能规模、职能强度3个指标,对太原市在省域、中西部、全国3个空间层次的城市主要职能进行了分析、力图通过该市的案例,探讨单个城市职能体系实证研究的思路和方法,从多角度较为系统地认识一个城市的职能体系,为城市定位和城市性质确定提供更丰富的基础依据。  相似文献   
820.
The nature and development of epikarst and soil development in aeolianites under a monsoonal climatic regime has not yet been described. Late Quaternary aeolianites of the southeastern coast of Saurashtra in western India show a wide array of epikarst and red-soil formation, and serve to typify the character of aeolianite weathering under a monsoonal climate. Three varieties of epikarst are identified that represent down-profile increase in groundwater flow. Five types of Terra Rossae represent a transition from the karstified limestone to soil development. Terra Rossae differ in the content of residual aeolianite and show both simple and complex profiles. The latter at places shows ped development. The results show that an ontogenetic (growth) sequence exists from incipient epikarst to complex palaeosols. This ontogenetic sequence represents an incremental increase in the groundwater budget of the region in response to changing intensification of the Indian monsoon rainfall. Magnetic properties of the Terra Rossae formed under a monsoonal climate are described for the first time. The magnetic susceptibility together with its frequency-dependent and anhysteretic remanent magnetization (ARM) represents both the concentration of single domain and ultrafine superparamagnetic (SP) magnetite. The formation of single domain and superparamagnetic magnetite and hematite are linked genetically to weathering of the aeolianite that leads to the formation of Terra Rossa.  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