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   2513篇
  免费   310篇
  国内免费   659篇
测绘学   237篇
大气科学   268篇
地球物理   393篇
地质学   1294篇
海洋学   353篇
天文学   485篇
综合类   133篇
自然地理   319篇
  2023年   17篇
  2022年   73篇
  2021年   97篇
  2020年   99篇
  2019年   111篇
  2018年   88篇
  2017年   88篇
  2016年   121篇
  2015年   136篇
  2014年   141篇
  2013年   143篇
  2012年   143篇
  2011年   172篇
  2010年   130篇
  2009年   186篇
  2008年   160篇
  2007年   173篇
  2006年   165篇
  2005年   168篇
  2004年   158篇
  2003年   165篇
  2002年   122篇
  2001年   104篇
  2000年   83篇
  1999年   80篇
  1998年   76篇
  1997年   33篇
  1996年   41篇
  1995年   38篇
  1994年   30篇
  1993年   20篇
  1992年   23篇
  1991年   20篇
  1990年   16篇
  1989年   12篇
  1988年   15篇
  1987年   6篇
  1986年   4篇
  1985年   4篇
  1984年   5篇
  1983年   1篇
  1982年   3篇
  1981年   2篇
  1980年   4篇
  1979年   1篇
  1978年   4篇
  1976年   1篇
排序方式: 共有3482条查询结果,搜索用时 31 毫秒
51.
Through a systematic observation of water level and temperature, and a comprehensive analysis of the data on major/trace elements, nitrite, hydrogen-oxygen isotopes, the conclusion has been drawn that there are two relatively independent groundwater systems (cool water and hot water), and the geochemical indicators of hot/cool waters are described. The cool water system is relatively enriched in Ca2 , Mg2 and HCO3-. Its TDS is relatively low, about 1400–1800 mg/L. The hot water system is relatively enriched in K , Na , Cl- and SO42-. Its TDS is relatively high, about 2200–2300 mg/L. The cool water system is enriched in Ba, Ga, Cd, and the hot water system is enriched in B, Ti, Cr, Ni, Cu, Mo, Rb, and Cs, relatively. Especially, the contents of Rb and Cs in the hot water system are more than five times as high as those in the cool water system. The NO3- contents of cool water discharged from the gold mine are relatively high, and those of hot water are extremely low. The δD and δ18O values follow an increasing order of surface water>mine cool water>mine hot water. The cool water comes mainly from the lateral supply of phreatic water, while the hot water comes mainly from the vertical supply of deeply circulating structure-fracture water. The ratio of cool water over hot water was estimated to be about 1:1 by a water quality model..  相似文献   
52.
Canopy water content (CWC) is important for mapping and monitoring the condition of the terrestrial ecosystem. Spectral information related to the water absorption features at 970 nm and 1200 nm offers possibilities for deriving information on CWC. In this study, we compare the use of derivative spectra, spectral indices and continuum removal techniques for these regions. Hyperspectral reflectance data representing a range of canopies were simulated using the combined PROSPECT + SAILH model. Best results in estimating CWC were obtained by using spectral derivatives at the slopes of the 970 nm and 1200 nm water absorption features. Real data from two different test sites were analysed. Spectral information at both test sites was obtained with an ASD FieldSpec spectrometer, whereas at the second site HyMap airborne imaging spectrometer data were also acquired. Best results were obtained for the derivative spectra. In order to avoid the potential influence of atmospheric water vapour absorption bands the derivative of the reflectance on the right slope of the canopy water absorption feature at 970 nm can best be used for estimating CWC.  相似文献   
53.
针对GPS定位测量的信号传播路径误差,分析了电离层电子浓度总含量梯度对差分定位精度的影响,利用L1载波重点讨论了1999年-2000年太阳活动期间低纬度赤道异常地区GPS差分定位精度的问题,同时验证对于长基线采用GPS广域差分技术可以使电离层定位误差得到明显提高。  相似文献   
54.
含盐量对松嫩平原碳酸盐渍土抗剪强度的影响   总被引:2,自引:0,他引:2  
松嫩平原分布着大量的碳酸盐渍土,其不良的工程性质给穿越其中的道路质量造成不同程度的影响。由于盐渍土所含易溶盐成分主要为碳酸氢钠,该研究通过向碳酸盐渍土内加入碳酸氢钠制备不同含盐量的盐渍土,进行室内直剪试验。试验表明:随着含盐量的增加土的粘聚力减小,内摩擦角有先减小后变大的趋势;随着含水量的增加,土的含盐量越高,土的粘聚力减小越明显。易溶盐在土中的状态形式是影响其抗剪强度参数变化的一个重要原因。  相似文献   
55.
膏溶角砾岩侧限膨胀特性试验研究   总被引:1,自引:0,他引:1  
研究了膏溶角砾岩的侧限膨胀特性及其影响因素。这种岩石受到扰动后可吸水发生膨胀变形使其结构发生变形甚至破坏。通过GDG高压固结仪采用平衡加压法和充分膨胀法研究了侧限情况下岩石的膨胀力指标, 解释了充分膨胀法膨胀力指标大于平衡加压法膨胀力指标的原因。通过试验研究了供水充足条件下膏溶角砾岩侧限自由膨胀变形演化规律, 阐述了膏溶角砾岩侧限膨胀变形的影响因素。岩石试样采自山西石太客运专线太行山隧道, 试验结果表明, 膏溶角砾岩的膨胀特性不同于常规的膨胀岩, 其轴向膨胀变形随含水率增加而增加, 随密度增大而减小。  相似文献   
56.
We have started a program of high-resolution (0.4content/n081663507896375/xxlarge8243.gif" alt="Prime" align="MIDDLE" BORDER="0">/pixel) 10content/n081663507896375/xxlarge956.gif" alt="mgr" align="MIDDLE" BORDER="0">m imaging of bipolar outflow sources using the 10content/n081663507896375/xxlarge956.gif" alt="mgr" align="MIDDLE" BORDER="0">m camera CAMIRAS. We present recent results obtained at the Canada France Hawaii Telescope which reveal extended emission or IR companions in several luminous objects. The extended emission we detected probably arises from transiently heated very small grains, while the newly discovered companions could contribute significantly to the outflow activity and extended far-IR emission usually attributed to the main optical source.  相似文献   
57.
We have used the ESO 10 content/q224v14h2828p456/xxlarge956.gif" alt="mgr" align="MIDDLE" BORDER="0">m camera, TIMMI, to image with a very high angular resolution (PFoV: 0.3content/q224v14h2828p456/xxlarge8243.gif" alt="Prime" align="MIDDLE" BORDER="0">; FWHM:0.9content/q224v14h2828p456/xxlarge8243.gif" alt="Prime" align="MIDDLE" BORDER="0">) several main-sequence star disk candidates. Information on the content/q224v14h2828p456/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-Pictoris dust disk has been obtained in a region largely inaccessible to previous observations: 0–80 AU, with a resolution of about 5 AU after deconvolution. Another promising target for 10 content/q224v14h2828p456/xxlarge956.gif" alt="mgr" align="MIDDLE" BORDER="0">m imaging, 51 Ophiuchi, appears point-like.based on data collected at the European Southern Observatory (ESO), La Silla, Chile  相似文献   
58.
A number of overflows from a large lava channel and tube system on the southwest rift zone of Mauna Loa were studied. Initial overflows were very low viscosity gas-rich pcontent/q6h925880rx28128/xxlarge257.gif" alt="amacr" align="BASELINE" BORDER="0">hoehoe evidenced by flow-unit aspect ratios and vesicle sizes and contents. Calculated volumetric flow-rates in the channel range between 80 and 890 m3/s, and those of the overflows between 35 and 110 m3/s. After traveling tens to hundreds of meters the tops of these sheet-like overflows were disrupted into a surface composed of clinker and pcontent/q6h925880rx28128/xxlarge257.gif" alt="amacr" align="BASELINE" BORDER="0">hoehoe fragments. After these 'a'content/q6h925880rx28128/xxlarge257.gif" alt="amacr" align="BASELINE" BORDER="0"> overflows came to rest, lava from the interiors was able to break out on to the surface as pcontent/q6h925880rx28128/xxlarge257.gif" alt="amacr" align="BASELINE" BORDER="0">hoehoe. The surface structure of a lava flow records the interaction between the differential shear rate (usually correlated with the volumetric flow-rate) and viscosity-induced resistance to flow. However, the interior of a flow, being better insulated, may react differently or record a later set of emplacement conditions. Clefts of toothpaste lava occurring within fields of clinker on proximal-type 'a'content/q6h925880rx28128/xxlarge257.gif" alt="amacr" align="BASELINE" BORDER="0"> flows also record different shear rates during different times of flow emplacement. The interplay between viscosity and shear rate determines the final morphological lava type, and although no specific portion of lava ever makes a transition from 'a'content/q6h925880rx28128/xxlarge257.gif" alt="amacr" align="BASELINE" BORDER="0"> back to pcontent/q6h925880rx28128/xxlarge257.gif" alt="amacr" align="BASELINE" BORDER="0">hoehoe, parts of a flow can appear to do so.  相似文献   
59.
Diagenetic changes are difficult to distinguish from variations in sources of organic matter to sediments. Organic geochemical comparisons of samples of wood, bark, and needles from a white spruce (Picea glauca) living today and one buried for 10,000 years in lake sediments have been used to identify the effects of diagenesis on vascular plant matter. Important biogeochemical changes are evident in the aged spruce components, even though the cellular structures of the samples are well preserved. Concentrations of total fatty acids dramatically diminish; unsaturated and shorter chainlength components are preferentially lost from the molecular distributions. Concentrations of total alcohols are similar in the modern and 10,000-year-old wood and bark but markedly lowered in the aged needles. Hydrocarbon concentrations and distributions show little diagenetic change in the 10,000-year-old plant materials. Cellulose components in the wood decrease relative to lignin components, although both types of materials remain in high concentration in comparison to other organic components. Aromatization of abietic acid proceeds more rapidly in buried spruce wood than in bark; retene is the dominant polyaromatic hydrocarbon in the aged wood. In contrast to the variety of changes evident in molecular compositions, neither content/x54x85n0361231l1/xxlarge963.gif" alt="sgr" align="BASELINE" BORDER="0">13C values nor C/N ratios differ significantly in the bulk organic matter of modern and aged spruce components.  相似文献   
60.
The Mariánské Lázncontent/m870k21588067504/xxlarge283.gif" alt="ecaron" align="BASELINE" BORDER="0"> complex (MLC) is located in the Bohemian Massif along the north-western margin of the Teplá-Barrandian microplate and consists of metagabbro, amphibolite and eclogite, with subordinate amounts of serpentinite, felsic gneiss and calcsilicate rocks. The MLC is interpreted as a metaophiolite complex that marks the suture zone between the Saxothuringian rocks to the north-west and the Teplá-Barrandian microplate to the south-east. Sm-Nd geochronology of garnet-omphacite pairs from two eclogite samples yields ages of 377±7, and 367±4 Ma. Samples of eclogite and amphibolite do not define a whole rock Sm-Nd isochron, even though there is a large range in Sm/Nd ratio, implying that the suite of samples may not be cogenetic. Eclogites do not have correlated content/m870k21588067504/xxlarge603.gif" alt="epsiv" align="BASELINE" BORDER="0"> Nd values and initial 87Sr/86Sr ratios. Five of the eight eclogite samples have high content/m870k21588067504/xxlarge603.gif" alt="epsiv" align="BASELINE" BORDER="0"> Nd values (+10.2 to +7.1) consistent with derivation from a MORB-like source, but variable 87Sr/86Sr ratios (0.7033 to 0.7059) which probably reflect hydrothermal seawater alteration. Three other eclogite samples have lower content/m870k21588067504/xxlarge603.gif" alt="epsiv" align="BASELINE" BORDER="0">Nd values (+ 5.4 to –0.8) and widely variable 87Sr/86Sr ratios (0.7033 to 0.7096). Such low content/m870k21588067504/xxlarge603.gif" alt="epsiv" align="BASELINE" BORDER="0">Nd values are inconsistent with derivation from a MORB, source and may reflect a subduction or oceanic island basalt component in their source. The MLC is an important petrotectonic element in the Bohemian Massif, providing evidence for Cambro-Ordovician formation of oceanic crust and interaction with seawater, Late Devonian (Frasnian-Famennian) high- and medium-pressure metamorphism related to closure of a Saxothuringian ocean basin, Early Carboniferous (Viséan) thrusting of the Teplá terrane over Saxothuringian rocks and Late Viséan extension.  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