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   2204篇
  免费   310篇
  国内免费   275篇
测绘学   161篇
大气科学   157篇
地球物理   665篇
地质学   984篇
海洋学   168篇
天文学   111篇
综合类   145篇
自然地理   398篇
  2024年   1篇
  2023年   18篇
  2022年   51篇
  2021年   85篇
  2020年   75篇
  2019年   93篇
  2018年   66篇
  2017年   95篇
  2016年   108篇
  2015年   97篇
  2014年   147篇
  2013年   276篇
  2012年   203篇
  2011年   99篇
  2010年   75篇
  2009年   131篇
  2008年   118篇
  2007年   90篇
  2006年   89篇
  2005年   100篇
  2004年   75篇
  2003年   65篇
  2002年   58篇
  2001年   46篇
  2000年   35篇
  1999年   33篇
  1998年   26篇
  1997年   24篇
  1996年   62篇
  1995年   118篇
  1994年   144篇
  1993年   14篇
  1992年   10篇
  1991年   11篇
  1990年   5篇
  1989年   10篇
  1988年   8篇
  1987年   6篇
  1986年   2篇
  1985年   8篇
  1984年   5篇
  1983年   2篇
  1982年   2篇
  1981年   2篇
  1980年   1篇
排序方式: 共有2789条查询结果,搜索用时 297 毫秒
941.
在山东新泰市新太古代泰山岩群变质岩系中,首次发现宝石级红刚玉,又称红宝石,商业名称暂定为"泰山红宝石"。红宝石产于鲁西地区新太古代二长花岗岩的壳源岩石包体之中,包体岩性以黑云更长变粒岩、黑云片岩、角闪石岩、斜长角闪岩、浅绿色含铬二云片岩为主,是新太古代古老地壳的深融残留;红刚玉与浅绿色含铬二云片岩关系密切,推测是新太古代变质岩中原岩残留斑晶矿物。  相似文献   
942.
泰安市旅游生态能值分析   总被引:1,自引:1,他引:0  
魏敏  冯永军  李芬  郑玉清 《地理学报》2012,67(9):1181-1189
运用能值理论, 从区域生态经济系统的视角, 提出旅游生态能值理论, 论述旅游输入能值、旅游输出能值、旅游能值交换率、旅游可持续发展指标等概念、测算依据和计算方法, 并对2010 年泰安区域生态经济系统内旅游可持续发展水平进行旅游生态能值分析。实证研究表明:(1) 泰安区域生态经济系统可利用能值为15867.32×1019 sej, 旅游输入能值为1766.59×1019 sej, 旅游输出能值为679.40×1019 sej, 旅游能值交换率为2.60。旅游可持续发展能值指标TSEI为1.76, 处在可持续发展阶段;(2) 运用旅游生态能值的理论评价旅游可持续发展水平是可行的。  相似文献   
943.
The long-term systematic errors of the analytical theories IAU 2000 and IAU 2006 of the Earth’s precession–nutational motion are studied making use of the VLBI data of 1984–2007. Several independent methods give indubitable evidence of the significant quadratic error in the IAU 2000 residuals of the precessional angle while the adopted value of the secular decrease /cy of the Earth’s ellipticity e (derived from Satellite Laser Ranging data) should manifest itself in the residuals of as the negative quadratic trend . The problem with the precession of the IAU 2006 theory adopted as a new international standard and based on the precession model P03 (Capitaine et al., Astron Astrophys 432:355–367, 2005) appears to be even more serious because the above mentioned quadratic term has already been incorporated into the P03 precession. Our analysis of the VLBI data demonstrates that the quadratic trend of the IAU 2006 residuals does amount to the expected value (30.0 ± 3) mas/cy2. It means, first, that the theoretical precession rate of IAU 2006 should be augmented by the large secular correction and, second, that the available VLBI data have potentiality of estimating the rate . And indeed, processing these data by the numerical theory ERA of the Earth’s rotation (Krasinsky, Celest Mech Dyn Astron 96:169–217, 2006, Krasinsky and Vasilyev, Celest Mech Dyn Astron 96:219–237, 2006) yields the estimate /cy statistically in accordance with the satellite-based . On the other hand, applying IAU 2000/2006 models, the positive value /cy is found which is incompatible with the SLR estimate and, evidently, has no physical meaning. The large and steadily increasing error of the precession motion of the IAU 2006 theory makes the task of replacing IAU 2006 by a more accurate model be most pressing.  相似文献   
944.
非黏性土泊松比试验研究   总被引:2,自引:0,他引:2  
土体泊松比是反映土体侧向变形的重要参数,但是目前的研究应用与模型参数的重要性不相符.通过基于局部变形测量的三轴试验,分析了泊松比的变化规律:在相同的成样方法和控制密度条件下,同一种材料的颗粒排列组合结构基本类似,在不同围压条件下的试样变形规律基本保持一致;随主应力比的增加,切线泊松比也随之增加,建立泊松比与主应力比之间的关系更符合泊松比取值规律.  相似文献   
945.
针对目前各大中城市私有汽车数量不断上升、城市停车资源趋于饱和甚至过载的现状,以西安市三环内停车资源为例,分析了其空间布局、配套设施以及建设投资主体等方面的资源分布发展现状,并制作了相关专题地图,为城市停车资源修建与管理提供参考.结果 表明,西安市停车资源整体呈聚集分布,在经济发展较好的区域聚集特征明显;由政府部门投资兴...  相似文献   
946.
In this study we introduce the first step towards a statistical model for the reliability of fisheries data. We applied Benford's Law to catch data from the Atlantic Canadian lobster (Homarus americanus) fishery's lobster fishery areas (LFAs) 33 and 34 and compared our results to those using observations from the “grey zone” (a highly regulated lobster fishery shared by Canada and United States) and a fishery with a different regulatory regime (snow crab, Chionoecetes opilio). Non‐conformity with Benford's Law is often considered as an indicator of human manipulation of accounting data. We found that observations from the grey zone conformed to the distribution predicted by Benford's Law, whereas observations from snow crab and both lobster fishery areas did not conform.  相似文献   
947.
Seasonal and tidal variations in the hydrology of Wellington harbour   总被引:1,自引:1,他引:0  
Observations were made on several hydrological features of Wellington Harbour, New Zealand (41° 16’ S, 174° 51’ E) during 1970 to 1972. These suggest that the harbour is topographically partially isolated from oceanic influences, and that waters within the harbour undergo efficient mixing.

Monthly mean sea‐surface temperatures ranged seasonally between 10.5°c and 18.5°c, and some stratification was observed during summer and winter. Salinities usually ranged from 33.5‰ to 34.5‰, and water transparency by Secchi disc from 3 m to 6 m. Dissolved oxygen content ranged from 96% to 127% saturation, usually exceeding 100% saturation in surface waters.

Under normal discharge conditions during winter, the Hutt River was observed to markedly affect surface temperatures and salinities as far south as Somes Island to a depth of about 5 m.  相似文献   
948.
Abstract

The San Diego Trough Geotechnical Test Area, located about 24 km southwest of San Diego in a water depth of about 1.2 km, lies near the base of the Coronado Escarpment directly north of the Coronado Fan. A new bathymetric map delineates a shallow basin in the soft, highly plastic, clayey silts flooring the Test Area. Measurements of shear strength by vane and static cone pene‐trometer, and bulk density by nuclear densitometer, were made in place from the submersible Deep Quest. Sixteen short (< 1.6 m) gravity cores were collected from ships.

The geotechnical properties show little areal variation and generally change uniformly with depth within the 55 km2 Test Area. Silt is the predominant grain size, averaging about 62%. In‐place bulk density shows little change with increasing depth, values range from 1.23 to 1.26 Mg/m3; laboratory density values increase with depth, ranging from 1.30 to 1.52 Mg/m3 between the surface and a depth of about 1.1 m. The difference between the in place and laboratory values may indicate sampling densification of the cored sediment. Water content in the cores decreases uniformly within the range of 249 to 43% dry weight. Shear strength increases linearly with depth. The laboratory shear strength values are lower than the in place values, which range from 4 kPa at the surface to about 29 kPa at a depth of 3.27 m. Predictor equations relate Atterberg limits, bulk density, water content, and laboratory and in place shear strength to depth. Sedimentation‐compression e log p curves have an equivalent compression index of 1.5 to nearly 2. Excluding rurbidite layers and sampling disturbance effects, all cores indicate a uniform depositional environment in the surface to 1.6 m of sediment sampled. The geotechnical properties indicate that the sediments in the west central and southwest parts of the Test Area exhibit vertical heterogeneity due to thin silt‐sand layers, presumably of turbidity current origin, that originated from the Coronado Canyon.  相似文献   
949.
Tropical cyclone ocean–wave model interactions are examined using an ESMF – (Earth System Modeling Framework) based tropical cyclone (TC) version of the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®1). This study investigates Hurricane Ivan, which traversed the Gulf of Mexico (GOM) in September 2004. Several oceanic and wave observational data sets, including Acoustic Doppler Current Profilers (ADCPs), National Oceanic and Atmospheric Administration (NOAA) buoys, satellite altimeter data, and Scanning Radar Altimeter (SRA) data, allow for a unique analysis of the coupled atmosphere, ocean (Navy Coastal Ocean Model, NCOM), and wave (Simulating WAves Nearshore, SWAN) models in COAMPS-TC. To determine the feasibility of coupling NCOM to SWAN in high-wind conditions during Hurricane Ivan, near-surface currents in NCOM were first compared to near-surface ADCP observations. Recent modifications to SWAN, including new wind-to-wave energy input and wave-breaking energy dissipation source functions, as well as a new ocean surface drag coefficient formulation appropriate for high-wind conditions, significantly improved the forecast wave field properties, such as significant wave height (SWH), in TC conditions. Further results show that the ocean-to-wave model coupling, which allows for the strong, hurricane-induced, surface currents in NCOM to interact with SWAN, provided additional improvements to the forecast SWH field. Additionally, wave-to-ocean model coupling, which included the input of the Stokes Drift Current (SDC) calculated from the SWAN wave spectra to NCOM, is examined. The models indicate that the SDC was on the order of 10–25% of the near-surface Eulerian current during Ivan. Recent studies of the importance of the SDC and the resulting Langmuir turbulence on vertical ocean mixing in TCs is also discussed.  相似文献   
950.
Drag forces acting on Schlegel‘s black rockfish Sebastes schlegeli were studied. A new drag force transducer was designed and used to measure the water drag on Schlegel‘s black rockfish in a vertical recirculating flume tank. Fourteen individuals were investigated, yielding two mean drag coefficients referred to the cross-sectional area and volume^2/3 respectively at water velocities ranging from 0.3 to 1.0 ms^-1 The drag coefficients can be used for estimating the drag forces acting on Sebastes schlegeli in water.  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