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   20477篇
  免费   3797篇
  国内免费   4779篇
测绘学   1021篇
大气科学   4137篇
地球物理   5294篇
地质学   10333篇
海洋学   2355篇
天文学   924篇
综合类   2416篇
自然地理   2573篇
  2024年   55篇
  2023年   304篇
  2022年   868篇
  2021年   992篇
  2020年   848篇
  2019年   959篇
  2018年   1163篇
  2017年   1039篇
  2016年   1235篇
  2015年   931篇
  2014年   1210篇
  2013年   1211篇
  2012年   1124篇
  2011年   1193篇
  2010年   1120篇
  2009年   1128篇
  2008年   1002篇
  2007年   977篇
  2006年   765篇
  2005年   821篇
  2004年   573篇
  2003年   611篇
  2002年   599篇
  2001年   568篇
  2000年   666篇
  1999年   1000篇
  1998年   802篇
  1997年   875篇
  1996年   810篇
  1995年   680篇
  1994年   560篇
  1993年   506篇
  1992年   391篇
  1991年   285篇
  1990年   228篇
  1989年   175篇
  1988年   174篇
  1987年   126篇
  1986年   104篇
  1985年   70篇
  1984年   51篇
  1983年   43篇
  1982年   36篇
  1981年   27篇
  1980年   35篇
  1979年   27篇
  1978年   15篇
  1976年   11篇
  1975年   14篇
  1958年   18篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
151.
南海囊糠虾亚科的初步研究   总被引:1,自引:0,他引:1  
囊糠虾亚科Gastrosaccinae 包括的种数并不很多,但有些种数量相当大,是经济鱼虾的天然饵料,在海洋食物链中占较重要的地位。例如黄海的漂浮囊糠虾Gastrosaccus pelagicus Ii 数量很大,普遍发现于底层鱼类胃含物中;此种及其他一些量大的种又可用作养殖鱼虾的饵料。这一亚科迄今已知共7属69种。本文根据中国科学院海洋研究所1958-60年在南海北部所采的底层浮游生物标本和中国科学院南海海洋研究所1974-81年采自西沙群岛、中沙群岛的标本写成,共发现4属12种,其中6种为中国近海的首次记录。  相似文献   
152.
对虾一种无包涵体杆状病毒病原的PCR检测   总被引:4,自引:0,他引:4  
为建立对虾无包涵体杆状病毒的快速诊断技术,应用PCR技术分别对暴发性流行病发生前、流行中和发病后的对虾样品进行了检测.根据已克隆的对虾杆状病毒部分基因组序列而设计的引物能特异性地扩增出靶DNA片段,最低可以检测1pg的病素DNA.由典型发病症状对虾的胃、腮、肝胰腺、中肠、游泳足、肌内和心脏等器官和组织中成功地检测到病毒.不同组织和器官经扩增得到的信号强弱不同:病虾的胃扩增得到的信号最强,中肠、肌肉、心脏和游泳足次之,腮和肝胰腺信号最弱,说明病毒在不同组织和器官中数量及感染程度不同.在对虾发病前及发病后,用二次PCR还能检测表面不发病呈隐性感染状态的对虾携带的病毒;而对野生健康虾的检测结果为阴性.研究表明,PCR是检测对虾暴发性流行病快速、灵敏而准确的方法,对病毒病的早期诊断、防治和高健康对虾品种的选育具有指导意义.  相似文献   
153.
I~IOXThe half-lives of natural radioactive nuclides "'Ra, 226Ra and 228Ra are 3. 66 d, 5. 76 a,1600 a respectively, hence, the marine process with various time scales can be researched by taking these nuclides as tracers. Because these nuclides can be determined by measuring them or theirdaughters' a,g, y-rays, they have been applied extensively in oceanography research. Huang(1996), Huang et al. (1996 a,b) and Chen (1996) have given reviews on it.Since the contents of "'Ra, 226Ra and 228…  相似文献   
154.
Temperature data at different layers of the past 45 years were studied and we found adiploe mode in the thermocline layer (DMT): anomalously cold sea temperature off the coast of Sumatra and warm sea temperature in the western Indian Ocean. First, we analyzed the temperature and the temperature anomaly (TA) along the equatorial Indian Ocean in different layers. This shows that stronger cold and warm TA signals appeared at subsurface than at the surface in the tropical Indian O-cean. This result shows that there may be a strong dipole mode pattern in the subsurface tropical Indian Ocean. Secondly we used Empirical Orthogonal Functions (EOF) to analyze the TA at thermocline layer. The first EOF pattern was a dipole mode pattern. Finally we analyzed the correlations between DMT and surface tropical dipole mode (SDM), DMT and Nino 3 SSTA, etc. and these correlations are strong.  相似文献   
155.
Using meteorological data of field observation in 1990 - 2000, especially polar orbit high-resolution NOAA satellite cloud maps received from the Antarctic expedition vessel since 1997, the formation and development of the Prydz Bay cyclone are studied in this paper. Some new viewpoints are suggested such as: when surround-polar cyclone enters the Prydz Bay, it can also intensify and develop in summer; cyclone can also develop in the easterlies in this bay. These view points revise old uncom-plete view point that the Prydz Bay is a burial ground of cyclone, and also further consummate formation-development theory of surround-cyclone in the Antarctic westerlies and cyclone in the Antarctic easterlies. In this paper, the mechanism of ice-air-sea interaction in the Prydz Bay is studied, and the physical process of cyclone formation-development is explained. By use of wholly dynamic transportation method, an energy exchange case of a cyclone, which explosively developed after entering the Prydz Bay, is calcu  相似文献   
156.
INTRODUCTIONTheradionuclidesofnaturaluraniumandthoriumseriescanbeusedtotracemarinepro cessofvarioustemporalandspatialscaleandpropertiesthatprovideplentifulinformationaboutphysics,chemistry ,geoscienceandbiologyofoceanbecausetheyhavedifferentchemicalpro…  相似文献   
157.
Abstract

Based on hydrological data obtained during the 7th to 9th Chinese National Arctic Research Expeditions in the summers of 2016–2018, the main water structure on the shelf of the northern Bering Sea and the volume and heat fluxes of the Bering Strait throughflow were analyzed. Distinct variability was identified in the three Pacific water masses feeding the strait - Anadyr Water (AW), Bering Shelf Water (BSW) and Alaskan coastal water (ACW). Overall, the temperature and salinity of the entire section increased each year, with 2018 showing significant anomalies, i.e., a temperature anomaly of up to 1?°C and a maximum salinity anomaly of 2. From 2016 to 2018, the extent of the ACW gradually narrowed in the eastern part of this section, while the AW expanded eastward each year. The net volume transport through each of the three sections increased poleward from 1.65?Sv to 2.76?Sv, with the AW increasing from 0?Sv to 1.03?Sv, the BSW varying between 0.52–1.65?Sv, and the ACW gradually decreasing from 1.04?Sv to disappearing completely. The net heat fluxes were also poleward, varying between 25.77 TW and 61.50 TW, and showing a significant increase. Significant variations in magnitude and extent were observed in each water mass of the Bering Strait throughflow, which could produce widespread effects in the Arctic Ocean and the global ocean beyond.  相似文献   
158.
In this study, unlike most previous investigations for wave-induced soil response, a simple semi-analytical model for the random wave-induced soil response is established for an unsaturated seabed of finite thickness. Two different wave spectra, the B-M and JONSWAP spectra, are considered in the new model. The influence of random wave loading on the soil response is investigated by comparing with the corresponding representative regular wave results through a parametric study, which includes the effect of the degree of saturation, soil permeability, wave height, wave period and seabed thickness. The maximum liquefaction depth under the random waves is also examined. The difference on the soil response under the two random wave types, B-M and JONSWAP frequency spectra, is also discussed in the present work.  相似文献   
159.
In this note we investigated the effects of a thin visco-elastic mud layer on wave propagation. Within the framework of linear water-wave theory, analytical solutions are obtained for damping rate, dispersion relation between wave frequency and wave number, and velocity components in the water column and mud layer. The wave attenuation rate reaches a maximum value when the mud layer thickness is about the same as the mud boundary layer thickness. Heavier mud has a weaker effect on the wave damping. However, the wave attenuation rate does not always decrease as the elastic shear modulus increases. In the range of small values for elastic shear modulus, the wave attenuation can be amplified quite significantly. The current solutions are compared with experimental data with different wave conditions and mud properties. In general, good agreements are observed.  相似文献   
160.
We present a new approach that incorporates two models to estimate the underwater light field from remote sensing of ocean color. The first employs a series of analytical, semi-analytical, and empirical algorithms to retrieve the spectrum of inherent optical properties (IOPs), including the absorption and the backscatter coefficients, from the spectrum of remote sensing reflectance. The second model computes the profile of photosynthetically available radiation E 0,PAR (z) for a vertically homogeneous water column using the information of the retrieved IOPs and the ambient optical environment. This computation is based on an improved look-up table technology that possesses high accuracy, comparable with the full solution of the radiative transfer equation, and meets the computational requirement of remote sensing application. This new approach was validated by in situ measurements and an extensive model-to-model comparison with a wide range of IOPs. We successfully mapped the compensation depth by applying this new approach to process the SeaWiFS imagery. This research suggests that E 0,PAR (z) can be obtained routinely from ocean-color data and may have significant implications for the estimation of global heat and carbon budget.  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