全文获取类型
收费全文 | 7605篇 |
免费 | 1153篇 |
国内免费 | 1868篇 |
专业分类
测绘学 | 432篇 |
大气科学 | 1945篇 |
地球物理 | 1871篇 |
地质学 | 3501篇 |
海洋学 | 1017篇 |
天文学 | 372篇 |
综合类 | 706篇 |
自然地理 | 782篇 |
出版年
2024年 | 27篇 |
2023年 | 112篇 |
2022年 | 304篇 |
2021年 | 344篇 |
2020年 | 281篇 |
2019年 | 345篇 |
2018年 | 364篇 |
2017年 | 321篇 |
2016年 | 421篇 |
2015年 | 353篇 |
2014年 | 427篇 |
2013年 | 452篇 |
2012年 | 348篇 |
2011年 | 378篇 |
2010年 | 348篇 |
2009年 | 416篇 |
2008年 | 330篇 |
2007年 | 307篇 |
2006年 | 277篇 |
2005年 | 279篇 |
2004年 | 209篇 |
2003年 | 234篇 |
2002年 | 205篇 |
2001年 | 222篇 |
2000年 | 278篇 |
1999年 | 417篇 |
1998年 | 322篇 |
1997年 | 321篇 |
1996年 | 322篇 |
1995年 | 273篇 |
1994年 | 226篇 |
1993年 | 203篇 |
1992年 | 163篇 |
1991年 | 136篇 |
1990年 | 113篇 |
1989年 | 116篇 |
1988年 | 101篇 |
1987年 | 55篇 |
1986年 | 60篇 |
1985年 | 44篇 |
1984年 | 39篇 |
1983年 | 20篇 |
1982年 | 20篇 |
1981年 | 24篇 |
1980年 | 18篇 |
1979年 | 11篇 |
1978年 | 8篇 |
1976年 | 6篇 |
1973年 | 7篇 |
1958年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
A change in the elevation of bare tidal flats outside a mangrove area is an indispensable factor for the sustainable development of mangroves. Waterline extraction, as an effective and economical tool used in reconstructing the terrain of an intertidal zone, has been widely applied to open-coast tidal flats by constructing a digital elevation model (DEM). However, mangrove wetlands are usually located in wave-sheltered sites, such as estuaries and bays that have narrow tidal channels flanked by tidal flats. Changes in water level are affected by the dry-wet processes of complex landforms caused by tides. This article takes as a study case the area of Yingluo Bay, which covers the core region of the Zhanjiang and Shankou National Mangrove National Nature Reserve in southwestern China. Waterline extraction based on seventeen multisource and multispectral satellite images obtained from December 2014 to April 2015, combining the finite-volume coastal ocean model (FVCOM) hydrodynamic model in an iterative process, was used to generate a topographical map of the bare tidal flat outside the mangrove area in Yingluo Bay. The quality of the iterative DEMs was evaluated via six transects of a ground-based survey using Real - time kinematic (RKT) GPS in May 2015. The mean absolute error (MAE) and root mean square error (RMSE) of the DEM decreased with an increase in the number of iterations. In this study, the DEM in the third iteration was used as the final output because the difference from the previous iterative DEM satisfied an inversion-stopping criterion. The MAE and RMSE of the final DEM with the measured data were 0.072 and 0.09?m, respectively, without considering small tidal creeks. The method used in this study can be an effective and highly precise approach for detecting and reconstructing the historical terrain of a bare tidal flat outside a mangrove area. This work also has great importance regarding intertidal resource management and the sustainable development of mangroves facing the vulnerable coastal ecological environment. 相似文献
72.
73.
GPS�����Ӳ�Ĺ�����Ԥ���о� 总被引:3,自引:1,他引:2
???GPS???????????????????ù??????????????????????????GPS?????????????????????????????????????????????????????????????????????????????????ó?Block IIR??M Rb???????????????Block IIR Rb?????Block IIR Rb???????????????Block IIA Rb???Cs?????????????????????Rb???2~3????????????????????????????????????????????????Rb?????1????????????1.9ns????????????С??????????? 相似文献
74.
In order to solve unsteady incompressible Navier–Stokes(N–S) equations, a new stabilized finite element method,called the viscous-splitting least square FEM, is proposed. In the model, the N–S equations are split into diffusive and convective parts in each time step. The diffusive part is discretized by the backward difference method in time and discretized by the standard Galerkin method in space. The convective part is a first-order nonlinear equation.After the linearization of the nonlinear part by Newton's method, the convective part is also discretized by the backward difference method in time and discretized by least square scheme in space. C~0-type element can be used for interpolation of the velocity and pressure in the present model. Driven cavity flow and flow past a circular cylinder are conducted to validate the present model. Numerical results agree with previous numerical results, and the model has high accuracy and can be used to simulate problems with complex geometry. 相似文献
75.
Zhi-Jia Tian Xiao-Wei Liu Hai-Bo Yuan Bing-Qiu Chen Mao-Sheng Xiang Yang Huang Chun Wang Hua-Wei Zhang Jin-Cheng Guo Juan-Juan Ren Zhi-Ying Huo Yong Yang Meng Zhang Shao-Lan Bi Wu-Ming Yang Kang Liu Xian-Fei Zhang Tan-Da Li Ya-Qian Wu Jing-Hua Zhang 《天文和天体物理学研究(英文版)》2018,(5)
Stellar systems composed of single, double, triple or higher-order systems are rightfully regarded as the fundamental building blocks of the Milky Way. Binary stars play an important role in formation and evolution of the Galaxy. Through comparing the radial velocity variations from multiepoch observations, we analyze the binary fraction of dwarf stars observed with LAMOST. Effects of different model assumptions, such as orbital period distributions on the estimate of binary fractions,are investigated. The results based on log-normal distribution of orbital periods reproduce the previous complete analyses better than the power-law distribution. We find that the binary fraction increases with Teff and decreases with [Fe/H]. We first investigate the relation between α-elements and binary fraction in such a large sample as provided by LAMOST. The old stars with high [α/Fe] dominate with a higher binary fraction than young stars with low [α/Fe]. At the same mass, earlier forming stars possess a higher binary fraction than newly forming ones, which may be related with evolution of the Galaxy. 相似文献
76.
77.
The late Quaternary shallow-water carbonates have been altered by a variety of diagenetic processes, and further influenced by high-amplitude global and regional sea level changes. This study utilizes a new borehole drilled on the Yongxing Island, Xisha Islands to investigate meteoric diagenetic alteration in the late Quaternary shallowwater carbonates. Petrographic, mineralogical, stable isotopic and elemental data provide new insights into the meteoric diagenetic processes of the reef limestone. The results show the variation in the distribution of aragonite,high-Mg calcite(HMC) and low-Mg calcite(LMC) divides the shallow-water carbonates in Core SSZK1 into three intervals, which are Unit I(31.20–55.92 m, LMC), Unit II(18.39–31.20 m, aragonite and LMC) and Unit III(upper 18.39 m of core, aragonite, LMC and HMC). Various degrees of meteoric diagenesis exist in the identified three units. The lowermost Unit I has suffered almost complete freshwater diagenesis, whereas the overlying Units II and III have undergone incompletely meteoric diagenesis. The amount of time that limestone has been in the freshwater diagenetic environment has the largest impact on the degree of meteoric diagenesis. Approximately four intact facies/water depth cycles are recognized. The cumulative depletion of elements such as strontium(Sr),sodium(Na) and sulphur(S) caused by duplicated meteoric diagenesis in the older reef sequences are distinguished from the younger reef sequences. This study provides a new record of meteoric diagenesis, which is well reflected by whole-rock mineralogy and geochemistry. 相似文献
78.
Huang Wen-xi 《岩土力学》1979,1(2):13-38
This paper first introduces the currently adopted stress-strain symbols,some specific terms and their definitions,the commonly used stress-strain testing methods and the formulations for test results。Then,the incremental elastoplastic theories upen which mathematical models are finally given。 相似文献
79.
80.
An experimental survey of the gold concentrated from well water by a hydroelectrical extraction (HEE) method has been carried out in the famous Au metallogenic province in the eastern part of Shandong Province in northeastern China. This method is based on the existence of ionic Au in groundwater around ore bodies. Under the action of an external electric field, ionic Au migrates toward the cathode where the metal is absorbed by a special absorbent attached to the cathode. Promising areas for further exploration can be evaluated from the amount of Au in the absorbent.Distinct Au anomalies were discovered by the experimental survey. The results from the follow-up study of some of these anomalies have shown that most of them are related to the known placer Au deposits and to the primary Au occurrences. However, two new Au prospects and several new Au sources have been found which offer new targets for Au hunting in the area.The hydroelectrical extraction method is an effective technique of looking for Au in areas covered with transported overburden. Since the Au in well water can be concentrated on the cathode with the absorbent, the sensitivity requirement for Au analysis could be reduced. The method also provides a useful tool for the exploration of blind placer Au. 相似文献