全文获取类型
收费全文 | 5526篇 |
免费 | 628篇 |
国内免费 | 630篇 |
专业分类
测绘学 | 164篇 |
大气科学 | 423篇 |
地球物理 | 1720篇 |
地质学 | 1247篇 |
海洋学 | 1218篇 |
天文学 | 781篇 |
综合类 | 182篇 |
自然地理 | 1049篇 |
出版年
2024年 | 16篇 |
2023年 | 65篇 |
2022年 | 94篇 |
2021年 | 127篇 |
2020年 | 137篇 |
2019年 | 176篇 |
2018年 | 126篇 |
2017年 | 168篇 |
2016年 | 175篇 |
2015年 | 178篇 |
2014年 | 241篇 |
2013年 | 440篇 |
2012年 | 246篇 |
2011年 | 205篇 |
2010年 | 173篇 |
2009年 | 336篇 |
2008年 | 367篇 |
2007年 | 378篇 |
2006年 | 302篇 |
2005年 | 270篇 |
2004年 | 252篇 |
2003年 | 261篇 |
2002年 | 223篇 |
2001年 | 166篇 |
2000年 | 176篇 |
1999年 | 191篇 |
1998年 | 171篇 |
1997年 | 90篇 |
1996年 | 159篇 |
1995年 | 193篇 |
1994年 | 212篇 |
1993年 | 80篇 |
1992年 | 67篇 |
1991年 | 60篇 |
1990年 | 51篇 |
1989年 | 43篇 |
1988年 | 29篇 |
1987年 | 29篇 |
1986年 | 20篇 |
1985年 | 21篇 |
1984年 | 12篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 4篇 |
1980年 | 20篇 |
1979年 | 3篇 |
1978年 | 7篇 |
1977年 | 5篇 |
1954年 | 4篇 |
排序方式: 共有6784条查询结果,搜索用时 109 毫秒
91.
Numerical analyses for the Bragg resonant reflection of carrier waves associated long waves due to sinusoidally varying seabeds are performed by using a set of coupled ordinary differential equations derived from the Boussinesq equations. The Boussinesq equations are firstly approximated with the Fourier decomposition. The coupled governing equations are then derived and used to simulate evolution of both short and long wave components. It is also found that wave groups are generated by two carrier waves with slightly different frequencies. The wave energy of the initial wave components is transferred to other harmonic components during propagation over a long distance. Evolution and reflection of both short and long waves were largely affected by nonlinearity. 相似文献
92.
In this paper, the theoretical analysis and experimental studies are employed to investigate the reflection characteris-tics of partial standing waves caused by wave overtopping and sloping top of structures. Based on the principle of conser-vation of wave energy flux, the third-order Stokes wave theory is used to formulate the reflection coefficient at wave overtopping; the calculation results are regressed into an applied expression. A series of experiments of wave reflection for a vertical-wall structure with chamfered and overhanging upper sections are carried out to investigate the influence of top slope on wave reflection. The regularity of variation of wave reflection in this case is analysed based on the experimental results. 相似文献
93.
The analytical method (AM) for separation of composite waves is presented based on the Hilbert transform. It is ap-plicable to both regular and irregular trains of waves. The wave data series measured with two wave gauges in the experi-ments are separated into two series of incident and reflected waves. Then, the reflection coefficient can be easily ob-tained. The arrival of reflected waves can also be detected for improveraent of the accuracy of the reflection coefficient. The reflection performance of the physical model can be estimated exactly without calculation of wave height and phase difference. Numerical samples developed to test the method are proved to be accurate. Physical experiments are conduct-ed and compared with Goda s method and satisfactory results are obtained. 相似文献
94.
Ocean Waves: Half-a-Century of Discovery 总被引:1,自引:0,他引:1
Paul H. LeBlond 《Journal of Oceanography》2002,58(1):3-9
While the nature of most ocean waves has long been known and their basic physics understood since the nineteenth century,
intense study of ocean waves during the second half of the twentieth century has taken the subject from the realm of mathematical
exercises to that of practical engineering. Modern understanding of the generation, propagation and interactions of ocean
waves with each other and with oceanic features has advanced to a quantitative level offering predictive capacity. This paper
presents a brief qualitative review of advances in knowledge of sound waves, wind waves, tsunamis, tides, internal waves and
long-period vorticity waves. The review is aimed at non-specialists who may benefit from an overview of the current state
of the subject and access to a bibliography of general-interest references.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
95.
环台湾岛海域半日潮波特征的三维模拟 总被引:10,自引:0,他引:10
用1997版POM海洋模式,首次应用于环台湾岛海域的潮波数值研究.得到该海域的半日潮波主要为23°N以南西太平洋传来的胁振潮.影响台湾海峡的半日潮波分别由海峡南北口传入的两支潮波,且北支强于南支.福建沿岸湄州湾-兴化湾为最强潮区,其M2分潮最大振幅可达240cm.最强潮流区位于澎湖水道,M2分潮最大潮流达196cm/s.环台湾岛海域潮波潮流水平结构上除海峡北部原有一个圆流点外,还发现另外存在4个新的圆流点.潮流垂直结构上主要为右偏,接近底层处为左偏. 相似文献
96.
97.
Molecular identification of ascaridoid nematodes from the deep-sea onion-eye grenadier (Macrourus berglax) from the East Greenland Sea 总被引:2,自引:0,他引:2
Esra Kellermanns Sven Klimpel Harry Wilhelm Palm 《Deep Sea Research Part I: Oceanographic Research Papers》2007,54(12):2194-2202
Macrourus berglax from the East Greenland Sea was studied for the presence of ascaridoid nematodes in 2001, 2002 and 2003. The fishes were collected between 278 and 413 m water depth using a benthopelagic net. Based on the amplification of the internal transcribed spacer ITS-1, 5.8S, ITS-2 and flanking sequences (=ITS+), three ascaridoid nematode species were identified. The prevalence of infestation during the 3 years ranged from 42.9% to 62.9% and 22.9% to 40.0% for the anisakids Anisakis simplex (s.s.) and Pseudoterranova decipiens (s.s.), respectively, and from 28.6% to 60.0% for the raphidascarid Hysterothylacium aduncum. A total of 18 specimens, two of each species and examination year, revealed no sibling species, suggesting a limited distribution of other ascaridoid siblings into the deep sea. The ITS-1, 5.8S and ITS-2 sequences of A. simplex (s.s.) from the East Greenland Sea did not differ from previously published sequence data (GenBank) from other regions in the Atlantic and Pacific oceans. The sequences of P. decipiens (s.s.) corresponded most closely to those of specimens from Richardson Bay, western Pacific, and differed in four positions (0.5%). They corresponded least to those of specimens from Japan (1.5%). The sequence data for H. aduncum differed in two positions in the ITS-1 (0.2%) and three positions in the ITS-2 (0.3%) from sequences from Japan. A high genetic similarity between the regions can be explained by (a) extensive final host migration in the case of A. simplex (s.s.), (b) an overlapping distribution of final host populations along the continental shelves for P. decipiens (s.s.) and (c) a low host specificity and large population size in the intermediate and final hosts for H. aduncum. The occurrence of the identified species in the macrourid fish underlines the potential of cosmopolitan ascaridoid nematodes to distribute not only horizontally but also vertically in the deep sea. 相似文献
98.
99.
The planet Mars lacks, today, a planetary, dynamic magnetic field, but strong, intense, localized magnetic fields of lithospheric origin, one to two orders of magnitude larger than the terrestrial lithospheric field, are present. This lithospheric magnetic field is the result of magnetization processes in the presence of a magnetic dynamo and of demagnetization processes after the dynamo shutdown, such as impact or volcanoes. This crude scenario can be more accurately specified by interpreting global and local models of the current magnetic field of Mars. Some specific areas are studied, including the intensely magnetized Terra Sirenum, as well as the magnetic anomaly associated with Apollinaris Patera. Magnetic minerals could be of primary and/or secondary origin; this latter would imply an early hydration of a basaltic crust. A scenario, in which Mars experienced a major polar wander due to the Tharsis bulge, prior to the cessation of its dynamo, is proposed and discussed. 相似文献
100.
Among numerous methods for cation exchange capacity (CEC) determination for soils and sediments, the cobaltihexamine chloride method is frequently used due to its ability to measure CEC at soil pH. After exchange with Co(NH3)63+ ions, CEC is estimated via the measurement of the Co remaining in solution. The modified method proposed allows a more rapid determination of CEC based on the measurement of the absorbance at 472 nm of the cobaltihexamine chloride solution before and after exchange. This method has been applied to various soil's horizons from four sites, selected to cover a wide range of CEC and pH values. The model obtained allows one to calculate CEC from absorbance at 472 nm with 95% confidence intervals. As CEC is of relevant meaning in agronomical and environmental purposes, and more recently in ecotoxicological studies, this modified method can be proposed as a rapid test for CEC evaluation. 相似文献