全文获取类型
收费全文 | 10650篇 |
免费 | 2250篇 |
国内免费 | 2868篇 |
专业分类
测绘学 | 1310篇 |
大气科学 | 1914篇 |
地球物理 | 2420篇 |
地质学 | 5598篇 |
海洋学 | 1585篇 |
天文学 | 479篇 |
综合类 | 1017篇 |
自然地理 | 1445篇 |
出版年
2024年 | 57篇 |
2023年 | 177篇 |
2022年 | 514篇 |
2021年 | 599篇 |
2020年 | 530篇 |
2019年 | 686篇 |
2018年 | 624篇 |
2017年 | 617篇 |
2016年 | 613篇 |
2015年 | 693篇 |
2014年 | 676篇 |
2013年 | 804篇 |
2012年 | 765篇 |
2011年 | 776篇 |
2010年 | 739篇 |
2009年 | 758篇 |
2008年 | 769篇 |
2007年 | 739篇 |
2006年 | 582篇 |
2005年 | 521篇 |
2004年 | 403篇 |
2003年 | 353篇 |
2002年 | 380篇 |
2001年 | 418篇 |
2000年 | 325篇 |
1999年 | 283篇 |
1998年 | 205篇 |
1997年 | 218篇 |
1996年 | 158篇 |
1995年 | 143篇 |
1994年 | 109篇 |
1993年 | 99篇 |
1992年 | 105篇 |
1991年 | 51篇 |
1990年 | 50篇 |
1989年 | 41篇 |
1988年 | 44篇 |
1987年 | 18篇 |
1986年 | 22篇 |
1985年 | 14篇 |
1984年 | 14篇 |
1983年 | 5篇 |
1982年 | 10篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1978年 | 8篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1958年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
61.
This article describes absolute calibration results for both JASON-1 and TOPEX Side B (TSB) altimeters obtained at the Lake Erie calibration site, Marblehead, Ohio, USA. Using 15 overflights, the estimated JASON altimeter bias at Marblehead is 58 ± 38 mm, with an uncertainty of 19 mm based on detailed error analysis. Assuming that the TSB bias is negligible, relative bias estimates using both data from the TSB-JASON formation flight period and data from 48 water level gauges around the entire Great Lakes confirmed the Marblehead results. Global analyses using both the formation flight data and dual-satellite (TSB and JASON) crossovers yield a similar relative bias estimate of 146 ± 59 mm, which agrees well with open ocean absolute calibration results obtained at Harvest, Corsica, and Bass Strait (e.g., Watson et al. 2003). We find that there is a strong dependence of bias estimates on the choice of sea state bias (SSB) models. Results indicate that the invariant JASON instrument bias estimated oceanwide is 71 mm, with additional biases of 76 mm or 28 mm contributed by the choice of Collecte Localisation Satellites (CLS) SSB or Center for Space Research (CSR) SSB model, respectively. Similar analysis in the Great Lakes yields the invariant JASON instrument bias at 19 mm, with the SSB contributed biases at 58 mm or 13 mm, respectively. The reason for the discrepancy is currently unknown and warrants further investigation. Finally, comparison of the TOPEX/POSEIDON mission (1992-2002) data with the Great Lakes water level gauge measurements yields a negligible TOPEX altimeter drift of 0.1 mm/yr. 相似文献
62.
63.
On low-pass digital filters in oceanography 总被引:1,自引:0,他引:1
-Two types of filters are widely used to remove semidirunal and diurnal tidal signals and other high frequency noises in oceanography. The first type of filters uses moving average with weights in time domain, and can be easily operated. Some data will be lost at each end of the time series, especially for the low low-pass filters. The second type of filters uses the discrete Fourier transform filter (DFTF) which operates in the frequency domain, and there are no data loss at the ends for the forward transform. However, owing to the Gibbs phenomenon and the discrete sampling (Nyquist effect) , ringing appears in the inverse transformed data, which is especially serious at each end. Thus some data at the ends are also discarded. The present study tries to find out what causes the ringing and then to seek for methods to overcome the ringing. We have found that there are two kinds of ringings, one is the Gibbs phenomenon, as defined before. The other is the "Nyquist"ringing due to sampling Nyquist critical 相似文献
64.
基于拉格朗日余流及其输运过程的一种三维空间弱非线性理论,进一步假定Si在河口的无因次化学转移项量级为k^2,给出了Si的长期输运方程。对于一种二维模型河口,数值求解了零阶天文潮,欧拉余流、斯托克斯漂移、拉格朗日余流和盐度的分布;分别计算了平均逼留时间为13,30,60,180d等所对应的Si浓度分布;给出了Si-s相关图,讨论了河口中Si的保守性问题。 相似文献
65.
66.
光合细菌在对虾育苗生产中的应用 总被引:6,自引:1,他引:6
1995年4月24日~5月8日,在文登高岛盐场育苗场50m3水体育苗池,进行了用几株光合细菌(RhodopseudomonasS.P.)混合菌液作为中国对虾(Penaeuschinensis)苗期水质净化剂及辅助饵料,取得了明显效果。育苗池水透明度提高约20cm,幼体挂脏率下降50%,变态提早1d左右,且多产仔虾百万尾 相似文献
67.
The colony-forming Phaeocystis species are causative agents of dense bloom occurrences in coastal waters worldwide. It is difficult to separate them because of the different morphologies associated with their colonial stages. In this study we applied molecular approaches to analyze the genetic variation of Phaeocystis globosa and Phaeocystis pouchetii from several geographic regions, and to assist in tracing the dispersal of bloom-forming Phaeocystis species in coastal waters of China. The sequences of the internal transcribed spacers (ITS1 and ITS2) of rDNA and the 5.8S ribosomal RNA gene of Phaeocystis strains were determined. Sequence comparison shows that P.globosa was the most divergent to P. pouchetii, exhibiting sequence divergence higher than 0.08. However, lower genetic divergences existed between strains of P.globosa. The sequence comparison of the Phaeocystis rDNA ITS clearly shows that the species isolated from the southeast coast of China is identified as P.globosa rather than P. cf. pouchetii or other species. Furthermore, the significance of rDNA variation in distinct global populations of P.globosa suggested it might have had sufficient time to accumulate detectable mutations at the rDNA locus, supporting the hypothesis of ancient dispersal of P.globosa to many areas, meaning that P.globosa blooms in the coastal waters of China are endemic rather than a newly introduced species or a foreign source. Finally, based on the high divergent region of rDNA ITS, a pair of species-specific primers for P.globosa were designed, they could be useful to detect the presence of this species in mixed plankton assemblages or flagellate stages that are recognized with diffculties by means of conventional microscopy. 相似文献
68.
工程风险分析中的风险当量及其评价标准 总被引:7,自引:2,他引:7
目前,量化、半量化风险分析是工程风险分析的主流。在对工程进行量化和半量化风险分析时,无论采用什么方法,风险评价指标和评价标准都是关键的重要问题。只有在建立了统一的评价指标和评价标准体系的基础上,对工程进行的风险分析能够得出正确和一致的结论。文章通过对国内外风险分析理论和实践的研究和总结,提出了在工程风险分析中确定风险评价指标和评价标准的原则和方法。 相似文献
69.
介绍了自适应电流保护原理在胜利海上平台供电系统中的应用。应用该原理的电流保护克服了传统电流保护一成不变的定值、受系统运行方式和短路类型的影响较大等缺点,它充分发挥了微机的记忆、逻辑判断、数学运算等强大的功能,提供实时计算保护装设处到系统等效电源间的阻抗,通过不断监视负荷电流来自动整定过流保护,并引入了反时限特性,解决了实际存在的技术问题,使得保护装置灵敏可靠。 相似文献
70.
Modelling of flow around a near-bed pipeline with a spoiler 总被引:1,自引:0,他引:1
Flow around a pipeline with and without a spoiler near a smooth wall is simulated by solving the Navier–Stokes equations. Finite-difference formulation with a second-order upwind scheme in a curvilinear coordinate system is employed. The influences of the spoiler on hydrodynamic forces, pressure distribution, vortex shedding frequency, velocity profile under the pipe, as well as shear stress on the wall are investigated. The attachment of a spoiler significantly increases drag, root-mean-square (RMS) lift, flow through the gap between the pipe and the wall and shear stress on the seabed around the pipe. The spoiler also generates a non-zero mean downward force on the pipeline, which may enhance the self-burial of the pipeline. 相似文献