全文获取类型
收费全文 | 6678篇 |
免费 | 1338篇 |
国内免费 | 2048篇 |
专业分类
测绘学 | 757篇 |
大气科学 | 883篇 |
地球物理 | 1179篇 |
地质学 | 4139篇 |
海洋学 | 1261篇 |
天文学 | 536篇 |
综合类 | 381篇 |
自然地理 | 928篇 |
出版年
2024年 | 31篇 |
2023年 | 134篇 |
2022年 | 319篇 |
2021年 | 402篇 |
2020年 | 304篇 |
2019年 | 354篇 |
2018年 | 377篇 |
2017年 | 321篇 |
2016年 | 391篇 |
2015年 | 420篇 |
2014年 | 417篇 |
2013年 | 562篇 |
2012年 | 572篇 |
2011年 | 583篇 |
2010年 | 608篇 |
2009年 | 568篇 |
2008年 | 540篇 |
2007年 | 498篇 |
2006年 | 453篇 |
2005年 | 388篇 |
2004年 | 313篇 |
2003年 | 255篇 |
2002年 | 223篇 |
2001年 | 260篇 |
2000年 | 193篇 |
1999年 | 97篇 |
1998年 | 83篇 |
1997年 | 65篇 |
1996年 | 58篇 |
1995年 | 34篇 |
1994年 | 46篇 |
1993年 | 30篇 |
1992年 | 29篇 |
1991年 | 20篇 |
1990年 | 15篇 |
1989年 | 7篇 |
1988年 | 12篇 |
1987年 | 11篇 |
1986年 | 7篇 |
1985年 | 7篇 |
1984年 | 11篇 |
1983年 | 6篇 |
1982年 | 6篇 |
1980年 | 3篇 |
1979年 | 5篇 |
1976年 | 2篇 |
1973年 | 2篇 |
1972年 | 3篇 |
1963年 | 3篇 |
1954年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
81.
82.
83.
The anoxic rates of heat dissipation by mussels (Mytilus edulis) were enhanced after exposure to pentachlorophenol (PCP) and tributyltin (TBT), both known uncouplers of oxidative phosphorylation. The degree of metabolic activation under anoxia was dependent upon the amount of toxicant accumulated in the tissues, but the concentration-response relationship was different to that under aerobic conditions. Biochemical measurements indicated that the anaerobic metabolic pathways were significantly disturbed by PCP and TBT. There was a decline in succinate, an increase in the fumarate: succinate ratio, and an increase in the accumulation of lactate, indicating a shift from succinate to lactate anaerobic pathways. A consequence of organic toxicant exposure was a reduction in anoxic survival time of mussels. 相似文献
84.
国外海岸沙丘形成与演化的研究 总被引:13,自引:0,他引:13
董玉祥 《海洋地质与第四纪地质》2001,21(2):93-98
自20世纪80年代以来,国外对海岸沙丘的研究已从主要对其地貌形态的研究转向海岸海丘与海岸变迁尤其是全球变化关系的研究上,海岸沙丘形成与演化的研究倍受关注,进行了大量研究并取得了一定的研究成果。这里介绍了国外在海岸沙丘形成与演化研究方面的主要进展及其成果,并以此探讨了国内今后开展海岸沙丘形成与演化研究的主要方向与问题。 相似文献
85.
采用同工酶电泳方法,对在不同温度处理下刺参(Apostichopus japonicus)的乳酸脱氢酶(LDH)、酯酶(EST)和超氧化歧化酶(SOD)的表达情况进行分析。本文结果显示刺参组织内无LDH同工酶的表达。在10℃到20℃的温度突变处理中,12 h内EST和SOD均有新增酶带,在20℃到10℃的温度突变1 h内EST表达有新增酶带。3 h内SOD表达有新增酶带。在不同变温幅度的处理中,EST和SOD只有1条酶带。这表明,刺参对温度突变可产生积极的响应,对于其适应环境温度的变化具有重要意义。 相似文献
86.
Problems with biogenic silica measurement in marginal seas 总被引:5,自引:0,他引:5
Surface sediment samples from the Bohai, Yellow Sea, and the Pacific were used to assess biogenic silica (BSi) content and to study uncertainties in BSi measurements. The contents of BSi in the Bohai and Yellow Sea are all less than 1%. The dissolution of BSi in sediments from the Bohai and Yellow Sea is very important to maintain high levels of silicate in the water column. The non-biogenic silica from clay minerals has an obvious effect on BSi of sediment samples in the Bohai and Yellow Sea with low BSi and high clay minerals. The solid to solution ratio was found to have a great influence on BSi measurement, which can induce uncertainties up to 75%. The effect of loss by sorption and centrifugation is negligible. Interlaboratory comparison of techniques for BSi measurement by the wet alkaline extraction technique of Mortlock and Froelich [Deep-Sea Res. 36 (1989) 1415-1426] with clay correction was suggested to give no significant differences. However, differences in sediment compositions and reagent to sample ratio may limit the application of the wet alkaline extraction method. 相似文献
87.
88.
海洋交通运输业碳排放研究对制定行之有效的低碳海洋交通政策具有重要意义。本文首先对2006—2015年中国沿海11个省(区、市)的海洋交通运输业碳排放量进行估算,然后运用超效率SBM模型对碳排放效率进行测度,最后运用Tobit模型探究碳排放效率的影响因素。结果表明:①中国海洋交通运输业碳排放效率整体上处于低效率状态,且各省(区、市)间差异明显,但差异正在逐渐缩小。②2006—2015年沿海各省(区、市)市海洋交通运输业碳排放效率变动呈现出“M”形波动上升趋势。③将各省(区、市)海洋交通运输业碳排放效率水平划分为不同类型,其中上海和天津属于高效率型,河北属于中效率型,其余省(区、市)市属于低效率型。④货运距离和能源结构对海洋交通运输业碳排放效率具有显著的正向作用,而能源强度对其具有显著的负向作用。 相似文献
89.
Processing of high-frequency multibeam echo sounder data for seafloor characterization 总被引:9,自引:0,他引:9
Processing simultaneous bathymetry and backscatter data, multibeam echosounders (MBESs) show promising abilities for remote seafloor characterization. High-frequency MBESs provide a good horizontal resolution, making it possible to distinguish fine details at the water-seafloor interface. However, in order to accurately measure the seafloor influence on the backscattered energy, the recorded sonar data must first be processed and cleared of various artifacts generated by the sonar system itself. Such a preprocessing correction procedure along with the assessment of its validity limits is presented and applied to a 95-kHz MBES (Simrad EM 1000) data set. Beam pattern effects, uneven array sensitivities, and inaccurate normalization of the ensonified area are removed to make possible further quantitative analysis of the corrected backscatter images. Unlike low-frequency data where the average backscattered energy proves to be the only relevant feature for discriminating the nature of the seafloor, high-frequency MBES backscatter images exhibit visible texture patterns. This additional information involves different statistical distributions of the backscattered amplitudes obtained from various seafloor types. Non-Rayleigh statistics such as K-distributions are shown to fit correctly the skewed distributions of experimental high-frequency data. Apart from the effect of the seafloor micro-roughness, a statistical model makes clear a correlation between the amplitude statistical distributions and the signal incidence angle made available by MBES bathymetric abilities. Moreover, the model enhances the effect of the first derivative of the seafloor backscattering strength upon statistical distributions near the nadir and at high incidence angles. The whole correction and analysis process is finally applied to a Simrad EM 1000 data set. 相似文献
90.
Autonomous vehicles are being developed to replace the conventional, manned surface vehicles that tow mine hunting towed platforms. While a wide body of work exists that describes numerical models of towed systems, they usually include relatively simple models of the towed bodies and neglect the dynamics of the towing vehicle. For systems in which the mass of the towing vehicle is comparable to that of the towed vehicle, it becomes important to consider the dynamics of both vehicles. In this work, we describe the development of a numerical model that accurately captures the dynamics of these new mine hunting systems. We use a lumped mass approximation for the towcable and couple this model to non-linear numerical models of an autonomous surface vehicle and an actively controlled towfish. Within the dynamics models of the two vehicles, we include non-linear controllers to allow accurate maneuvering of the towed system. 相似文献