全文获取类型
收费全文 | 4613篇 |
免费 | 531篇 |
国内免费 | 920篇 |
专业分类
测绘学 | 88篇 |
大气科学 | 446篇 |
地球物理 | 735篇 |
地质学 | 1677篇 |
海洋学 | 1073篇 |
天文学 | 1424篇 |
综合类 | 153篇 |
自然地理 | 468篇 |
出版年
2024年 | 23篇 |
2023年 | 59篇 |
2022年 | 106篇 |
2021年 | 125篇 |
2020年 | 156篇 |
2019年 | 182篇 |
2018年 | 141篇 |
2017年 | 150篇 |
2016年 | 158篇 |
2015年 | 180篇 |
2014年 | 220篇 |
2013年 | 220篇 |
2012年 | 181篇 |
2011年 | 256篇 |
2010年 | 182篇 |
2009年 | 354篇 |
2008年 | 316篇 |
2007年 | 356篇 |
2006年 | 380篇 |
2005年 | 273篇 |
2004年 | 289篇 |
2003年 | 241篇 |
2002年 | 218篇 |
2001年 | 188篇 |
2000年 | 188篇 |
1999年 | 175篇 |
1998年 | 177篇 |
1997年 | 95篇 |
1996年 | 87篇 |
1995年 | 69篇 |
1994年 | 66篇 |
1993年 | 39篇 |
1992年 | 34篇 |
1991年 | 28篇 |
1990年 | 33篇 |
1989年 | 23篇 |
1988年 | 14篇 |
1987年 | 11篇 |
1986年 | 13篇 |
1985年 | 20篇 |
1984年 | 16篇 |
1983年 | 6篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1978年 | 2篇 |
1977年 | 1篇 |
排序方式: 共有6064条查询结果,搜索用时 31 毫秒
121.
The physical and chemical variability of the water column at subtidal station of an estuary in the Seto Inland Sea, Japan,
was studied over a 24-hour period during a spring tide (tidal range ca. 2 m) in May 1995. Surface water and several depths through the water column were monitored every one and two hours, respectively.
At each occasion, water temperature, salinity and dissolved oxygen concentration were measured and water samples were collected
for the determination of nutrients and suspended particulate matter (SPM). Disruptive changes in the physical and chemical
characteristics of the water was produced by the tidal cycle and the mixing of water masses of different origin. These changes
were highly significant both spatially and temporally, yet with varying effects on physical parameters, nutrients and the
different components of SPM. Significant differences in nutrient concentrations were also observed when the data-set was divided
into ebb and flood components, irrespective of the depth. Nitrate and nitrite rose to 1.8 times higher during the flood. Spatial
differences of SPM were less marked than those of nutrients, only particulate organic carbon (POC) being significantly higher
at the surface than in the intermediate and the lower layer. Both POC and pheopigment concentrations increased markedly through
the water column, being highest shortly before the lower low tide. In contrast, suspended solid (SS) content increased sharply
after the lower low tide (>40 mg l−1) and this coincided with a marked decrease of the C/SS content (<20 mg g−1). The lagtime between POC and SS tidal transport was caused by particle resuspension from the exposed intertidal sediments
as the tidal level rose, and particle transport selection in relation to the tidal state.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
122.
通过室内模拟试验,在坡度为3°和9°、流量为20和60L/min条件下研究了不同浓度(0~350kg/m3)含沙水流流经草地的泥沙沉积过程及其水力学特性。结果表明,坡面泥沙沉积量随含沙量的增加而增大,3°时泥沙沉积率与含沙量呈正相关,而9°时沉积率与含沙量呈反势。坡度对泥沙沉积影响显著,而在相同坡度条件下,两种流量试验的泥沙沉积量无明显差异。相同坡面坡上部位流速小于坡下部位,且含沙量对坡面流速影响较小。在相同坡度和流量条件下,水流雷诺数随含沙量的增大而减小。3°时水流阻力系数和曼宁糙率均随含沙量的增加而增大,而9°时含沙量对阻力影响不明显,因此在土壤侵蚀较严重地区进行坡面水文过程演算时需考虑含沙量对缓坡糙率的影响。 相似文献
123.
Calcimicrobialites across the Permian-Triassic boundary in Huayingshan (华蓥山) region were investigated using the fluorescence microscopic measurements to understand the occurrence of organic matter. The microbialites are composed of micrite matrix and coarse spar cement. Abundant rhombic or magnetic needle-like carbonate minerals were observed adrift within the cement. The fluorescence microscopic measurement indicates the micrite matrix in microbialites shows the most abundant organic matter, with the rhombic or magnetic needle-like carbonate minerals and coarse spar cement coming to the 2nd and the 3rd, respectively. Organic matter is mainly preserved in the space between the grains of the micrite minerals but almost evenly distributed in the rhombic or magnetic needle-like carbonate minerals. As one of the common diagenesis types, dolomitization is observed to occur in the microbialites in Huayingshan. However, the carbonate cement in microbialites still has high content of element Sr as shown by the microprobe analysis, reflecting that the dolomitization might have happened in a restricted environment. Observation under the fluorescence microscope shows that dolomitization just led to the redistribution of organic matter in the grain space of dolomite minerals, inferring that the diagenesis has a slight effect on the preservation, and thus on the content of organic matter in the microbialites. 相似文献
124.
黄河内蒙古段上覆水、悬浮物和底泥重金属特征及生态风险研究 总被引:24,自引:12,他引:12
系统地采集黄河内蒙古段上覆水、悬浮物和底泥样品,分析其中重金属元素的含量。采用地质累积指数法和潜在生态危害系数法,研究了黄河干流内蒙古段的上覆水、悬浮物和底泥的重金属沿程分布特征、污染程度和生态风险。分析结果显示:除了As元素,悬浮物中重金属元素含量总体上高于底泥中的含量。地质累积指数法评价结果表明:悬浮物和底泥属于无污染—中度污染,悬浮物总体上比底泥污染严重;潜在生态危害系数法评价结果表明:底泥和悬浮物中重金属的生态危害总体上属轻微状态;因此,生态风险指数法和地质累积指数评价结果基本一致。 相似文献
125.
This paper studies a landfill where there are three faults running through.As serious pollution has occurred to the geological environment,the landfill is to be closed up and renovated.The paper aims to explore the role of fracture structure in leachate pollution at the landfill.The research was carried out in several stages.First,mathematical models of the pre-renovation landfill with three faults running through and the landfill after renovation were established.And then,the boundary conditions and parameters of the two mathematical models were determined.The groundwater level of the landfills was simulated in order to modify the two mathematical models.As a result,a feasible mathematical model was achieved.Based on this model,a comparison was made of the COD concentration variations in the inside leachate and outside leachate between the two landfills. Accordingly,the impacts of the fracture structure on the pollution of leachate at the landfills could be identified.The study results show that while faults contribute to the migration of leachate,they also serve as a confluence of leachate,thus further deteriorating the environment.The COD concentrations of the inside leachate and the outside leacbate of the pre-renovation landfill are respectively 800 mg/L and 220 mg/L higher than those of the post.renovation landfill.Therefore,measures must be taken to handle the leachate seepage in areas where there are faults as well as the neighboring areas so as to get the environmental pollution under control. 相似文献
126.
This paper studies a landfill where there are three faults running through. As serious pollution has occurred to the geological environment, the landfill is to be closed up and renovated. The paper aims to explore the role of fracture structure in leachate pollution at the landfill. The research was carried out in several stages. First, mathematical models of the pre-renovation landfill with three faults running through and the landfill after renovation were established. And then, the boundary conditions and parameters of the two mathematical models were determined. The groundwater level of the landfills was simulated in order to modify the two mathematical models. As a result, a feasible mathematical model was achieved. Based on this model, a comparison was made of the COD concentration variations in the inside leachate and outside leachate between the two landfills. Accordingly, the impacts of the fracture structure on the pollution of leachate at the landfills could be identified. The study results show that while faults contribute to the migration of ieachate, they also serve as a confluence of leachate, thus further deteriorating the environment. The COD concentrations of the inside leachate and the outside leachate of the pre-renovation landfill are respectively 800 mg/L and 220 mg/L higher than those of the post-renovation landfill. Therefore, measures must be taken to handle the ieachate seepage in areas where there are faults as well as the neighboring areas so as to get the environmental pollution under control. 相似文献
127.
128.
129.
水泥粉煤灰加固有机质土的试验研究 总被引:3,自引:0,他引:3
对于高有机质含量的泻湖相软土,单纯采用水泥不能有效提高该软土的力学性能,因此提出了采用水泥和粉煤灰作为固化剂的加固方法。通过不同水泥掺入量、粉煤灰掺入量和龄期下水泥土的无侧限抗压强度试验,分析了水泥粉煤灰固化土的强度规律和变形规律,探讨了水泥和粉煤灰加固高有机质含量软土的机理。结果表明,粉煤灰对于水泥试块的早期强度影响较小,对后期强度影响较大;粉煤灰最佳掺入量为12%,超过此掺入量水泥土强度反而会降低,粉煤灰水泥土的破坏应变、E50也在粉煤灰掺量为12%时分别达到最低值和最大值。水泥掺加粉煤灰可有效地提高高有机质含量软土的强度。 相似文献
130.