全文获取类型
收费全文 | 3909篇 |
免费 | 1401篇 |
国内免费 | 1873篇 |
专业分类
测绘学 | 638篇 |
大气科学 | 437篇 |
地球物理 | 727篇 |
地质学 | 4320篇 |
海洋学 | 318篇 |
天文学 | 39篇 |
综合类 | 206篇 |
自然地理 | 498篇 |
出版年
2024年 | 33篇 |
2023年 | 122篇 |
2022年 | 363篇 |
2021年 | 283篇 |
2020年 | 338篇 |
2019年 | 458篇 |
2018年 | 373篇 |
2017年 | 390篇 |
2016年 | 380篇 |
2015年 | 446篇 |
2014年 | 429篇 |
2013年 | 474篇 |
2012年 | 404篇 |
2011年 | 456篇 |
2010年 | 392篇 |
2009年 | 370篇 |
2008年 | 246篇 |
2007年 | 328篇 |
2006年 | 262篇 |
2005年 | 141篇 |
2004年 | 80篇 |
2003年 | 75篇 |
2002年 | 49篇 |
2001年 | 40篇 |
2000年 | 39篇 |
1999年 | 56篇 |
1998年 | 16篇 |
1997年 | 20篇 |
1996年 | 20篇 |
1995年 | 12篇 |
1994年 | 11篇 |
1993年 | 9篇 |
1992年 | 10篇 |
1991年 | 9篇 |
1990年 | 6篇 |
1989年 | 6篇 |
1988年 | 10篇 |
1987年 | 3篇 |
1986年 | 6篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1964年 | 1篇 |
1958年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有7183条查询结果,搜索用时 234 毫秒
61.
长江三角洲是研究古海岸沉积环境演化的理想区域。选取长江三角洲平原一支长岩芯钻孔上部55 m层位的沉积物进行光释光(optically stimulated luminescence,OSL)测年研究。根据沉积物粒度特性,在实验前处理过程中,提取100~200 μm或63~100 μm粗颗粒石英矿物进行OSL测年。测年样品的预热坪与剂量恢复实验表明选择180℃作为预热条件较为合适,石英OSL信号衰退曲线以快组分为主。条件实验结果、等效剂量分布以及各测片的循环比、热转移等方法学层面的实验结果表明,OSL测年技术对该孔的沉积物测年具有适应性和可靠性。通过年代—深度关系模型,建立该段地层中全新世以来的年龄框架。依据该孔的年代地层序列并结合前人工作,探讨全新世以来长江三角洲的沉积演化。 相似文献
62.
63.
Jamaledin Baniamerian Shuang Liu Xiangyun Hu Maurizio Fedi Mahak Singh Chauhan Mahmoud Ahmed Abbas 《Geophysical Prospecting》2020,68(7):2320-2342
Inversion of magnetic data is complicated by the presence of remanent magnetization, and it provides limited information about the magnetic source because of the insufficiency of data and constraint information. We propose a Fourier domain transformation allowing the separation of magnetic anomalies into the components caused by induced and remanent magnetizations. The approach is based on the hypothesis that each isolated source is homogeneous with a uniform and specific Koenigsberger ratio. The distributions of susceptibility and remanent magnetization are subsequently recovered from the separated anomalies. Anomaly components, susceptibility distribution and distribution of the remanent and total magnetization vectors (direction and intensity) can be achieved through the processing of the anomaly components. The proposed method therefore provides a procedure to test the hypotheses about target source and magnetic field, by verifying these models based on available information or a priori information from geology. We test our methods using synthetic and real data acquired over the Zhangfushan iron-ore deposit and the Yeshan polymetallic deposit in eastern China. All the tests yield favourable results and the obtained models are helpful for the geological interpretation. 相似文献
64.
65.
基于MODIS数据,以湖北省地级以上城市城区为研究对象,通过对湖北省13个地级以上城市城区边界矢量数据与地表温度因子进行套合处理,采用叠置分析方法对2000、2015、2017年湖北省地级以上城市城区的地表温度进行统计,并分析城镇化建设对城区地表温度产生的影响。 相似文献
66.
Pinus taiwanensis is a species endemic to China. This study selected four typical plots of Pinus taiwanensis in the natural secondary forest around Macheng City, in order to reveal the characteristics of and the relationships between different diameter classes (determined based on the diameter at breast height or DBH), forest densities and species diversity, as well as the similarities of species diversity of different plots within the community. The result showed that Pinus taiwanensis was the dominant species in the community. The ratio of Pinus taiwanensis trees of diameter class IV reached a peak of 19.46% of the total followed by diameter class VII at 18.92%. The study recorded 156 species of vascular plants from 130 genera of 71 families; Pinus taiwanensis was the dominant species in the community. When the forest density was 1200 trees ha -1 with the largest average diameter of DBH=36.779±4.444 cm, the diversity (Shannon index H'=1.6716) and the evenness (Pielou index E=0.6727) of the tree layer was the highest. When the forest density reached 1525 trees ha -1 with the lowest average diameter of DBH=18.957±5.141 cm, the richness (Dma=5.4308), the diversity (H'=2.9612) and the evenness (E=0.8985) of all shrub layers climbed to the maximum. When the forest density was 1325 trees ha -1, the richness (Dma=5.8132), the diversity (H'=3.0697) and the evenness (E=0.9025) of all herb layers peaked. In terms of vertical structure, the average diversity indexes were herb layer>shrub layer>tree layer. High canopy density weakened light intensity in the community, causing a reduction in the species diversities of herbs and shrubs. The average similarity coefficient between the sample plots was 0.3356, which was at the medium dissimilarity level. External disturbances and improper management were major contributors to the low species diversity of the community. The implementation of scientific management measures is urgently needed to optimize the forest structures of Pinus taiwanensis, create a benign community environment, and promote species diversities and establish a stable forest community structure. 相似文献
67.
为解决虚拟地理环境缺乏有效组织、规划、管理群体协同的问题,研究并构建角色模型及角色支持下的虚拟地理环境群体协同方法。构建了包含角色扮演、角色权限、角色感知、角色思维、角色行为和角色表达等模块的角色模型,设计了基于角色的虚拟地理环境群体协同框架,提出了多角色群体协同规划方法,建立了角色支撑下的群体协同冲突检测与消除策略。针对人类活动与全球变化相互影响问题,构建多领域人员参与下虚拟地理环境群体协同模拟与评估原型系统。经系统测试发现,通过角色模型能够较好地实现协同参与者与虚拟地理环境之间的耦合,同时能够为虚拟地理环境的群体协同提供系统性方法支撑。 相似文献
68.
Carbon dioxide (CO2) is one of the major gases that contribute to the global warming. Therefore, studying the distribution of CO2 can help people understand the carbon cycle. Based on the GOSAT retrieved CO2 products, the temporal and spatial distribution and seasonal variation of CO2 concentration were analyzed from 2011 to 2015. CO2 concentration has obvious seasonal variation. It was low in summer, and was high in spring, and the annual increase was about 2 ppm. Nevertheless, the annual growth rate of CO2 concentration in summer was higher than that in spring, it was 0.5425% in summer and was 0.46% in spring. CO2 concentration was low in the northwest and was high in the southeast. The growth rate of CO2 was 2.8 ppm in the northwest and was 3.42 ppm in the southeast. More human’s activities made CO2 concentration higher in the southeast than that in other regions. 相似文献
69.
在虚拟地理环境(virtual geographic environments,VGE)的研究中,城市公共安全作为一种典型的应用,致力于实现风险的认知与表达。然而,针对城市突发公共事件风险评估的共享管理需求与即时性需求,尚缺乏一种面向过程的风险监测与评估的表达方法,难以提供快速、全面的决策支持。针对此问题,本文首先基于元对象设施(meta object facility,MOF)建模架构,提出统一时空框架下面向过程的风险评估建模方法,实现风险监测评估方法的共享与重用,并提供面向过程的即时风险评估个性化信息服务。其次,设计了风险评估过程管理原型系统,实现风险评估过程建模、注册、执行和可视化等功能。最后,通过试验模拟太原市燃气泄漏事件作为应用案例,验证了方法的可行性。 相似文献
70.
利用1991—2020年我国中北亚热带不同烤烟烟区180个国家级气象站气候因子数据,根据影响优质烟叶生长发育的气候条件,分析气候差异性,探究烟叶生长发育的气象规律,应用主成分分析计算各气候因子的权重,依据气候相似原理,对湖北省两大烟区与周边烟区采用改进的欧氏距离作为相似度量指标,并与系统聚类结果进行相似性对比。结果表明:鄂西南、鄂西北、豫西、豫南、豫中、湘西、湘中烟区基本具备了生产优质烟叶的气候条件,除降水量外,其余各气候因子在不同发育期的差值伸根期最大,旺长期次之,成熟期最小。鄂西北和陕南烟区的气候因子最为接近,鄂西南和湘西、川东南烟区的气候因子具有高度相似性,从而为不同区域间借鉴生产经验、优化烤烟种植布局和开发特色优质烟叶提供理论依据。 相似文献