全文获取类型
收费全文 | 1990篇 |
免费 | 567篇 |
国内免费 | 723篇 |
专业分类
测绘学 | 37篇 |
大气科学 | 23篇 |
地球物理 | 304篇 |
地质学 | 2300篇 |
海洋学 | 136篇 |
天文学 | 15篇 |
综合类 | 169篇 |
自然地理 | 296篇 |
出版年
2024年 | 3篇 |
2023年 | 24篇 |
2022年 | 68篇 |
2021年 | 97篇 |
2020年 | 88篇 |
2019年 | 111篇 |
2018年 | 72篇 |
2017年 | 55篇 |
2016年 | 104篇 |
2015年 | 120篇 |
2014年 | 153篇 |
2013年 | 149篇 |
2012年 | 126篇 |
2011年 | 132篇 |
2010年 | 166篇 |
2009年 | 157篇 |
2008年 | 166篇 |
2007年 | 175篇 |
2006年 | 145篇 |
2005年 | 148篇 |
2004年 | 130篇 |
2003年 | 101篇 |
2002年 | 113篇 |
2001年 | 96篇 |
2000年 | 87篇 |
1999年 | 98篇 |
1998年 | 66篇 |
1997年 | 62篇 |
1996年 | 57篇 |
1995年 | 40篇 |
1994年 | 36篇 |
1993年 | 38篇 |
1992年 | 17篇 |
1991年 | 22篇 |
1990年 | 7篇 |
1989年 | 10篇 |
1988年 | 16篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 2篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1979年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有3280条查询结果,搜索用时 15 毫秒
91.
92.
93.
94.
Jiang Jiayu Fu Shouxian Li Jiuling 《应用地球物理》2005,2(4):241-246
Branching river channels and the coexistence of valleys, ridges, hiils, and slopes'as the result of long-term weathering and erosion form the unique loess topography. The Changqing Geophysical Company, working in these complex conditions, has established a suite of technologies for high-fidelity processing and fine interpretation of seismic data. This article introduces the processes involved in the data processing and interpretation and illustrates the results. 相似文献
95.
Characteristics and genesis of maghemite in Chinese loess and paleosols: Mechanism for magnetic susceptibility enhancement in paleosols 总被引:4,自引:0,他引:4
Tianhu Chen Huifang Xu Qiaoqin Xie Jun Chen Junfeng Ji Huayu Lu 《Earth and Planetary Science Letters》2005,240(3-4):790-802
Morphological characteristics and microstructures of magnetic minerals extracted from Chinese loess and paleosols were investigated using powder X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Our results indicate that maghemite in loess–paleosol sequences was transformed from magnetite through oxidation of magnetite. Maghemite transformed from eolian magnetite during chemical weathering has low-angle grain boundaries among maghemite nano-crystals. Some nano-crystalline maghemites with nanoporous texture resulted from microbe-induced precipitation of magnetite or transformation of poorly crystalline ferric Fe (oxy)hydroxides in presence of Fe-reducing bacteria. Aggregates of euhedral maghemite nano-crystals were transformed from magnetite magnetosomes. Both microbe-induced nanoporous magnetite and microbe-produced magnetite magnetosomes are directly related to microbial activities and pedogenesis of the paleosols. It is proposed that the formation of nano-crystalline maghemite with superparamagnetic property in paleosol results in the enhancement of magnetic susceptibility, although the total amount (weight percent) of magnetic minerals in both paleosol and loess units is similar. Our results also show that nano-crystalline and nanoporous magnetite grains prefer to transform into maghemite in semi-arid soil environments instead of hematite, although hematite is a thermodynamically stable phase. This result also indicates that a decrease in crystal size will increase stability of maghemite. It is also inferred that surface energy of maghemite is lower than that of hematite. 相似文献
96.
97.
98.
From water to tillage erosion dominated landform evolution 总被引:3,自引:1,他引:3
While water and wind erosion are still considered to be the dominant soil erosion processes on agricultural land, there is growing recognition that tillage erosion plays an important role in the redistribution of soil on agricultural land. In this study, we examined soil redistribution rates and patterns for an agricultural field in the Belgian loess belt. 137Cs derived soil erosion rates have been confronted with historical patterns of soil erosion based on soil profile truncation. This allowed an assessment of historical and contemporary landform evolution on agricultural land and its interpretation in relation to the dominant geomorphic process. The results clearly show that an important shift in the relative contribution of tillage and water erosion to total soil redistribution on agricultural land has occurred during recent decades. Historical soil redistribution is dominated by high losses on steep midslope positions and concavities as a result of water erosion, leading to landscape incision and steepening of the topography. In contrast, contemporary soil redistribution is dominated by high losses on convex upperslopes and infilling of slope and valley concavities as a result of tillage, resulting in topographic flattening. This shift must be attributed to the increased mechanization of agriculture during recent decades. This study shows that the typical topographical dependency of soil redistribution processes and their spatial interactions must be accounted for when assessing landform and soil profile evolution. 相似文献
99.
内蒙古大青山-乌拉山地区孔兹岩系的早期变质地层结构——下地壳近水平顺层滑脱变形的产物 总被引:4,自引:0,他引:4
近年来孔兹岩系作为高级变质岩区一套特征性的变质沉积岩石组合而受到广泛关注,但其变质地层结构及其相关的构造作用和构造样式还不为人所了解。通过对内蒙古大青山一乌拉山地区以孔兹岩系发育为特征的高级变质杂岩的研究发现,区内孔兹岩系自下而上由3个岩石地层单位组成:榴云片麻岩岩组、透辉片麻岩岩组和大理岩岩组,表现出由细碎屑沉积-碎屑沉积和钙质化学沉积的混合物-镁质碳酸盐组成的沉积旋回。这些变质地层不仅遭受到具有近等温降压顺时针P-T演化特征的麻粒岩相变质作用的改造,同时,还遭受到近水平顺层滑脱变形构造、穹-褶构造和近东西向陡倾叶理带这三种构造形式为主的多期变形的改造。由此,这些变质地层在空间上多以不规则条带状、不规则团块状或透镜状形式出露,同时,各地层单位在空间上具有“犬牙交错,参差产出”,“岩层缺失,隔层相触”的特征。研究结果表明,区内孔兹岩系目前的产出状态及“犬牙交错,参差产出”的特征是穹-褶构造尤其是近东西向陡倾叶理带叠加改造的结果,在消除其改造的影响后可以确定,由于近水平顺层滑脱变形改造的结果,这些变质地层发生“岩层缺失,隔层相触”,在空间上表现为以近水平的长轴近东西向的扁豆体堆叠而成的早期变质地层结构,这是高级变质地层所特有的变质地层结构。 相似文献
100.
旋挖钻进技术作为一种高效优质的钻进方法近年来广泛地被用于公路、铁路及大型水电站基础工程施工建设中,取得了良好的社会效益和经济效益。探讨了在卵砾石、漂石等复杂地层以及坚硬基岩中进行旋挖钻进所用钻头的结构及其施工工艺。 相似文献