全文获取类型
收费全文 | 639篇 |
免费 | 135篇 |
国内免费 | 180篇 |
专业分类
测绘学 | 67篇 |
大气科学 | 118篇 |
地球物理 | 181篇 |
地质学 | 356篇 |
海洋学 | 79篇 |
天文学 | 19篇 |
综合类 | 54篇 |
自然地理 | 80篇 |
出版年
2024年 | 2篇 |
2023年 | 11篇 |
2022年 | 43篇 |
2021年 | 39篇 |
2020年 | 26篇 |
2019年 | 34篇 |
2018年 | 32篇 |
2017年 | 35篇 |
2016年 | 33篇 |
2015年 | 29篇 |
2014年 | 40篇 |
2013年 | 49篇 |
2012年 | 35篇 |
2011年 | 39篇 |
2010年 | 37篇 |
2009年 | 53篇 |
2008年 | 31篇 |
2007年 | 27篇 |
2006年 | 27篇 |
2005年 | 24篇 |
2004年 | 12篇 |
2003年 | 13篇 |
2002年 | 17篇 |
2001年 | 24篇 |
2000年 | 37篇 |
1999年 | 26篇 |
1998年 | 20篇 |
1997年 | 25篇 |
1996年 | 32篇 |
1995年 | 21篇 |
1994年 | 12篇 |
1993年 | 15篇 |
1992年 | 10篇 |
1991年 | 10篇 |
1990年 | 10篇 |
1989年 | 1篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 7篇 |
1985年 | 3篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有954条查询结果,搜索用时 359 毫秒
31.
Research on land use cover change (LUCC) has reached a mature stage and has formed a relatively complete scientific system. However, most studies analyzed the LUCC process from the view of land use types ignoring the integrality and systematicness of the land use system, which brought certain constraints to understanding the complexity and systematicness of land use process scientifically. Using the Modern Yellow River Delta (MYRD) of China as the study area, this research introduced the complex network analysis method to the study of the LUCC process by using 9 periods of land use data between 1976 and 2014 to build 8 stages of complex networks. The values of node degree, betweenness, changing proportions, and average shortest path in the networks were calculated to identify the key land use types, changing models of land use types, and the stability of land use system. Additionally, main parts of complex networks of the period from 1976 to 1995 and the period from 1995 to 2014 were selected to evaluate the changing characteristics of the LUCC process. The results indicated the area and proportion of natural wetland kept reducing, but the area and proportion of artificial wetland or non-wetland continued rising. The bare land, reed, bush, and cultivated land were the key land use types of the LUCC process. In the past 38 years, beach, bare land, reed and bush had been the output types, and the building land and salt pan had been the input types. The LUCC process has been a transfer process of natural wetland to artificial wetland and non-wetland in the past 38 years, which could be divided into land accretion process (1976 to 1995) and construction process (1995 to 2014). The land ecosystem was unstable for the period from 1990 to 2006. 相似文献
32.
33.
34.
提出了一种利用无人机快速获取的视频数据的可量测立体模型构建方法。首先,通过无人机飞行平台快速获取视频数据,基于改进平板摄像机智能标定技术,获取摄像头的内方位元素,结合摄像机内方位参数、视频帧率、飞行高度、速度、影像分辨率等参数实现了基于实时视频流的关键帧影像自适应自动提取算法,并对提取关键帧影像进行畸变差改正;其次,通过对校正后的影像进行光束法空中三角测量加密处理,计算视频关键帧影像的外方位元素信息。最后,利用北京地区的试验数据进行了验证分析。结果表明利用能快速实现可量测立体模型构建,具有较高的量测精度,能有效的提高应急测绘条件下的灾情地理信息获取,为灾情评估和辅助决策提供依据。 相似文献
35.
Tao Zhang Wutian Cai Yingzhi Li Tingting Geng Zhiyin Zhang Yonggao Lv Miao Zhao Jinwei Liu 《Arabian Journal of Geosciences》2018,11(17):510
In order to study the major ion chemistry and controls of groundwater, 65 groundwater samples were collected and their major ions measured from wells within Lhasa River Basin. Groundwater has the characteristics of slightly alkaline and moderate total dissolved solid (TDS). TDS concentration ranged from 122.0 to 489.9 mg/L with a median value of 271.2 mg/L. Almost all the groundwater samples suited for drinking and irrigation. The major cations of groundwater are Ca2+ and Mg2+, accounting for 59.6 and 31.3% of the cations, respectively. Meanwhile, HCO3? and SO42? constituted about 56.7 and 36.9% of the anions, respectively, in Lhasa River Basin. The hydrochemical type of groundwater is HCO3-SO4-Ca-Mg. The chemical composition of groundwater samples located in the middle of Gibbs model, which indicates that the major chemical process of groundwater is controlled by rock weathering. Carbonate weathering was the dominant hydro-geochemical process controlling the concentration of major ions in groundwater within Lhasa River Basin, but silicate weathering also plays an important role. 相似文献
36.
Precipitation runoff is a critical hillslope hydrological process for downslope streamflow and piedmont/floodplain recharge. Shimen hillslope micro‐catchment is strategically located in the central foothill region of Taihang Mountains, where runoff is crucial for water availability in the piedmont corridors and floodplains of north China. This study analyzes precipitation‐runoff processes in the Shimen hillslope micro‐catchment for 2006–2008 using locally designed runoff collection systems. The study shows that slope length is a critical factor, next only to precipitation, in terms of runoff yield. Regression analysis also shows that runoff is related positively to precipitation, and negatively to slope length. Soil mantle in the study area is generally thin and is therefore not as critical a runoff factor as slope length. The study shows a significant difference between overland and subsurface runoff. However, that between the 0–10 and 10–20 cm subsurfaces is insignificant. Runoff hardly occurs under light rains (<10 mm), but is clearly noticeable under moderate‐to‐rainstorm events. In the hillslope catchment, vertical infiltration (accounting for 42–84% of the precipitation) dominates runoff processes in subsurface soils and weathered granite gneiss bedrock. A weak lateral flow (at even the soil/bedrock interface) and the generally small runoff suggest strong infiltration loss via deep percolation. This is critical for groundwater recharge in the downslope piedmont corridors and floodplains. This may enhance water availability, ease water shortage, avert further environmental degradation, and reduce the risk of drought/flood in the event of extreme weather conditions in the catchment and the wider north China Plain. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
37.
38.
39.
40.
最近完成的岩石力学三轴实验表明,岩石的破坏既能由最大主应力的增加引起,又能由中等主应力的变化(增加或减小)或最小主应力的减小引起,因此联系到地震的发生也是如此。地震前必须有一个应力和能量的增加过程,但地震不一定发生在应力和能量的增加过程中,地震也可以发生在应力和能量的减小过程中。文章还讨论了能够引起地壳中应力减小的作用,並从地震活动性方面探索应力减小引起地震的可能性。 相似文献