全文获取类型
收费全文 | 8250篇 |
免费 | 1417篇 |
国内免费 | 2001篇 |
专业分类
测绘学 | 416篇 |
大气科学 | 1979篇 |
地球物理 | 2092篇 |
地质学 | 4212篇 |
海洋学 | 865篇 |
天文学 | 387篇 |
综合类 | 805篇 |
自然地理 | 912篇 |
出版年
2024年 | 33篇 |
2023年 | 170篇 |
2022年 | 379篇 |
2021年 | 420篇 |
2020年 | 338篇 |
2019年 | 342篇 |
2018年 | 448篇 |
2017年 | 412篇 |
2016年 | 467篇 |
2015年 | 356篇 |
2014年 | 471篇 |
2013年 | 464篇 |
2012年 | 363篇 |
2011年 | 391篇 |
2010年 | 433篇 |
2009年 | 425篇 |
2008年 | 419篇 |
2007年 | 374篇 |
2006年 | 342篇 |
2005年 | 309篇 |
2004年 | 219篇 |
2003年 | 264篇 |
2002年 | 263篇 |
2001年 | 226篇 |
2000年 | 291篇 |
1999年 | 412篇 |
1998年 | 373篇 |
1997年 | 385篇 |
1996年 | 342篇 |
1995年 | 279篇 |
1994年 | 254篇 |
1993年 | 187篇 |
1992年 | 168篇 |
1991年 | 128篇 |
1990年 | 85篇 |
1989年 | 97篇 |
1988年 | 84篇 |
1987年 | 57篇 |
1986年 | 42篇 |
1985年 | 36篇 |
1984年 | 21篇 |
1983年 | 25篇 |
1982年 | 10篇 |
1981年 | 14篇 |
1980年 | 21篇 |
1979年 | 10篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1958年 | 9篇 |
1954年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
选取浮游植物初级产量、浮游植物现存量、有机物耗氧量、总氮、无机氮、总磷、活性磷等七项指标,由德尔菲法确定各指标的类别界限值和权重,用灰色系统理论的定权灰色聚类方法,借用计算机对23个湖泊和水库营养类型主要参数作灰色聚类分析,以建立一种划分我国湖泊和水库营养类型的有效方法,作为渔业生产的依据。 相似文献
32.
Farmers along the Amazon River each year face multiple natural hazards that threaten crop production and limit the potential for agricultural development of the expansive floodplain and active channel. In this paper we report the findings of a study of natural hazard-related risk associated with rice production on silt bars in the active channel of the Amazon River near Iquitos, Peru. Data were gathered in four rice producing communities in 2014 using household surveys (n = 83 households), focus group discussions, surveying of land elevations along the Amazon River, and interpretation of remote sensing imagery. The probability, extent, and severity of rice crop shortfalls were estimated for recent production years and the economic losses to farming households were also assessed. Our findings point to a very high risk of crop shortfalls due to natural hazards, suggesting that a good year brings rice farmers bounty and a bad year, near penury. River stage reversals (repiquetes) and edaphic conditions were found to be more problematic than the often cited hazard of high and/or early floods. Also surprisingly, farmers’ perceptions of hazards and risk diverged markedly from actual shortfalls experienced during the production years studied. Our results provide the first quantitative estimates of risk due to the multiple natural hazards along the Amazon River and point to the need to assist lowland farmers with risk mitigation so as to unlock the considerable potential of Amazon floodlands for agricultural production. 相似文献
33.
利用巴丹吉林沙漠北缘拐子湖流沙下垫面2013年7、10月和2014年1、4月的湍流通量资料,计算并分析了研究区近地层湍流强度,同时针对风速分量、温度、水汽和CO2归一化标准差随稳定度的变化关系和总体输送系数等陆面过程特征进行分析。结果表明:(1)风速各分量的湍流强度均随风速的增加逐渐减小,风速处于2 m·s-1以下时湍流发展最为旺盛。湍流强度主要由水平方向风速分量决定,垂直方向风速的作用较小,且近中性和不稳定层结利于湍流的发展。与其他地区相比,平坦且没有建筑物的沙漠地区,机械湍流较弱,湍流强度相应较小。(2)风速各分量的归一化标准差与稳定度(z/L)均满足1/3次方函数规律,其中垂直方向风速分量的拟合曲线方程较好。(3)动量输送系数Cd具有明显的夏季高、冬季低的变化状态且各月的日变化形态均呈夜间低、日间高的循环形态。热量输送系数Ch的不同月份日变化间并没有明显的排列次序,且日出日落前后具有明显的波动。不稳定层结时,Cd和Ch均随风速的增加逐渐减小;稳定层结时,Cd和Ch均随着风速的增加逐渐上升。 相似文献
34.
STUDY ON GIS FOR YIELD ESTIMATION BY REMOTE SENSING IN JILIN MAIZE BELTSTUDYONGISFORYIELDESTIMATIONBYREMOTESENSINGINJILINMAIZ... 相似文献
35.
使用紫金山天文台青海观测站13.7米毫米波射电望远镜,于1996年12月10日至1997年1月2日和1997年3月25日至1997年4月4日对海尔-波普彗星的CO分子J=1—0转动跃迁谱线(频率为115.27120GHz)进行了观测.观测谱线表明,CO分子相对于地心的速度比彗星整体相对于地心的速度要小些,即有蓝移现象.这反映了CO分子是由该彗星迎着太阳的面以一定的速度产生出来的.从观测谱线中还初步估算了该彗星CO分子的产生速率. 相似文献
36.
本文主要论述用25米射电望远镜和自相关频谱仪于1986年4月13—17日(UT)期间在我国陕西眉县首次对哈雷彗星羟基分子1667 MHz射电话线的观测及归算结果。由谱线轮廓的幅度、宽度、形状和面积相继导出的谱线强度为-2.4Jy,膨胀速度为0.92kms~(-1),速度的一级矩为0.20kms~(-1),以及羟基的母分子产生率为1.7×10~(29)mols~(-1)。文中所给出的宇宙羟基源W3和W12的观测结果,分别用来检验设备系统探测射电谱线的功能及对哈雷彗星的羟基谱线进行强度定标。这些结果均同理论预期或以往的有关观测相符合。 相似文献
37.
Regional land use changes are an important part of global changes. The research on land use changes in the Three Gorges Reservoir
Area of China attracts a lot of attention owing to the Three Gorges Dam building. The Three Gorges Reservoir Area becomes
one of the important research areas. This study analyzed the transforming processes and traits of each land use type and the
regional differences of land use changes during the past 30 years, summarized the distribution of different land use types
in different buffer zones and regresses the equation areas and different buffer distances based on buffer analyses and regression
analyses, and then analyzed the transforming rules in different buffer distances, got the optimal influence distances. The
research results indicate that, (1) cultivated land lies at the northwest of the reservoir and was decreasing, however, the
construction land was increasing, especially the urban construction land, a large number of land was flooded because of the
reservoir water level rise; (2) urban area was sprawling quickly in developed and neighboring areas, and a great deal of cultivated
land and a considerable amount of grassland were occupied; in the earlier time, rural settlements occupied lots of cultivated
land and a sum of forestry land in the later time; (3) the optimum influenced distances for cultivated land and forestry land
were 10–35 km, and for urban and rural settlements were in 5–20 km. Overall, this research can reflect the spatial-temporal
characteristics of land use changes during the 30 years, and it is helpful for urban planning and land use planning in the
reservoir area. 相似文献
38.
安徽铜陵新桥矿区二叠系栖霞组底部和石炭系黄龙组—船山组之间产出层状、似层状菱铁矿矿层。开展菱铁矿矿层成因研究对于深入剖析区域层控矽卡岩型铜铁矿床成矿机制具有重要意义。本文利用粉晶X射线衍射(XRD)、扫描电镜(SEM)对菱铁矿矿石进行矿物学研究,结果发现菱铁矿矿石主要由菱铁矿、石英、伊利石和有机质等组成,菱铁矿颗粒粒径较小,表面具有成岩自生的自形石英硬模的微结构,SEM原位微区成分分析显示菱铁矿中除了主量元素铁,还含有大量的锰、锌和钙。矿石中存在两种微结构和不同成因的石英:表面具菱铁矿硬模和次生加大结构的碎屑石英;具六方双锥、单锥以及生物成因球形的自生石英。菱铁矿矿石的组成和矿物表面微结构表明其为沉积成因,非岩浆热液起源。富有机质和亚铁的沉积菱铁矿层和沉积胶状黄铁矿层协同作用,可能是铜陵地区乃至长江中下游成矿带层状铜铁矿床层控性重要制约因素,以及可能作为燕山期中酸岩浆演化的氧化性含铜成矿流体卸载成矿的地球化学还原障。
相似文献39.
With the rapid development of the marine economy industry, human exploitation of marine resources is increasing, which is contributing to the growing trend of eutrophication and frequent occurrence of red tide. Accordingly, investigations of seawater quality have attracted a great deal of attention. This study was conducted to construct a seawater environmental quality assessment model based on the variable fuzzy recognition model. The uncertainty and ambiguity of the seawater quality assessment were then considered, combining the monitoring values of evaluation indicators with the standard values of seawater quality. Laizhou Bay was subsequently selected for a case study. In this study, the correct variable model for different parameters was obtained according to the linear and nonlinear features of evaluation objects. Application of the variable fuzzy recognition model for Laizhou Bay, water quality evaluation and comparison with performance obtained using other approaches revealed that the generated model is more reliable than traditional methods, can more reasonably determine the water quality of various samples, and is more suitable for evaluation of a multi-index, multi-level, nonlinear marine environment system; accordingly, the generated model will be an effective tool for seawater quality evaluation. 相似文献
40.
Near-source noise suppression of AMT by compressive sensing and mathematical morphology filtering 总被引:5,自引:0,他引:5
Guang Li Xiao Xiao Jing-Tian Tang Jin Li Hui-Jie Zhu Cong Zhou Fa-Bao Yan 《应用地球物理》2017,14(4):581-589
In deep mineral exploration, the acquisition of audio magnetotelluric (AMT) data is severely affected by ambient noise near the observation sites; This near-field noise restricts investigation depths. Mathematical morphological filtering (MMF) proved effective in suppressing large-scale strong and variably shaped noise, typically low-frequency noise, but can not deal with pulse noise of AMT data. We combine compressive sensing and MMF. First, we use MMF to suppress the large-scale strong ambient noise; second, we use the improved orthogonal match pursuit (IOMP) algorithm to remove the residual pulse noise. To remove the noise and protect the useful AMT signal, a redundant dictionary that matches with spikes and is insensitive to the useful signal is designed. Synthetic and field data from the Luzong field suggest that the proposed method suppresses the near-source noise and preserves the signal well; thus, better results are obtained that improve the output of either MMF or IOMP. 相似文献