全文获取类型
收费全文 | 2248篇 |
免费 | 100篇 |
国内免费 | 522篇 |
专业分类
测绘学 | 62篇 |
大气科学 | 250篇 |
地球物理 | 1202篇 |
地质学 | 708篇 |
海洋学 | 298篇 |
天文学 | 93篇 |
综合类 | 99篇 |
自然地理 | 158篇 |
出版年
2023年 | 18篇 |
2022年 | 19篇 |
2021年 | 29篇 |
2020年 | 18篇 |
2019年 | 19篇 |
2018年 | 24篇 |
2017年 | 7篇 |
2016年 | 33篇 |
2015年 | 24篇 |
2014年 | 49篇 |
2013年 | 46篇 |
2012年 | 119篇 |
2011年 | 166篇 |
2010年 | 42篇 |
2009年 | 61篇 |
2008年 | 144篇 |
2007年 | 145篇 |
2006年 | 142篇 |
2005年 | 149篇 |
2004年 | 135篇 |
2003年 | 90篇 |
2002年 | 102篇 |
2001年 | 92篇 |
2000年 | 100篇 |
1999年 | 56篇 |
1998年 | 49篇 |
1997年 | 63篇 |
1996年 | 63篇 |
1995年 | 71篇 |
1994年 | 50篇 |
1993年 | 86篇 |
1992年 | 74篇 |
1991年 | 94篇 |
1990年 | 82篇 |
1989年 | 56篇 |
1988年 | 58篇 |
1987年 | 42篇 |
1986年 | 35篇 |
1985年 | 31篇 |
1984年 | 24篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 13篇 |
1980年 | 22篇 |
1978年 | 5篇 |
1976年 | 6篇 |
1974年 | 8篇 |
1948年 | 8篇 |
1947年 | 8篇 |
1943年 | 4篇 |
排序方式: 共有2870条查询结果,搜索用时 31 毫秒
971.
Coastal wetlands present particular challenges for coastal governance and for the implementation of the Ramsar Convention, not least because coastal areas are focal points of human activity and of governance ambiguity. Through the evaluation of Ramsar delivery at both national and local levels in Japan and England, the relationship between Ramsar implementation and coastal governance was examined. In England, Ramsar status is primarily treated as a nature conservation designation which limits the wider opportunities inherent in the designation. In contrast, in Japan, the Ramsar Convention is used as a policy driver at the national level and as a leverage to encourage citizen engagement, economic benefit, and wetland conservation at the local level. It was concluded that through the implementation of the Ramsar Convention in important coastal wetland areas, significant steps can be taken towards delivering integrated approaches to coastal governance. 相似文献
972.
This paper proposes and tests a method of producing macrofauna habitat potential maps based on a weights-of-evidence model (a probabilistic approach) for the Hwangdo tidal flat, Korea. Samples of macrobenthos were collected during field work, and we considered five mollusca species for habitat mapping. A weights-of-evidence model was used to calculate the relative weights of 10 control factors that affect the macrobenthos habitat. The control factors were compiled as a spatial database from remotely sensed data combined with GIS analysis. The relative weight of each factor was integrated as a species potential index (SPI), which produced habitat potential maps. The maps were compared with the surveyed habitat locations, revealing a strong correlation between the potential maps and species locations. The combination of a GIS-based weights-of-evidence model and remote sensing techniques is an effective method in determining areas of macrobenthos habitat potential in a tidal flat setting. 相似文献
973.
ANN application for prediction of atmospheric nitrogen deposition to aquatic ecosystems 总被引:1,自引:0,他引:1
The occurrences of increased atmospheric nitrogen deposition (ADN) in Southeast Asia during smoke haze episodes have undesired consequences on receiving aquatic ecosystems. A successful prediction of episodic ADN will allow a quantitative understanding of its possible impacts. In this study, an artificial neural network (ANN) model is used to estimate atmospheric deposition of total nitrogen (TN) and organic nitrogen (ON) concentrations to coastal aquatic ecosystems. The selected model input variables were nitrogen species from atmospheric deposition, Total Suspended Particulates, Pollutant Standards Index and meteorological parameters. ANN models predictions were also compared with multiple linear regression model having the same inputs and output. ANN model performance was found relatively more accurate in its predictions and adequate even for high-concentration events with acceptable minimum error. The developed ANN model can be used as a forecasting tool to complement the current TN and ON analysis within the atmospheric deposition-monitoring program in the region. 相似文献
974.
最近,在苏门答腊俯冲带展开了一系列相互关联的海上及陆地研究,重点针对印度尼西亚2004年和2005年的板块边界地震破裂。这项由英国(英国苏门答腊联盟)、印度尼西亚、美国、法国和德国的科学家联手开展的研究计划旨在对这一边缘的地壳结构进行成像,以分析沿走向和上倾方向地震破裂 相似文献
975.
引言
2011年3月11日日本本州岛东海岸附近海域发生的M9.0地震(Tohoku earthquake,以下称“东日本大地震”——译者注)及其引发的海啸造成数以万计的人员死亡,造成的财产损失可能超过1万亿美元,这是有历史记录以来最严重的自然灾害之一。 相似文献
976.
The present work explores relations between stream power,braiding intensities and bank erosion in certain stretches of the Brahmaputra River.In this paper,an objective approach is presented to enable quantitative assessment of spatio-temporal behaviour of channel braiding process of the Brahmaputra River by using the Plan Form Index and corresponding estimation of stream power to establish a behavioural pattern of variability of potential energy expenditure.The braiding index is compared for discrete years to understand the morphological behaviour.Subsequently,a real time estimation of stream power for certain stretches of Brahmaputra River is done in order to analyse its variability in braiding intensity and bank erosion.The paper presents the dynamic behaviour of the channel pattern of the Brahmaputra River System in Assam valley of India over a time span of 18 years.The procedure addresses the selection of input parameters from digital satellite images,comprising scenes for the years 1990,1997 and 2007 with specific dates,from Dhubri near Indo-Bangladesh Border to Upper Assam.Deployment of GIS technique has been made to extract the required parameters to derive Plan Form Indices for the entire study reach.Stream power estimation is done for corresponding latest floods and for corresponding dates of image scenes.The study indicated that due to consistent aggradation of riverbed inducing temporal declination of stream power,there is an occurrence of wide spread braiding.This in turn incurs substantial yearly land loss due to bank erosion,caused by flow concentrations due to temporal evolution of multiple channels in the Brahmaputra River. 相似文献
977.
总结了数字化预测钢框架结构倒塌和预测精度通过两个4层标准抗弯框架的1∶8缩尺模型的地震模拟器试验得到证实的研究计划。文中论证了以下几点:(1)由于结构框架和地震地面运动的实际综合作用,可能发生侧倾倒塌;(2)P-Δ效应和构件损坏接近倒塌的结构框架的性能起主导作用;(3)只要构件的损坏在分析模型中有足够的代表性,那么就可以用相对简单的分析模型来合理地预测结构的倒塌;(4)接近倒塌的框架体系对于框架中每个重要构件所经历的受力过程非常敏感,意味着通常用于构件试验的对称循环加载过程并不能为模拟接近倒塌的损坏提供足够的信息。 相似文献
978.
主要研究地面运动的频率成分在非弹性需求下对单自由度(SDF)和多自由度(MDF)钢架结构的作用。通过对文献进行详细的审查,识别出最能代表地面运动频率成分的指标。选择地面运动的平均周期(Tm)是由于它能区别各种不同地面运动的频谱形态及其与震级、距离和场地特征的关系。首先使用一组128次地面运动的记录根据L对单自由度体系在特定延性水平时的非弹性位移要求进行研究。然后通过采用相同的地面运动资料做增量动力分析,把研究扩展到多自由度系统,评价Tm对各种工程需求参数的影响。得到的结果表明,对于单自由度体系,当弹性周期(Te)与Tm的比值低于1时,位移将增大。对于多自由度体系,结果证明了高阶振型对于基底剪力的影响和当Tm接近结构的高阶振型周期时,对最大层间侧移的影响变得更加显著。对单自由度体系和多自由度体系,在非弹性水平较高时这些观测更为明显。研究结果对于欧洲抗震设计法是尤为重要的。 相似文献
979.
Experimental study of friction dissipators for seismic protection of building structures 总被引:1,自引:1,他引:0
This paper presents the results from unidirectional shaking table tests of two reduced scale steel models of a building frame, with one and two floors, respectively. These frames incorporate friction dissipators at every floor. The inputs are sine-dwells and artificial and registered earthquakes. This study is part of a larger research project aiming to assess the seismic efficiency of friction dissipators by means of an integrated numerical and experimental approach. Inside this framework, the main objectives of these experiments are to: (i) collect a wide range of results to calibrate a numerical model derived within the project, (ii) clarify some of the most controversial issues about friction dissipators (including behavior for inputs containing pulses, capacity to reduce resonance peaks, introduction of high frequencies in the response, and self-generated eccentricities), (iii) better understand their dynamic behavior, (iv) provide insight on the feasibility and reliability of using simple friction dissipators for seismic protection of building structures and (v) characterize the hysteretic behavior of these devices. Most of these objectives are satisfactorily reached and relevant conclusions are stated. 相似文献
980.
The study reports for the first time on the heavy metal contamination of the waters surrounding a shipwreck lying on the sea floor. Square wave anodic stripping voltammetry has been used for a survey of the total and dissolved Cd, Pb and Cu contents of the seawater at the site of the sinking of the Nicole M/V (Coastal Adriatic Sea, Italy). Results show that the hulk has a considerable impact as regards all three metals in the bottom water, especially for the particulate fraction concentrations, which increased by factors of ∼9 (Cd), ∼3 (Pb) and ∼5 (Cu). The contaminated plume extended downstream for about 2 miles. Much lower contamination was observed for dissolved bottom concentrations; nevertheless Pb (0.56 ± 0.03 nmol/L) is higher than the Italian legal limits established for 2015 and Cd (0.23 ± 0.03 nmol/L) is very close the limit of Cd will be exceeded if the hulk is not removed. 相似文献