全文获取类型
| 收费全文 | 96篇 |
| 免费 | 12篇 |
| 国内免费 | 18篇 |
专业分类
| 测绘学 | 19篇 |
| 大气科学 | 21篇 |
| 地球物理 | 17篇 |
| 地质学 | 27篇 |
| 海洋学 | 28篇 |
| 天文学 | 7篇 |
| 综合类 | 1篇 |
| 自然地理 | 6篇 |
出版年
| 2023年 | 3篇 |
| 2022年 | 5篇 |
| 2021年 | 6篇 |
| 2020年 | 5篇 |
| 2019年 | 9篇 |
| 2018年 | 4篇 |
| 2017年 | 8篇 |
| 2016年 | 2篇 |
| 2015年 | 8篇 |
| 2014年 | 9篇 |
| 2013年 | 9篇 |
| 2012年 | 5篇 |
| 2011年 | 12篇 |
| 2010年 | 8篇 |
| 2009年 | 2篇 |
| 2008年 | 6篇 |
| 2007年 | 4篇 |
| 2006年 | 4篇 |
| 2005年 | 2篇 |
| 2004年 | 1篇 |
| 2003年 | 1篇 |
| 2002年 | 5篇 |
| 2001年 | 3篇 |
| 2000年 | 1篇 |
| 1999年 | 2篇 |
| 1996年 | 1篇 |
| 1995年 | 1篇 |
排序方式： 共有126条查询结果，搜索用时 15 毫秒
71.
Successful development of approaches to quantify impacts of diverse landuse and associated agricultural management practices on ecosystem services is frequently limited by lack of historical and contemporary landuse data. We hypothesized that ground truth data from one year could be used to extrapolate previous or future landuse in a complex landscape where cropping systems do not generally change greatly from year to year because the majority of crops are established perennials or the same annual crops grown on the same fields over multiple years. Prior to testing this hypothesis, it was first necessary to classify 57 major landuses in the Willamette Valley of western Oregon from 2005 to 2011 using normal same year ground-truth, elaborating on previously published work and traditional sources such as Cropland Data Layers (CDL) to more fully include minor crops grown in the region. Available remote sensing data included Landsat, MODIS 16-day composites, and National Aerial Imagery Program (NAIP) imagery, all of which were resampled to a common 30 m resolution. The frequent presence of clouds and Landsat7 scan line gaps forced us to conduct of series of separate classifications in each year, which were then merged by choosing whichever classification used the highest number of cloud- and gap-free bands at any given pixel. Procedures adopted to improve accuracy beyond that achieved by maximum likelihood pixel classification included majority-rule reclassification of pixels within 91,442 Common Land Unit (CLU) polygons, smoothing and aggregation of areas outside the CLU polygons, and majority-rule reclassification over time of forest and urban development areas. Final classifications in all seven years separated annually disturbed agriculture, established perennial crops, forest, and urban development from each other at 90 to 95% overall 4-class validation accuracy. In the most successful use of subsequent year ground-truth data to classify prior year landuse, an overall 57-class accuracy of 75% was achieved despite the omission of 10 entire classes, most of which were annually disturbed or perennial crops grown on very few fields. Synthetic ground-truth data for the 2004 harvest year based on the most common landuse classes over the following 7 years classified 49 of 57 categories at an overall accuracy of 96% in a final version that included CLU polygon majority rule, default smoothing and aggregation, and forcing of urban development and forest from multi-year majority-rule. 相似文献
72.
为了评价MEMS(Micro Electro Mechanical Systems)固体化学微推进阵列的推进性能,对其单元微冲量的精确测试显得尤为重要。在传统冲击摆的基础上,考虑微推进器推力和燃气射流冲击力之间的比例关系,设计了一套适用于MEMS固体化学微推进阵列单元微冲量的间接测试装置,并成功用于6×6规格(单元集成度为36个/cm2)微推进阵列的实际测试中。结果表明:典型实验数据下的待测微冲量为2.5442×10-4Ns,相对误差小于5%;实测8个单元的微冲量平均值为2.5574×10-4 N·s,相对偏差较小,具有很好的重复性。 相似文献
73.
利用2011年春季(5-6月)、2009年夏季(8月)、2010年秋季(11-12月)和2009 年冬季(12月一次年1月)4个航次实测的悬浮体质量浓度资料,分析了东海陆架表层水体总悬浮体、有机悬浮体和无机悬浮体质量浓度的季节分布特征。结果表明,东海陆架表层水体总悬浮体质量浓度冬半年高于夏半年,等值线基本平行于岸线,在浙江沿岸29°N附近有一浑水舌向东南方向延伸,常年存在2个高值中心和2个次高值中心。有机悬浮体质量浓度则为夏半年高于冬半年,存在2个高值中心和2个次高值中心,季节变化显著。无机悬浮体质量浓度分布趋势相似于总悬浮体质量浓度,但整体轮廓线向近岸收缩约0.5°,常年存在2个高值中心和1个次高值中心。有机悬浮体占总悬浮体百分比按季节从高到低依次为春季、夏季、冬季和秋季,其中长江口、杭州湾外近海海域和黑潮区为东海有机悬浮体百分含量高值区。 相似文献
74.
含厚层软弱夹层硬质围岩的变形破坏特性 总被引：1，自引：0，他引：1
以某隧道为背景,通过地质条件分析并结合FLAC3D数值模拟手段,采用对比含有厚层软弱夹层和不含软弱夹层的2个不同模型的方法,从地应力变化、塑性区开展、变形、下覆岩体受压和位移等角度分析了含厚层软弱夹层硬质围岩的变形破坏特性。发现对于含厚层软弱夹层和不含软弱夹层的这2个不同模型,地应力的差别较小,但都为不含节理的岩体的0.5倍或更小,因此认为节理是影响含厚层软弱夹层硬质围岩变性破坏的因素之一;而2个模型在塑性区开展、变形量、下覆岩体受压和位移等方面差别较大,因此认为厚层软弱夹层对硬质围岩的变形破坏起着显著作用。同时通过创建三维模型采用分步开挖的方法,模拟了含厚层软弱夹层硬质围岩的变形破坏过程,分析认为其具有隐蔽性和发生时间不确定性的特点。研究结果可为类似地质情况的隧道建设提供参考。 相似文献
75.
针对一种基于图像处理的岩体结构面迹线半自动检测的方法进行了研究。该法的基本思路是在图像中人工指定少量难以正确识别的主要裂隙控制点,然后将图像一分为二,分别对二者进行边缘检测,得到两张包含结构面迹线和伪边缘的分段线图片,之后根据距离、角度原则分别对两张图片中的分段线进行迭代连接,最终将检测结果叠加,得到完整裂隙迹线检测结果。该法将主要裂隙与次要裂隙区域分开处理,能够更好地减少二者之间的干扰,所以能够更好地保证主次裂隙识别的完整性与正确性,因而具有更高的精确性。 相似文献
76.
为了评价MEMS(Micro Electro Mechanical Systems)固体化学微推进阵列的推进性能,对其单元微冲量的精确测试显得尤为重要。在传统冲击摆的基础上,考虑微推进器推力和燃气射流冲击力之间的比例关系,设计了一套适用于MEMS固体化学微推进阵列单元微冲量的间接测试装置,并成功用于6×6规格(单元集成度为36个/cm2)微推进阵列的实际测试中。结果表明:典型实验数据下的待测微冲量为2.5442×10-4Ns,相对误差小于5%;实测8个单元的微冲量平均值为2.5574×10-4 N·s,相对偏差较小,具有很好的重复性。 相似文献
77.
近年来,多智能体系统的协调控制在多机器人合作控制、交通车辆控制、无人飞机编队和网络的资源分配等领域有着广泛的应用,成为当前控制学科的一个热点问题.首先介绍了多智能体系统的研究背景、智能体的概念和相关的图论知识;然后从多智能体系统协调控制包含的几个问题入手,即群集问题、编队控制问题、一致性问题和网络优化问题等,对其国内外的发展现状进行了总结和分析;最后,给出了多智能体系统有待解决的一些问题,以促进对多智能体系统协调控制理论与应用的进一步研究. 相似文献
78.
无线隐蔽通信是将消息隐藏在无线通信数据帧和信号中进行隐蔽传输的技术,属于无线通信和信息隐藏技术的交叉领域,其理论与技术的研究方兴未艾.本文分析了无线隐蔽通信的系统模型和主要能力要素,总结归纳了噪声式隐蔽信道容量的理论以及链路层、编码层、调制层和信号层中无线隐蔽通信技术的研究进展,并指出了需要进一步研究的问题. 相似文献
79.
State parameter, ψ, has been widely used to combine the influence of void ratio, e, and stress level, p′, on the soils behavior. Stress ratio, Rs, and modified state parameter, ψm, have also been proposed for the same purpose. This paper aims to evaluate and compare the different state indices in combining the effect of fines content, density and stress level for five different types of sands, by processing a large number of previously published experimental data. The use of the recently established concept of equivalent interparticle void ratio, e⁎, in definition of the state indices is also evaluated. The results indicate that the influence of fines presence, in addition to the e and p′, on the behavior is favorably reflected by the state indices. Unique correlations were derived between the cyclic or monotonic strength and each of the state indices, independent of the fines content. The correlations, for all the different types of soils, fell into limited types of common formulations. ψ and Rs worked generally better than ψm, whether defined in terms of e or e⁎. The extension of straight part of critical state line was found to be an appropriate reference line for calculating Rs used in conjunction with e or e⁎. 相似文献
80.
In most countries worldwide regulatory bodies set effluent discharge limits into rivers and other natural water bodies. These limits specify the maximum permissible concentration of defined pollutants that may be discharged into the water body. This limit is conceptually based on the self-purification (assimilative) capacity of the receiving water. However, this self-purification constant is itself a function of the water's pollutant loading. Umguza River situated south west of Zimbabwe, is fed by tributaries that drain an urban catchment and as such is prone to pollution due to human activities in the catchment. This study investigated the levels of pollution in Umguza River that would affect its self-purification capacity. This was achieved by characterising the spatial distribution of a selected range of water quality parameters as well as determining the self-purification capacity of a stretch of the river. Critical pollutant concentrations were determined for some of the parameters that showed high values along the stretch. The selected parameters of interest were dissolved oxygen, suspended solids, phosphates, nitrates, COD, turbidity, ammonia, pH, alkalinity and temperature. The study was carried out from January 2014 to April 2014. The self-purification capacity was determined using a formula that compares the mass flux of a pollutant upstream and downstream of the selected stretch of the river. Statistical analysis was used to establish relationships between the pollutants and the self-purification capacity of the river. The study found that the levels of ammonia and phosphates were very high compared to the regulated limits (2 mg/l vs 0.5 mg/l; and 8 mg/l vs 0.5 mg/l respectively). It was also found that the self-purification capacity varied significantly across pollutants. It was therefore concluded that a critical pollutant concentration exists above which the river completely loses its natural ability to assimilate and decrease its pollutant load over time. It was also concluded that the self-purification capacity depends on the pollutant of concern in the river. It is recommended that the self-purification capacity of a river be determined before regulatory bodies set effluent discharge limits. It is also recommended that the water quality of water bodies draining pollution prone catchments be monitored regularly, besides just monitoring the discharge points. 相似文献