全文获取类型
收费全文 | 4564篇 |
免费 | 496篇 |
国内免费 | 597篇 |
专业分类
测绘学 | 662篇 |
大气科学 | 416篇 |
地球物理 | 1127篇 |
地质学 | 1246篇 |
海洋学 | 392篇 |
天文学 | 1283篇 |
综合类 | 171篇 |
自然地理 | 360篇 |
出版年
2024年 | 31篇 |
2023年 | 53篇 |
2022年 | 95篇 |
2021年 | 108篇 |
2020年 | 153篇 |
2019年 | 152篇 |
2018年 | 121篇 |
2017年 | 106篇 |
2016年 | 151篇 |
2015年 | 156篇 |
2014年 | 225篇 |
2013年 | 296篇 |
2012年 | 183篇 |
2011年 | 249篇 |
2010年 | 223篇 |
2009年 | 326篇 |
2008年 | 296篇 |
2007年 | 334篇 |
2006年 | 346篇 |
2005年 | 288篇 |
2004年 | 251篇 |
2003年 | 227篇 |
2002年 | 205篇 |
2001年 | 169篇 |
2000年 | 159篇 |
1999年 | 144篇 |
1998年 | 152篇 |
1997年 | 75篇 |
1996年 | 84篇 |
1995年 | 64篇 |
1994年 | 44篇 |
1993年 | 47篇 |
1992年 | 39篇 |
1991年 | 12篇 |
1990年 | 25篇 |
1989年 | 13篇 |
1988年 | 11篇 |
1987年 | 12篇 |
1986年 | 8篇 |
1985年 | 5篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 6篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1977年 | 2篇 |
1973年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有5657条查询结果,搜索用时 15 毫秒
991.
It is the goal of remote sensing to infer information about objects or a natural process from a remote location. This invokes that uncertainty in measurement should be viewed as central to remote sensing. In this study, the uncertainty associated with water stages derived from a single SAR image for the Alzette (G.D. of Luxembourg) 2003 flood is assessed using a stepped GLUE procedure. Main uncertain input factors to the SAR processing chain for estimating water stages include geolocation accuracy, spatial filter window size, image thresholding value, DEM vertical precision and the number of river cross sections at which water stages are estimated. Initial results show that even with plausible parameter values uncertainty in water stages over the entire river reach is 2.8 m on average. Adding spatially distributed field water stages to the GLUE analysis following a one-at-a-time approach helps to considerably reduce SAR water stage uncertainty (0.6 m on average) thereby identifying appropriate value ranges for each uncertain SAR water stage processing factor. For the GLUE analysis a Nash-like efficiency criterion adapted to spatial data is proposed whereby acceptable SAR model simulations are required to outperform a simpler regression model based on the field-surveyed average river bed gradient. Weighted CDFs for all factors based on the proposed efficiency criterion allow the generation of reliable uncertainty quantile ranges and 2D maps that show the uncertainty associated with SAR-derived water stages. The stepped GLUE procedure demonstrated that not all field data collected are necessary to achieve maximum constraining. A possible efficient way to decide on relevant locations at which to sample in the field is proposed. It is also suggested that the resulting uncertainty ranges and flood extent or depth maps may be used to evaluate 1D or 2D flood inundation models in terms of water stages, depths or extents. For this, the extended GLUE approach, which copes with the presence of uncertainty in the observed data, may be adopted. 相似文献
992.
A parametric study of 13 608 ductile moment‐resisting steel frames designed according to Eurocodes 3 and 8 is performed. A flowchart for the evaluation of the seismic‐resistant capacity of the designed frames is developed based on the N2 method. The design structural overstrength, ductility supply, plastic redistribution parameter, supply reduction factor and performance ratio of the frames are analysed. We determine that the frames have performance ratios higher than 1, mostly due to high values of design structural overstrength, showing that the seismic supply produced by the restraints of Eurocodes 3 and 8 is always higher than the seismic demand. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
993.
Within the framework of ecosystem-based management, we focused on the use of seasonal closures as effective measures to minimise the degradation of benthic communities by trawling. These closures imply the complete cessation of trawling fleet activity and are commonly used in the Mediterranean to reduce the annual fishing effort, with the ultimate goal of effective resource management. In this study, we aimed to investigate how epibenthic communities respond to seasonal closures. The potential benefits of short-term annual closures in two Mediterranean fishing grounds were evaluated by analysing changes in community structure and composition that were linked to the closure. A decrease of faunal abundance was observed with the resumption of fishing activity after the closure at both fishing grounds. Remarkably, results indicated that some large and mobile fauna were able to respond to these closures. We concluded that the currently planned closures are too short to benefit benthic communities. 相似文献
994.
Using aeromagnetic data acquired in the area from the Cerro Prieto geothermal field, we estimated the depth to the Curie point isotherm, interpreted as the base of the magnetic sources, following statistical spectral-based techniques. According to our results the Curie point isotherm is located at a depths ranging from 14 to 17 km. Our result is somewhat deeper than that obtained previously based only in 2-D and 3-D forward modeling of previous low-quality data. However, our results are supported by independent information comprising geothermal gradients, seismicity distribution in the crust, and gravity determined crustal thickness. Our results imply a high thermal gradient (ranging between 33 and 38 °C/km) and high heat flow (of about 100 mW/m2) for the study area. The thermal regime for the area is inferred to be similar to that from the Salton trough. 相似文献
995.
陕西省地震局研制的一套软件解决了地震前兆数字化和大华北资料共享后的大量数据在分析预报中的快速应用问题。内容有“九五”、“十五”、模拟和大华北资料的下载、处理、保存、计算、统计和制图等功能。采取了简化操作、成批处理、自动成图等方法,有效地提高了分析速度,与Mapsis互相兼容,取长补短,能在20分钟内完成地震趋势会商前的准备工作,在震情分析中发挥了良好的作用。 相似文献
996.
Bellie Sivakumar Ashish Sharma 《Stochastic Environmental Research and Risk Assessment (SERRA)》2008,22(4):451-459
High-resolution temporal rainfall data sequences serve as inputs for a range of applications in planning, design and management
of small (especially urban) water resources systems, including continuous flow simulation and evaluation of alternate policies
for environmental impact assessment. However, such data are often not available, since their measurements are costly and time-consuming.
One alternative to obtain high-resolution data is to try to derive them from available low-resolution information through
a disaggregation procedure. This study evaluates a random cascade approach for generation of high-resolution rainfall data
at a point location. The approach is based on the concept of scaling in rainfall, or, relating the properties associated with
the rainfall process at one temporal scale to a finer-resolution scale. The procedure involves two steps: (1) identification
of the presence of scaling behavior in the rainfall process; and (2) generation of synthetic data possessing same/similar
scaling properties of the observed rainfall data. The scaling identification is made using a statistical moment scaling function,
and the log–Poisson distribution is assumed to generate the synthetic rainfall data. The effectiveness of the approach is
tested on the rainfall data observed at the Sydney Observatory Hill, Sydney, Australia. Rainfall data corresponding to four
different successively doubled resolutions (daily, 12, 6, and 3 h) are studied, and disaggregation of data is attempted only
between these successively doubled resolutions. The results indicate the presence of multi-scaling behavior in the rainfall
data. The synthetic data generated using the log–Poisson distribution are found to exhibit scaling behaviors that match very
well with that for the observed data. However, the results also indicate that fitting the scaling function alone does not
necessarily mean reproducing the broader attributes that characterize the data. This observation clearly points out the extreme
caution needed in the application of the existing methods for identification of scaling in rainfall, especially since such
methods are also prevalent in studies of the emerging satellite observations and thus in the broader spectrum of hydrologic
modeling. 相似文献
997.
脆性岩石侧向变形特征及损伤机理研究 总被引:4,自引:1,他引:3
进行了三峡花岗岩常规三轴压缩、保持轴向应变和保持轴向应力的卸围压试验,研究了脆性岩石在不同应力路径和不同加载控制方式下的侧向变形特征,可见临界侧向应变均稳定在(?0.004 ? 0.000 5)范围内。进一步进行三峡花岗岩的全过程应力-应变曲线和损伤力学分析,发现脆性岩石在不同应力路径和不同加载控制方式下均以侧向损伤为主,且侧向损伤曲线的形态近似,达到临界破坏状态时损伤值稳定在0.7?0.8左右。最后以侧向损伤变量表征花岗岩脆性破坏过程,建立了基于应变空间的、可以考虑卸荷应力路径的损伤模型和应变型破坏准则。 相似文献
998.
999.
石英尾砂对旱地表土磁性的影响及其环境意义 总被引:1,自引:0,他引:1
对安徽省凤阳县石英尾砂、受尾砂影响的表土、旱地表土和城镇表土进行了磁学测量,结果表明这4类样品的磁性特征明显不同。尽管这4类样品的磁化率均由磁铁矿主导,但旱地表土中可能含少量磁赤铁矿,石英尾砂、受尾砂影响的表土和城镇表土中可能含少量铁的氢氧化物(如纤铁矿),受尾砂影响的表土中还含少量赤铁矿。城镇表土中磁铁矿的含量明显高于其他3类样品,受尾砂影响的表土中的磁铁矿含量介于石英尾砂和旱地表土之间。城镇表土中磁铁矿的粒径范围较宽,小于1 μm,石英尾砂中磁铁矿的粒径大于0.1 μm,旱地表土中磁铁矿的粒径小于0.1 μm,受尾砂影响的表土中磁铁矿的粒径介于旱地表土与石英尾砂的粒径之间。城镇表土的磁化率与频率磁化率呈负相关,旱地表土的磁化率与频率磁化率呈正相关,而受石英尾砂影响的表土的磁化率与频率磁化率没有明显的相关关系。在城市工业和交通等人类活动的影响下,城镇表土的磁性较旱地表土增强。而在石英砂生产这一人类活动的影响下,受尾砂影响的表土的磁性较旱地表土减弱,这是由石英尾砂中磁性矿物的含量较旱地表土4的少所致。 相似文献
1000.