全文获取类型
收费全文 | 1524篇 |
免费 | 301篇 |
国内免费 | 250篇 |
专业分类
测绘学 | 227篇 |
大气科学 | 288篇 |
地球物理 | 694篇 |
地质学 | 452篇 |
海洋学 | 134篇 |
天文学 | 24篇 |
综合类 | 69篇 |
自然地理 | 187篇 |
出版年
2024年 | 7篇 |
2023年 | 14篇 |
2022年 | 33篇 |
2021年 | 36篇 |
2020年 | 69篇 |
2019年 | 74篇 |
2018年 | 82篇 |
2017年 | 102篇 |
2016年 | 94篇 |
2015年 | 89篇 |
2014年 | 91篇 |
2013年 | 196篇 |
2012年 | 97篇 |
2011年 | 88篇 |
2010年 | 81篇 |
2009年 | 76篇 |
2008年 | 92篇 |
2007年 | 122篇 |
2006年 | 100篇 |
2005年 | 70篇 |
2004年 | 59篇 |
2003年 | 39篇 |
2002年 | 38篇 |
2001年 | 33篇 |
2000年 | 38篇 |
1999年 | 33篇 |
1998年 | 33篇 |
1997年 | 39篇 |
1996年 | 29篇 |
1995年 | 23篇 |
1994年 | 21篇 |
1993年 | 11篇 |
1992年 | 12篇 |
1991年 | 15篇 |
1990年 | 10篇 |
1989年 | 8篇 |
1988年 | 7篇 |
1987年 | 3篇 |
1984年 | 1篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有2075条查询结果,搜索用时 0 毫秒
1.
Average velocity in streams is a key variable for the analysis and modelling of hydrological and hydraulic processes underpinning water resources science and practice. The present study evaluates the impact of the sampling duration on the quality of average velocity measurements acquired with contemporary instruments such as Acoustic Doppler Velocimeters (ADV) an Acoustic Doppler Current Profilers (ADCP). The evaluation combines considerations on turbulent flows and principles and configurations of acoustic instruments with practical experience in conducting customized analysis for uncertainty analysis purposes. The study sheds new insights on the spatial and temporal variability of the uncertainty in the measurement of average velocities due to variable sampling durations acting in isolation from other sources of uncertainties. Sampling durations of 90 and 150 s are found sufficient for ADV and ADCP, respectively, to obtain reliable average velocities in a flow affected only by natural turbulence and instrument noise. Larger sampling durations are needed for measurements in most of the natural streams exposed to additional sources of data variability. 相似文献
2.
Subsurface flow measurements using passive flux meters in variably-saturated cold-regions landscapes
To date, passive flux meters have predominantly been applied in temperate environments for tracking the movement of contaminants in groundwater. This study applies these instruments to reduce uncertainty in (typically instantaneous) flux measurements made in a low-gradient, wetland dominated, discontinuous permafrost environment. This method supports improved estimation of unsaturated and over-winter subsurface flows which are very difficult to quantify using hydraulic gradient-based approaches. Improved subsurface flow estimates can play a key role in understanding the water budget of this landscape. 相似文献
3.
4.
寇新建 《大地测量与地球动力学》1994,(3)
由于观测条件的变化,观测值方差将产生随机波动。本文根据Bayes估计理论,推导了线性模型待估参数的分布函数及观测值方差估计公式,并将其用于位移检验。计算结果表明,在同样的置信度下,本方法更符合实际。文中还对参数估值的统计性质进行了讨论。 相似文献
5.
Qin Chengzhi 《中国地震研究》2004,18(2):178-187
The migration of strong earthquakes is an important research topic because the migration phenomena reflect partly the seismic mechanism and involve the prediction of tendency of seismic activity. Research on migration of strong earthquakes has mostly focused on finding the phenomena. Some attempts on getting regularity were comparatively subjective. This paper suggests that there should be indices of migration in earthquake dataset and the indexes should have statistical meaning if there is regularity in the migration of strong earthquakes. In this study, three derivative attributes of migration, i.e., migration orientation, migration distance and migration time interval, were statistically analyzed. Results in the North China region show that the migration of strong earthquakes has statistical meaning. There is a dominant migration orientation (W by S to E by N), a dominant distance (≤100km and on the confines of 300~700km), and a dominant time interval (≤1a and on the confines of 3~4a). The results also show that the migration will differ slightly with different magnitude range or earthquake activity phase. 相似文献
6.
7.
气相色谱法测定地下水中六六六结果的不确定度评定 总被引:7,自引:4,他引:3
依照《测量不确定度评定与表示》,对气相色谱法测定地下水中六六六(HCH)四种单体结果进行了不确定度评定。分析了测量过程中引入的不确定度来源,包括提取液体积的量取、样品提取溶液的定容体积、分析仪器的进样量、标准系列溶液的测量以及仪器重复测定等分量引入不确定度及其各参数的采集和计算方法,最后合成标准不确定度,通过乘以95%概率下的扩展因子2,获得测量结果的扩展不确定度。 相似文献
8.
9.
10.