全文获取类型
收费全文 | 4272篇 |
免费 | 975篇 |
国内免费 | 1301篇 |
专业分类
测绘学 | 845篇 |
大气科学 | 238篇 |
地球物理 | 421篇 |
地质学 | 2471篇 |
海洋学 | 114篇 |
天文学 | 4篇 |
综合类 | 1065篇 |
自然地理 | 1390篇 |
出版年
2024年 | 6篇 |
2023年 | 56篇 |
2022年 | 158篇 |
2021年 | 199篇 |
2020年 | 206篇 |
2019年 | 209篇 |
2018年 | 194篇 |
2017年 | 233篇 |
2016年 | 241篇 |
2015年 | 266篇 |
2014年 | 357篇 |
2013年 | 303篇 |
2012年 | 446篇 |
2011年 | 485篇 |
2010年 | 413篇 |
2009年 | 304篇 |
2008年 | 323篇 |
2007年 | 315篇 |
2006年 | 337篇 |
2005年 | 284篇 |
2004年 | 278篇 |
2003年 | 194篇 |
2002年 | 147篇 |
2001年 | 131篇 |
2000年 | 99篇 |
1999年 | 91篇 |
1998年 | 55篇 |
1997年 | 54篇 |
1996年 | 31篇 |
1995年 | 21篇 |
1994年 | 34篇 |
1993年 | 21篇 |
1992年 | 12篇 |
1991年 | 8篇 |
1990年 | 5篇 |
1989年 | 6篇 |
1988年 | 8篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1983年 | 2篇 |
排序方式: 共有6548条查询结果,搜索用时 953 毫秒
21.
青岛胶州湾3.2级地震构造背景与控震断裂 总被引:1,自引:0,他引:1
本文应用构造分析的方法 ,对青岛胶州湾 3.2级地震发生的地震地质背景和构造背景进行分析。结果表明 :胶州湾 3.2级地震主要受 NEE向郝官庄断裂和 NNW向大沽河断裂控制 ,并根据现代活动断裂的标志 ,对上述 2组断裂活动性特征作了阐述 相似文献
22.
The dimensions of sand ripples in full-scale oscillatory flows 总被引:1,自引:0,他引:1
T. O'Donoghue J.S. Doucette J.J. van der Werf J.S. Ribberink 《Coastal Engineering》2006,53(12):997-1012
New large-scale experiments have been carried out in two oscillatory flow tunnels to study ripple regime sand suspension and net sand transport processes in full-scale oscillatory flows. The paper focuses on ripple dimensions and the new data are combined with existing data to make a large dataset of ripple heights and lengths for flows with field-scale amplitudes and periods. A feature of the new experiments is a focus on the effect of flow irregularity. The combined dataset is analysed to examine the range of hydraulic conditions under which oscillatory flow ripples occur, to examine the effects of flow irregularity and ripple three-dimensionality on ripple dimensions and to test and improve existing methods for predicting ripple dimensions.The following are the main conclusions. (1) The highest velocities in a flow time-series play an important role in determining the type of bedform occurring in oscillatory flow. Bedform regime is well characterised by mobility number based on maximum velocity in the case of regular flow and based on the mean of the highest one tenth peak velocities in the case of irregular flow. (2) For field-scale flows, sand size is the primary factor determining whether equilibrium ripples will be 2D or 3D. 2D ripples occur when the sand D50 ≥ 0.30 mm and 3D ripples occur when D50 ≤ 0.22 mm (except when the flow orbital diameter is low). (3) Ripple type (2D or 3D) is the same for regular and irregular flows and ripple dimensions produced by equivalent regular and irregular flows follow a similar functional dependence on mobility number, with mobility number based on maximum velocity in the case of regular flow and based on the mean of the highest one tenth velocities in the case of irregular flow. For much of the ripple regime, ripple dimensions have weak dependency on mobility number and ripple dimensions are similar for regular and irregular flows with the same flow orbital amplitude. However, differences in ripples produced by equivalent regular and irregular flows become significant at the high mobility end of the ripple regime. (4) Ripple dimensions predicted using the Wiberg and Harris formulae are in poor agreement with measured ripple dimensions from the large-scale experiments. Predictions based on the Mogridge et al. and the Nielsen formulae show better overall agreement with the data but also show systematic differences in cases of 3D ripples and ripples generated by irregular flows. (5) Based on the combined large-scale data, modifications to the Nielsen ripple dimension equations are proposed for the heights and lengths of 2D ripples. The same equations apply to regular and irregular flows, but with mobility number appropriately defined. 3D ripples are generally smaller than 2D ripples and estimates of 3D ripple height and length may be obtained by applying multipliers of 0.55 and 0.73 respectively to the 2D formulae. The proposed modified Nielsen formulae provide an improved fit to the large-scale data, accounting for flow irregularity and ripple three-dimensionality. 相似文献
23.
24.
25.
26.
27.
28.
以烟台市为例,从生态环境协调性、经济可行性、社会可接受性等方面,构建了2004年城市土地可持续利用评价的指标体系,采用特尔斐等方法计算了其可持续利用度。结果表明,烟台城市土地利用目前仍处于可持续利用起步阶段。 相似文献
29.
GM(1,1)动态模型在吴江市地下水水位预测中的应用 总被引:1,自引:0,他引:1
以吴江市地下水水位预测为例,详细阐述了地下水水位时间序列的GM(1,1)动态模型的原理和建立过程,并根据模型的预测值和实测值,对模型的精度进行了检验,结果表明,模型的预测精度达到了99.27%,等级属于Ⅰ级,具有实际的应用价值,为地下水资源的科学管理提供了依据。 相似文献
30.