全文获取类型
收费全文 | 121篇 |
免费 | 16篇 |
国内免费 | 13篇 |
专业分类
测绘学 | 14篇 |
大气科学 | 36篇 |
地球物理 | 52篇 |
地质学 | 18篇 |
海洋学 | 12篇 |
天文学 | 1篇 |
综合类 | 2篇 |
自然地理 | 15篇 |
出版年
2023年 | 1篇 |
2022年 | 2篇 |
2021年 | 3篇 |
2020年 | 1篇 |
2019年 | 2篇 |
2018年 | 2篇 |
2017年 | 4篇 |
2016年 | 2篇 |
2015年 | 4篇 |
2014年 | 4篇 |
2013年 | 7篇 |
2012年 | 5篇 |
2011年 | 5篇 |
2010年 | 10篇 |
2009年 | 6篇 |
2008年 | 11篇 |
2007年 | 10篇 |
2006年 | 7篇 |
2005年 | 5篇 |
2004年 | 5篇 |
2003年 | 6篇 |
2002年 | 7篇 |
2001年 | 3篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1998年 | 4篇 |
1997年 | 4篇 |
1996年 | 5篇 |
1995年 | 3篇 |
1994年 | 1篇 |
1993年 | 4篇 |
1992年 | 1篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有150条查询结果,搜索用时 156 毫秒
91.
Sediment incipient motion is a fundamental issue in sediment transport theory and engineering practice. Silt has transitional behavior between cohesive and non-cohesive sediment and its incipient motion is still poorly understood. This study aims to find an expression for incipient motion from silt to sand from a unified perspective and analysis. From the analysis of forces, using the derivation method for the Shields curve, an expression for sediment incipient motion is proposed for both silt and sand under conditions of combined waves and currents. The differences and similarities in the sediment motion threshold were analyzed under the effects of waves and currents, and fine and coarse sediment. The Shields number was revised by introducing the cohesive force and additional static water pressure, which indicates that this study could be seen as an extension of the Shields curve method for silt. A number of experimental datasets as well as field data were used to verify the formula. The effect of bulk density on fine sediment was discussed and tested using experimental data. 相似文献
92.
中国西北不同沙源地区起沙阈值的对比分析与研究 总被引:8,自引:1,他引:7
沙尘粒子进入大气所受到的空气动力因子和阴力因子分别用摩擦速度u,和临界摩擦速度u(?)来定量表示。临界摩擦速度是影响沙尘释放和输送的重要参数。文中利用2004年春季浑善达克沙地和2006年春季黄土高原与戈壁3种不同沙源地区开展的微气象学和沙尘暴观测资料,分析了沙尘天气过程中沙尘浓度随摩擦速度的变化关系,估算了临界起沙摩擦速度u(?),并与不同学者给出的结果进行对比。结果表明:沙尘天气过程中,临近起沙阶段,沙尘浓度较低,摩擦速度增加快,为起沙阶段提供了利的动力条件;起沙阶段,动力和热力湍流混合使得沙尘浓度迅速升高;沙尘天气减弱阶段,由于重力沉隆作用,沙尘浓度随摩擦速度近似呈线性递减关系。以空气中的沙尘浓度大于200μg/m~3作为起沙的判据条件,得到浑善达克沙地和戈壁地区的临界起沙摩擦速度分别为0.6和0.45 m/s。由于受地形及非均匀下垫面的影响,黄土高原的临界起沙摩擦速度与风向有关。西北风反映了来自西北方向毛乌素沙地的影响,u(?)为0.35 m/s,偏南风时对应观测站点南部黄土丘陵沟壑区的起沙特征,u(?)为0.7 m/s。黄土丘陵沟壑区的地表粗糙度值和土壤粒子间的相互作用力较大,从而导致了较大的临界起沙摩擦速度。不同的起沙阈值条件和风速频率分布与沙尘释放能力的不同相对应。 相似文献
93.
A statistical assessment on the stochastic relationship between biomarker concentrations and environmental exposures 总被引:1,自引:1,他引:0
C.-C.?Chen K.-Y.?WuEmail author M. J. W.?Chang 《Stochastic Environmental Research and Risk Assessment (SERRA)》2004,18(6):377-385
Biological monitoring has gradually developed into a powerful tool in the identification and quantification of exposures to occupational and/or environmental hazards in environmental and occupational health studies. Aggregate individual exposure to pollutants and evidence for exploring dose-response relationship in the human bodies can be assessed through biomarker measurements. The existence of inter-individual differences among a study population, however, often hampers the relationship assessment between exposure and the biomarker. In this paper, a statistical random effects model identified from a simplified one-compartment pharmacokinetic model is applied to establish the dynamic relationship between a biomarker and its corresponding external exposure. This model avoids the complex parameter estimation problem encountered using a physiologically based pharmacokinetic (PBPK) model, and incorporates inter-individual variations often ignored in the usual regression approach. In addition, the relevant parameters for the generic kinetic process can be estimated directly. As a guideline for preliminary sampling strategy, tables of required sample sizes and the number of repeated measurements to achieve the desired statistical power and test efficiency are given. The currently established biological exposure indices (BEIs) for benzene and methyl chloroform are employed to illustrate the impact of inter- individual variations on the percentages of protection for workers exposed to the threshold limit value (TLV) of the corresponding chemical.This research was partially supported by the Environmental Protection Agency of Taiwan (Contract No. EPA-88-E102–03–003). 相似文献
94.
This paper presents a way to empirically fit experimental data for the horizontal flux of various sizes of wind-blown dry sand using data from wind tunnel experiments. We measured vertical wind profiles to derive threshold shear velocity and estimated shear velocity and the flux of sand mass as a function of the height for nine different grain sizes. We propose a fitting model based on the experimental data and a least-squares method and derive an explicit form of sand flux as a function of height and shear velocity for these grain sizes. We also obtained an explicit form of the empirical equation for the measurement of sand transport per unit width and unit time by integrating the empirical equation as a function of height. Finally, we compared the effectiveness of Bagnold's equation, Kawamura's expression and Lettau and Lettau's equation, for predicting sand transport with the results of our empirical equation. The results show that the transport predicted by all of the equations were always lower than the measured results from the empirical equation for all grain sizes and shear velocities. However, the empirical equation matched Bagnold's equation, Kawamura's equation, and Lettau and Lettau's equation if the coefficients in these equations were adjusted instead of using their original coefficients. The empirical equation for sand transport in the present study contradicts previous conclusions generated by Bagnold's equation, which predict that for a given wind drag, the transport of a coarse sand is greater than that of a fine sand. 相似文献
95.
A model for presentation of seismic pore water pressures 总被引:3,自引:0,他引:3
Tomislav Ivi 《Soil Dynamics and Earthquake Engineering》2006,26(2-4):191-199
A model for presentation of pore water pressures induced in sand samples during cyclic undrained testing is described. The proposed model belongs to a class of so-called ‘damage parameter models’, which correlate the pore pressure rise with a parameter based on an accumulated variable during testing. The concept of threshold strain is also incorporated in the model. The model has been verified on several series of published cyclic test data. Its parameters lie in a narrow band for a wide range of sand properties. The empirical functions that represent the common shape of individual curves for interpreted pore pressure data are also suggested. The proposed procedure has been adapted for presentation of other cyclic soil tests, and examples of interpretation of cyclic stress-controlled as well as cyclic drained tests are included. 相似文献
96.
自动提取土地利用变化信息后处理方法的研究 总被引:1,自引:1,他引:1
研究目的是对计算机自动提取的土地利用变化信息进行后处理,以提高信息提取精度; 研究方法是模板判断法,该方法有效
地改善了单像元判断的效果,提高了变化信息的提取精度,可以应用于土地利用动态遥感监测。 相似文献
97.
98.
Profile change in a lacustrine multiple-barred nearshore was investigated over the ice-free season of 2001/2002 at Burley Beach on the southeastern shore of Lake Huron in order to identify the feedback mechanisms between the pre-existing morphology and the wave forcing and the consequence of those feedbacks to the behaviour of the nearshore environment. The characteristics of the offshore wave field were monitored using a Falmouth Scientific combined 3D-ACM wave recorder and pressure transducer. Supplemental wave data were downloaded from a 3-m discus buoy operated by the National Data Buoy Center, 75 km to the NW of the study site. The three nearshore bars were in a quasi-equilibrium state through a large part of the ice-free season, with dramatic changes occurring during relatively moderate storm events that followed much larger storms in late October. A comparison of the incident wave field with changes in the nearshore profile through canonical correlation analysis indicates that the morphology responds to the distribution of the significant, root-mean-square (rms) and average wave heights between surveys. The threshold between bar decay and onshore bar migration and growth is associated with the onset of breaking of the rms wave at the bar crest (Hrms hcr−1 ≈0.3–0.4). The threshold between onshore and offshore migration is associated with the onset of breaking of the average wave at the bar crest (Havg hcr−1 ≈0.3–0.4), coincident with complete dissipation of the significant wave over the lakeward slope of the bar (Hs hcr−1 >0.6). Inshore wave data collected during an instrumented study at the same site revealed that the middle and inner bars remained at the threshold of onshore and offshore migration over a wide range of offshore significant wave heights (0.8 to 2.4 m) prior to the October storms. This self-organised equilibrium is a result of changes to the incident wave distribution through breaking on the outer bar. It is concluded that the prediction of bar response requires an understanding of the feedback between the bar and the local wave distribution in addition to an understanding of the feedback associated with the profile as a whole. 相似文献
99.
The objective of this study was the formulation of fluvial morphological regularities for small rivers with a wide range of morphological and geological characteristics in North-Rhine-Westphalia (Germany) based on a statistical research methodology. Such empirical quantitative information on reference conditions is required for the restoration of small rivers in the former highly industrialised Ruhr-Area. Following the approach of some classic empirical works in fluvial morphology of the last century, several natural reference rivers in the entire research area have been observed in order to provide a statistical correlation between independent and dependent morphological variables. Regressions between valley-floor slope, bankfull discharge and stream power on the one hand and several variables describing the longitudinal profile and river planform on the other hand have shown some significant results. The regularities found are a quantitative contribution to the establishment of reference conditions as well as a useful tool for the restoration of small rivers, if the specific properties and values of the underlying random sampling are taken into account. In addition, the relation between stream power and sinuosity shows the likely existence of a threshold: Exceeding a stream power of 100 W/m, the sinuosity decreases after an increase for lower stream power values. Comparable thresholds were found for the relation between stream power and pool depth as well as stream power and step steepness. The thresholds could be explained by a change in the type of energy dissipation, due to different physio-geographical settings in highland rivers within forested v-shaped valleys. Here, large-woody debris seems to increase the channel roughness and possibly replaces the significance of coarse-grained bed material, pool depth and step steepness as contributors to energy dissipation. 相似文献
100.