全文获取类型
收费全文 | 355篇 |
免费 | 20篇 |
国内免费 | 13篇 |
专业分类
测绘学 | 18篇 |
大气科学 | 22篇 |
地球物理 | 119篇 |
地质学 | 180篇 |
海洋学 | 10篇 |
天文学 | 14篇 |
综合类 | 5篇 |
自然地理 | 20篇 |
出版年
2023年 | 2篇 |
2022年 | 10篇 |
2021年 | 13篇 |
2020年 | 30篇 |
2019年 | 19篇 |
2018年 | 40篇 |
2017年 | 37篇 |
2016年 | 42篇 |
2015年 | 23篇 |
2014年 | 25篇 |
2013年 | 30篇 |
2012年 | 30篇 |
2011年 | 24篇 |
2010年 | 16篇 |
2009年 | 8篇 |
2008年 | 6篇 |
2007年 | 6篇 |
2006年 | 8篇 |
2004年 | 3篇 |
2003年 | 2篇 |
2002年 | 3篇 |
2000年 | 2篇 |
1996年 | 1篇 |
1993年 | 1篇 |
1991年 | 1篇 |
1989年 | 3篇 |
1988年 | 1篇 |
1982年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有388条查询结果,搜索用时 31 毫秒
21.
Tuna Eken Z. Hossein Shomali Roland Roberts Reynir Bödvarsson 《Geophysical Journal International》2007,169(2):617-630
Upper-mantle structure under the Baltic Shield is studied using non-linear high resolution teleseismic P -phase tomography. Observed relative arrival-time residuals from 52 teleseismic earthquakes recorded by the Swedish National Seismological Network (SNSN) are inverted to delineate the structure of the upper mantle. The network consists of 47 (currently working) three-component broad-band stations located in an area about 450 km wide and 1450 km long. In order to reduce complications due to possible significant three-dimensionality of Earth structure, events chosen for this study lay close to in-line with the long-axis of the array (±30°) . Results indicate P -wave velocity perturbations of ±3 per cent down to at least 470 km below the network. The size of the array allows inversion for structures even at greater depths, and lateral variations of velocity at depths of up to 680 km appear to be resolved. Below the central part of the array (60°–64° N), where ray coverage is best, the data reveals a large region of relatively low velocity at depths of over about 300 km. At depths less than about 250–300 km, the models include a number of features, including an apparent slab-like structure dipping gently towards the north. 相似文献
22.
Hydro-mechanical evaluation of stabilized mine tailings 总被引:3,自引:0,他引:3
23.
The scaled boundary finite‐element method has been developed for the dynamic analysis of unbounded domains. In this method only the boundary is discretized resulting in a reduction of the spatial dimension by one. Like the finite‐element method no fundamental solution is required. This paper extends the scaled boundary finite‐element method to simulate the transient response of non‐homogeneous unbounded domains with the elasticity modulus and mass density varying as power functions of spatial coordinates. To reduce the number of degrees of freedom and the computational cost, the technique of reduced set of base functions is applied. The scaled boundary finite‐element equation for an unbounded domain is reformulated in generalized coordinates. The resulting acceleration unit‐impulse response matrix is obtained and assembled with the equation of motion of standard finite elements. Numerical examples of non‐homogeneous isotropic and transversely isotropic unbounded domains demonstrate the accuracy of the scaled boundary finite‐element method. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
24.
Sedimentary facies and sequence stratigraphy of the Asmari Formation at Chaman-Bolbol, Zagros Basin, Iran 总被引:1,自引:0,他引:1
Mahnaz Amirshahkarami Hossein Vaziri-Moghaddam Azizolah Taheri 《Journal of Asian Earth Sciences》2007,29(5-6):947-959
The Oligocene–Miocene Asmari Formation of the Zagros Basin is a thick sequence of shallow water carbonate. In the study area, it is subdivided into 14 microfacies that are distinguished on the basis of their depositional textures, petrographic analysis and fauna. Based on the paleoecology and lithology, four distinct depositional settings can be recognized: tidal flat, lagoon, barrier, and open marine. The Asmari Formation represents sedimentation on a carbonate ramp. In the inner ramp, the most abundant lithofacies are medium grained wackestone–packstone with imperforated foraminifera. The middle ramp is represented by packstone–grainstone to floatstone with a diverse assemblage of larger foraminifera with perforate wall, red algae, bryozoa, and echinoids. The outer ramp is dominated by argillaceous wackestone characterized by planktonic foraminifera and large and flat nummulitidae and lepidocyclinidae. Three third-order depositional sequences are recognized from deepening and shallowing trends in the depositional facies, changes in cycle stacking patterns, and sequence boundary features. 相似文献
25.
Comparison between Shannon and Tsallis entropies for prediction of shear stress distribution in open channels 总被引:2,自引:1,他引:1
Hossein Bonakdari Zohreh Sheikh Mohtaram Tooshmalani 《Stochastic Environmental Research and Risk Assessment (SERRA)》2015,29(1):1-11
The concept of Tsallis entropy was applied to model the probability distribution functions for the shear stress magnitudes in circular channels (with filling ratios of 0.506, 0.666, 0.826), circular with flat bed (filling ratios of 0.333, 0.666), rectangular channel (1.34, 2, 3.94, 7.37 aspect ratios) and compound channel (with relative depths of 0.324, 0.46). The equation for the shear stress distribution was obtained according to the entropy maximization principle, and is able to estimate the shear stress distribution as much on the walls as the channel bed. The approach is also compared with the predictions obtained based on the Shannon entropy concept. By comparing the two prediction models, this study highlights the application of Tsallis entropy to estimate the shear stress distribution of open channels. Although the results of the two models are similar in the circular cross-section, the differences between them are more significant in circular with flat bed and rectangular channels. For a wide range of filling ratio values, experimental data are used to illustrate the accuracy and reliability of the proposed model. 相似文献
26.
Analysis of spatial and temporal variations of reference evapotranspiration (ETo) is important in arid and semi‐arid regions where water resources are limited. The main aim of this study was to analyse the spatial distribution and the annual, seasonal and monthly trends of the Penman–Monteith ETo for 21 stations in the arid and semi‐arid regions of Iran. Three statistical tests the Mann‐Kendall, Sen's slope estimator and linear regression were used for the analysis. The analysis revealed that ETo increased from January to July and deceased from July to December at almost all stations. Additionally, higher annual ETo values were found in the southeast of the study region and lower values in the northwest of the region. Although the results showed both positive and negative trends in annual ETo series, ETo generally increased, significantly so in six (~30%) of the stations. Analysis of the impacts of meteorological variables on the temporal trends of ETo indicated that the increasing trend of ETo was most likely due to a significant increase in minimum air temperature, while decreasing trend of ETo was mainly caused by a significant decrease in wind speed. At the sites where increasing ETo trends were statistically significant, the rate of increase varied from (+)8·36 mm/year at Mashhad station to (+)31·68 mm/year at Iranshahr station. On average, an increasing trend of (+)4·42 mm/year was obtained for the whole study area during the last four decades. Seasonal and monthly ETo have also tended to increase at the majority of the stations. The greatest numbers of significant trends were observed in winter on the seasonal time‐scale and in September on the monthly time‐scale. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
27.
The load transfer capacity and failure mechanism of a fully grouted bolt installed across a joint in shear is investigated,
both experimentally and numerically, in five types of bolt. The double-shearing testing of bolts were studied in concrete
blocks of 20, 40 and 100 MPa strengths, subjected to different pretension loads of 0, 5, 10, 20, 50 and 80 KN, respectively.
The parameters examined include: shear resistance, shear displacement, induced strains and stresses during the bolt-bending
process, and its ultimate failure across the sheared joint planes. The conclusions drawn from the study were that the strength
of the concrete, bolt profile configuration and bolt pretension load played a significant influence on the shear resistance,
shear displacement and failure mechanism of the reinforced medium. Experimental and numerical simulations showed that the
failure occurs as a result of the induced axial and shear stresses acting between the hinge point distances in the vicinity
of the shear joint plane. 相似文献
28.
Interactive effect of nitrogen fertilizer and hydrocarbon pollution on soil biological indicators 总被引:2,自引:1,他引:1
Somayeh Emami Ahmad Ali Pourbabaei Hossein Ali Alikhani 《Environmental Earth Sciences》2014,72(9):3513-3519
The investigation of the impact of different forms of nitrogen fertilizer (NO3-N and NH4-N) on microbial parameters, enzyme activities and phytotoxicity in a petroleum-contaminated soil was evaluated by an incubation study. The tested enzymes, microbial activity and seed germination index showed different patterns in response to both petroleum and nitrogen fertilizer addition and time of incubation. The results apparently showed that the contamination of soil with petroleum has a negative effect on soil ecosystem. Nitrogen fertilizer could improve inhibition of petroleum hydrocarbons in soil. Nevertheless, nitrogen fertilizer had no significant effect on urease activity in the petroleum-contaminated soil. As compared to NO3-N, the addition of NH4-N to the soil resulted in a greater impact on soil performance as attested by the recovery of the soil germination capability and higher values of the respiration. The application of nitrogen fertilizer may be suggested as a good strategy for restoring soils in regions affected by the same problem. 相似文献
29.
Meteorological drought is a natural climatic phenomenon that occurs over various time scales and may cause significant economic, environmental and social damages. Three drought characteristics, namely duration, average severity and peak intensity, are important variables in water resources planning and decision making. This study presents a new method for construction of three-dimensional copulas to describe the joint distribution function of meteorological drought characteristics. Using the inference function for margins, the parameters for six types of copulas were tested to select the best-fitted copulas. According to the values of the log-likelihood function, Galambos, Frank and Clayton were the selected copula models to describe the dependence structure for pairs of duration–severity, severity–peak and duration–peak, respectively. Trivariate cumulative probability, conditional probability and drought return period were also investigated based on the derived copula-based joint distributions. The proposed model was evaluated over the observed data of a Qazvin synoptic station, and the results were compared with the empirical probabilities. For measuring the model accuracy, R 2, root mean square error (RMSE) and the Nash–Sutcliffe efficiency (NSE) criteria were used. Results indicated that R 2, RMSE and NSE were equal to 0.91, 0.098 and 0.668, respectively, which demonstrate sufficient accuracy of the proposed model. Drought probabilistic characteristics can provide useful information for water resource planning and management. 相似文献
30.
Hossein Basser Shahaboddin Shamshirband Dalibor Petković Hojat Karami Shatirah Akib Afshin Jahangirzadeh 《Natural Hazards》2014,73(3):1439-1449
This study proposes a new approach for determining the optimum dimensions of a protective spur dike to mitigate the amount of scour around existing spur dikes. Several parameters of a protective spur dike were studied to determine their optimum values, including length, angle, and distance. Also the effect of changes of flow intensity and sediment size were examined. The main objective of this article was to predict the optimum values of protective spur dikes to attain the best performance. To predict the parameters of protective spur dikes for controlling the scour around spur dikes, we used the adaptive neuro-fuzzy inference system method to construct a process that simulates the optimal parameters of a protective spur dike, including the actual length of the protective spur dike, the actual length of the main spur dikes, the distance between the protective spur dike and the first spur dike, the angle between the protective spur dike and the direction of flow, the intensity of the flow, and median size of the bed sediments. This intelligent estimator was implemented using MATLAB/Simulink, and the performances were investigated. The simulation results presented in this paper show the effectiveness of the developed method. 相似文献