全文获取类型
收费全文 | 4870篇 |
免费 | 213篇 |
国内免费 | 94篇 |
专业分类
测绘学 | 155篇 |
大气科学 | 534篇 |
地球物理 | 1371篇 |
地质学 | 2125篇 |
海洋学 | 204篇 |
天文学 | 575篇 |
综合类 | 82篇 |
自然地理 | 131篇 |
出版年
2022年 | 42篇 |
2021年 | 76篇 |
2020年 | 64篇 |
2019年 | 40篇 |
2018年 | 165篇 |
2017年 | 159篇 |
2016年 | 238篇 |
2015年 | 169篇 |
2014年 | 256篇 |
2013年 | 312篇 |
2012年 | 251篇 |
2011年 | 190篇 |
2010年 | 235篇 |
2009年 | 245篇 |
2008年 | 183篇 |
2007年 | 141篇 |
2006年 | 118篇 |
2005年 | 117篇 |
2004年 | 90篇 |
2003年 | 75篇 |
2002年 | 81篇 |
2001年 | 87篇 |
2000年 | 82篇 |
1999年 | 75篇 |
1998年 | 102篇 |
1997年 | 66篇 |
1996年 | 78篇 |
1995年 | 61篇 |
1994年 | 66篇 |
1993年 | 51篇 |
1992年 | 37篇 |
1991年 | 36篇 |
1990年 | 39篇 |
1989年 | 41篇 |
1988年 | 27篇 |
1987年 | 41篇 |
1986年 | 35篇 |
1985年 | 37篇 |
1984年 | 44篇 |
1983年 | 56篇 |
1982年 | 43篇 |
1981年 | 35篇 |
1980年 | 39篇 |
1979年 | 44篇 |
1978年 | 42篇 |
1977年 | 38篇 |
1976年 | 28篇 |
1975年 | 46篇 |
1974年 | 38篇 |
1973年 | 40篇 |
排序方式: 共有5177条查询结果,搜索用时 46 毫秒
171.
Climatic Change - The assessment of ecological impacts of pumped-storage (PS) hydropower plants on the two connected water bodies is usually based on present climatic conditions. However,... 相似文献
172.
A semi-active strategy for model predictive control (MPC), in which magneto-rheological dampers are used as an actuator, is presented for use in reducing the nonlinear seismic response of high-rise buildings. A multi-step predictive model is developed to estimate the seismic performance of high-rise buildings, taking into account of the effects of nonlinearity, time-variability, model mismatching, and disturbances and uncertainty of controlled system parameters by the predicted error feedback in the multi-step predictive model. Based on the predictive model, a Kalman-Bucy observer suitable for semi-active strategy is proposed to estimate the state vector from the acceleration and semi-active control force feedback. The main advantage of the proposed strategy is its inherent stability, simplicity, on-line real-time operation, and the ability to handle nonlinearity, uncertainty, and time-variability properties of structures. Numerical simulation of the nonlinear seismic responses of a controlled 20-story benchmark building is carried out, and the simulation results are compared to those of other control systems. The results show that the developed semi-active strategy can efficiently reduce the nonlinear seismic response of high-rise buildings. 相似文献
173.
The Upper Quaternary seismic stratigraphy and active faults of the Gulf of İzmit were investigated by means of high-resolution
shallow seismic profiling data in the source region of 1999 İzmit earthquake. High-resolution seismic reflection data correlated
with borehole data indicate that the stratigraphy of İzmit Bay consists of three distinct depositional sequences formed in
response to middle Pleistocene-Holocene sea-level changes. Reflector R, separating the pre-Holocene sequences (1 and 2) from
the Holocene sequence (3), represents an erosional unconformity produced by the subaerial fluvial erosion of the continental
shelves at the time of the last glacial maximum. Occasional, anomalous reflections (acoustic turbidity) observed within the
Holocene sequence are interpreted as gas accumulations. The maximum thickness of the Holocene sediments is found to be about
25 m. The isopach map of Holocene sediment implies that the thickness of the Holocene decreases from the east towards the
central and western basins of İzmit Bay. Two distinct fault systems are interpreted in İzmit Bay. The main fault system extending
roughly in an E-W direction along the Gulf of İzmit is an active right lateral strike slip fault with a normal component.
The secondary faults are normal faults striking in different directions and these are identified as being both active and
inactive. In addition, prominent compressive features are identified in the seismic cross-sections of some profiles acquired
to the east of Hersek Peninsula where the focal mechanisms of the aftershocks of the 1999 İzmit earthquake also reveal predominantly
reverse faulting mechanisms, as identified by a local dense seismic network. 相似文献
174.
INTRODUCTIONWeatherforecastingasamajorsubjectinatmosphericsciencehasdevelopedsincethe 1 950’sintoamodernscience .Numericalweatherforecastmodelsareextensivelyandfrequentlyusedtocheckthetheories,ruleouttheoldincorrectonesandpresentnewideas,andsuggestmethods… 相似文献
175.
The thermal strati?cation of the lakes impedes the transfer of atmospheric oxygen into the lower layers of the lake. In lakes which are af fected by diverse anthropogenic in?uences, the increasing organic matter amounts lead to a sharp decrease in hypolimnetic oxygen amounts, aided by thermal strati?cation,and anaerobic conditions arise. The determination of hypolimnetic oxygen demand(HOD) and areal hypolimnetic oxygen demand(AHOD) and their monitoring represent an integrated approach to investigate the oxygenation of lakes, the nutrient conditions, and the physicochemical dynamics. In this study, two lakes dif fering by size and af fected by dif ferent anthropogenic sources, are investigated in this respect. At?rst, bathymetric studies were conducted to determine the depth, surface area, and volume relationships.Then, based on monitoring studies conducted in 2013 and 2014, the thermal strati?cation dynamics and layer properties were established using the relative thermal resistance to mixing(RTRM) index based on temperature and density pro?les. Following this, the oxygen depletion rates were determined by oxygen and temperature pro?ling in the hypolimnion. For the years of 2013 and 2014, the AHOD values for the Borabey Pond which is far from anthropogenic in?uences, were found to be 0.848 and 0.569 g O 2/(m·d), respectively.The AHOD values for the Porsuk Reservoir which was overburdened for years by industrial and domestic pollution were found to be 4.263 and 5.099 g O 2/(m·d), larger than its counterpart by almost sevenfold. The HOD and AHOD monitoring can be considered to a valuable tool for assessing the ecological and chemical status of lakes within Annex 5 of the Water Framework Directive and as an integrated approach to assess and monitor the status of lakes. 相似文献
176.
The thermal strati?cation of the lakes impedes the transfer of atmospheric oxygen into the lower layers of the lake. In lakes which are af fected by diverse anthropogenic in?uences, the increasing organic matter amounts lead to a sharp decrease in hypolimnetic oxygen amounts, aided by thermal strati?cation,and anaerobic conditions arise. The determination of hypolimnetic oxygen demand(HOD) and areal hypolimnetic oxygen demand(AHOD) and their monitoring represent an integrated approach to investigate the oxygenation of lakes, the nutrient conditions, and the physicochemical dynamics. In this study, two lakes dif fering by size and af fected by dif ferent anthropogenic sources, are investigated in this respect. At?rst, bathymetric studies were conducted to determine the depth, surface area, and volume relationships.Then, based on monitoring studies conducted in 2013 and 2014, the thermal strati?cation dynamics and layer properties were established using the relative thermal resistance to mixing(RTRM) index based on temperature and density pro?les. Following this, the oxygen depletion rates were determined by oxygen and temperature pro?ling in the hypolimnion. For the years of 2013 and 2014, the AHOD values for the Borabey Pond which is far from anthropogenic in?uences, were found to be 0.848 and 0.569 g O 2/(m·d), respectively.The AHOD values for the Porsuk Reservoir which was overburdened for years by industrial and domestic pollution were found to be 4.263 and 5.099 g O 2/(m·d), larger than its counterpart by almost sevenfold. The HOD and AHOD monitoring can be considered to a valuable tool for assessing the ecological and chemical status of lakes within Annex 5 of the Water Framework Directive and as an integrated approach to assess and monitor the status of lakes. 相似文献
177.
Astronomy Letters - We present LTE analysis of high resolution optical spectra for the B-type abundance standard candidate HD 35039 (HR 1765, 22 Ori) with an ambigous binary nature. The spectra... 相似文献
178.
最小二乘估计和部分变量误差模型的总体最小二乘估计不具备抵御粗差的能力。鉴于粗差可能同时出现在灰色白化微分方程的观测值和系数矩阵中,本文提出基于IGGⅢ抗差方案的部分变量总体最小二乘稳健估计。结合仿真数据和高铁路基观测数据,系统地比较稳健最小二乘、部分变量总体最小二乘、本文算法参数估计结果和算法稳定性。结果表明,本文算法预测精度高,可以应用到高铁路基沉降预测中。 相似文献
179.
Artificial ground motion compatible with specified ground shaking peaks and target response spectrum 总被引:1,自引:0,他引:1
1 Introduction Time history analysis is usually needed not only for the seismic design of important structures, such as nuclear plants, large concrete dams and super high-rise buildings, but also for seismic performance based design. Performance-based design requires tha knowledge of the entire process of nonlinear seismic responses of structures be obtained, which depends on the ground motion time history as input for this analysis. However, because the observed strong-motion recordings are l… 相似文献
180.
China Ocean Engineering - Turkey has announced its plan to construct a new waterway, Canal Istanbul, parallel to the Bosphorus. In this study, the influence of Canal Istanbul on salinity... 相似文献