全文获取类型
收费全文 | 43932篇 |
免费 | 4127篇 |
国内免费 | 5390篇 |
专业分类
测绘学 | 3672篇 |
大气科学 | 4159篇 |
地球物理 | 6830篇 |
地质学 | 12468篇 |
海洋学 | 3441篇 |
天文学 | 16724篇 |
综合类 | 1819篇 |
自然地理 | 4336篇 |
出版年
2024年 | 153篇 |
2023年 | 471篇 |
2022年 | 869篇 |
2021年 | 1036篇 |
2020年 | 1183篇 |
2019年 | 1460篇 |
2018年 | 971篇 |
2017年 | 1267篇 |
2016年 | 1296篇 |
2015年 | 1421篇 |
2014年 | 1730篇 |
2013年 | 2003篇 |
2012年 | 1760篇 |
2011年 | 1877篇 |
2010年 | 1567篇 |
2009年 | 3194篇 |
2008年 | 3085篇 |
2007年 | 3410篇 |
2006年 | 3356篇 |
2005年 | 2865篇 |
2004年 | 2883篇 |
2003年 | 2439篇 |
2002年 | 2037篇 |
2001年 | 1801篇 |
2000年 | 1513篇 |
1999年 | 1444篇 |
1998年 | 1625篇 |
1997年 | 718篇 |
1996年 | 517篇 |
1995年 | 632篇 |
1994年 | 585篇 |
1993年 | 425篇 |
1992年 | 283篇 |
1991年 | 256篇 |
1990年 | 207篇 |
1989年 | 251篇 |
1988年 | 182篇 |
1987年 | 140篇 |
1986年 | 125篇 |
1985年 | 88篇 |
1984年 | 55篇 |
1983年 | 35篇 |
1982年 | 26篇 |
1981年 | 19篇 |
1980年 | 25篇 |
1979年 | 15篇 |
1977年 | 25篇 |
1976年 | 27篇 |
1973年 | 18篇 |
1971年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
海洋平台结构振动的AMD主动控制参数优化分析 总被引:5,自引:0,他引:5
本文针对海洋平台结构的冰激振动和地震反应控制问题,提出了采用AMD主动控制的控制策略,结合JZ20-2MUQ平台结构进行了AMD控制系统的硬参数和软参数的优化分析,并就相应于最优参数下的AMD控制海洋平台结构冰激振动和地震反应的几种代表性工况进行了时程分析,得到了一些定性和定量的结论,为实际工程的控制设计提供了基础。本文提出的AMD主动控制方法对类似的海洋平台结构的控制问题也有参考价值。 相似文献
972.
20 0 1年 11月 14、 15日新疆喀什台记录到 86次振动事件 (“奇异震群”事件 )。为识别其性质 ,应用波谱分析、偏振分析等方法 ,对该事件和同范围构造地震的地震波特征进行了对比研究 ,发现 :(1)喀什“震群”的全波列衰减比构造地震的全波列衰减快很多 ;(2 )与构造地震的速度谱相比 ,“震群”的频谱相对较为“集中”,而构造地震的则较为“发散”;(3)“震群”位移谱的拐角频率和卓越频率均较构造地震的相应量高出一倍多 ,且离散较小。通过对喀什“奇异震群”这些特征进行分析研究 ,认为这一事件极有可能是人工事件 相似文献
973.
This paper presents the results of a study undertaken todetermine the seismic hazard of Lebanon. The seismic hazard evaluation wasconducted using probabilistic methods of hazard analysis. Potential sourcesof seismic activities that affect Lebanon were identified and the earthquakerecurrence relationships of these sources were developed from instrumentalseismology data, historical records, and earlier studies undertaken toevaluate the seismic hazard of neighboring countries. The sensitivityof the results to different assumptions regarding the seismic sources in theLebanese segment and choice of the attenuation relationship wasevaluated. Maps of peak ground acceleration contours, based on 10percent of probability of exceedance in 50 years and 100 years time spans,were developed. 相似文献
974.
Nonlinear earthquake response analysis and energy calculation for seismic slit shear wall structures
Based on the concept of structural passive control, a new type of slit shear wall, with improved seismic performance when
compared to an ordinary solid shear wall, was proposed by the authors in 1996. The idea has been verified by a series of pseudo-static
and dynamic tests. In this paper a macro numerical model is developed for the wall element and the energy dissipation device.
Then, nonlinear time history analysis is carried out for a 10-story slit shear wall model tested on a shaking table. Furthermore,
the seismic input energy and the individual energy dissipated by the components are calculated by a method based on Newmark-β
assumptions for this shear wall model, and the advantages of this shear wall are further demonstrated by the calculation results
from the viewpoint of energy. Finally, according to the seismic damage criterion on the basis of plastic accumulative energy
and maximum response, the optimal analysis is carried out to select design parameters for the energy dissipation device. 相似文献
975.
Positive tectonic inversion is related to the transmission of compressional stresses along a décollement into the foreland of an orogenic zone. This stress and strain concentration in regions remote from the main orogenic front is commonly related to the presence of pre-existing rheological heterogeneities such as normal syn-depositional faults. During inversion, these pre-existing normal faults are reactivated as reverse faults. Tectonic inversion in the Rhenohercynian fold-and-thrust belt during the Variscan Orogeny shows that inversion is likely synchronous with the onset of collision in the hinterland. Here, we present the results of a simplified thermo-mechanical model (STM) which allows one to study strain partitioning between two orogenic zones. We show that, if the two orogenic zones have the same mechanical properties, the viscosity of the décollement, which links them, controls the initial strain partitioning. During subsequent finite shortening, erosional processes determine the partitioning of strain rate. The presence of a weak structure in the inverted zone and of a low-viscosity décollement leads to initial strain concentration in the inverted track rather than in the collision zone and a progressive decrease in strain partitioning between the two orogenic zones. The STM results are in good agreement with results of a 2D finite-element model. We conclude that, in the western part of the Rhenohercynian Massif, simultaneous uplift and deformation within the Mid-German Crystalline Rise (the main collision zone) and the Ardenne Anticlinorium (the inverted zone) lead to interpreting this orogenic event as a case of vice tectonic rather than the propagation of a ‘wave of folding’ towards the Variscan front, as suggested by previous authors. 相似文献
976.
选取了河南郑州台数字化 DSQ型水管倾斜仪观测资料较好的 2 0 0 1年 10月 1日至 12月 31日的数据进行了分析处理 ,对该仪器的观测精度进行了评定。经计算 ,其北南向和东西向日飘移分别为 0 .0 0 0 7s和 0 .0 0 2 6 s,M2 的振幅因子为 0 .714 9和 0 .6 4 14 ,振幅因子中误差为 0 .0 0 5 5和0 .0 0 4 4。 相似文献
977.
978.
The garnet-muscovite geothermometer was refined through empirical calibration by using natural rocks metamorphosed under the physical conditions of 238—1306 MPa and 490—700℃. Input temperatures and pressures were determined through simultaneously applying the garnet-biotite geothermometer and the garnet-biotite-plagioclase-quartz barometer, assuming that all FeO in muscovite and garnet be ferrous. Garnet was treated as the asymmetric quaternary solid solution, and muscovite as the symmetric binary solid solution. Input muscovite compositions include Fe atoms between 0.03—0.19 and Mg atoms between 0.04—0.16 on the basis of 11 oxygen atoms, and input garnet compositions include spessartine fractions between 0.01—0.289, grossular fractions between 0.028—0.273, and the Fe/Mg ratio between 3.387-18.986. The resulting garnet-muscovite geothermometer reproduces temperatures within (50℃ compared with the garnet-biotite thermometer. Total random error of ±37℃ of the new thermometer may stem from the pressure uncertainty of ±200 MPa, and uncertainties of ±5% of Fe and Mg components in muscovite, and ±5% of Fe, Mg, Mn and Ca components in garnet, altogether. When there exist 10%, 20%, 30%, 40% and 50% Fe3+ in muscovite, respectively, the computed garnet-muscovite temperatures will be 1—6℃, 2—12℃, 3—16℃, 5—24℃ and 7—29℃, respectively, lower than those obtained when assuming that all FeO be ferrous. The new garnet-muscovite geothermometer can efficiently reflect temperature change of typical prograde sequences and contact aureole rocks, and may be applied to low- to high-grade and low- to high-pressure metamorphic rocks. 相似文献
979.
The non-parametric Mann–Whitney (MW) statistic test has been popularly used to assess the significance of a shift in median
or mean of hydro-meteorological time series. It has been considered that the test is more suitable for non-normally distributed
data and it may be not sensitive to the distribution type of sample data. However, no evidence has been provided to demonstrate
these. This study investigates the power of the test in various circumstances by means of Monte Carlo simulation. Simulation
results demonstrate that the power of the test is very sensitive to various properties of sample data. The power depends on
the pre-assigned significance level, magnitude of a shift, sample size, and its occurrence position within a time series;
and it is also strongly affected by the variation, skewness, and distribution type of a time series. The bigger the magnitude
of a shift, the more powerful the test is; the larger the sample size, the more powerful the test is; and the bigger the variation
within a time series, the less power the test has. The test has the highest power if a shift occurs at the midpoint of a time
series. For the samples with different distribution types, the power of the test is dramatically different. The test has the
highest power for time series with the extreme value type III (EV3) distribution while it indicates the lowest power for time
series with the lognormal distribution. 相似文献
980.
天山西部中山带积雪变化趋势与气温和降水的关系——以巩乃斯河谷为例 总被引:12,自引:9,他引:12
根据位于巩乃斯河谷的天山积雪雪崩研究站近30年来的年最大雪深、月平均气温、月降水量观测记录,用平均差值法、最小二乘法、自回归滑动平均法检验了天山西部中山带积雪、冷季降水、冷季平均气温的变化趋势,结果表明,天山西部中山带积雪呈增加趋势,近30年来年平均增加1.43%,与青藏高原、南极大陆及格陵兰冰盖表面积雪积累增加相一致。天山西部中山带冷季气温和降水的变化趋势也是增加的,其中冷季降水平年平均增加0.12%,而冷季气温升高了0.8℃,积雪与冷季气温之间存在着弱的负相关关系,而与冷季降水呈显著的正相关关系。积雪的增加主要是因为气候变暖引起的冷季降水的增加对积雪增加的贡献大于由于冷季气温升高而造成积雪减少的贡献的结果。 相似文献