全文获取类型
收费全文 | 1540篇 |
免费 | 88篇 |
国内免费 | 346篇 |
专业分类
测绘学 | 60篇 |
大气科学 | 147篇 |
地球物理 | 605篇 |
地质学 | 599篇 |
海洋学 | 335篇 |
天文学 | 69篇 |
综合类 | 56篇 |
自然地理 | 103篇 |
出版年
2023年 | 12篇 |
2022年 | 20篇 |
2021年 | 16篇 |
2020年 | 22篇 |
2019年 | 17篇 |
2018年 | 26篇 |
2017年 | 9篇 |
2016年 | 14篇 |
2015年 | 16篇 |
2014年 | 31篇 |
2013年 | 32篇 |
2012年 | 77篇 |
2011年 | 111篇 |
2010年 | 43篇 |
2009年 | 44篇 |
2008年 | 90篇 |
2007年 | 80篇 |
2006年 | 92篇 |
2005年 | 84篇 |
2004年 | 100篇 |
2003年 | 52篇 |
2002年 | 60篇 |
2001年 | 54篇 |
2000年 | 66篇 |
1999年 | 33篇 |
1998年 | 49篇 |
1997年 | 41篇 |
1996年 | 41篇 |
1995年 | 41篇 |
1994年 | 53篇 |
1993年 | 41篇 |
1992年 | 43篇 |
1991年 | 67篇 |
1990年 | 51篇 |
1989年 | 78篇 |
1988年 | 45篇 |
1987年 | 29篇 |
1986年 | 27篇 |
1985年 | 27篇 |
1984年 | 29篇 |
1983年 | 18篇 |
1982年 | 16篇 |
1981年 | 10篇 |
1980年 | 17篇 |
1964年 | 5篇 |
1935年 | 5篇 |
1934年 | 4篇 |
1926年 | 3篇 |
1925年 | 3篇 |
1924年 | 3篇 |
排序方式: 共有1974条查询结果,搜索用时 31 毫秒
921.
引言
北磁极位于地磁场竖直向下的地球表面.虽然地核通过内部对流的地球发电机过程维持了地球的主磁场,但却引起了北磁极随时间的不断漂移.北磁极的漂移速度从20世纪90年代初的15 km/a突然增加到90年代末的55 km/a. 相似文献
922.
东海陆架盆地南部构造样式及分布特征 总被引:2,自引:0,他引:2
以东海陆架盆地南部为重点研究对象,基于区域地震反射资料,结合盆地的结构特征,分析、总结了其构造样式特征。结果表明:根据构造样式形成的动力学机制,可以将东海陆架盆地南部划分为伸展型构造样式、挤压型构造样式和复合型构造样式3类;伸展型构造样式可继续分为半地堑(箕状断陷)构造组合、复式地堑式构造组合、多米诺式断裂构造组合、掀斜断块和火成岩构造;挤压型构造样式分为挤压背斜、断背斜和叠瓦式断裂组合;复合型构造样式分为正反转构造和负花状构造。 相似文献
923.
924.
张宝成 Teunissen J G Peter Odijk Dennis 欧吉坤中国科学院测量与地球物理研究所 动力大地测量学重点实验室 蒋振伟 《地球物理学报》2012,55(7):2203-2211
提出利用GPS参考网估计电离层延迟、卫星相位偏差的算法,用于实现区域内精密单点定位(Precise Point Positioning, PPP)的整周模糊度快速固定.利用站间距约为100~200 km的参考网进行实验,结果表明:电离层延迟的内插和外推精度均优于1 dm,卫星相位偏差估值的日内变化不超过0.2周;此外,单天内不同时刻始,固定PPP整周模糊度所需时长最多不超过10 min,且当模糊度成功固定后,三维位置解较之相应浮点解的精度改善优于80%.新算法可望解决PPP普遍存在的收敛时间过长问题,增强了PPP技术的实用性. 相似文献
925.
This article presents the analytical solution of Laplace equation for the steady flow around open-ended cylindrical piezometers located in an infinite, isotropic, and incompressible saturated soil. Shape factors are obtained for piezometers with varying length-to-diameter ratios. Comparisons are made with published factors obtained by means of approximate analytical solutions, numerical approaches, and measurements in electric analog models. It is shown that some expressions that are currently used in practice are inadequate and should be abandoned. 相似文献
926.
Fresh water resources within the Kingdom of Saudi Arabia are a rare and precious commodity that must be managed within a context of integrated water management. Wadi aquifers contain a high percentage of the naturally occurring fresh groundwater in the Kingdom. This resource is currently overused and has become depleted or contaminated at many locations. One resource that could be used to restore or enhance the fresh water resources within wadi aquifers is treated municipal waste water (reclaimed water). Each year about 80 percent of the country's treated municipal waste water is discharged to waste without any beneficial use. These discharges not only represent a lost water resource, but also create a number of adverse environmental impacts, such as damage to sensitive nearshore marine environments and creation of high-salinity interior surface water areas. An investigation of the hydrogeology of wadi aquifers in Saudi Arabia revealed that these aquifers can be used to develop aquifer recharge and recovery (ARR) systems that will be able to treat the impaired-quality water, store it until needed, and allow recovery of the water for transmittal to areas in demand. Full-engineered ARR systems can be designed at high capacities within wadi aquifer systems that can operate in concert with the natural role of wadis, while providing the required functions of additional treatment, storage and recovery of reclaimed water, while reducing the need to develop additional, energy-intensive desalination to meet new water supply demands. 相似文献
927.
G. V. WILSON Mingxiang XU Yunming CHEN Guobin LIU M. J. M. ROEMKENS 《国际泥沙研究》2005,20(3):249-258
Due to the extensive gullying from historically excessive erosion in the loess plateau of China, much of this region is being converted to native grass and shrub vegetation. Tunnel scour and mass wasting are important gully erosion processes resulting from preferential flow through macropores ( pores 〉 1 mm diameter). The objective of this study is to assess the changes with time in macropore flow characteristics of soils on the Loess Plateau following conversion to grass vegetation and the associated degree of mass wasting of gully faces. Ridge areas that had been revegetated for 1 year, 6 years, and 〉 15 years following tilling, and for 6 years following contour-ditching and the adjacent gully faces were characterized for their macropore and soil matrix properties on a 50 cm by 50 cm area. The total number of macropores increased from 11.6/m^2 to 39.6/m2 from 1 to 6 years and to 51.6/m2 after 15 years of revegetation following tillage. The macroporosity increased from 0.0008 m^3/m^3 to 0.0018 m^3/m^3 from 1 to 6 years of revegetation following tillage but the lowest macroporosity (0.0005 m3/m3) was 6 years of revegetation following contour-ditching. The contourditched area had the lowest infiltration rate (95 m/d) through the soil matrix (areas without macropores) with the tilled areas having similar infiltration rates regardless of the number of years of revegetation (averaged 146 m/d). Due to tunnel scour erosion of macropores during infiltration into the area revegetated for 1 year, pore diameters enlarged by more than 200% resulting in this condition having the highest individual macropore infiltration rates (7967 m/d). Macropores in all other areas were stable with no tunnel scour erosion of macropores. The total capacity for infiltration through macropores increased significantly with time following revegetation. The number of macropores on the gully faces was triple (92.8/m2) and the macroporosity quadruple (0.004 m3/m3) that of the ridge surfaces. The upper gully faces exhibited 1.1 slumps m^-1 for a total soil loss of 48622 kg per ha. 相似文献
928.
特大地震的同震及震后滑动能引起地壳介质的时间变化,这是因为强地面震动破坏了近地表的沉积层,或者应力扰动改变了裂隙密度和/或深部流体的移动。这种随时间变化的地壳属性可引起地震波速度出现部分变化,并可根据互相关波形和测量重复地震序列内它们的时间滞后探测到。本文根据2004年苏门答腊—安达曼特大地震和2005年尼亚斯—锡默卢特大地震的重复余震序列,分析了高频(0.5~2.0Hz)P尾波和S尾波以及长周期(0.03~0.1Hz)面波的滞后时间。观测到的滞后时间揭示出几个主要特征:(1)2004年地震序列的S尾波滞后时间系列!(t)基本上在0值附近波动,有时随消逝时间而变为负值,而2005年地震序列的S尾波的!(t)则表现为随消逝时间而单调增长;(2)S尾波的平均速度降低(-δV S)是P尾波平均速度降低(-δV P)的2倍,瑞利波的平均速度降低(-δV LR)比勒夫波的平均速度降低(-δV LQ)大3~4倍;(3)δV S和δV LR显示了速度随时间的恢复,特别是2005年地震的序列。2005年地震序列δV S速度随时间恢复的形状与附近大地测量台站得到的位移时间系列类似。尽管我们讨论了可能的人为结果,如震源间距、噪声场的时间变化和仪器响应,但观测结果均表明地壳内的非均匀波速时间降低是由2004年和2005年的特大地震的同震和/或震后滑动引起的。 相似文献
929.
Intense research and refinement of the tools used in performance-based seismic engineering have been made,but the maturity and accuracy of these methods have not been adequately confirmed with actual data from the field. The gap between the assumed characteristics of actual building systems and their idealized counterparts used for analysis is wide. When the randomly distributed flaws in buildings as they exist in urban areas and the extreme variability of ground motion patterns combine,the conventional procedures used for pushover or dynamic response history analyses seem to fall short of reconciling the differences between calculated and observed damage. For emergency planning and loss modeling purposes,such discrepancies are factors that must be borne in mind. Two relevant examples are provided herein. These examples demonstrate that consensus-based analytical guidelines also require well-idealized building models that do not lend themselves to reasonably manageable representations from field data. As a corollary,loss modeling techniques,e.g.,used for insurance purposes,must undergo further development and improvement. 相似文献
930.
G. S. GOLITSYN 《大气科学进展》2009,26(3):585-598
The goal of this paper is to quantitatively formulate some necessary conditions for the
development of intense atmospheric vortices. Specifically, these criteria are discussed for tropical
cyclones (TC) and polar lows (PL) by using bulk formulas for fluxes of momentum, sensible heating,
and latent heating between the ocean and the atmosphere. The velocity scale is used in two forms: (1)
as expressed through the buoyancy flux b and the Coriolis parameter lc for rotating fluids convection,
and (2) as expressed with the cube of velocity times the drag coefficient through the formula for
total kinetic energy dissipation in the atmospheric boundary layer. In the quasistationary case the
dissipation equals the generation of the energy. In both cases the velocity scale can be expressed
through temperature and humidity differences between the ocean and the atmosphere in terms of the
reduced gravity, and both forms produce quite comparable velocity scales. Using parameters b and lc,
we can form scales of the area and, by adding the mass of a unit air column, a scale of the total
kinetic energy as well. These scales nicely explain the much smaller size of a PL, as compared to
a TC, and the total kinetic energy of a TC is of the order 1018-1019 J. It will be shown
that wind of 33 m s-1 is produced when the total enthalpy fluxes between the ocean and the
atmosphere are about 700 W m-2 for a TC and 1700 W m-2 for a PL, in association with
the much larger role of the latent heat in the first case and the stricter geostrophic constraints
and larger static stability in the second case. This replaces the mystical role of 26oC as
a criterion for TC origin.
The buoyancy flux, a product of the reduced gravity and the wind speed, together with the atmospheric
static stability, determines the rate of the penetrating convection. It is known from the observations
that the formation time for a PL reaching an altitude of 5--6 km can be only a few hours, and a day, or
even half a day, for a TC reaching 15--18 km. These two facts allow us to construct curves on the plane
of Ts and ΔT=Ts-Ta to determine possibilities for forming an intense vortex. Here, Ta is the atmospheric temperature at the height z=10 m. A PL should have ΔT>20oC in accordance
with the observations and numerical simulations. The conditions for a TC are not so straightforward but
our diagram shows that the temperature difference of a few degrees, or possibly even a fraction of a
degree, might be sufficient for TC development for a range of static stabilities and development times. 相似文献