全文获取类型
收费全文 | 30563篇 |
免费 | 6412篇 |
国内免费 | 6919篇 |
专业分类
测绘学 | 2122篇 |
大气科学 | 5125篇 |
地球物理 | 8572篇 |
地质学 | 16271篇 |
海洋学 | 3596篇 |
天文学 | 1596篇 |
综合类 | 3004篇 |
自然地理 | 3608篇 |
出版年
2024年 | 156篇 |
2023年 | 558篇 |
2022年 | 1232篇 |
2021年 | 1471篇 |
2020年 | 1257篇 |
2019年 | 1547篇 |
2018年 | 1764篇 |
2017年 | 1699篇 |
2016年 | 1984篇 |
2015年 | 1646篇 |
2014年 | 2017篇 |
2013年 | 2109篇 |
2012年 | 1915篇 |
2011年 | 1996篇 |
2010年 | 1880篇 |
2009年 | 1774篇 |
2008年 | 1721篇 |
2007年 | 1576篇 |
2006年 | 1315篇 |
2005年 | 1258篇 |
2004年 | 947篇 |
2003年 | 1036篇 |
2002年 | 1166篇 |
2001年 | 1086篇 |
2000年 | 1016篇 |
1999年 | 1191篇 |
1998年 | 874篇 |
1997年 | 938篇 |
1996年 | 863篇 |
1995年 | 737篇 |
1994年 | 597篇 |
1993年 | 552篇 |
1992年 | 420篇 |
1991年 | 315篇 |
1990年 | 247篇 |
1989年 | 196篇 |
1988年 | 181篇 |
1987年 | 132篇 |
1986年 | 112篇 |
1985年 | 79篇 |
1984年 | 54篇 |
1983年 | 48篇 |
1982年 | 37篇 |
1981年 | 29篇 |
1980年 | 43篇 |
1979年 | 30篇 |
1978年 | 16篇 |
1976年 | 11篇 |
1975年 | 14篇 |
1958年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
21.
A. P. M. Vaughan L. A. Dowling F. J. G. Mitchell S.‐E. Lauritzen A. M. McCabe P. Coxon 《第四纪科学杂志》2004,19(6):577-590
Organic‐rich deposits, uncovered during overburden removal from mantled gypsum karst at Knocknacran opencast gypsum mine, Co. Monaghan, are the best candidate to date for a last interglacial record in Ireland. The two till and organic‐rich deposits (preserved at different quarry elevations) were emplaced on to a Tertiary dolerite surface during high‐energy flood events and subsequently folded and faulted by movement towards sinkholes in underlying gypsum. Uranium–thorium disequilibrium dating suggests that the organic‐rich deposits in the upper section were hydrologically isolated at ca. 41 ka and those in the lower section at ca. 86 ka. Interpretation of the pollen content, although tentative because of the depositional and post‐depositional history of the material, suggests that the organic material originated in a warm stage possibly warmer than the post‐Eemian interstadials. The unusual setting of preservation may indicate that in situ, last interglacial deposits have generally been removed by erosion in Ireland. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
22.
This paper presents a numerical model for predicting the dynamic response of rock mass subjected to large‐scale underground explosion. The model is calibrated against data obtained from large‐scale field tests. The Hugoniot equation of state for rock mass is adopted to calculate the pressure as a function of mass density. A piecewise linear Drucker–Prager strength criterion including the strain rate effect is employed to model the rock mass behaviour subjected to blast loading. A double scalar damage model accounting for both the compression and tension damage is introduced to simulate the damage zone around the charge chamber caused by blast loading. The model is incorporated into Autodyn3D through its user subroutines. The numerical model is then used to predict the dynamic response of rock mass, in terms of the peak particle velocity (PPV) and peak particle acceleration (PPA) attenuation laws, the damage zone, the particle velocity time histories and their frequency contents for large‐scale underground explosion tests. The computed results are found in good agreement with the field measured data; hence, the proposed model is proven to be adequate for simulating the dynamic response of rock mass subjected to large‐scale underground explosion. Extended numerical analyses indicate that, apart from the charge loading density, the stress wave intensity is also affected, but to a lesser extent, by the charge weight and the charge chamber geometry for large‐scale underground explosions. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
23.
THECONSTRUCTIONANDITSDEVELOPMENTOFTHEOVERSEASTRANSPORTSYSTEMINNORTHEASTCHINAGaoShali(高莎丽)(DepartmentofGeography,NortheastNorm... 相似文献
24.
Teruyuki Maruoka Christian Koeberl Jun‐ichi Matsuda Yasuhiko Syono 《Meteoritics & planetary science》2003,38(8):1255-1262
Abstract— Carbon isotopic compositions were measured for shock‐produced diamond and shocked graphite formed at peak pressures ranging from 37 to 52 GPa. The δ13C values of diamonds produced in a sealed container were generally lower than that of the initial graphite. The differences in the carbon isotopic composition between initial graphite and shocked graphite/diamond may reflect kinetic isotopic fractionation during the oxidation of the graphite/diamond and/or analytical artifacts possibly induced by impurities in the samples. The pressure effect on the isotopic fractionations between graphite and diamond can be estimated from the δ13C values of impurity‐free diamonds produced using a vented container from which gases, including oxygen, in pore spaces escaped during or after the diamond formation (e.g., 0.039 ± 0.085‰ at a peak pressure of 52 GPa). Any isotopic fractionation induced by shock conversion of graphite to diamond is too small to be detected in natural shock‐induced diamond‐graphite systems related to terrestrial impact cratering processes. 相似文献
25.
APPLICATION OF GEOGRAPHICAL PARAMETER DATABASE TO ESTABLISHMENT OF UNIT POPULATION DATABASE 总被引:1,自引:1,他引:0
Now GIS is turning into a good tool in handling geographical, economical, and population data, so we can obtain more and more information from these data. On the other hand, in some cases, for a calamity, such as hurricane, earthquake, flood, drought etc., or a decision-making, such as setting up a broadcasting transmitter, building a chemical plant etc., we have to evaluate the total population in the region influenced by a calamity or a project. In this paper, a method is put forward to evaluate the population in such special region. Through exploring the correlation of geographical parameters and the distribution of people in the same region by means of quantitative analysis and qualitative analysis, unit population database (1km× 1km) is established. In this way, estimating the number of people in a special region is capable by adding up the population in every grid involved in this region boundary. The geographical parameters are obtained from topographic database and DEM database on the scale of 相似文献
26.
A THREE-DIMENSIONAL ELASTIC NESTED-GRID MESO-(β-γ)SCALE ATMOSPHERIC MODEL——PARTI:MODEL DESCRIPTION 下载免费PDF全文
A three-dimensional elastic nonhydrostatic mesoscale(β-γ)model with nested-grid is presented.It uses a set of fullequations in terrain-following coordinates as its basic dynamic frame,which is solved with a time-splitting algorithmfor acoustic and gravity waves.The model physical parameterization includes a K-theory subgrid eddy mixing for cloudand free atmosphere,a bulk planetary boundary layer parameterization,and three types of sofisticated cloudmicrophysics schemes with double-parameters for hail-bearing clouds,warm clouds and snowing clouds respectively.The model is designed to be used flexibly for simulations of a variety of meso-and small-scale atmospheric processes,and can be improved as a regional and local operational NWP system in future. 相似文献
27.
本文对徐州地震台JB—72水平摆倾斜仪1988年11月至1989年5月间所记录到的24次同震阶变进行了深入分析,认为:a.造成同震阶变的原因是多方面的,而反应了地球应变场准静态调整的同震阶变才是有价值的价变异常;b.多观测手段的地震台站,同震阶变应该在不止一台仪器上得到响应。 相似文献
28.
29.
We studied the behavior of magnetic field, horizontal motion and helicity in a fast emerging flux region NOAA 10488 which
eventually forms a δ spot. It is found that the rotation of photospheric footpoints forms in the earlier stage of magnetic
flux emergence and the relative shear motion of different magnetic flux systems appears later in this active region (AR).
Therefore the emerging process of the AR can be separated into two phases: rotation and shear. We have computed the magnetic
helicity injected into the corona using the local correlation tracking (LCT) technique. Furthermore we determined the vertical
component of current helicity density and the vertical component of induction electric fields Ez = (V× B)z in the photosphere. Particularly we have presented the comparison of the injection rate of magnetic helicity and the variation
of the current helicity density. The main results are as follows: (1) The strong shear motion (SSM) between the new emerging
flux system and the old one brings more magnetic helicity into the corona than the twisting motions. (2) After the maturity
of the main bipolar spots, their twist decreases and the SSM becomes dominant and the major contributor of magnetic non-potentiality
in the solar atmosphere in this AR. (3) The positions of the maxima of Ez (about 0.1 ∼ 0.2 V cm−1) shift from the twisting areas to the areas showing SSMs as the AR evolved from the rotation phase to the shear one, but
no obvious correlation is found between the kernels of Hα flare and Ez for the M1.6 flare in this AR. (4) The coronal helicity inferred from the horizontal motion of this AR amounts to −6 × 1043 Mx2. It is comparable with the coronal helicity of ARs producing flares with coronal mass ejections (CMEs) or helicity carried
away by magnetic clouds (MCs) reported in previous studies (Nindos, Zhang, and Zhang, 2003; Nindos and Andrews, 2004). In
addition, the formation of the δ configuration in this AR belongs to the third formation type indicated by Zirin and Liggett
(1987), i.e., collision of opposite polarities from different dipoles, and can be naturally explained by the SSM. 相似文献
30.
A coupled deterministic hydrological and water temperature model, CEQUEAU, was modified to include soil temperature and crown closure in its calculation of local advective terms in the heat budget. The modified model was than tested to verify its sensitivity to these modifications. An analysis of the heat budget of a small forested catchment in eastern Canada revealed that the advective term related to interflow plays a significant role in the daily water heat budget, providing on average 28% of the local advective budget (which also includes advective heat terms from surface runoff and groundwater) and nearly 14% of the total heat budget (which includes all radiative terms at the water surface, convection and evaporation, as well as the local advective terms). Relative sensitivity indices (RSIs) were used to verify the impact of the newly introduced parameters and variables. Among them, parameters related to the forest cover (crown closure and leaf area index) have a maximum RSI of ?0·6; i.e. a 100% increase in value produces a 60% decrease in the local advective term. Parameters with the greatest influence are the volume of water contributing to interflow and the amplitude of the net radiative flux at the soil surface, which, if doubled, would double the contribution of the local interflow advective term to the heat budget. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献