全文获取类型
收费全文 | 113392篇 |
免费 | 1935篇 |
国内免费 | 790篇 |
专业分类
测绘学 | 2878篇 |
大气科学 | 8480篇 |
地球物理 | 22542篇 |
地质学 | 40899篇 |
海洋学 | 9811篇 |
天文学 | 24161篇 |
综合类 | 314篇 |
自然地理 | 7032篇 |
出版年
2021年 | 668篇 |
2020年 | 812篇 |
2019年 | 861篇 |
2018年 | 4188篇 |
2017年 | 3960篇 |
2016年 | 3529篇 |
2015年 | 1535篇 |
2014年 | 2197篇 |
2013年 | 4741篇 |
2012年 | 3252篇 |
2011年 | 5670篇 |
2010年 | 5271篇 |
2009年 | 6447篇 |
2008年 | 5441篇 |
2007年 | 5823篇 |
2006年 | 3538篇 |
2005年 | 3426篇 |
2004年 | 3339篇 |
2003年 | 3224篇 |
2002年 | 2942篇 |
2001年 | 2451篇 |
2000年 | 2392篇 |
1999年 | 2008篇 |
1998年 | 2019篇 |
1997年 | 1998篇 |
1996年 | 1737篇 |
1995年 | 1647篇 |
1994年 | 1493篇 |
1993年 | 1363篇 |
1992年 | 1299篇 |
1991年 | 1133篇 |
1990年 | 1323篇 |
1989年 | 1148篇 |
1988年 | 1069篇 |
1987年 | 1261篇 |
1986年 | 1141篇 |
1985年 | 1405篇 |
1984年 | 1598篇 |
1983年 | 1500篇 |
1982年 | 1382篇 |
1981年 | 1381篇 |
1980年 | 1172篇 |
1979年 | 1125篇 |
1978年 | 1153篇 |
1977年 | 1082篇 |
1976年 | 994篇 |
1975年 | 944篇 |
1974年 | 949篇 |
1973年 | 964篇 |
1972年 | 591篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
Analytical study of ground motion caused by seismic wave propagation across faulted rock masses 下载免费PDF全文
Studying seismic wave propagation across rock masses and the induced ground motion is an important topic, which receives considerable attention in design and construction of underground cavern/tunnel constructions and mining activities. The current study investigates wave propagation across a rock mass with one fault and the induced ground motion using a recursive approach. The rocks beside the fault are assumed as viscoelastic media with seismic quality factors, Qp and Qs. Two kinds of interactions between stress waves and a discontinuity and between stress waves and a free surface are analyzed, respectively. As the result of the wave superposition, the mathematical expressions for induced ground vibration are deduced. The proposed approach is then compared with the existing analysis for special cases. Finally, parametric studies are carried out, which includes the influences of fault stiffness, incident angle, and frequency of incident waves on the peak particle velocities of the ground motions. 相似文献
32.
The present study used two mitochondrial markers (16S rRNA and COI) to assess the genetic diversity of a newly founded Lessepsian migrant mussel, Brachidontes pharaonis, in Tunisian waters. The species appears to be restricted to only one population in Rades Harbour, in the northern part of the country. Phylogenetic analyses revealed the monophyly of B. pharaonis in Tunisia. Both molecular markers revealed high genetic variability of the B. pharaonis population. Haplotype networks and demographic analyses confirmed the recent expansion events within this population. Multiple human-mediated introduction events involving several founder populations and intensive population growth rates are probably the main causes of the high polymorphism observed within this invasive mollusc. 相似文献
33.
34.
Analytical solutions for generalizing the Ekman stationary flow of a viscous incompressible fluid in an infinite layer are obtained. The solution of an overdetermined system of the Oberbeck–Boussinesq equations is considered. It is suggested to use a class of exact solutions for this problem. It is shown that the structure of the solutions allows one to preserve the advective derivative in the heat-conductivity equation; this makes it possible to model the stratification of the temperature and pressure fields and describe the oceanic countercurrents. 相似文献
35.
Boyarskikh I. G. Agatova A. R. Kulikova A. I. Bakiyanov A. I. Skaptsov M. V. Mazeika J. 《Izvestiya Atmospheric and Oceanic Physics》2018,54(8):805-825
Izvestiya, Atmospheric and Oceanic Physics - Areas with large gradients and anomalous variations in geomagnetic and radiation fields have been revealed and mapped at the northwestern foot of the... 相似文献
36.
This paper examines the small-scale solar wind turbulence driven in view of the Alfvén waves subjected to ponderomotive nonlinearity. Filamentation instability is known to take place for the case of dispersive Alfvén wave (DAW) propagating parallel to the ambient magnetic field. The ponderomotive force associated with DAW is responsible for wave localization and these webs of filaments become more intense and irregular as one proceeds along the spatial domain. The ponderomotive force associated with pump changes with pump parameters giving rise to different evolution patterns. This paper studies in detail the nonlinear evolution of filamentation instability introduced by dispersive Alfven waves (DAWs) which becomes dispersive on account of the finite frequency of DAW i.e., pump frequency is comparable to the ion cyclotron frequency. We have explicitly obtained the perturbation dynamics and then examined the impact of pump magnitude on the driven magnetic turbulence using numerical simulation. The results show steepening at small scales with increasing pump amplitude. The compressibility associated with acoustic fluctuations may explain the variation in spectral scaling of solar wind turbulence as observed by Alexandrova et al. (Astrophys. J. 674:1157, 2008). 相似文献
37.
Ophioleila elegans是一种非常少见的蛇尾。2014年大洋35航次中利用“蛟龙”号载人潜器在西北太平洋麦哲伦海山链的采薇平定海山水深1660m处采集到2株该种标本。该种首次发现于夏威夷海山相似的生境当中,本次采集到的样品是继首次发现之后的再次公开报道。本文中对该种的形态学特征,如腕部骨骼等,进行了更加详细的研究,同时基于COI序列进行系统发育分析。形态和系统发育分析的结果都表明,Ophioleila属与辐蛇尾科更加接近。 相似文献
38.
Factors influencing biodeposit production by the New Zealand freshwater mussel Echyridella menziesii
Kevin J. Collier Susan J. Clearwater Paula H. M. W. Neijenhuis Susanna A. Wood 《新西兰海洋与淡水研究杂志》2018,52(1):1-15
Laboratory experiments on the New Zealand freshwater mussel Echyridella menziesii were used to investigate the short-term effects (7–8 days) of food type on rates of biodeposition and benthic substrate respiration. Post-feeding biodeposition rates ranged from 0.34 to 1.52?mg?g?1?h?1 (mean?=?0.50?mg g?1?h?1) and were unaffected by the addition of toxin-producing Microcystis. Addition of suspended sediment (30?mg?L?1) visibly altered substrate composition, and increased total and inorganic biodeposit production rates by 24–33% compared to mussels fed commercial phytoplankton stock. Biodeposition rates of mussels in lake bed substrates were 38% higher than those in silica sand for identical feeding regimes, suggesting that a significant proportion of material produced in this experiment could have been derived from feeding on organic matter in the lake bed sediments. Respiration rates were higher in treatments with Microcystis but were unaffected by the presence of mussels. This laboratory study suggests that biodeposition by E. menziesii is resilient to short-term exposure to Microcystis, and highlights the ability of mussels to alter benthic substrate composition by incorporating suspended sediment into substrates. 相似文献
39.
Here, we describe a methodology for quantifying the spawning habitat of īnanga (Galaxias maculatus), a protected native fish species. Our approach is demonstrated with a survey of the Heathcote/ōpāwaho following the Canterbury earthquakes that produced unexpected findings. Spawning habitat was detected over a 2.5?km reach and the area occupied by spawning sites (75m2) was much larger than in previous records (ca.?21m2). Sites dominated by the invasive Phalaris arundinaceae were found to support high egg numbers. Spawning has not previously been recorded on this species and it is identified in the literature as a threat to spawning habitat. Considerable spatio-temporal variation was also detected in the location of spawning sites and pattern of egg production. Together, these aspects illustrate the need for a comprehensive survey methodology to reliably quantify spawning habitat. The Heathcote/ōpāwaho example shows the utility of our census approach for achieving this, and supporting habitat conservation objectives. 相似文献
40.
L. F. Chernogor 《Solar System Research》2018,52(3):206-222
System spectral analysis of temporal variations in the level of acoustic signals recorded at a number of European infrasound stations is carried out. The prevailing periods that varied within 3–5 s are found. Initial kinetic and acoustic energies of the Romanian meteoroid and infrasound parameters are calculated: celerity (280 m/s), acoustic efficiency, and stratospheric wind velocity (about 20 m/s). Parameters of cylindrical and explosive shock waves are found: duration, characteristic size, and amplitude. The dependence of the attenuation of infrasonic waves on distance is estimated. The results of the estimates are in good agreement with the results of the observations. 相似文献