全文获取类型
收费全文 | 72285篇 |
免费 | 940篇 |
国内免费 | 462篇 |
专业分类
测绘学 | 1805篇 |
大气科学 | 5372篇 |
地球物理 | 13603篇 |
地质学 | 28542篇 |
海洋学 | 5666篇 |
天文学 | 14626篇 |
综合类 | 250篇 |
自然地理 | 3823篇 |
出版年
2021年 | 412篇 |
2020年 | 478篇 |
2019年 | 492篇 |
2018年 | 3856篇 |
2017年 | 3597篇 |
2016年 | 2762篇 |
2015年 | 981篇 |
2014年 | 1361篇 |
2013年 | 2621篇 |
2012年 | 2520篇 |
2011年 | 4548篇 |
2010年 | 4067篇 |
2009年 | 4697篇 |
2008年 | 3837篇 |
2007年 | 4456篇 |
2006年 | 2039篇 |
2005年 | 1990篇 |
2004年 | 1884篇 |
2003年 | 1876篇 |
2002年 | 1633篇 |
2001年 | 1337篇 |
2000年 | 1267篇 |
1999年 | 1053篇 |
1998年 | 1025篇 |
1997年 | 1061篇 |
1996年 | 848篇 |
1995年 | 833篇 |
1994年 | 792篇 |
1993年 | 668篇 |
1992年 | 662篇 |
1991年 | 595篇 |
1990年 | 647篇 |
1989年 | 562篇 |
1988年 | 536篇 |
1987年 | 625篇 |
1986年 | 550篇 |
1985年 | 709篇 |
1984年 | 746篇 |
1983年 | 771篇 |
1982年 | 704篇 |
1981年 | 642篇 |
1980年 | 689篇 |
1979年 | 566篇 |
1978年 | 574篇 |
1977年 | 524篇 |
1976年 | 503篇 |
1975年 | 499篇 |
1974年 | 470篇 |
1973年 | 473篇 |
1971年 | 312篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
62.
63.
64.
The structure, functioning and hydrodynamic properties of aquifers can be determined from an analysis of the spatial variability of baseflow in the streams with which they are associated. Such analyses are based on simple low‐cost measurements. Through interpreting the hydrological profiles (Q = f(A)) it is possible to locate the aquifer(s) linked to the stream network and to determine the type of interrelated flow, i.e. whether the stream drains or feeds the aquifer. Using an analytical solution developed for situations with a positive linear relationship, i.e. where the baseflow increases linearly with increasing catchment size, it is also possible to estimate the permeability of the aquifer(s) concerned at catchment scale. Applied to the hard‐rock aquifers of the Oman ophiolite, this method shows that the ‘gabbro’ aquifer is more permeable than the ‘peridotite’ aquifer. As a consequence the streams drain the peridotites and ‘leak’ into the gabbro. The hydrological profiles within the peridotite are linear and positive, and indicate homogeneity in the hydrodynamic properties of these formations at the kilometre scale. The permeability of the peridotite is estimated at 5 · 10?7 to 5 · 10?8 m/s. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
65.
B. A. Ivanov 《Solar System Research》2005,39(5):381-409
Multi-ring impact basins have been found on the surfaces of almost all planetary bodies in the Solar system with solid crusts. The details of their formation mechanism are still unclear. We present results of our numerical modeling of the formation of the largest known terrestrial impact craters. The geological and geophysical data on these structures accumulated over many decades are used to place constraints on the parameters of available numerical models with a dual purpose: (i) to choose parameters in available mechanical models for the crustal response of planetary bodies to a large impact and (ii) to use numerical modeling to refine the possible range of original diameters and the morphology of partially eroded terrestrial craters. We present numerical modeling results for the Vredefort, Sudbury, Chicxulub, and Popigai impact craters and compare these results with available geological and geophysical information. 相似文献
66.
Parametric transduction offers valuable advantages for underwater acoustic communications. Perhaps the most significant benefit is the fact that high directivity is achieved by means of a physically small transmit transducer. This feature may, ultimately, be employed to permit long-range, low-frequency communication using a compact source. The high directivity is desirable to combat multipath propagation and to achieve data communications in water which is shallow by comparison with range. A real-time, high data-rate “model” differential phase shift keying (DPSK) communication system has been constructed and demonstrated. This system uses parametric transduction, with a 300-kHz primary frequency and a 50-kHz secondary frequency. Experimental results show that the system can be employed to combat multipath propagation in shallow water and can achieve high data-rate text and color image transmission at 10 and 20 kb s-1 for 2-DPSK and 4-DPSK, respectively, through a transmission bandwidth of 10 kHz. The “model” system was developed to confirm performance predictions for a future, operational long-range link employing a 50-kHz primary frequency and a 5-kHz secondary frequency 相似文献
67.
68.
V. Delouille J. De Patoul J. F. Hochedez L. Jacques J. P. Antoine 《Solar physics》2005,228(1-2):301-321
The extreme ultraviolet imaging telescope (EIT) of SOHO offers a unique record of the solar atmosphere for its sampling in
temperature, field of view, resolution, duration, and cadence. To investigate globally and locally its topology and evolution
during the solar cycle, we consider a multi-scale approach, and more precisely we use the wavelet spectrum.
We present three results among the applications of such a procedure. First, we estimate the typical dimension of the supergranules
as seen in the 30.4 nm passband, and we show that the evolution of the characteristic network scale is almost in phase with
the solar cycle. Second, we build pertinent time series that give the evolution of the signal energy present in the corona
at different scales. We propose a method that detects eruptions and post-flaring activity in EUV image sequences. Third, we
introduce a new way to extract active regions in EIT images, with perspectives in, e.g., long-term irradiance analysis. 相似文献
69.
70.
The orbits of (69230) Hermes and 2002 SY50 are similar and the Earth approaches both of them twice: at the end of October the local orbital minimum distances are smaller
than 0.007 AU, and at the end of April the distances are smaller than 0.04 AU. This gives us opportunities to observe the
meteors associated with these asteroids. Using the geocentric parameters of the orbital close encounters (the theoretical
radiants) and our D
N
distance function (Valsecchi et al. Mon. Not. R. Astron. Soc. 304 (1999) 743), we searched for meteoroids originated by Hermes and 2002 SY50. A search among 1830 good quality photographic meteors gave negative results: we found no meteor dynamically similar to Hermes
or 2002 SY50. In a second search, done in a set of 62150 radio meteors, we applied two methods (M1, M2) and in both cases we found two
streams; the streams found with the M1 method had 43 and 30 members, those found with the M2 method had 39 and 14 members.
However, these results do not look convincing, due to the small number of common members in the corresponding streams. We
therefore conclude that amongst the IAU meteors used in our search there are no compact streams associated with Hermes and
2002 SY50. 相似文献