全文获取类型
收费全文 | 254792篇 |
免费 | 4979篇 |
国内免费 | 3348篇 |
专业分类
测绘学 | 6847篇 |
大气科学 | 18876篇 |
地球物理 | 53473篇 |
地质学 | 88153篇 |
海洋学 | 21159篇 |
天文学 | 55801篇 |
综合类 | 1005篇 |
自然地理 | 17805篇 |
出版年
2021年 | 2230篇 |
2020年 | 2584篇 |
2019年 | 2835篇 |
2018年 | 3186篇 |
2017年 | 2820篇 |
2016年 | 5539篇 |
2015年 | 4184篇 |
2014年 | 6879篇 |
2013年 | 14189篇 |
2012年 | 6360篇 |
2011年 | 7562篇 |
2010年 | 6570篇 |
2009年 | 9218篇 |
2008年 | 8113篇 |
2007年 | 7488篇 |
2006年 | 9624篇 |
2005年 | 7605篇 |
2004年 | 7561篇 |
2003年 | 7056篇 |
2002年 | 6688篇 |
2001年 | 5982篇 |
2000年 | 5925篇 |
1999年 | 5196篇 |
1998年 | 5216篇 |
1997年 | 5026篇 |
1996年 | 4671篇 |
1995年 | 4423篇 |
1994年 | 4090篇 |
1993年 | 3841篇 |
1992年 | 3624篇 |
1991年 | 3583篇 |
1990年 | 3749篇 |
1989年 | 3508篇 |
1988年 | 3301篇 |
1987年 | 3844篇 |
1986年 | 3404篇 |
1985年 | 4216篇 |
1984年 | 4733篇 |
1983年 | 4400篇 |
1982年 | 4307篇 |
1981年 | 3923篇 |
1980年 | 3634篇 |
1979年 | 3504篇 |
1978年 | 3478篇 |
1977年 | 3275篇 |
1976年 | 3044篇 |
1975年 | 2956篇 |
1974年 | 2914篇 |
1973年 | 3073篇 |
1972年 | 2024篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
821.
C. Christiansen F. Gertz M. J. C. Laima L. C. Lund-Hansen T. Vang C. Jürgensen 《Environmental Geology》1997,29(1-2):66-77
The yearly nutrient supply from land and atmosphere to the study area in SW Kattegat is 10 900 tons of N and 365 tons of
P. This is only few percent of the supply from adjacent marine areas, as the yearly transport through the study area is 218 000
tons of N and 18 250 tons of P. Yearly net deposition makes up 1340 tons of N (on average 2.5 g m–2 yr–1) and 477 ton of P (on average 0.9 g m–2 yr–1). Shallow-water parts of the study area have no net deposition because of frequent (>35% of the year) resuspension. Resuspension
frequency in deep water is <1% of the year. Resuspension rates, as averages for the study area, are 10–17 times higher than
net deposition rates. Because of resuspension, shallow-water sediments are coarse lag deposits with small amounts of organic
matter (1.1%) and nutrients (0.04% N and 0.02% P). Deep-water sediments, in contrast, are fine grained with high levels of
organic matter (11.7%) and nutrients (0.43% N and 0.15% P). Laboratory studies showed that resuspension changes the diffusive
sediment water fluxes of nutrients, oxygen consumption, and penetration into the sediment. Fluxes of dissolved reactive phosphate
from sediment to water after resuspension were negative in organic-rich sediments (13.2% organic matter) with low porosity
(56) and close to zero in coarse sediments with a low organic matter content (2.3%) and high porosity (73). Fluxes of inorganic
N after resuspension were reduced to 70% and 0–20% in relation to the rates before resuspension, respectively.
Received: 10 July 1995 · Accepted: 19 January 1996 相似文献
822.
A new method of initial orbit determination 总被引:2,自引:0,他引:2
Up to now we have been dealing with the construction of entirely analytical planetary theories such as VSOP82 (Bretagnon, 1982) and TOP82 (Simon, 1983). These theories take into account the whole of the Newtonian perturbations of nine point masses: the Sun, the Earth-Moon barycentre, the planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune. They also take into account perturbations due to some minor planets, to the action of the Moon and the relativistic effects. The perturbations of these last three types are in a very simple way under analytical form but they considerably increase the computations when introduced in the numerical integration programs.In the present paper we thus study a solution in which the Newtonian perturbations for the ten point masses are treated through numerical integration, the other perturbations being analytically added. 相似文献
823.
L. Zaninetti 《Astrophysics and Space Science》1985,117(2):287-292
We present optical photometric observations of the cataclysmic variables AO Psc and V 1223 Sgr, both of which are believed to contain slowly-rotating white-dwarf pulsars. These objectes belong to the new class of intermediate polars in which the magnetized-white-dwarf rotation period lies in between synchrous and extremely fast rotation periods. The observations reported here were carried out from the Kavalur Observatory using a 1 m reflector. High-time resolution light curves depicting a rich variety of flicker phenomena are presented. Results of a periodic analysis to search for coherent periodic modulations in the light curves are also presented. We have also derived the optical pulse profile of AO Psc and point out its similarities to the X-ray pulse profile. 相似文献
824.
Berkson J. Kloosterman H. Akal T. Berrou J. 《Oceanic Engineering, IEEE Journal of》1985,10(3):299-302
Signals from an explosive source backscattered from the seafloor and received at long range by hydrophones of a towed array are processed to estimate the directional distribution of energy for a given time increment. As assembly of these data shows the time and amplitude of scattering features, and after conversion to distance, the geographic location of the return. A frequency-domain beam-forming procedure is used in which beam levels are averaged over a given band of a broad-band source. The processing is applied to experimental data obtained in the southern Tyrrhenian Sea. The major backscattering occurred at the Baconi Seamounts and the coastal margin of Sardinia. 相似文献
825.
The three-dimensional geometry of the heliospheric current sheet seen from fixed points in interplanetary space is constructed for idealized (sinusoidal) magnetic neutral lines (equators) and for an observed magnetic equator on the basis of the “kinematic method” developed by Hakamada and Akasofu (1982). The cross-sections of the wavy current sheet at distances 1, 2 and 5 a.u. are also constructed for the idealized magnetic neutral lines. 相似文献
826.
Forward modeling of zero-offset data is performed in the frequency-space domain using a one-way extrapolation equation. The use of the frequency domain offers several advantages over conventional time domain methods. The greatest advantage of the frequency domain is that all time derivatives are evaluated exactly by a simple multiplication. Synthetic zero-offset sections are computed with a high degree of accuracy for arbitrary velocity and reflectivity structures. Examples are shown for realistic complicated models and compared with results from physical modeling. 相似文献
827.
828.
829.
830.
C. Giraudi 《第四纪科学杂志》2004,19(6):537-545
The island of Lampedusa lies on the northern edge of the African continental shelf, but during some Quaternary marine lowstands it was joined to the African continent. The study and dating of the aeolian, alluvial, detrital sediments, calcareous crusts and speleothems have established that the climatic–environmental variations recorded on the island can be related chronologically to those known for northern Libya, Tunisia and the Italian peninsula. During the Last Glacial Maximum, phases of Saharan dust accumulation on Lampedusa occurred, and were coeval with dust accumulation in crater lakes and on high mountains in central‐southern Italy, and with phases of glacial advance in the Apennines and in the Alps. During the late Holocene, accumulation of Saharan dust on Lampedusa occurred but there was little accumulation of dust on the northern side of the Mediterranean Sea. With the new data from Lampedusa, it is possible to envisage two different scenarios of atmospheric circulation relating to the Last Glacial Maximum and to the late Holocene. During the Last Glacial Maximum, southerly atmospheric circulation brought rainfall to the southern slopes of the Alps and to the Apennines. During the late Holocene, a prevalent westerly atmospheric circulation became established in the northern Mediterranean. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献