全文获取类型
收费全文 | 78271篇 |
免费 | 1435篇 |
国内免费 | 743篇 |
专业分类
测绘学 | 2050篇 |
大气科学 | 5656篇 |
地球物理 | 15917篇 |
地质学 | 27891篇 |
海洋学 | 6686篇 |
天文学 | 17865篇 |
综合类 | 250篇 |
自然地理 | 4134篇 |
出版年
2022年 | 455篇 |
2021年 | 817篇 |
2020年 | 884篇 |
2019年 | 932篇 |
2018年 | 2119篇 |
2017年 | 2022篇 |
2016年 | 2565篇 |
2015年 | 1479篇 |
2014年 | 2410篇 |
2013年 | 4093篇 |
2012年 | 2609篇 |
2011年 | 3317篇 |
2010年 | 2906篇 |
2009年 | 3780篇 |
2008年 | 3205篇 |
2007年 | 3236篇 |
2006年 | 3046篇 |
2005年 | 2326篇 |
2004年 | 2362篇 |
2003年 | 2240篇 |
2002年 | 2086篇 |
2001年 | 1891篇 |
2000年 | 1804篇 |
1999年 | 1480篇 |
1998年 | 1545篇 |
1997年 | 1450篇 |
1996年 | 1214篇 |
1995年 | 1190篇 |
1994年 | 1077篇 |
1993年 | 924篇 |
1992年 | 894篇 |
1991年 | 852篇 |
1990年 | 914篇 |
1989年 | 763篇 |
1988年 | 732篇 |
1987年 | 823篇 |
1986年 | 740篇 |
1985年 | 934篇 |
1984年 | 1037篇 |
1983年 | 960篇 |
1982年 | 893篇 |
1981年 | 820篇 |
1980年 | 786篇 |
1979年 | 763篇 |
1978年 | 781篇 |
1977年 | 643篇 |
1976年 | 607篇 |
1975年 | 645篇 |
1974年 | 559篇 |
1973年 | 598篇 |
排序方式: 共有10000条查询结果,搜索用时 658 毫秒
991.
992.
The paper presents a theoretical and an experimental investigation into the buckling of seven oblate hemi-ellipsoidal dome shells under external hydrostatic pressure. Four of the shells were made in glass reinforced plastic and three were made from a thermosetting plastic called solid urethane plastic. All the vessels were tested to destruction. The theoretical study was made with the aid of a non-linear finite element solution, where both geometrical and material non-linearity were allowed for. Good agreement was found between experiment and theory for all the vessels. The very oblate domes failed axisymmetrically. Theoretical convergence was good for the more oblate domes but it was not as good as for the less oblate domes. This may have been because the less oblate domes did not fail in a classical axisymmetric manner as was expected. This work is of much importance in ocean engineering. 相似文献
993.
The mechanism of the effect of a collapsing turbulent eddy on diapycnal transport in a stably stratified fluid is considered. It is shown that at small Richardson turbulent numbersRi
0 the mixing efficiency increases asRi
0, and at large numbers it decreases in proportion toRi
0
–1/2
.Translated by Mikhail M. Trufanov. UDK 551.465.15. 相似文献
994.
Peculiarities of the dynamics and the structure formation in the estuary run-off lenses have been analysed based on field data obtained at the Guinean coast (the central eastern Atlantic Ocean). Attention was paid to the processes of restratification, internal mixing, and exchange in the areas of the frontal zone of the run-off lens and the tidal frontal zone. The stabilization of the front of the estuary run-off lens by means of the periodic washing out of its forepart by tidal upwelling is described. The mechanisms of the energy transformation in the leading part of the lens have been revealed. Effects of run-off and loczalization of transformed waters in the small-scale frontal zones are found. The energy estimations represented will be useful in considering analogous phenomena in other coastal regions.Translated by Mikhail M. Trufanov. 相似文献
995.
996.
The data on the species composition, trophic structure, and distribution of macrozoobenthos in Amur Bay obtained in 2001 are presented. Long-term changes in the benthos are analyzed. In 2001, the total benthos biomass significantly increased, although the parameters of the species richness declined as compared to 1986–1989. In 2001, as well as in the 1970s and 1980s, the benthos trophic structure was characterized by the prevalence of deposit feeders. The entire structure of the benthos is evaluated as an eutrophic one. Eutrophication of the bay is considered to be the most probable cause of the negative changes in the benthic communities of the bay in 2001, as well as 15 years ago. 相似文献
997.
V. N. Aref’ev F. V. Kashin V. K. Semenov R. M. Akimenko N. E. Kamenogradskii N. I. Sizov V. P. Sinyakov L. B. Upenek V. P. Ustinov 《Izvestiya Atmospheric and Oceanic Physics》2006,42(6):739-751
Refined data of systematic measurements of total water vapor in the atmosphere from May 1980 to April 2005 are presented. The data were obtained at the Issyk Kul atmospheric-monitoring station by the method of solar molecular-absorption spectroscopy. Over 25 years, the annual mean water-vapor content in the atmosphere increased by 4.5% at a mean rate of increase of 0.18% per year. However, the water-vapor content decreased in the last five years. The results of statistical processing of experimental data (general statistical characteristics, correlation coefficients, composite oscillations) are described. A refined model is proposed for forecasts of temporal variations in the monthly mean and annual mean water-vapor contents for the coming years. The model includes a linear trend and the sum of oscillations with periods close to the periods of a number of well-known geophysical phenomena. Regression equations are proposed to relate the water-vapor content in the atmospheric column to the surface temperature and absolute humidity. 相似文献
998.
Martina A. Doblin Stephen B. Baines Lynda S. Cutter Gregory A. Cutter 《Estuarine, Coastal and Shelf Science》2006,67(4):681-694
As part of a study of estuarine selenium cycling, we measured the concentration, chemical form (speciation), and distribution of particulate selenium under various river flow conditions in the North San Francisco Bay (from the Golden Gate to the Sacramento and San Joaquin Rivers). We also conducted laboratory studies on the accumulation of selenium by phytoplankton, the critical first step in the transformation of dissolved to particulate selenium. Total particulate selenium concentration in the North SF Bay was relatively constant between high and low flow periods, ranging spatially from 0.05 to 0.35 nmol l−1 and comprising between 5 and 12% of the total water column selenium inventory. Mean concentrations were generally highest in the Carquinez Strait–Suisun Bay region (salinity 0–17) and lowest in Central Bay. However, selenium content of suspended particles varied with river flow, with higher content during low flow (9.76 ± 4.17 nmol g−1; mean ± sd; n = 67) compared to high flow (7.10 ± 4.24 nmol g−1; n = 39). Speciation analyses showed that most particulate selenium is organic selenide (45 ± 27%), with a smaller proportion (typically <30%) of adsorbed selenite + selenate and a varying proportion (35 ± 28%) of elemental selenium. Based on the amount of elemental selenium in the seston (total suspended material), we calculate that resuspension of estuarine sediments could contribute 29–100% of particulate selenium in the water column. While selenium content of SF Bay seston (>0.4 μm) is relatively unenriched compared to phytoplankton (13.6–155 nmol g−1 dry weight) on a mass basis, when normalized to carbon or nitrogen, seston contains a similar selenium concentration to SF Bay sediments or phytoplankton cultures. SF Bay seston is thus comprised of selenium-rich phytoplankton and phyto-detritus, but also inorganic clay mineral particles that effectively “dilute” total particulate selenium. Selenium concentrations in algal cultures (11 species) exposed to 90 nmol l−1 selenite show relatively large differences in selenium accumulation, with the diatoms, chlorophytes and cryptophytes generally having lower selenium cell content (3.8 ± 2.7 × 10−9 nmol selenium cell−1) compared to the dinoflagellates (193 ± 73 × 10−9 nmol selenium cell−1). Because phytoplankton are such a rich (but variable) source of selenium, their dynamics could have a profound effect on the particulate selenium inventory in the North SF Bay. 相似文献
999.
Multi-channel seismic reflection data from the western continental margin of India (WCMI) have been analyzed to construct a plausible model for gas hydrate formation. A reflector at 2950 ms two way travel time (TWT) on one of the sections is interpreted to represent the base of the layer of the methane hydrate, identified by a bottom simulating reflector (BSR) that lies almost 500 ms beneath the sea floor. BSRs of similar origin are common world wide, where they are usually interpreted to mark the base of gas hydrate bearing clastic sediment, with or without underlying free gas. In this study we present a model with the contrasting physical properties that produce synthetic wavelets that match with the observed BSR amplitude and waveforms for varying source-receiver offsets of multi-channel seismic reflection data. The preliminary results presented here put important constraints on models that predict the distribution and formation of hydrate. Offset-dependent amplitude recovery also gives an appropriate response for hydrate characterization. 相似文献
1000.
Using ultrasonic scanning it is possible to display the position of an acoustic discontinuity within a sediment core and the amplitude of the reflected signal which results from it. From these results it has been possible to examine the effect of variations in the geotechnical properties on the reflection of compressional waves at normal incidence within the core at a frequency of 100 kHz. The occurrence of acoustic reflectors in the surface layers of North Atlantic sediments has also been examined.Two ultrasonic scanning systems for the non-destructive testing of marine sediment cores have been developed. The performance of the two systems has been evaluated using the results from measurements made on a suite of marine sediment cores from the North Atlantic in conjunction with X-ray scanning and geological analysis of the same cores. 相似文献