全文获取类型
收费全文 | 49080篇 |
免费 | 472篇 |
国内免费 | 385篇 |
专业分类
测绘学 | 1236篇 |
大气科学 | 3247篇 |
地球物理 | 8388篇 |
地质学 | 20965篇 |
海洋学 | 4056篇 |
天文学 | 10032篇 |
综合类 | 216篇 |
自然地理 | 1797篇 |
出版年
2022年 | 297篇 |
2021年 | 455篇 |
2020年 | 508篇 |
2019年 | 556篇 |
2018年 | 3751篇 |
2017年 | 3428篇 |
2016年 | 2559篇 |
2015年 | 653篇 |
2014年 | 1073篇 |
2013年 | 1687篇 |
2012年 | 2087篇 |
2011年 | 3838篇 |
2010年 | 3413篇 |
2009年 | 3794篇 |
2008年 | 3077篇 |
2007年 | 3709篇 |
2006年 | 1348篇 |
2005年 | 1173篇 |
2004年 | 1040篇 |
2003年 | 1071篇 |
2002年 | 935篇 |
2001年 | 651篇 |
2000年 | 603篇 |
1999年 | 461篇 |
1998年 | 477篇 |
1997年 | 453篇 |
1996年 | 390篇 |
1995年 | 349篇 |
1994年 | 377篇 |
1993年 | 294篇 |
1992年 | 294篇 |
1991年 | 286篇 |
1990年 | 322篇 |
1989年 | 220篇 |
1988年 | 225篇 |
1987年 | 268篇 |
1986年 | 209篇 |
1985年 | 314篇 |
1984年 | 276篇 |
1983年 | 258篇 |
1982年 | 275篇 |
1981年 | 216篇 |
1980年 | 263篇 |
1979年 | 195篇 |
1978年 | 211篇 |
1977年 | 165篇 |
1976年 | 167篇 |
1975年 | 172篇 |
1974年 | 164篇 |
1973年 | 164篇 |
排序方式: 共有10000条查询结果,搜索用时 765 毫秒
971.
N. I. Bondar’ 《Astronomy Reports》2002,46(6):489-496
Variability of the photospheric radiation of 40 (dKe-dMe) dwarfs in the solar neighborhood due to variations in the spottedness of their surfaces is analyzed based on the behavior of their mean annual brightnesses over long time intervals. The amplitudes and characteristic time scales of the variations of the mean annual brightness are taken to be indicators of photospheric activity and were used to infer the levels of photospheric activity in the stars studied. The influence of axial rotation on the development of cyclic activity in young red dwarfs and F-M main-sequence stars is analyzed. The durations and amplitudes of the photospheric variability of rapidly rotating (dK0e-dK5e) stars testifies to a higher level of photospheric activity among red dwarfs and solar-type stars. The X-ray luminosities of these stars grow with the amplitude of the variations of the mean annual brightness. However, this is not typical of rapidly rotating M dwarfs, for which the X-ray emission varies by more than two orders of magnitude, although their degrees of spottedness are all virtually the same. A linear relationship between the X-ray and bolometric luminosities is observed for young (dKe-dMe) stars, with their ratios log(L x/L bol) being about ?3. These properties can be used to determine whether a red dwarf is a young star or is already on the main sequence. 相似文献
972.
S. Shibuya 《Geotechnical and Geological Engineering》2002,20(4):333-369
A double exponential fitting model (DEFM) capable of expressing the non-linear stress-stiffness relationship of geomaterials has been proposed by Shibuya et al. (1997). The model comprises two material constants; the elastic stiffness at very small strains and the strength, together with other free parameters to determine the complete stress-stiffness relationship. In this paper, the capability of the original function used for DEFM in simulating the tangent stiffness-stress relationship of geomaterials is first discussed. Second, the methods for determining the free model parameters, as well as its conversion to obtain a stress-strain relationship are proposed. The applicability of DEFM to predicting non-linear stress-stiffness relationship is examined in detail in a total of forty-nine fitting cases of compression test data on sedimentary rock, artificial soft rock and soft clay. It is found that the DEFM is effective in expressing the non-linear stress-stiffness relationship of various kinds of geomaterials at small to intermediate strains, say less than 0.5%. The superiority of this model compared to other fitting models currently in use is also demonstrated in some of the fitting cases. 相似文献
973.
Lithology and Mineral Resources - The chemical composition and Nd isotope systematics were obtained for mudrocks (mudstones) from sections of the Siberian hypostratotype of the Riphean and Vendian... 相似文献
974.
Morphology, microstructure, and chemical composition of the nodular fraction of phosphatic sand from the outer Namibia shelf are considered. The nodules are divided into three varieties: concretions, phosphatic casts of shells pelecipod and gastropod, and phosphatized sea lion coprolites. A part of nodules shows massive homogeneous structure, whereas other nodules contain sand-size phosphatic grains derived from the host phosphatic sand. All nodules display the same spectrum of microstructures, including colloform, granular-globular, and crystallomorphic ones. The crystallomorphic nodules are composed of scattered and aggregated crystallites of isometric, rod-shaped, and dumbbell-shaped forms, as well as crystallized shells on grains incorporated into nodules. Phosphatized remains of diatoms, foraminifers, and embryonic gastropods, as well as vermicular and filamentous bacterial forms occur among biogenic components of the nodules. Framboidal pyrite is present in all nodules. In terms of phosphate microstructure, character of organic remains, and chemical composition, the nodules are similar to phosphate grains of the host sand, recent phosphorites from the inner Namibia shelf, and some ancient phosphorites. 相似文献
975.
N. Al-Shayea 《Rock Mechanics and Rock Engineering》2002,35(4):271-297
Summary
A fracture toughness study was conducted on a limestone rock formation from a petroleum reservoir in Saudi Arabia, and results
were compared with those for outcrop specimens from the same geological formation. The objective was to investigate the possibility
of using outcrop specimens to estimate the fracture toughness behavior of reservoir rock at in-situ conditions of temperature and confining pressure. The study was made on reservoir specimens from a depth of about 3.5 km,
at both ambient and reservoir conditions. Mixed mode I–II fracture toughness at reservoir conditions of high temperature and
confining pressure was studied using straight notched Brazilian disk (SNBD) specimens under diametrical compression. Tests
were conducted at ambient conditions, at an effective confining pressure (σ3) of 28 MPa (4000 psi), and at a temperature of 116°C. The results showed a substantial increase in fracture toughness under confining pressure. Under σ3=28 MPa, the pure mode-I fracture toughness (K
IC), increased by a factor of about 3.2, and the pure mode-II fracture toughness (K
IIC) increased by a factor of 4.4, compared to those under ambient conditions. On the other hand, K
IC at 116°C was only 25% more than that at ambient conditions. These results were compared with recent results for outcrop specimens
from the same geological formation. The results reveal that outcrop specimens can be successfully used to predict the fracture
behavior of reservoir specimens at in-situ conditions, in spite of some differences at ambient conditions. Additionally, fracture toughness envelopes were obtained
for reservoir specimens at ambient and high pressure conditions, in both positive and negative regions.
Received September 14, 2000; accepted February 22, 2002 Published online September 2, 2002 相似文献
976.
977.
Papadimitriou P. Voulgaris N. Kassaras I. Kaviris G. Delibasis N. Makropoulos K. 《Natural Hazards》2002,27(1-2):15-33
On 7 September 1999 at 11:56 GMT a destructive earthquake (Mw = 6.0) occurred close to Athens (Greece). The rupture process is examined using data from the Cornet local permanent network, as well as teleseismic recordings. Data recorded by a temporary seismological network were analyzed to study the aftershock sequence. The mainshock was relocated at 38.105°N, 23.565°E, about 20 km northwest of Athens. Four foreshocks were also relocated close to the mainshock. The modeling of teleseismic P and SH waves provides a well-constrained focal mechanism of the mainshock (strike = 105°, dip = 55° and rake = -80°) at a depth of 8 km and a seismic moment M0 = 1.01025 dyn·cm. The obtained fault plane solution represents normal faulting indicating an almost north-south extension. More than 3500 aftershocks were located, 1813 of which present RMS < 0.1 s and ERH, ERZ < 1.0 km. Two main clusters were distinguished, while the depth distribution is concentrated between 2 and 11 km. Over 1000 fault plane solutions of aftershocks were constrained, the majority of which also correspond to N–S extension. No surface breaks were observed but the fault plane solution of the mainshock is in agreement with the tectonics of the area and with the focal mechanisms obtained by aftershocks. The hypocenter of the mainshock is located on the deep western edge of the fault plane. The relocated epicenter coincides with the fringe that represents the highest deformation observed on the differential interferometric image. The calculated source duration is 5 sec, while the estimated dimensions of the fault are 15 km length and 10 km width. The source process is characterized by unilateral eastward rupture propagation, towards the city of Athens. An evident stop phase observed in the recordings of the Cornet local stations is interpreted as a barrier caused by the Aegaleo Mountain. 相似文献
978.
Identifying and quantifying urban recharge: a review 总被引:12,自引:1,他引:12
David N. Lerner 《Hydrogeology Journal》2002,10(1):143-152
The sources of and pathways for groundwater recharge in urban areas are more numerous and complex than in rural environments.
Buildings, roads, and other surface infrastructure combine with man-made drainage networks to change the pathways for precipitation.
Some direct recharge is lost, but additional recharge can occur from storm drainage systems. Large amounts of water are imported
into most cities for supply, distributed through underground pipes, and collected again in sewers or septic tanks. The leaks
from these pipe networks often provide substantial recharge. Sources of recharge in urban areas are identified through piezometry,
chemical signatures, and water balances. All three approaches have problems. Recharge is quantified either by individual components
(direct recharge, water-mains leakage, septic tanks, etc.) or holistically. Working with individual components requires large
amounts of data, much of which is uncertain and is likely to lead to large uncertainties in the final result. Recommended
holistic approaches include the use of groundwater modelling and solute balances, where various types of data are integrated.
Urban recharge remains an under-researched topic, with few high-quality case studies reported in the literature.
Electronic Publication 相似文献
979.
Yu. N. Efremov 《Astronomy Reports》2002,46(10):791-804
The nature of stellar complexes with peculiar populations and morphologies is investigated. The existence in the LMC of complexes made up of isolated stars, on the one hand, and consisting exclusively of clusters, on the other hand, could be due to different turbulence patterns in the initial gaseous medium. Arc-shaped stellar complexes are unlikely to be the result of star formation in a gaseous shell swept up by a central source of pressure, and instead probably reflect the shape of a bow shock that develops when a sufficiently dense cloud is subject to dynamical pressure. A peculiar arc-shaped complex in NGC 6946, which contains a young, massive cluster, may be the result of an oblique infall of a high-velocity cloud onto a region of the gaseous disk of the Galaxy with a strong, regular magnetic field; the properties of this complex can be explained as the result of a collision of the resulting shocks. The arc-shaped complexes in the LMC were also probably produced by high-velocity clouds moving obliquely through the more tenuous gas of the LMC disk. A similar complex in NGC 300 may owe its origin to the effect produced on a dense cloud by the shock from an extremely powerful external explosion, whose stellar remnant may have survived as an X-ray source now located along the line of symmetry of the arc of the complex. The rareness of such structures can be explained by the narrow range of conditions under which they can develop. 相似文献
980.
O. V. Verkhodanov A. I. Kopylov Yu. N. Pariiskii N. S. Soboleva A. V. Temirova O. P. Zhelenkova 《Astronomy Reports》2002,46(7):531-542
BV RI data are presented for the majority of steep-spectrum objects in the RC catalog with m R <23.5m. Previously developed programs are applied to these data to estimate the redshifts and ages of the stellar systems of the host galaxies. Applying this program to the color data (BV RI JHK) for distant radio galaxies with spectroscopic redshifts indicates that this approach provides accurate estimates of the redshifts of such radio galaxies, close to those obtained using field galaxies (~20%). The age estimates are much less trustworthy, but a lower limit to the ages of objects that are not very distant (z<1.5) can be determined with certainty. We have identi fied several galaxies whose formal ages exceed the age of the Universe at the corresponding z in simple Cold Dark Matter models for the Universe. The possibility of using such objects to elucidate the role of “dark energy” is discussed. This paradox disappears in models with cosmological constants (Λ terms) equal to 0.6–0.8. 相似文献