全文获取类型
收费全文 | 52581篇 |
免费 | 1237篇 |
国内免费 | 880篇 |
专业分类
测绘学 | 1679篇 |
大气科学 | 4048篇 |
地球物理 | 10476篇 |
地质学 | 19154篇 |
海洋学 | 4419篇 |
天文学 | 11696篇 |
综合类 | 309篇 |
自然地理 | 2917篇 |
出版年
2022年 | 368篇 |
2021年 | 618篇 |
2020年 | 600篇 |
2019年 | 661篇 |
2018年 | 1316篇 |
2017年 | 1222篇 |
2016年 | 1552篇 |
2015年 | 1020篇 |
2014年 | 1499篇 |
2013年 | 2730篇 |
2012年 | 1791篇 |
2011年 | 2341篇 |
2010年 | 1967篇 |
2009年 | 2534篇 |
2008年 | 2170篇 |
2007年 | 2180篇 |
2006年 | 2144篇 |
2005年 | 1615篇 |
2004年 | 1508篇 |
2003年 | 1445篇 |
2002年 | 1364篇 |
2001年 | 1200篇 |
2000年 | 1178篇 |
1999年 | 997篇 |
1998年 | 998篇 |
1997年 | 966篇 |
1996年 | 865篇 |
1995年 | 822篇 |
1994年 | 720篇 |
1993年 | 646篇 |
1992年 | 590篇 |
1991年 | 621篇 |
1990年 | 656篇 |
1989年 | 571篇 |
1988年 | 535篇 |
1987年 | 665篇 |
1986年 | 553篇 |
1985年 | 638篇 |
1984年 | 768篇 |
1983年 | 692篇 |
1982年 | 692篇 |
1981年 | 588篇 |
1980年 | 599篇 |
1979年 | 516篇 |
1978年 | 494篇 |
1977年 | 488篇 |
1976年 | 429篇 |
1975年 | 423篇 |
1974年 | 449篇 |
1973年 | 484篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
142.
Global sea-level pressure distribution has been analysed for the months of April and July for 5 years of contrasting situations of Indian summer monsoon, comprising of two drought years (1972 and 1974), a flood year (1975) and two normal monsoon years (1970 and 1973). Mean monthly sea-level pressure data at about 400 stations have been used in the study. Prominent features of pressure departures from long-term normals have also been noted. It is observed that the month of April shows more prominent contrasting features than July. In April, the high pressure centres over USSR and the North Pacific move considerably eastward during poor monsoon years, while a breakaway cell of Icelandic Low goes deep south. Both the high pressure areas over south Indian Ocean and Australia are stronger in good monsoon years. In July, the subtropical high pressure zone over the southern Indian Ocean is stronger and the Australian high is more eastward, in good monsoon years. 相似文献
143.
V. V. Seredin Yu. A. Danilcheva L. O. Magazina I. G. Sharova 《Lithology and Mineral Resources》2006,41(3):280-301
Results of the study of a new Ge-bearing area of the Pavlovka brown coal deposit are presented. Ge is accumulated in bed III2 lying at the bottom of the Late Paleogene-Early Neogene coal-bearing sequence adjacent to the Middle Paleozoic granite basement. The Ge content in coals and coal-bearing rocks varies in different sections from 10 to 200–250 ppm, reaching up to 500–600 ppm in the highest-grade lower part of the bed. The metalliferous area reveals a geochemical zoning: complex Ge-Mo-W anomalies subsequently grades along the depth and strike into Mo-W and W anomalies. Orebodies, like those at many Ge-bearing coal deposits, are concentric in plan and dome-shaped in cross-section. Coals in their central parts, in addition to Ge, W, and Mo, are enriched in U, As, Be, Ag, and Au. Distribution of Ge and other trace elements in the metalliferous sequence and products of gravity separation of Ge-bearing coals is studied. These data indicate that most elements (W, Mo, U, As, Be) concentrated like Ge in the Ge-bearing bed relative to background values are restricted to the organic matter of coals. The electron microscopic study shows that Ge-bearing coals contain native metals and intermetallic compounds in association with carbonates, sulfides, and halogenides. Coal inclusions in the metalliferous and barren areas of the molasse section strongly differ in contents of Ge and associated trace elements. Ge was accumulated in the coals in the course of the interaction of ascending metalliferous solutions with organic matter of the buried peat bogs in Late Miocene. The solutions were presumably represented by N2-bearing thermal waters (contaminated by volcanogenic CO2) that are typical of granite terranes. 相似文献
144.
145.
The mechanisms of reduction of hexavalent chromium by green rust sodium sulphate: Formation of Cr-goethite 总被引:1,自引:0,他引:1
The molecular-level processes that control green rust sodium sulphate (GRNa,SO4) reaction with chromate were studied using high-resolution techniques. Changes in solid morphology, structure and composition were observed with atomic force microscopy, transmission electron microscopy and X-ray diffraction. The results suggest the following mechanisms: Chromate replaces sulphate in the GR interlayer and is reduced by Fe(II). Formation of sparingly soluble Cr(III)-solid blocks further chromate entry, but Cr(VI) reduction continues at the GR solid/solution interface. Electron transfer from the centre of the GR crystals to the surface facilitates rapid reaction. Less stable zones of the reacted GRNa,SO4 dissolve and amorphous Cr(III),Fe(III)-solid forms. During equilibration, Cr-substituted goethite evolves in association with remaining GRNa,SO4, fed by material from the amorphous phase and dissolving oxidised GR. In contrast, previous Cr(VI) experiments with the carbonate form of GR, GRCO3, have suggested only reaction and deposition at the surface. From the perspective of environmental protection, these results have important implications. Goethite is sparingly soluble and the inclusion of Cr(III) as a solid-solution makes it even less soluble. Compared to Cr adsorbed at the surface of an Fe(III)-phase, Cr(III) incorporated in goethite is much less likely to be released back to groundwater. 相似文献
146.
S. Tadisetty K. Matsui H. Shimada R. N. Gupta 《Rock Mechanics and Rock Engineering》2006,39(4):383-393
Summary. Discontinuous manual observations and irregular caving characteristics of roof rocks often lead to improper decisions resulting
in accidents and production loss. Hence, systematic monitoring of the hanging roof behind the chock shields is necessary for
safe and productive mining operations. A real-time application was successfully implemented in an Indian mine for forecasting
of hanging roof behaviour to enhance safety and productivity. This paper reports the functioning of real-time TWAP (time weighted
average pressure) analysis in the forecasting of hanging roof behaviour in real time. 相似文献
147.
Naidu L.S. G. Rao V.V.S. T. Rao G. Mahesh J. Padalu G. Sarma V.S. Prasad P.R. Rao S.M. R. Rao B.M. 《Arabian Journal of Geosciences》2013,6(10):3709-3724
The Central Godavari delta is located along the Bay of Bengal Coast, Andhra Pradesh, India, and is drained by Pikaleru, Kunavaram and Vasalatippa drains. There is no groundwater pumping for agriculture as wells as for domestic purpose due to the brackish nature of the groundwater at shallow depths. The groundwater table depths vary from 0.8 to 3.4 m and in the Ravva Onshore wells, 4.5 to 13.3 m. Electrical Resistivity Tomography (ERT) surveys were carried out at several locations in the delta to delineate the aquifer geometry and to identify saline water aquifer zones. Groundwater samples collected and analyzed for major ions for assessing the saline water intrusion and to identify the salinity origin in the delta region. The results derived from ERT indicated low resistivity values in the area, which can be attributed to the existence of thick marine clays from ground surface to 12–15 m below ground level near the coast and high resistivity values are due to the presence of coarse sand with freshwater away from the coast. The resistivity values similar to saline water <0.01 Ω m is attributed to the mixing of the saline water along surface water drains. In the Ravva Onshore Terminal low resistivity values indicated up coning of saline water and mixing of saline water from Pikaleru drain. The SO 4 ?2 /Cl?and Na+2/Cl?ratios did not indicate saline water intrusion and the salinity is due to marine palaeosalinity, dilution of marine clays and dissolution of evaporites. 相似文献
148.
Stokes' formula from 1849 is still the basis for the gravimetric determination of the geoid. The modification of the formula,
originating with Molodensky, aims at reducing the truncation error outside a spherical cap of integration. This goal is still
prevalent among various modifications. In contrast to these approaches, some least-squares types of modification that aim
at reducing the truncation error, as well as the error stemming from the potential coefficients, are demonstrated. The least-squares
estimators are provided in the two cases that (1) Stokes' kernel is a priori modified (e.g. according to Molodensky's approach)
and (2) Stokes' kernel is optimally modified to minimize the global mean square error. Meissl-type modifications are also
studied. In addition, the use of a higher than second-degree reference field versus the original (Pizzetti-type) reference
field is discussed, and it is concluded that the former choice of reference field implies increased computer labour to achieve
the same result as with the original reference field.
Received: 14 December 1998 / Accepted: 4 October 1999 相似文献
149.
Yu. V. Amelin L. M. Heaman V. M. Verchogliad V. M. Skobelev 《Contributions to Mineralogy and Petrology》1994,116(4):411-419
U–Pb zircon/baddeleyite ages obtained for the Korosten anorthosite-rapakivi granite complex, Ukrainian shield, suggest that different magmatic phases were emplaced during a period of ca. 30 million years as a series of distinct igneous episodes. The earliest 1789.1±2.0 Ma anorthosites were followed by 1781.3±3.2 Ma dykes of plagiogranite porphyries. The emplacement of a major rapakivi granite phase took place at 1767.4±2.2 Ma, and was followed by emplacement of layered intrusions of anorthosites, gabbronorites, diabases and ultrabasic rocks between 1761 and 1758 Ma. The minimum duration of magmatism of about 30 million years, the 6–15 million years interval between igneous pulses, and alternation of discrete episodes of basic and felsie magmatism are common features of major anorthositemangerite-charnockite-rapakivi granite complexes. Temporal distribution of igneous activity in the Korosten complex shows that the gabbro-anorthosites and the granites are not comagmatic, although they are possibly cogenetic, and that at least four portions of granitic and basic magmas were generated during a relatively long period of at least 30 million years. The time gap of about 20–25 million years between early basic and later and more voluminous granitic magmatism, characteristic of the Korosten pluton, Wiborg and Salmi batholiths, probably reflects the duration of extensional processes before the generation of large volumes of magma in the lower crust. 相似文献
150.
Site response studies in Agartala Urban agglomeration 总被引:1,自引:0,他引:1
A systematic investigation using digital microearthquake recorders with short period SS-1 seismometers, covering 76 sites
in and around Agartala city, has been carried out for site response (SR) studies in the area. In the northern part of the
area, SR varies from 1.15 to 1.85 corresponding to peak frequency 0.76 to 0.93 Hz where soil is mostly semi-consolidated and
stiffer than recent Quaternary deposits (Haora River formation). In the southern part of the area, SR varies from 1.12 to
2.42 corresponding to peak frequency from 0.71 to 0.85 Hz within the Dupitila formation (early Quaternary). It is observed
that estimated SR from H/V increases from edges to middle of the Haora River valley and impedance contrast fallows the similar
trend. This reflects that site response by H/V is influenced by impedance contrast, whereas computed amplification from 1-D
model shows opposite trend. The maximum amplification at fundamental frequency of resonance, 1.04 Hz estimated from H/V by
near to BH-7, is 2.5 times greater than the impedance contrast/ratio derived from 1-D model for same location. Relationship
between resonance frequency and depth was obtained by applying quarter wavelength and Bard (2000) methodologies, which shows linearity, whereas H/V shows its nonlinearity characteristic in soil across the valley part of
Haora River. Shear wave velocities, and subsequently, SPT index and factor of safety (by cyclic stress approach) were estimated
from geotechnical parameters. Vs30 and site response data were used in this study for getting a first hand information about
soil stiffness condition in the area. The estimation of SPT index and factor of safety could be a useful tool for delineating
liquefied and none liquefied zones at various depth levels, especially where water table exists at a very shallow level. The
expected liquefiable zone was observed at depths varying from 6–25 m beneath the soil bearing zones where percentage of fines
is estimated to be more than 35% for the area. This knowledge about subsurface soil characteristics will be useful for the
civil engineers/city planners, which can be taken into account at the time of constructing earthquake-resistant structures
in the area. 相似文献