全文获取类型
收费全文 | 132218篇 |
免费 | 1936篇 |
国内免费 | 942篇 |
专业分类
测绘学 | 3043篇 |
大气科学 | 9223篇 |
地球物理 | 26284篇 |
地质学 | 46450篇 |
海洋学 | 11910篇 |
天文学 | 29586篇 |
综合类 | 329篇 |
自然地理 | 8271篇 |
出版年
2021年 | 1284篇 |
2020年 | 1470篇 |
2019年 | 1606篇 |
2018年 | 3191篇 |
2017年 | 2993篇 |
2016年 | 3669篇 |
2015年 | 2075篇 |
2014年 | 3569篇 |
2013年 | 6813篇 |
2012年 | 3839篇 |
2011年 | 5207篇 |
2010年 | 4682篇 |
2009年 | 6153篇 |
2008年 | 5422篇 |
2007年 | 5329篇 |
2006年 | 5130篇 |
2005年 | 3928篇 |
2004年 | 3899篇 |
2003年 | 3671篇 |
2002年 | 3578篇 |
2001年 | 3136篇 |
2000年 | 3086篇 |
1999年 | 2563篇 |
1998年 | 2588篇 |
1997年 | 2438篇 |
1996年 | 2158篇 |
1995年 | 2104篇 |
1994年 | 1841篇 |
1993年 | 1741篇 |
1992年 | 1636篇 |
1991年 | 1559篇 |
1990年 | 1682篇 |
1989年 | 1483篇 |
1988年 | 1327篇 |
1987年 | 1603篇 |
1986年 | 1414篇 |
1985年 | 1728篇 |
1984年 | 2002篇 |
1983年 | 1906篇 |
1982年 | 1743篇 |
1981年 | 1648篇 |
1980年 | 1438篇 |
1979年 | 1400篇 |
1978年 | 1409篇 |
1977年 | 1262篇 |
1976年 | 1215篇 |
1975年 | 1183篇 |
1974年 | 1166篇 |
1973年 | 1217篇 |
1972年 | 747篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
881.
P. A. Lozovik A. K. Morozov M. B. Zobkov T. A. Dukhovicheva L. A. Osipova 《Water Resources》2007,34(2):204-216
A relationship between indirect chemical indices of organic matter content of surface waters and light absorption in the visible and ultraviolet ranges is discussed. The allochthonous and autochthonous types of organic matter are found to essentially differ in the rate of light absorption. Techniques are proposed for the calculation of the concentrations of allochthonous and autochthonous organic matter by the integral rate of light consumption in the visible region and the bichromate oxidability of water, as well as by three indirect indices: chemical oxygen demand, permanganate oxidability, and water color index. The mean concentrations of allochthonous and autochthonous organic matter in large lakes (Ladoga, Onega, and Baikal) and in rivers and smaller water bodies in Karelia (>300 water objects) are analyzed. Allochthonous organic matter was found to predominate in most surface waters of Karelia (>80%), while allochthonous organic matter predominates only in lakes with a small specific catchment area (supposedly, <5) and in highly eutrophic lakes. 相似文献
882.
For first time, during 1991, seismic activity was recorded during an eruption at Colima volcano. We analyze these data to
obtain a stress pattern using a composite focal mechanism technique. From the analysis of regional seismicity, the Tamazula
Fault and the Armeria River appear as active features and the dip of the slab east of the Jalisco Block is approximately 12°.
Southwest of Colima volcano a vertical alignment of seismic events was observed. We estimate five different composite focal
mechanism solutions from our data set, which indicate a change of the stress field at the volcano after the 1991 eruption.
These solutions suggest that the stress field in the volcanic edifice was controlled by stresses related to the emplacement
of magma superimposed on the regional stress field. No evidence of active local faults in the volcanic edifice was found.
We propose a model for the eruptive process that involves tilting of the volcanic edifice.
Received: 15 October 1995 / Accepted: 26 October 1998 相似文献
883.
Experimental investigations and modeling of nonlinear elasticity of fiber-reinforced soil under cyclic loading at small strain are conducted in this paper. The investigations include three aspects. First, cyclic shear tests are conducted using conventional triaxial apparatus. Twenty-seven specimens with three different fiber contents are employed to conduct triaxial cyclic shear tests under different confining pressure and loading repetition. Effects of geofiber, confining pressure and loading repetition on elastic shear modulus of reinforced soil are studied and analyzed. Second, a hyperbolic function is introduced to describe the nonlinear stress–strain skeletal curve under cyclic loading. Nonlinear elastic modulus is expressed as a function of shear strain and two variables A and B that are related to the initial tangential modulus and ultimate cyclic loading stress, respectively. In the present paper, variables A and B both are further assumed to be functions of geofiber content, confining pressure and loading repetition. Finally, eight constitutive coefficients of the nonlinear elastic model are calibrated using stress–strain curves from cyclic triaxial shear tests. The calibration of parameters is conducted using the technique of the linear regression for multiple variables. Impacts and effects of geofiber, confining pressure and loading repetitions on soil nonlinear elastic behavior are discussed. 相似文献
884.
Summary The morphology of the Wadati-Benioff zone in the Kermadec region, based on the distribution of 1100 earthquake foci, verified the existence of an intermediate aseismic gap and its relation to active andesitic volcanism, and the non-uniformity of subduction due to the hampering effect of the main structural features of the subducting Pacific plate. Two cycles of the recently active subduction in the Tonga-Kermadec island arc were found. 相似文献
885.
V. B. Ershova A. V. Prokopiev A. K. Khudoley V. F. Proskurnin T. Andersen K. Kullerud M. A. Stepunina D. A. Kolchanov 《Doklady Earth Sciences》2017,474(2):613-616
This work presents the U–Pb (LA–ICP–MS) data of detrital zircons from metasedimentary sequences of northwestern Taimyr. An analysis of the youngest populations of detrital zircons testifies to the wide distribution of Cambrian sequences in the study area, but not Precambrian sequences as was considered earlier, and the need for a substantial revision of the stratigraphic scheme of this area. The detrital zircon age distribution shows that the Timan margin of the Baltic paleocontinent was a major provenance area of the Cambrian sediments in northern and northwestern Taimyr, as well as the coeval sediments of the Severnaya Zemlya archipelago. 相似文献
886.
The spacetime gravitational field of a deformable body 总被引:3,自引:0,他引:3
The high-resolution analysis of orbit perturbations of terrestrial artificial satellites has documented that the eigengravitation
of a massive body like the Earth changes in time, namely with periodic and aperiodic constituents. For the space-time variation
of the gravitational field the action of internal and external volume as well as surface forces on a deformable massive body
are responsible. Free of any assumption on the symmetry of the constitution of the deformable body we review the incremental
spatial (“Eulerian”) and material (“Lagrangean”) gravitational field equations, in particular the source terms (two constituents:
the divergence of the displacement field as well as the projection of the displacement field onto the gradient of the reference
mass density function) and the `jump conditions' at the boundary surface of the body as well as at internal interfaces both
in linear approximation. A spherical harmonic expansion in terms of multipoles of the incremental Eulerian gravitational potential
is presented. Three types of spherical multipoles are identified, namely the dilatation multipoles, the transport displacement
multipoles and those multipoles which are generated by mass condensation onto the boundary reference surface or internal interfaces.
The degree-one term has been identified as non-zero, thus as a “dipole moment” being responsible for the varying position
of the deformable body's mass centre. Finally, for those deformable bodies which enjoy a spherically symmetric constitution,
emphasis is on the functional relation between Green functions, namely between Fourier-/ Laplace-transformed volume versus
surface Love-Shida functions (h(r),l(r) versus h
′(r),l
′(r)) and Love functions k(r) versus k
′(r). The functional relation is numerically tested for an active tidal force/potential and an active loading force/potential,
proving an excellent agreement with experimental results.
Received: December 1995 / Accepted: 1 February 1997 相似文献
887.
Doklady Earth Sciences - The paper presents the results of chronological and lithostratigraphic studies of five small lakes located in the northern part of the Karelian Isthmus, where the... 相似文献
888.
Petrogenesis of Peraluminous Granites, Monashee Mountains, Southeastern Canadian Cordillera 总被引:2,自引:2,他引:2
Two-mica granites that locally contain garnet and sillimaniteoccur as dikes, sills, and sheets up to 50 m thick within thesillimanite zonc of the Monashee Mountains in the southeasternCanadian Cordillera of British Columbia. Syn-kinematic and post-kinematicgranites are recognized. U-Pb dating of zircon demonstrates that the syn-kinematic granitesare 100.4?0.3 Ma old, based on duplicate concordant single abradedzircon analyses. Other zircons have slightly older Pb/Pb dates,indicating small amounts of inherited zircons. Monazites are99?10 Ma old. Post-kinematic granites have 62.5?0.2 Ma zirconages and 634+0.1 Ma monazite ages. High initial 87 ratios (0.714920.74181)and evidence of Precambrian Pb inheritance indicates that bothsyn- and post-kinematic granites were derived from a crustalsource. Geobarometric estimates suggest that both generationsof granites equilibrated at 68 kb (2230 km). Zirconand monazite saturation temperatures range from 660824?Cand indicate that these minerals were liquidus phases earlyin the crystallization history of the granites. Because monazitesaturation temperatures generally exceed those of zircon, itis possible that some monazites may be inherited. Apatite saturationtemperatures in excess of 900?C suggest that both generationsof granites contain source inherited apatite. Syn- and post-kinematic granites have essentially identicalmajor and trace element chemistries. Syn-kinematic graniteshave steep light rare earth element (LREE) enriched patternswith pronounced negative Eu anomalies. The REE patterns of post-kinematicgranites range from steep LREE enriched patterns with negativeNd and Eu anomalies to flat patterns with low LREE contents,negative Nd anomalies, and both positive and negative Eu anomalies.Modelling of REE, Rb, Sr, and Ba contents demonstrates thatsyn-kinematic gramtes could have been generated by a low degreeof partial melting (with 1025% feldspar fractionationof the melt) of Late Proterozoic Horsethief Creek Group metapelitesleaving a monazite-bearing upper amphibolite facies residue.Post-kinematic granites were produced by partial melting ofa geochemically and isotopically similar metapelitic source.The suite of post-kinematic granites can be related by a smallamount (up to 0.1%) of monazite crystal fractionation. 相似文献
889.
NiAl2O4 is a largely inverse spinel, which in detail shows increasing randomisation with temperature of Ni and Al between the octahedral and tetrahedral cation sites of the spinel structure. We have used powder XRD to determine this cation distribution in various samples of NiAl2O4 quenched after annealing between 700 and 1400° C. The inversion parameter (x) can be measured with a precision of ± 0.004 (one standard deviation), and a comparison of different methods of synthesis, X-ray diffraction and refinement techniques, suggests a probable accuracy of better than 0.01. The results are supported by some preliminary single crystal refinements on flux-grown samples.Below 800° C the rate of cation ordering becomes very slow, and, despite reaching an apparently steady state, it is doubtful if our samples attained complete internal equilibrium. Above 1250° C the cation redistribution becomes so fast that the quenching method becomes unreliable. Between 800 and 1250° C inclusive, the degree of inversion changes smoothly from 0.87 at 800° C to 0.79 at 1250° C, and is accompanied by linear changes in u, the oxygen parameter, from 0.2555 to 0.2563 (±0.0002), and a0, the lattice parameter, from 8.0462 to 8.0522 Å (±0.0002 Å). 相似文献
890.
Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas 总被引:70,自引:0,他引:70
The composition of chromian spinel in alpine-type peridotites has a large reciprocal range of Cr and Al, with increasing Cr# (Cr/(Cr+Al)) reflecting increasing degrees of partial melting in the mantle. Using spinel compositions, alpine-type peridotites can be divided into three groups. Type I peridotites and associated volcanic rocks contain spinels with Cr#<0.60; Type III peridotites and associated volcanics contain spinels with Cr#>0.60, and Type II peridotites and volcanics are a transitional group and contain spinels spanning the full range of spinel compositions in Type I and Type II peridotites. Spinels in abyssal peridotites lie entirely within the Type I spinel field, making ophiolites with Type I alpine-type peridotites the most likely candidates for sections of ocean lithosphere formed at a midocean ridge. The only modern analogs for Type III peridotites and associated volcanic rocks are found in arc-related volcanic and intrusive rocks, continental intrusive assemblages, and oceanic plateau basalts. We infer a sub-volcanic arc petrogenesis for most Type III alpine-type peridotites. Type II alpine-type peridotites apparently reflect composite origins, such as the formation of an island-arc on ocean crust, resulting in large variations in the degree and provenance of melting over relatively short distances. The essential difference between Type I and Type III peridotites appears to be the presence or absence of diopside in the residue at the end of melting.Based on an examination of co-existing rock and spinel compositions in lavas, it appears that spinel is a sensitive indicator of melt composition and pressure of crystallization. The close similarity of spinel composition fields in genetically related basalts, dunites and peridotites at localities in the oceans and in ophiolite complexes indicates that its composition reflects the degree of melting in the mantle source region. Accordingly, we infer from the restricted range of spinel compositions in abyssal basalts that the degree of mantle melting beneath mid-ocean ridges is generally limited to that found in Type I alpine-type peridotites. It is apparent, therefore, that the phase boundary OL-EN-DI-SP +meltOL-EN-SP+melt has limited the degree of melting of the mantle beneath mid-ocean ridges. This was clearly not the case for many alpine-type peridotites, implying very different melting conditions in the mantle, probably involving the presence of water. 相似文献