全文获取类型
收费全文 | 29392篇 |
免费 | 502篇 |
国内免费 | 222篇 |
专业分类
测绘学 | 736篇 |
大气科学 | 2257篇 |
地球物理 | 5752篇 |
地质学 | 10775篇 |
海洋学 | 2548篇 |
天文学 | 6231篇 |
综合类 | 65篇 |
自然地理 | 1752篇 |
出版年
2021年 | 269篇 |
2020年 | 297篇 |
2019年 | 297篇 |
2018年 | 727篇 |
2017年 | 725篇 |
2016年 | 840篇 |
2015年 | 469篇 |
2014年 | 803篇 |
2013年 | 1558篇 |
2012年 | 925篇 |
2011年 | 1223篇 |
2010年 | 1052篇 |
2009年 | 1363篇 |
2008年 | 1184篇 |
2007年 | 1172篇 |
2006年 | 1161篇 |
2005年 | 841篇 |
2004年 | 839篇 |
2003年 | 766篇 |
2002年 | 768篇 |
2001年 | 712篇 |
2000年 | 701篇 |
1999年 | 550篇 |
1998年 | 518篇 |
1997年 | 598篇 |
1996年 | 468篇 |
1995年 | 470篇 |
1994年 | 465篇 |
1993年 | 372篇 |
1992年 | 382篇 |
1991年 | 353篇 |
1990年 | 358篇 |
1989年 | 327篇 |
1988年 | 334篇 |
1987年 | 358篇 |
1986年 | 317篇 |
1985年 | 417篇 |
1984年 | 408篇 |
1983年 | 441篇 |
1982年 | 410篇 |
1981年 | 360篇 |
1980年 | 398篇 |
1979年 | 320篇 |
1978年 | 299篇 |
1977年 | 293篇 |
1976年 | 260篇 |
1975年 | 259篇 |
1974年 | 267篇 |
1973年 | 254篇 |
1971年 | 172篇 |
排序方式: 共有10000条查询结果,搜索用时 804 毫秒
941.
Si K- and L-edge ELNES spectroscopy and multiple-scattering (MS) calculations are used to examine mixed Si coordination compounds
varying in SiVI:SiIV ratio. As in previous studies, the edges are influenced mainly by silicon coordination (tetrahedral vs. octahedral), as supported
by the MS calculations. We demonstrate two methods semi-quantitatively to extract the value of SiVI/(SiVI+SiIV): (1) A linear relationship between the L2,3-L1 splitting and SiVI/(SiVI+SiIV) is observed, (2) a fitting method based on the coaddition of reference tetrahedral and octahedral Si spectra is applied
to both Si K- and L-edge ELNES spectra.
Received: February 10, 1997 / Revised, accepted: May 23, 1997 相似文献
942.
943.
944.
C. Peirce R. B. Whitmarsh R. A. Scrutton B. Pontoise F. Sage J. Mascle 《Geophysical Journal International》1996,125(3):781-795
During May 1990 and January-February 1991, an extensive geophysical data set was collected over the Côte d'Ivoire-Ghana continental margin, located along the equatorial coast of West Africa. The Ghana margin is a transform continental margin running subparallel to the Romanche Fracture Zone and its associated marginal ridge—the Côte d'Ivoire-Ghana Ridge. From this data set, an explosive refraction line running ∼ 150 km, ENE-WSW between 3°55'N, 3°21'W and 4°23'N, 2°4'W, has been modelled together with wide-angle airgun profiles, and seismic reflection and gravity data. This study is centred on the Côte d'Ivoire Basin located just to the north of the Côte d'Ivoire-Ghana Ridge, where bathymetric data suggest that a component of normal rifting occurred, rather than the transform motion observed along the majority of the equatorial West African margin.
Traveltime and amplitude modelling of the ocean-bottom seismometer data shows that the continental Moho beneath the margin rises in an oceanward direction, from ∼ 24 km below sea level to ∼ 17 km. In the centre of the line where the crust thins most rapidly, there exists a region of anomalously high velocity at the base of the crust, reaching some 8 km in thickness. This higher-velocity region is thought to represent an area of localized underplating related to rifting. Modelling of marine gravity data, collected coincident with the seismic line, has been used to test the best-fitting seismic model. This modelling has shown that the observed free-air anomaly is dominated by the effects of crustal thickness, and that a region of higher density is required at the base of the crust to fit the observed data. This higher-density region is consistent in size and location with the high velocities required to fit the seismic data. 相似文献
Traveltime and amplitude modelling of the ocean-bottom seismometer data shows that the continental Moho beneath the margin rises in an oceanward direction, from ∼ 24 km below sea level to ∼ 17 km. In the centre of the line where the crust thins most rapidly, there exists a region of anomalously high velocity at the base of the crust, reaching some 8 km in thickness. This higher-velocity region is thought to represent an area of localized underplating related to rifting. Modelling of marine gravity data, collected coincident with the seismic line, has been used to test the best-fitting seismic model. This modelling has shown that the observed free-air anomaly is dominated by the effects of crustal thickness, and that a region of higher density is required at the base of the crust to fit the observed data. This higher-density region is consistent in size and location with the high velocities required to fit the seismic data. 相似文献
945.
Periodicity of magnetic intensities in magnetic anomaly profiles: the Cenozoic of the South Atlantic
Spectral analyses of several published magnetic anomaly profiles from Candé & Kent (1992a) were undertaken prior to analysing, in the same way, raw magnetic anomaly data from similar parts of the South Atlantic. It was found that similar and distinct medium and short wavelengths were present in both the published and raw data. When these are converted into the time domain using the average rate of spreading for each profile, these periodicities appear similar, possibly identical, to those expected from the long-term eccentricity orbital parameters (Fischer, DeBoer & Premoli Silva 1990). While such correlations are not necessarily causative, they suggest that magnetohydro-dynamical processes near the core-mantle boundary may be affected by gravitational changes due to planetary orbital perturbations. 相似文献
946.
Vineyards in New Zealand are pushing further south than anywhere else in the Southern Hemisphere, this despite poor understanding of the latitudinal limits of grape growing. European experience shows a limit of 50oN. Valuable pointers and cautionary tales from Germany encourage a view that the southern limit in New Zealand may be 45oS. This is broadly confirmed by comparative climate data from Germany and New Zealand. 相似文献
947.
Both ‘tanka’ and ‘beri’, the well-like structures made for storing drinking water fetched from distant places or superficially charged from the runoff water of the monsoon rains, are considered integral components of rural communities in the Thar Desert of Rajasthan State in north-western India.A. stephensi, the confirmed malaria vector in desert regions, breeds in these structures predominantly. 相似文献
948.
Coloradoapos;s 1992 vote on Amendment Two was the first meaningful, broad-based test of attitudes on gay rights within the United States. Analysis of voting results distinguishes places of tolerance from places of rejection. Social differences separating these places reflect typical contrasts between “traditionalists” and “modernizers” found in cultural conflicts at the national level. Places showing support for gay rights are recreational-based mountain communities, urban areas, and university communities. Those showing opposition are rural, agriculturally based communities. Suburban areas produced a more ambivalent vote. 相似文献
949.
The preparation of the preliminary seismic hazard maps of the territory of Slovenia has been based on an expansion of the basic approach laid out by Cornell in 1968. Three seismic source models were prepared. Two of them are based mainly on the earthquake catalogue using the Poissonian probability model. A map of seismic energy release and a map of earthquake epicenter density are used to delineate seismic sources in these models. The geometry of the third model which is based on a rough estimate of seismotectonic setting is taken from the probabilistic seismic hazard analysis of a nuclear power plant in Slovenia. Published ground motion attenuation models based on strong motion records of recent strong earthquakes in Italy are used. Test maps for variable and uniform b-values are presented. The computer program, Seisrisk III, developed by the U.S. Geological Survey is used. 相似文献
950.
T. J. B. Holland E. V. S. S. K. Babu D. J. Waters 《Contributions to Mineralogy and Petrology》1996,124(3-4):383-394
Thermodynamic modelling of (1) osumilite solid solutions and (2) dehydration melting in pelitic compositions within the KFMASH
system is quite successful in reproducing the invariant and univariant reactions determined in experimental studies. Even
though rather preliminary, such melt thermodynamic models may be very useful in interpolating and extrapolating the limited
information available from a small number of experimental runs. These methods allow the compositions of all phases to be monitored
as a function of pressure, temperature and equilibrium phase assemblage for any desired bulk composition. Locating the higher
variance phase fields (e.g. quadrivariant, quinivariant) is often difficult or impossible by inspection, but is made relatively
easy using thermodynamic software such as thermocalc. In the KFMASH system the calculated partition of Fe and Mg between osumilite,
garnet, cordierite, orthopyroxene and biotite are shown to be in good agreement with experimental and natural data and allow
reliable calculation of mineral compositions coexisting with quartz-saturated and H2O-undersaturated melts for a variety of bulk compositions. These phase diagram calculations allow quite tight limits to be
placed on the pressure, temperature and water activity conditions which accompanied metamorphism of natural osumilite occurrences
in Nain, Namaqualand, and Rogaland. At fixed bulk composition, the initial melting of pelites by dehydration of biotite can
occur via univariant, divariant or trivariant equilibria depending upon pressure of metamorphism. Of particular interest is
that, for low pressures or more magnesian bulk compositions, fluid-absent melting begins by generating liquid from the high-variance
assemblage biotite+cordierite+K-feldspar+ quartz. This type of modelling allows investigation, at least qualitatively, of
the fine scale details of melt production as a function of changes in pressure, temperature and bulk composition.
Received: 29 November 1995 / Accepted: 22 April 1996 相似文献