全文获取类型
收费全文 | 545篇 |
免费 | 25篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 8篇 |
大气科学 | 30篇 |
地球物理 | 122篇 |
地质学 | 192篇 |
海洋学 | 53篇 |
天文学 | 115篇 |
自然地理 | 53篇 |
出版年
2021年 | 2篇 |
2020年 | 6篇 |
2019年 | 8篇 |
2018年 | 9篇 |
2017年 | 11篇 |
2016年 | 15篇 |
2015年 | 8篇 |
2014年 | 13篇 |
2013年 | 28篇 |
2012年 | 20篇 |
2011年 | 17篇 |
2010年 | 16篇 |
2009年 | 29篇 |
2008年 | 33篇 |
2007年 | 15篇 |
2006年 | 29篇 |
2005年 | 19篇 |
2004年 | 13篇 |
2003年 | 23篇 |
2002年 | 19篇 |
2001年 | 8篇 |
2000年 | 11篇 |
1999年 | 15篇 |
1998年 | 11篇 |
1997年 | 14篇 |
1996年 | 5篇 |
1995年 | 9篇 |
1994年 | 4篇 |
1993年 | 6篇 |
1992年 | 4篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1989年 | 4篇 |
1988年 | 5篇 |
1987年 | 7篇 |
1986年 | 7篇 |
1985年 | 11篇 |
1984年 | 10篇 |
1983年 | 11篇 |
1982年 | 21篇 |
1981年 | 6篇 |
1980年 | 9篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1977年 | 7篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1974年 | 6篇 |
1973年 | 7篇 |
1972年 | 3篇 |
排序方式: 共有573条查询结果,搜索用时 500 毫秒
41.
Double-packer injection tests have been carried out in a borehole in the Chalk of Hampshire and a borehole in the Permian Penrith Sandstone of Cumbria, England. The tests have been supplemented by borehole logging and television inspections and by laboratory measurements on cores. A new mathematical treatment has enabled results to be calculated for injection test intervals containing fissures. In both formations, transmissivity immediately around the borehole is dominated by a few major fissures. In the Penrith Sandstone there is good agreement between intergranular hydraulic conductivity measurements and packer-test results in the portions of the borehole which do not contain major fissures. In the Chalk, the packer-test values are always higher than the intergranular measurements, suggesting that some degree of fissuring is present throughout. 相似文献
42.
Manganese oxides precipitated by bubbling air through 0.01 molar solutions of MnCl2, Mn(NO3)2, MnSO4, or Mn(ClO4)2 at a constantly maintained pH of 8.5 to 9.5 at temperatures of 25°C or higher consisted mainly of hausmannite, Mn3O4. At temperatures near 0°C, but with other conditions the same, the product is feitknechtite, βMnOOH, except that if the initial solution is MnSO4 and the temperature is near 0°C the product is a mixture of manganite, γMnOOH and groutite, αMnOOH.All these oxides are metastable in aerated solution and alter by irreversible processes to more highly oxidized species during aging. A two-step nonequilibrium thermodynamic model predicts that the least stable species, βMnOOH, should be most readily converted to MnO2. Some preparations of βMnOOH aged in their native solution at 5°C attained a manganese oxidation state of +3.3 or more after 7 months. Hausmannite aged at 25°C altered to γMnOOH. The latter is more stable than a or βMnOOH, and manganese oxidation states above 3.0 were not reached in hausmannite precipitates during 4 months of aging. Initial precipitation of MnCO3 rather than a form of oxide is likely only where oxygen availability is very low.Composition of solutions and oxidation state and morphology of solids were determined during the aging process by chemical analyses, X-ray and electron diffraction and transmission electron micrographs. 相似文献
43.
Lance A. Waller Li Zhu Carol A. Gotway Dennis M. Gorman Paul J. Gruenewald 《Stochastic Environmental Research and Risk Assessment (SERRA)》2007,21(5):573-588
Past studies consistently indicate measurable local associations between alcohol distribution and the incidence of violence.
These results, coupled with measurements of spatial correlation, reveal the importance of spatial analysis in the study of
the interaction of alcohol and violence. While studies increasingly incorporate spatial correlation among model residuals
to improve precision and reduce bias, to date, most analyses assume associations that are constant and independent of location,
an assumption coming under increasing scrutiny in the quantitative geography literature. In this paper, we review and contrast
two approaches for the estimation of and inference for spatially heterogeneous effects (i.e., associative factors whose impacts on the outcome of interest vary throughout geographic space). Specifically,
we provide an in-depth comparison of “geographically weighted regression” models (allowing covariate effects to vary in space
but only allowing relatively ad hoc inference) with “variable coefficient” models (allowing varying effects via spatial random
fields and providing model-based estimation and inference, but requiring more advanced computational techniques). We compare
the approaches with respect to underlying conceptual structures, computational implementation, and inferential output. We
apply both approaches to violent crime, illegal drug arrest, and alcohol distribution data from Houston, Texas and compare
results in light of the differing methodological structures of the two approaches. 相似文献
44.
45.
In the prognostic stratiform cloud scheme used in the UK Meteorological Office Unified Model, mixed phase clouds are assumed to exist between 0 and –15 °C. Recent observations of such clouds, carried out using the C-130 aircraft of the Meteorological Research Flight, suggest that a smaller range of 0 to –9°C may be more appropriate. The sensitivity of cloud and radiation fields as simulated by a 5 × 7.5° latitude-longitude version of the Unified Model to such a reduction in the temperature range of mixed phase clouds is considered. Using a smaller temperature range systematic errors in the radiation budget of the model are reduced in mid-latitudes, bringing the model into closer agreement with ERBE data. The sensitivity of model albedo to an increase in the temperature range over which mixed phase clouds are assumed to exist, suggested by previous observational studies, is also considered together with the impact of removing the mixed phase part of the precipitation parametrization altogether. 相似文献
46.
Organic-rich samples derived from a Middle Cambrian Formation in the Georgina Basin, and from the Middle Proterozoic of the McArthur Basin in northern and central Australia, yielded alginite ranging from immature oil shale material to overmature residue. A maturation scale has been developed based on the thermal evolution of alginite as determined from reflectance and fluorescence. The coalification path of alginite is marked by jumps in contrast to the linear path of wood-derived vitrinite. Six zones have been recognised, ranging from undermature (zone I), through the mature (zones II/III), followed by a stable stage of no change (zone IV) to the overmature (zones V and VI). The onset of oil generation in alginite as evident from the present study is at 0.3% Ro Alg. and is expressed in a change of fluorescence from yellow to brown, and a coalification jump from 0.3 to 0.6% Ro of Alg. In many boreholes zone III can be distinguished between 0.6 and 0.8% Ro of Alg. where subsequent oil generation occurs. Zones II and III represent the oil window.A zone of little or no change designated zone IV, at
of alginite follows zones II/III. A marked coalification jump characterises zone V, where a pronounced change in reflectance occurs to >1.0% Ro Alg., signifying peak gas generation. The border of oil preservation lies at the transition of zone V and VI, at 1.6% Ro Alg. In zone VI gas generation only occurs.Comparison of reflectance results with experimental and geochemical pyrolysis data supports high activation energies for hydrocarbon generation from alginite, and therefore a later onset of oil generation than other liptinite macerals (i.e. cutinite, exinite, resinite) as well as a narrow oil window.Transmission electron microscopy (TEM) confirms that alginite does not go through a distinct intermediate stage but that the percentage of unreacted organic matter decreases as maturation proceeds. A clear distinction can be made in TEM between immature alginite, alginite after oil generation, and alginite residue following gas generation. Alginite beyond 1.6% Ro acquires very high densities and the appearance of inertinite in TEM.Bitumens/pyrobitumens make a pronounced contribution to the organic matter throughout the basins and have been shown to effect pyrolysis results by suppressing Tmax. The bitumens/pyrobitumens have been divided into four groups, based on their reflectance and morphology, which in turn appears to be an expression of their genetic history. Their significance is in aiding the understanding of the basins' thermal history, and the timing of oil and gas generation. 相似文献
47.
Metals in the sea may form complexes with organic compounds, a process known as chelation, and this may modify their properties very greatly. Here the effect of chelation on the toxicity of copper to algae has been studied in the laboratory using the powerful chelating agent EDTA. Similar phenomena in the sea have implications both for anti-fouling paints, which usually contain copper, and for metal pollution in waters with a high organic content. 相似文献
48.
Butler Shearer Redfern Colhoun O'Kane Penny Morris Griffiths & Cullum 《Monthly notices of the Royal Astronomical Society》1998,296(2):379-391
The Galway/DIAS Image Sharpening Camera, TRIFFID, has been used to make observations in two colours of the centre of the post-core-collapse globular cluster M15. We present here our analysis of the photometry in B over two seasons. We have combined the complementary qualities of the HST 's high astrometric precision and TRIFFID's extended coverage and photometric precision, to perform crowded-field photometry in the innermost region of M15. Our technique virtually eliminates the problem of extreme crowding which has hitherto hampered studies of the variable star populations in globular cluster cores, and thereby provides an extension of the HST 's capability. Candidate variables detected with the HST have been confirmed and monitored over longer periods. We show that most of these are RR Lyrae stars, and that one is a short-period Type II Cepheid (the third to be discovered in M15). Our photometric study also produced evidence of a similar number of new variables. These also appear to be RR Lyrae stars, except for a possible eclipsing system. Further data from an upgraded version of TRIFFID have recently been obtained to help to refine the light curves of all these objects. 相似文献
49.
Robert J. Morris 《Geochimica et cosmochimica acta》1975,39(3):381-388
The distribution of amino acids with depth have been described for three samples of a core of recent sediment (< 1000–8000 yr) from the continental slope off N.W. Africa. Basic amino acids are more abundant than acidic amino acids in all samples. The amino acid composition of the oldest sample resembles most closely the amino acid composition of planktonic protein. It is suggested that the differences in amino acid composition of the three samples reflect diagenetic changes which are probably controlled by the environment of deposition. 相似文献
50.
Compositional profiles of a series of garnets, extracted from a pelitic schist occurring in the Moinian rocks of Northern Sutherland, have been established. The profiles are complex and do not conform to a simple fractionation model. A model to explain them is suggested. 相似文献