全文获取类型
收费全文 | 953篇 |
免费 | 29篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 17篇 |
大气科学 | 78篇 |
地球物理 | 246篇 |
地质学 | 273篇 |
海洋学 | 99篇 |
天文学 | 132篇 |
综合类 | 3篇 |
自然地理 | 137篇 |
出版年
2020年 | 8篇 |
2019年 | 9篇 |
2018年 | 27篇 |
2017年 | 15篇 |
2016年 | 21篇 |
2015年 | 16篇 |
2014年 | 21篇 |
2013年 | 43篇 |
2012年 | 38篇 |
2011年 | 46篇 |
2010年 | 36篇 |
2009年 | 32篇 |
2008年 | 41篇 |
2007年 | 35篇 |
2006年 | 34篇 |
2005年 | 34篇 |
2004年 | 24篇 |
2003年 | 25篇 |
2002年 | 34篇 |
2001年 | 19篇 |
2000年 | 18篇 |
1999年 | 14篇 |
1998年 | 19篇 |
1997年 | 16篇 |
1996年 | 9篇 |
1995年 | 15篇 |
1994年 | 11篇 |
1993年 | 18篇 |
1992年 | 11篇 |
1991年 | 4篇 |
1990年 | 13篇 |
1989年 | 9篇 |
1988年 | 9篇 |
1987年 | 11篇 |
1986年 | 18篇 |
1985年 | 19篇 |
1984年 | 17篇 |
1983年 | 23篇 |
1982年 | 18篇 |
1981年 | 8篇 |
1980年 | 18篇 |
1979年 | 15篇 |
1978年 | 20篇 |
1977年 | 12篇 |
1976年 | 16篇 |
1975年 | 7篇 |
1974年 | 11篇 |
1973年 | 12篇 |
1972年 | 5篇 |
1971年 | 5篇 |
排序方式: 共有985条查询结果,搜索用时 15 毫秒
91.
By the method of electron reflection, we have identified seven well-defined magnetized regions in the equatorial belt of the lunar far side sampled by the Apollo 16 Particles and Fields subsatellite. Most of these surface magnetic fields lie within one basin radius from the rim of a ringed impact basin, where thick deposits of basin ejecta are observed or inferred. The strongest of the seven magnetic features is linear, at least 250 km long, and radial to the Freundlich-Sharonov basin. The apparent correlation with basin ejecta suggests some form of impact origin for the observed permanently magnetized regions. 相似文献
92.
93.
94.
95.
Richard B. Wheeler John B. Anderson Rudy R. Schwarzer Claudia L. Hokanson 《Environmental Geology》1980,3(3):163-175
Sediments are the ultimate sink for contaminants in the marine environment, and physical processes of sedimentation influence
the distribution and accumulation of these contaminants. Evaluation of contaminant levels in sediments is one approach to
assessing environmental impact; data interpretation depends on consideration of sediment texture and mineralogy, however,
which profoundly influence chemical composition. In this study, comparison of potentially contaminated sediments from the
production field with control populations was done only within the context of similar (as to texture and organic carbon and
carbonate content) sample groups as determined by cluster analysis. Ba, Cd, and Sr are identified as contaminants. Supported
by the identification of a well-crystallized expandable clay—possibly bentonite—drilling fluids are a potential source of
Ba. Ba and Sr may be unnaturally high because of their abundance in discharged produced formation waters, but may also be
naturally controlled by the unique faunal assemblage associated with the structures. Cd is probably derived from corrosion
of the structures and assorted debris on the seafloor. In general, contamination is limited to an area within 100 m of the
platforms. Furthermore, substantial erosion around platforms has probably effectively removed and dispersed the bulk of the
contaminants introduced into the marine environment by the offshore exploration/production operations. 相似文献
96.
The thermodynamic properties of the lower mantle are determined from the seismic profile, where the primary thermodynamic variables are the bulk modulus K and density ρ. It is shown that the Bullen law (K ∝ P) holds in the lower mantle with a high correlation coefficient for the seismic parametric Earth model (PEM). Using this law produces no ambiguity or trade-off between ρ0 and K0, since both K0 and K′0 are exactly determined by applying a linear K?ρ relationship to the data. On the other hand, extrapolating the velocity data to zero pressure using a Birch-Murnaghan equation of state (EOS) results in an ambiguous answer because there are three unknown adjustable parameters (ρ0, K0, K′0) in the EOS.From the PEM data, K = 232.4 + 3.19 P (GPa). The PEM yields a hot uncompressed density of 3.999 ± 0.0026 g cm?3 for material decompressed from all parts of the lower mantle. Even if the hot uncompressed density were uniform for all depths in the lower mantle, the cold uncompressed mantle would be inhomogeneous because the decompression given by the Bullen law crosses isotherms; for example, the temperature is different at different depths. To calculate the density distribution correctly, an isothermal EOS must be used along an isotherm, and temperature corrections must be placed in the thermal pressure PTH.The thermodynamic parameters of the lower mantle are found by iteration. Values of the three uncompressed anharmonic parameters are first arbitrarily selected: α0 (hot), the coefficient of thermal expansion; γ0, the Grüneisen parameter; and δ, the second Grüneisen parameter. Using γ0 and the measured ρ0 (hot) and K0 (hot), the values of θ0 (Debye temperature) and q = dlnγ/dlnρ are found from the measured seismic velocities. Then from (αKT)0 and q the thermal pressure PTH at all high temperatures is found. Correlating PTH against T to the geotherm for the lower mantle, PTH is found at all depths Z. The isothermal pressure, along the 0 K isotherm, at every Z is found by subtracting PTH from the measured P given by the seismic model. Using the isothermal pressure at depth Z, the solution for the cold uncompressed density ρ0C and the cold uncompressed bulk modulus, KT0 is found as a trace in the KT0?ρ0C plane. A narrow band of solutions is then found for ρ0C and KT0 at all depths.The thermal expansion at all T is found from [ρ0C ? ρ0 (hot)/ρ0C. From Suzuki's formula, the best fit to the thermal expansion determines γ0 and α0 (hot). When the values of these two parameters do not agree with the original assumptions, the calculation is repeated until they do agree. In this way all the important thermodynamic parameters are found as a self-consistent set subject only to the assumptions behind the equations used. 相似文献
97.
The salt marsh cord grass,Spartina alterniflora Loisel., occurs in markedly distinct short and tall growth forms. Both forms have the same number of chromosomes, although polyploidy is well established in the genus. Previous studies have shown that height is primarily affected by nitrate availability and environmental stresses such as increased soluble salt concentrations. These studies have shown that, within a marsh, height differences cannot be attributed to genetic, chromosomal differences or electrophoretic banding. However, more subtle genetic differences may be involved. Other studies suggest that between marshes,S. alterniflora, in response to a latitudinal gradient on the Atlantic coast, has evolved ecotypes differing in height, color and flowering period. This review paper points out that plant height inS. alterniflora may be determined by a combination of environmental and genetic factors. 相似文献
98.
99.
Marine mammals are probably frequently exposed to floating oil but little is known about the effect oil pollution has on them. Oil stranded on the shore in Pembrokeshire, West Wales in September 1974 coincided with the start of the Grey Seal breeding season there. Observations have been made on the effect the oil had on the newborn pups and their mothers. 相似文献
100.
Jean-Claude C. Mercier Douglas A. Anderson Neville L. Carter 《Pure and Applied Geophysics》1977,115(1-2):199-226
Mechanical data and flow processes from steady state deformation experiments may be used to infer the state of stress in the lithosphere and asthenosphere. Extrapolations of flow equations to a representative geologic strain rate of 10–14/sec. for halite, marble, quartzite, dolomite, dunite and enstatolite are now warranted because the steady state flow processes in the experiments are identical to those in rocks and because the geotherms are reasonably well established. More direct estimates are obtained from free dislocation densities, subgrain sizes and recrystallized grain sizes all of which are functions only of stress. Using the last of these techniques, we have estimated stress profiles as a function of depth from xenoliths in basalts and kimberlites, whose depths of equilibration were determined by pyroxene techniques, from four different areas of subcontinental and suboceanic upper mantle. The results are similar and indicate stress differences of about 200 to 300 bars at 40 to 50 km, decaying to a few tens of bars at depths betow 100 km. These stresses are reasonable and are in accord with extrapolations of the mechanical data provided that allowance is made for a general increase in strain rate and decrease in viscosity with depth. 相似文献