全文获取类型
收费全文 | 1117篇 |
免费 | 182篇 |
国内免费 | 197篇 |
专业分类
测绘学 | 48篇 |
大气科学 | 193篇 |
地球物理 | 331篇 |
地质学 | 434篇 |
海洋学 | 173篇 |
天文学 | 81篇 |
综合类 | 50篇 |
自然地理 | 186篇 |
出版年
2024年 | 3篇 |
2023年 | 11篇 |
2022年 | 38篇 |
2021年 | 28篇 |
2020年 | 32篇 |
2019年 | 46篇 |
2018年 | 33篇 |
2017年 | 45篇 |
2016年 | 38篇 |
2015年 | 36篇 |
2014年 | 55篇 |
2013年 | 87篇 |
2012年 | 53篇 |
2011年 | 51篇 |
2010年 | 48篇 |
2009年 | 69篇 |
2008年 | 63篇 |
2007年 | 70篇 |
2006年 | 58篇 |
2005年 | 69篇 |
2004年 | 50篇 |
2003年 | 52篇 |
2002年 | 50篇 |
2001年 | 43篇 |
2000年 | 43篇 |
1999年 | 35篇 |
1998年 | 50篇 |
1997年 | 43篇 |
1996年 | 33篇 |
1995年 | 26篇 |
1994年 | 27篇 |
1993年 | 25篇 |
1992年 | 19篇 |
1991年 | 12篇 |
1990年 | 9篇 |
1989年 | 7篇 |
1988年 | 7篇 |
1987年 | 8篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 5篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有1496条查询结果,搜索用时 46 毫秒
21.
地震次生毒气泄漏与扩散的数值模拟与动态仿真 总被引:16,自引:1,他引:16
文章对破坏性地震中可能发生的次生毒气泄漏与扩散灾害进行了数值模拟与动态仿真研究,设定了几种有实际意义的数值分析模型,分别进行了结构地震破坏,有毒有害气体泄漏,在一定泄漏和气象条件下的向周围空间扩散的动态仿真研究,给出了典型毒气扩散过程中可致人员死亡,严重危险和有感区域的范围的动态变化结果,作者认为地震次生毒气泄漏与扩散研究对大地震应急、城市和大型企业防灾都有重要的意义和广阔的应用前景。 相似文献
22.
23.
24.
最优化的矿物扩散地质速率计:北大别黄土岭麻粒岩缓慢冷却的证据 总被引:2,自引:0,他引:2
对北大别黄土岭麻粒岩中石榴子石进行了详细的电子探针工作,并根据一颗石榴子石的成分环带,建立了扩散模型并进行了数值模拟,结合最优化的方法,得到了极其缓慢的冷却速率0.1℃/a。从扩散动力学数值模拟的角度对大别山造山带的冷却速率进行了探索性研究。该结果表明,黄土岭麻粒岩在受麻粒岩相改造后经历的是一个极其缓慢、持续时间很长的冷却过程。 相似文献
25.
文中研究了耦合海-气环流模式中的双热带辐合带(Double ITCZ)现象,并对这一现象的成因从海洋热量收支的角度进行了初步分析。Double ITCZ现象是在热带太平洋赤道南北两侧各出现一条ITCZ的现象,这是耦合海-气环流模式中的较为普遍的一种异常现象,与实际气候中出现的Double ITCZ现象并非指同一问题。文中对比观测和模式结果,指出了Double ITCZ现象的主要特征,针对它的出现过程进行细致分析,再利用模式输出的热量收支各项进行统计,得到了从海洋热收支分析得到的海表温度变化原因。与观测到的正常模态相比,Double ITCZ是一个异常的模态,它的特征突出地表现为西太平洋暖池区的降温和东南太平洋10°S附近的升温。海洋热量收支分析表明,暖池区的降温主要是受到扩散的作用,而表层平流的异常增暖在决定异常辐合带区升温过程中占第一位的作用。 相似文献
26.
利用三维非静力能量闭合(E-ε)的边界层模式,以深圳海岸复杂地形进行了实际模拟。结果给出了该地区海陆风情形下气流和湍流孤变化特征。在海风发展盛期,气流方向由大面积的水域和内陆的位置决定,不规则海岸线对局地气流影响不大,夜间陆风时,不规则海岸线对局地气流影响较大,湍流能量高值颁在陆地上空的不稳定层内,水面上湍能很小。模拟结果与实测结果上比较吻合。 相似文献
27.
Diffusion modelling of growth-zoned garnet is used in combination with standard geothermometric and geobarometric techniques to estimate cooling and denudation rates from the mafic eclogites of the Red Cliff area, Great Caucasus, Russia. Euhedral garnet porphyroblasts exhibit different degrees of prograde growth zoning depending on the size of the grain (100 μm to several mm in diameter). Zoning patterns are mainly expressed in terms of Fe–Mg exchange, with 100*Mg/(Mg+Fe) increasing from 18–20 to 33–37 from core to rim. Geothermobarometry yields conditions of 680±40 °C and a minimum of 1.6±0.2 GPa and of 660±40 °C and 0.8±0.2 GPa for the high-pressure and retrograde stages of equilibration, respectively. A temperature of 600±40 °C has been recorded for the late-stage metamorphic overprint in the mica schists surrounding the eclogites. Relaxation of garnet zoning profiles was modelled for three different hypothetical P–T –t trajectories, all with an initial temperature of 680 °C and a pressure change of 0.8 GPa. The first two trajectories involve decompression associated with regular cooling down to 660 °C (near isothermal) and 600 °C. The third path is a two-step trajectory comprising near-isobaric cooling down to 600 °C followed by isothermal decompression to 0.8 GPa. These P–T trajectories cover as wide a range of pressure and temperature changes endured by the rocks as possible, thus representing extreme cases for calculating cooling and exhumation rates. Calculations indicate that the zoning pattern of the smallest garnet (i.e. garnet for which the zoning is most easily eliminated during post-growth processes) along the different paths can be preserved for the following average exhumation and cooling rates: path 1, 143 mm a?1 and 102 °C Ma?1; path 2, 60 mm a?1 and 171 °C Ma?1; path 3, 11–30 mm a?1 and 200–400 °C Ma?1. These results are discussed in light of theoretical P–T–t paths extracted from thermal models of regions of thickened crust, and from analogue models of accretionary wedge and continental lithosphere subduction. 相似文献
28.
Disequilibrium for Ca during growth of pelitic garnet 总被引:14,自引:1,他引:14
Compositional zoning in hundreds of almandine-rich garnets in amphibolite by facies micaceous quartzites from the Picuris Range, north-central New Mexico USA, indicates that although Mn, Mg and Fe achieve chemical equilibrium at hand-sample scale during garnet growth, Ca does not. Instead, Ca concentrations at the surface of growing garnets appear to depend strongly on kinetic factors that govern the local chemical environment, yielding disequilibrium for Ca at scales larger than the region immediately surrounding an individual porphyroblast. Detailed zoning profiles were obtained for 371 garnet crystals in a small volume of a single sample of garnetiferous quartzite, and core analyses were made of 97 additional crystals. Each analysis was made on a section that passed precisely through the morphological centre of the crystal, located by means of 3-D imagery from computed X-ray tomography. The data reveal strong correlations between crystal size and concentrations of Mn, Mg and Fe (but not Ca) in garnet cores; a relationship between crystal size and isolation; rigorous cross-correlations among concentrations of Mn, Mg and Fe (but not Ca); and systematic variations in Ca concentrations as a function of crystal size and core composition that are anomalous in comparison to the behaviour of the other divalent cations. We interpret these observations as the result of thermally accelerated diffusion-controlled garnet growth, in circumstances that promoted rapid intergranular diffusion and thus rock-wide equilibration of Mn, Mg and Fe, but that prevented equilibration at similar scale for Ca because of its more sluggish intergranular diffusion. The anomalous behaviour of Ca is made evident in these garnets by the presence of sharp spikes in Ca concentration, which are demonstrably not a consequence of any simultaneous rock-wide event, such as a change in pressure, temperature, or some other intensive parameter. Instead, Ca concentrations probably reflect the local extent of reaction in the immediate vicinity of each porphyroblast. To the degree that such kinetic factors introduce departures from chemical equilibrium for Ca, thermobarometric estimates that involve grossular contents of pelitic garnet will be in error. 相似文献
29.
A general correspondence principle is presented that relates any time-domain electromagnetic diffusion field to an electromagnetic wavefield in a 'corresponding' configuration. The principle applies to arbitrarily inhomogeneous and anisotropic media and arbitrary transmitters and receivers. For the correspondence between the two types of electromagnetic fields to hold, the electric conductivity in the diffusive case and the permittivity in the wavefield case should have the same spatial variation, while the permeability distributions in space in the two cases are to be identical. Essential steps in the derivation of the correspondence principle are the use of the time Laplace transformation of causal signals, taken at real, positive values of the transform parameter, the Schouten-Van der Pol theorem in the theory of the Laplace transformation, and the reliance upon Lerch's theorem of the uniqueness of the interrelation between causal field quantities and their time-Laplace-transform representations at real, positive values of the transform parameter. Correspondence is then established between the tensorial Green's functions in the two cases, where the Green's functions are the point-receiver responses (either electric or magnetic field) to point-transmitter excitations (either electric- or magnetic-current source).
Through the correspondence principle, all transient electromagnetic wavefields (where losses are neglected) have as a counterpart a transient diffusive electromagnetic field (where the electric displacement current is neglected). The interrelation yields the tool to compare quantitatively the potentialities of the two types of fields in transient electromagnetic geophysical prospecting.
Finally, a general medium-parameter scaling law for time-domain electromagnetic wavefields is presented. 相似文献
Through the correspondence principle, all transient electromagnetic wavefields (where losses are neglected) have as a counterpart a transient diffusive electromagnetic field (where the electric displacement current is neglected). The interrelation yields the tool to compare quantitatively the potentialities of the two types of fields in transient electromagnetic geophysical prospecting.
Finally, a general medium-parameter scaling law for time-domain electromagnetic wavefields is presented. 相似文献
30.
We measured the partial pressure of oxygen (PO2) in the interstitial gas surrounding the sand-swimming Namib moleEremitalpa granti namibensis. At a sand temperature of 26 °C, which produced a nearly maximal rate of oxygen consumption, thePO2near the noses of the animals averaged only 0·9 kPa (6·7 Torr) below the level in the free atmosphere. High oxygen availability was a result of the notably low metabolic rate in the 20 g mammals and the dry, porous and metabolically inactive nature of dune sand. A mathematical model indicated that normal mammals weighing 200 g or more could comfortably exist completely encased in dune sand. We concluded that the moles' small size and low metabolic rate are not adaptations to hypoxia or hypercapnia underground but are probably related to low food availability and the energetic cost of foraging in their desert environment. 相似文献