全文获取类型
收费全文 | 10167篇 |
免费 | 2810篇 |
国内免费 | 3573篇 |
专业分类
测绘学 | 297篇 |
大气科学 | 1535篇 |
地球物理 | 2202篇 |
地质学 | 8929篇 |
海洋学 | 1074篇 |
天文学 | 115篇 |
综合类 | 1174篇 |
自然地理 | 1224篇 |
出版年
2024年 | 64篇 |
2023年 | 163篇 |
2022年 | 336篇 |
2021年 | 406篇 |
2020年 | 390篇 |
2019年 | 510篇 |
2018年 | 449篇 |
2017年 | 518篇 |
2016年 | 512篇 |
2015年 | 606篇 |
2014年 | 706篇 |
2013年 | 690篇 |
2012年 | 826篇 |
2011年 | 794篇 |
2010年 | 751篇 |
2009年 | 757篇 |
2008年 | 687篇 |
2007年 | 830篇 |
2006年 | 770篇 |
2005年 | 651篇 |
2004年 | 608篇 |
2003年 | 530篇 |
2002年 | 478篇 |
2001年 | 540篇 |
2000年 | 461篇 |
1999年 | 419篇 |
1998年 | 370篇 |
1997年 | 299篇 |
1996年 | 276篇 |
1995年 | 232篇 |
1994年 | 196篇 |
1993年 | 174篇 |
1992年 | 138篇 |
1991年 | 99篇 |
1990年 | 70篇 |
1989年 | 61篇 |
1988年 | 50篇 |
1987年 | 32篇 |
1986年 | 20篇 |
1985年 | 17篇 |
1984年 | 12篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1980年 | 9篇 |
1979年 | 4篇 |
1978年 | 11篇 |
1977年 | 14篇 |
1976年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
961.
962.
963.
Silica mobility and fluid movement during metamorphism of the Connemara schists, Ireland 总被引:5,自引:1,他引:5
Quartz veins are developed in a wide range of metasediment types in the upper amphibolite facies rocks of Connemara, and attest to considerable migration of silica. Contrary to common assumptions, there is clear evidence that these veins do not primarily result from movement of fluid to regions of lower P–T down the regional geothermal gradient. Under amphibolite facies conditions, a dilute chloride fluid moving down temperature has the potential to alter 60g of plagioclase to muscovite for each gram of vein quartz precipitated, while cooling over the temperature interval from 650 to 500° C. The absence of significant metasomatic effects in the vein walls effectively precludes a simple origin from such through-flowing, externally derived fluids. The oxygen isotopic composition of matrix quartz shows considerable differences between different rock types (quartzite, pelite and marble), with a range of δ18OSMOW from c.+ 11.5% (quartzite) to + 18.5% (marble). In each rock type, vein quartz compositions closely match those of the matrix quartz. These results demonstrate the importance of local segregation processes in the formation of veins, and suggest that fluid convection cells were not developed during metamorphism on a scale larger than the individual sedimentary formations, if at all. Both oxygen isotope data and the absence of metasomatism indicate that veins form primarily by segregation of quartz from the host lithologies, with only a relatively minor component of through flow of externally derived fluid. Veins are clearly not the major pathways of metamorphic dewatering. It is proposed that abundant veins in the predominantly pelitic Ballynakill Formation formed during peak metamorphic D3 folding because the formation was embrittled by high fluid pressures but was capped by impermeable marble. Hence the pelitic formation fractured repeatedly and the pore fluid drained through the fractures to form veins, while irreversible loss through the rest of the succession was a much less important process. In the central mountains of Connemara, rather pure, unreactive quartzites are cut by widely spaced, laterally extensive quartz veins that are axial planar to D3 folds. These veins may mark pathways whereby metamorphic fluid made its way through the massive impermeable quartzite from lower parts of the nappe pile, but here too, oxygen isotope data indicate considerable segregation of locally derived quartz, reflecting the importance of pumping of fluid between wail rocks and fractures relative to the component of through flow. 相似文献
964.
965.
滑坡位于汶川县城关镇南沟,属深层牵引式土质滑坡。该滑坡正处于活动期,一旦大规模活动,将危及县政府等几十个机关、学校和居民的安全,给数千人的生命、财产带来极大的损失。本文提出了筑坝、排水、封山育林等具体防治方案和措施。 相似文献
966.
Spatial averaging of hydraulic conductivity in three-dimensional heterogeneous porous media 总被引:4,自引:0,他引:4
A. J. Desbarats 《Mathematical Geology》1992,24(3):249-267
Numerical models of groundwater flow require the assignment of hydraulic conductivities to large grid blocks discretizing the flow domain; however, conductivity data is usually available only at the much smaller scale of core samples. This paper describes a geostatistical model for hydraulic conductivity at both the core or point scale and that of grid blocks. Conductivity at the block scale is obtained empirically as a spatial power-average of point scale values. Assuming a multivariate Gaussian model for point log-conductivity, expressions are derived for the ensemble mean and variance of block conductivity. The expression for the ensemble mean of block scale conductivity is found to be similar to an expression for the ensemble effective conductivity of an infinite field derived analytically by earlier authors. Here, block conductivities obtained by power averaging are compared with effective conductivities obtained from a numerical flow model and are found to be in excellent agreement for a suitably chosen averaging exponent. This agreement deteriorates gradually as the log variance of conductivity increases beyond 2. For arbitrary flow field geometry and anisotropic conductivity covariances, the averaging exponent can be calibrated by recourse to numerical flow experiments. For cubic fields and an isotropic spatial covariance, the averaging exponent is found to be 1/3. In this particular case, it was found that flow field discretization at the block scale through local averaging of point conductivities gave similar results to those obtained directly using a point scale discretization of the flow field. 相似文献
967.
用因子分析及方差最大原则下的正交旋转技术,计算并识别长江中下游汛期(5~9月)降水场的四个空间高载荷区及相对应的因子得分系数.它们的空间、时间变化,反映了长江中下游区域的实际降水特点.赤道东太平洋海温、区域平均海温及ENSO事件都与高载荷区的汛期降水有一定的联系,所得结果可作为汛期降水短期气候预报的参考因子. 相似文献
968.
工业区与非工业区辐射雾水的化学组成 总被引:9,自引:0,他引:9
1988年冬季,你们对云南省安宁工业区和西双版纳非工业区雾水进行采集,工业区雾水混浊肮脏,含有大量固体颗粒,雾水中离子浓度值与附近工厂排向大气的污染物有关,但雾水酸度值并不高。非工业区雾水比较清洁,雾水中离子浓度值平均比工业区低一个数量级。本文还讨论了一个雾过程离子浓度变化与温度层结的关系。 相似文献
969.
970.