全文获取类型
收费全文 | 1984篇 |
免费 | 259篇 |
国内免费 | 286篇 |
专业分类
测绘学 | 111篇 |
大气科学 | 270篇 |
地球物理 | 710篇 |
地质学 | 619篇 |
海洋学 | 405篇 |
天文学 | 111篇 |
综合类 | 79篇 |
自然地理 | 224篇 |
出版年
2024年 | 5篇 |
2023年 | 14篇 |
2022年 | 37篇 |
2021年 | 29篇 |
2020年 | 41篇 |
2019年 | 53篇 |
2018年 | 40篇 |
2017年 | 77篇 |
2016年 | 62篇 |
2015年 | 53篇 |
2014年 | 98篇 |
2013年 | 138篇 |
2012年 | 83篇 |
2011年 | 96篇 |
2010年 | 102篇 |
2009年 | 156篇 |
2008年 | 125篇 |
2007年 | 143篇 |
2006年 | 120篇 |
2005年 | 102篇 |
2004年 | 91篇 |
2003年 | 87篇 |
2002年 | 84篇 |
2001年 | 61篇 |
2000年 | 73篇 |
1999年 | 61篇 |
1998年 | 81篇 |
1997年 | 55篇 |
1996年 | 45篇 |
1995年 | 45篇 |
1994年 | 53篇 |
1993年 | 45篇 |
1992年 | 31篇 |
1991年 | 20篇 |
1990年 | 24篇 |
1989年 | 26篇 |
1988年 | 14篇 |
1987年 | 22篇 |
1986年 | 7篇 |
1985年 | 7篇 |
1984年 | 3篇 |
1983年 | 7篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1973年 | 2篇 |
排序方式: 共有2529条查询结果,搜索用时 125 毫秒
41.
42.
43.
44.
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
45.
Coupled inverse modelling of groundwater flow and mass transport and the worth of concentration data 总被引:1,自引:0,他引:1
Harrie-Jan Hendricks Franssen Jaime Gómez-Hernández Andrés Sahuquillo 《Journal of Hydrology》2003,281(4):281-295
This paper presents the extension of the self-calibrating method to the coupled inverse modelling of groundwater flow and mass transport. The method generates equally likely solutions to the inverse problem that display the variability as observed in the field and are not affected by a linearisation of the state equations. Conditioning to the state variables is measured by an objective function including, among others, the mismatch between the simulated and measured concentrations. Conditioning is achieved by minimising the objective function by gradient-based methods. The gradient contains the partial derivatives of the objective function with respect to: log conductivities, log storativities, prescribed heads at boundaries, retardation coefficients and mass sources. The derivatives of the objective function with respect to log conductivity are the most cumbersome and need the most CPU-time to be evaluated. For this reason, to compute this derivative only advective transport is considered. The gradient is calculated by the adjoint-state method. The method is demonstrated in a controlled, synthetic study, in which the worth of concentration data is analysed. It is shown that concentration data are essential to improve transport predictions and also help to improve aquifer characterisation and flow predictions, especially in the upstream part of the aquifer, even in the case that a considerable amount of other experimental data like conductivities and heads are available. Besides, conditioning to concentration data reduces the ensemble variances of estimated transmissivity, hydraulic head and concentration. 相似文献
46.
指出了共线条件方程式教学中应注意的一些问题:共线条件方程式是联立的两个平面方程式,存在双主距(fx,fy)时的几何概念,以及它的变换式与直接线性变换关系式的异同点。 相似文献
47.
During the past three decades, damage due to swelling action of Ankara Clay has been observed more clearly in some parts of Ankara where rapid expansion of the city led to the construction of various kinds of structures. In this study, a comprehensive research program has been conducted (i) to investigate the effect of remoulding and desiccation on the swelling behavior of Ankara Clay and its swelling anisotropy, (ii) to estimate depth of active zone, (iii) to develop a simple technique in determining the magnitude of swelling based on water content of the soaked specimen after 24 and 72 h (wmax24,72), and (iv) to produce predictive models which could be used to estimate the swelling potential of Ankara Clay from its mineralogical and simply measured engineering characteristics. A laboratory testing program was carried out using both undisturbed, and remoulded and desiccated samples selected from 20 different locations. Montmorillonite was identified as being the main clay mineral present. Based on the moisture content variation with depth, the active expansive zone was considered to be about 2 m. The test results suggest that swelling pressure of the clay considerably decreases and/or dies out when the water content is greater than 30%. It is also noted that the measured lateral swelling is significantly in excess of the vertical equivalent indicating an anisotropy with respect to swelling. Statistical assessments indicate that a new parameter (wmax24,72) from a simple test, suggested in this study, is a very strong parameter for predicting swelling parameters of Ankara Clay. Based on 60 empirical predictive equations with coefficients of correlation between 0.96 and 0.66 from multiple regression analyses, wmax24,72, methylene blue value, liquid limit, dry unit weight and smectite content are the most important index and mineralogical properties to predict the swelling parameters of Ankara clay with small deviations from the measured values. 相似文献
48.
GPS载波相位三差观测方程的改化 总被引:3,自引:0,他引:3
本文根据相关测量平差原理推导出改化GPS载波相位三差观测方程的算法,使改化后的协因数呈分块对角体。这种改化算法不但顾及观测值的相关性,而且节省大量的存储单元。使采用严密平差法平差三差观测值成为一种最简单的数据处理方法之一。 相似文献
49.
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
50.
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