全文获取类型
收费全文 | 4899篇 |
免费 | 947篇 |
国内免费 | 1156篇 |
专业分类
测绘学 | 397篇 |
大气科学 | 759篇 |
地球物理 | 1120篇 |
地质学 | 2700篇 |
海洋学 | 584篇 |
天文学 | 314篇 |
综合类 | 417篇 |
自然地理 | 711篇 |
出版年
2024年 | 27篇 |
2023年 | 79篇 |
2022年 | 225篇 |
2021年 | 269篇 |
2020年 | 225篇 |
2019年 | 291篇 |
2018年 | 276篇 |
2017年 | 280篇 |
2016年 | 268篇 |
2015年 | 266篇 |
2014年 | 268篇 |
2013年 | 302篇 |
2012年 | 263篇 |
2011年 | 297篇 |
2010年 | 287篇 |
2009年 | 280篇 |
2008年 | 311篇 |
2007年 | 286篇 |
2006年 | 181篇 |
2005年 | 208篇 |
2004年 | 186篇 |
2003年 | 170篇 |
2002年 | 191篇 |
2001年 | 217篇 |
2000年 | 164篇 |
1999年 | 176篇 |
1998年 | 135篇 |
1997年 | 152篇 |
1996年 | 110篇 |
1995年 | 102篇 |
1994年 | 74篇 |
1993年 | 71篇 |
1992年 | 79篇 |
1991年 | 38篇 |
1990年 | 43篇 |
1989年 | 36篇 |
1988年 | 39篇 |
1987年 | 16篇 |
1986年 | 19篇 |
1985年 | 14篇 |
1984年 | 13篇 |
1983年 | 5篇 |
1982年 | 7篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 8篇 |
1977年 | 7篇 |
1976年 | 5篇 |
1958年 | 9篇 |
排序方式: 共有7002条查询结果,搜索用时 31 毫秒
991.
992.
993.
最优性价比的观测系统设计方法研究 总被引:1,自引:1,他引:0
三维观测系统优化设计是野外地震采集工作的一个重要环节,影响着后期地震资料采集处理解释的各个工作环节.因此,高质量的观测系统设计是地震勘探工作的关键,然而在无限制的追求高信噪比、高分辨率的地震资料,高难度的野外施工的同时,这往往会带来较大的成本投入.鉴于以上存在的问题,文章着重分析质量最优和成本最优的观测系统设计,通过寻找观测系统参数与采集质量和采集成本的关系,结合二者的优点,提出了最优性价比的观测系统优化设计方法,并通过一个实际工区的试验,体现出该方法的应用效果,为高质量的观测系统设计提供新的思路和方法. 相似文献
994.
CT断层重建中滤波函数设计的新方法 总被引:1,自引:0,他引:1
滤波函数在CT图像重建过程中起着非常重要的作用,直接关系到重建图像质量。为了提高CT重建图像质量,本文从加权平均的思想出发,根据FBP重建算法的理论基础,提出一种设计滤波函数的新思路,并分析了五点加权平均滤波函数性能比较。最后针对Shepp-Logan模型数据和实际的海螺投影数据,设计出一种新的滤波函数并与S-L和R-L滤波函数的重建效果进行了比较。从比较结果可以看出,新设计的滤波函数重建的图像效果在整体性能上最好,在局部地方,其密度分辨率有所提高。本文为滤波函数的设计提出了一种新的想法和思路。 相似文献
995.
One-dimensional flow experiments on biological clogging were carried out by biostimulating columns packed with glass beads, sterilized, and inoculated with toluene-utilizing bacteria. Biostimulation consisted of continuously injecting toluene at four concentrations (3.0 ± 0.9, 6.1 ± 0.8, 8.7 ± 1.6, and 11.3 ± 0.8 mg/L). The results of column flow experiments indicated that a threshold concentration of toluene exists below which the total biomass in the column can be kept at a steady-state level. The column cores were extruded and segmented to determine the biomass distribution throughout the column at clogging. Clogging resulted in a significant buildup of filamentous bacteria close to the inlet end. Based on the nucleotide sequence of 16S rRNA genes, the dominant filamentous bacteria were identified as Nocardia farcinica. A 235-d column experiment demonstrated that the clogging near injection points can be controlled by keeping the influent concentration of toluene below the threshold. 相似文献
996.
After the Yutian M_S7.3 earthquake,the authors instantly collected 1Hz high frequency data of the 4 reference stations within 350 km around the epicenter,and calculated the GNSS data with the TRACK module. The results showed that:( 1) The co-seismic displacement of Yutian station,about 54 km from the epicenter,is the most obvious,particularly in the EW component,with a change of about 52.5 ± 11mm,which is more than three times the mean-square error of calculating precision.( 2) In the Yutian reference station,the biggest variation in the EW component appeared within 1 minute after the earthquake.( 3) The change in the NS component is not great. 相似文献
997.
Challenges to the representation of suspended sediment transfer using a depth‐averaged flux 下载免费PDF全文
The sediment saturation recovery process (i.e. the adaptation of suspended sediment concentration [SSC] to local forcing) is the main feature of the non‐equilibrium suspended sediment transport (SST) frequently occurring in fluvial, estuarine and coastal waters. In order to quantitatively describe this phenomenon, a series solution is analytically derived, including the evolution of both vertical SSC profile and near‐bed sediment flux (NBSF), and is verified by net erosion and net deposition experiments, respectively. The results suggest that the sediment saturation recovery process involves vertically varying fluxes that are not represented correctly by depth‐averaging. Consequently, a vertical two‐dimensional (2D) combined scheme is established and applied respectively in to a dredged trench and to a sand wave feature to demonstrate this argument. By analyzing the variations of the calculated depth‐averaged SSC and NBSF we reveal that the equilibrium state presented by the sediment carrying capacity (SCC) form of the NBSF, which is usually applied in depth‐integrated SST models, lags behind the actual dynamic bed equilibrium state. Moreover, the key factor α, the so‐called saturation recovery coefficient within this form, is not only a function of local Rouse number but also is influenced by the local SSC profile. Finally, a three‐dimensional (3D) non‐orthogonal curvilinear body‐fitted SST model is developed and validated in the Yangtze estuary, China, combined with the in situ hourly hydrographic data from August 14–15, 2007 during spring tide in the wet season. Model results confirm that the vertically varying sediment saturation recovery process, the discrepancies between the actual and SCC form of NBSF and non‐constant value of α are significant in actual real geomorphic cases. The quantitative morphological change resulting from variations in environmental conditions may not be correctly represented by uncorrected depth‐integrated SST models if they do not treat the effects of vertical motion on the sediment saturation recovery process. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
998.
999.
Contribution of climatic and technological factors to crop yield: empirical evidence from late paddy rice in Hunan Province,China 总被引:2,自引:1,他引:1
Zhu Wang Tao Ye Jun Wang Zhuo Cheng Peijun Shi 《Stochastic Environmental Research and Risk Assessment (SERRA)》2016,30(7):2019-2030
Climatic and technological factors are two remarkable aspects that are thought to contribute to crop yield change. However, the most significant factors and their contribution rate remain debatable. Selecting Hunan Province, which is one of the largest paddy rice producing regions in China as the research area, the marginal contributions of climatic and technological factors to late paddy yield change are estimated using a county-level panel data regression model with explicit consideration of technological variables during 2000–2011. The results indicate that the mean daily temperature and adoption rate of hybrid rice have significant positive impacts on late paddy rice yield. During the research period, 1 °C temperature increase will cause 7.220 t/km2 increase in late paddy rice yield, and 1 % increase in the adoption rate of hybrid rice will cause 0.212 t/km2 increase. Climatic factors, especially their annual variability, exhibit a stronger overall linkage to changes in late paddy rice yield in the study area compared with the technological factors. Climatic factors accounted for 67.645 and 74.585 % of the trend and variability in late paddy rice yield, respectively. The results of this study can provide valuable information for stakeholders to adjust the input on late paddy rice production and better adapt to the effect of climate change. 相似文献
1000.
Xue-Qing Liu Yan-Chun Wang Gui-Bing Zhang Sheng-Li Ma Li-Fang Cheng Wen-Wu Yu 《应用地球物理》2016,13(4):641-648
Frequency attenuation occurs when seismic waves propagate through the porous reservoirs containing hydrocarbons. Current researches on the seismic frequency attenuation mainly focus on the post-stack domain instead of the prestack domain. Here we propose the frequency attenuation gradient vs. offset (FAGVO) based on the amplitude variation with offset and frequency attenuation integral equations. We derive the FAGVO equation that equals to zero in a full-elastic medium and is negative in a viscoelastic medium. FAGVO is affected by the viscosity of the medium, the coefficients of reflection, the frequency variation, and high-frequency attenuation. FAGVO uses the differences of partially stacked data to decrease the interference caused by subsurface strata affecting the frequency attenuation, highlights the frequency attenuation gradient anomalies in hydrocarbon-bearing reservoir pores, and finally realizes the hydrocarbon fluid identification. The method was verified using a two-dimensional wave equation forward model and was found to be cost effective. Furthermore, the method does not require well information, which can be applied in the stage of seismic exploration, especially, in the exploration of a none-well project. 相似文献