全文获取类型
收费全文 | 701篇 |
免费 | 98篇 |
国内免费 | 163篇 |
专业分类
测绘学 | 82篇 |
大气科学 | 80篇 |
地球物理 | 187篇 |
地质学 | 285篇 |
海洋学 | 69篇 |
天文学 | 149篇 |
综合类 | 55篇 |
自然地理 | 55篇 |
出版年
2024年 | 3篇 |
2023年 | 5篇 |
2022年 | 14篇 |
2021年 | 12篇 |
2020年 | 26篇 |
2019年 | 34篇 |
2018年 | 20篇 |
2017年 | 24篇 |
2016年 | 24篇 |
2015年 | 30篇 |
2014年 | 36篇 |
2013年 | 46篇 |
2012年 | 42篇 |
2011年 | 33篇 |
2010年 | 41篇 |
2009年 | 42篇 |
2008年 | 49篇 |
2007年 | 44篇 |
2006年 | 45篇 |
2005年 | 40篇 |
2004年 | 50篇 |
2003年 | 41篇 |
2002年 | 26篇 |
2001年 | 35篇 |
2000年 | 21篇 |
1999年 | 20篇 |
1998年 | 17篇 |
1997年 | 21篇 |
1996年 | 12篇 |
1995年 | 26篇 |
1994年 | 20篇 |
1993年 | 14篇 |
1992年 | 7篇 |
1991年 | 11篇 |
1990年 | 3篇 |
1989年 | 12篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有962条查询结果,搜索用时 640 毫秒
721.
软土固结过程中展现出明显的非线性压缩和渗透特性,同时竖井的淤堵效应常导致井阻在固结过程中随深度和时间不断演化,但目前能考虑井阻随时空演化的竖井地基非线性固结解析解还很鲜见。通过引入孔隙比与有效应力及孔隙比与渗透系数间的半对数模型描述了土体的非线性固结特性,建立了能同时考虑井阻随时空变化和涂抹影响的竖井地基非线性固结模型,并采用分离变量法获得了固结模型的解析解。将特定参数下固结解的计算结果与实测数据、已有的竖井地基固结解答进行了对比分析以验证其可靠性。最后,对竖井地基的非线性固结性状开展了大量计算分析。结果表明:竖井渗透系数随深度线性衰减越明显则地基固结速率越慢;外荷载一定时,随着软土压缩指数cc与渗透率指数ck之比的增大,竖井地基固结速度减慢;在cc /ck值不变的情况下,外荷载增加,地基固结速率加快。在涂抹区的3种径向渗透系数变化模式中,抛物线变化模式下的地基固结速度最快,线性变化模式下的地基固结速度次之,恒定模式下的地基固结速度最慢,且这种性状并不因为考虑井阻变化或土体非线性固结特性而发生改变。 相似文献
722.
微震定位方法是微震监测技术的重要组成部分,其关键是定位震源位置。利用空间网格划分并计算网格交点目标函数值,对微震定位目标函数二维及三维空间分布进行了分析,并据此获取了目标函数连续且极小值唯一、单轴收敛范围逐步减小、各轴收敛范围不一的规律。利用以上规律及模式搜索法、网格搜索法的优缺点,探索出了基于连续比较模块、变步长模块、加速模块的变步长加速搜索法。通过模拟算例与工程数据下收敛稳定性、结果精确度、计算速度以及参数初始值影响程度4个指标的效果对比,结果表明:模拟算例下,对比模拟退火算法、遗传算法,变步长加速搜索法的目标函数值标准差、定位误差标准差、波速误差标准差均为0;该算法的定位误差平均值分别为其余二者的0.7%、1.9%;该算法的计算时间平均值分别为其余二者的6.9%、33.2%。该算法单独更改各参数对定位误差的影响在0.005~0.025 m之间;减小搜索步长下限可有效提高结果精确度,并增加相应的计算时间。在规定初至到时与目标函数模型及检波器位置坐标下,搜索算法对定位精度无实质影响。 相似文献
723.
以黎明镇境内34 km2的范围为研究区,以1 161个表层土壤样本K,N,P元素含量为研究对象,以1∶2 000航摄影像为基础地理信息数据,利用地统计方法分析研究区内表层土壤养分元素的空间分布特征,并针对分析结果为研究区制作变量处方图。在"天地图·黑土地"平台实现空间插值图、变量处方图的在线展示,有利于提高农业生产效率和耕地的单位面积产量。 相似文献
724.
725.
攀西地区层状岩体套叠式韵律的分形研究 总被引:1,自引:0,他引:1
套叠式韵律是攀西层状岩体的重要特征,它在岩相、矿物、结构、元素含量和斜长石牌号等各方面都有表现。本文阐述了套叠式韵律最为明显的岩相和元素含量特征,并采用分形中的关联维方法对白马岩体数据进行计算。从计算结果可知:形成MgO的套叠式韵律需要有5个状态变量控制:SiO2、TiO2、V2O3、S等元素的套叠式韵律的形成受7个状态变量的控制,推测岩体韵律的形成受7个状态变量控制。其中有两个状态变量对MgO影响不大。InC(r)—Inr曲线近似为一直线,表明该岩体韵律可用分形的方法研究 相似文献
726.
727.
对适用于非等间隔时间序列的一维CLEAN谱分析方法进行了研究。用模拟信号进行了检验,证实了它也适用于有噪声的序列,但谱质量和噪声强度密切相关,把这种方法应用到激变变变星TT Ari的测光资料分析中,检测到近20min周期的准周期振荡。 相似文献
728.
J. P. Narayan 《Geophysical Journal International》1999,139(3):879-887
This paper presents the development of a 2.5-D simulation technique for acoustic wave propagation in media with variable density and velocity. A comparative study of the 2-D and 2.5-D responses of a model reveals the spatially and temporally damped nature of the 2.5-D acoustic wave equations. The simulated results for constant and variable density models show that the density variation affects only the reflectivity of the layer. The computational cost for variable density models is 2.17 and 2.26 times that for constant density models for the 2.5-D and 2-D cases, respectively. Furthermore, the 2.5-D computational cost in the time domain is only about 10–15 per cent more than that for two dimensions, so this modest increase in computational cost can avoid the exorbitant 3-D computational cost.
Snapshots for a crosshole geometry were computed at various times in order to study the effect of heterogeneity on the amplitude and shape of the wave front. Extensive analysis of an oil-bearing reservoir with and without the inclusion of a gas zone was performed using a point source as well as multiple sources. In addition, the effects of the thickness of a low-velocity layer (oil-bearing) and of the location of the source have been studied. It is concluded from the numerical response that the waveguide action of the low-velocity layer depends on its thickness in terms of the dominant wavelength. Trapping of waves was not observed when the source was outside the low-velocity layer. Furthermore, the presence of heterogeneity in the low-velocity layer contributes considerably to the leakage of energy in the adjacent layers due to scattering/diffraction. It was found that, in the 2.5-D numerical simulation, the stability condition and the requirement of the number of grid points per wavelength to avoid grid dispersion are the same as for the 2-D case. 相似文献
Snapshots for a crosshole geometry were computed at various times in order to study the effect of heterogeneity on the amplitude and shape of the wave front. Extensive analysis of an oil-bearing reservoir with and without the inclusion of a gas zone was performed using a point source as well as multiple sources. In addition, the effects of the thickness of a low-velocity layer (oil-bearing) and of the location of the source have been studied. It is concluded from the numerical response that the waveguide action of the low-velocity layer depends on its thickness in terms of the dominant wavelength. Trapping of waves was not observed when the source was outside the low-velocity layer. Furthermore, the presence of heterogeneity in the low-velocity layer contributes considerably to the leakage of energy in the adjacent layers due to scattering/diffraction. It was found that, in the 2.5-D numerical simulation, the stability condition and the requirement of the number of grid points per wavelength to avoid grid dispersion are the same as for the 2-D case. 相似文献
729.
730.
Discriminant analysis of the geomorphic characteristics and stability of landslide dams 总被引:1,自引:0,他引:1
Jia-Jyun Dong Yu-Hsiang Tung Chien-Chih Chen Jyh-Jong Liao Yii-Wen Pan 《Geomorphology》2009,110(3-4):162-171
Landslides can cause the formation of dams, but these dams often fail soon after lake formation. Thus, rapidly evaluating the stability of a landslide dam is crucial for effective hazard mitigation. This study utilizes discriminant analysis based on a Japanese dataset consisting of 43 well documented landslide dams to determine the significant variables, including log-transformed peak flow (or catchment area), and log-transformed dam height, width and length in hierarchical order, which affect the stability of a landslide dam. The high overall prediction power (88.4% of the 43 training cases are correctly classified) and the high cross-validation accuracy (86%) demonstrate the robustness of the proposed discriminant models PHWL (with variables including log-transformed peak flow, and log-transformed dam height, width and length) and AHWL (with variables including log-transformed catchment area, and log-transformed dam height, width and length). Compared to a previously proposed “DBI” index-based graphic approach, the discriminant model AHV – which uses the log-transformed catchment area, dam height, and dam volume as relevant variables – shows better ability to evaluate the stability of landslide dams. Although these discriminant models are established using a Japanese dataset only, the present multivariate statistical approach can be applied for an expanded dataset without any difficulty when more completely documented worldwide landslide-dam data are available. 相似文献