全文获取类型
收费全文 | 1886篇 |
免费 | 497篇 |
国内免费 | 294篇 |
专业分类
测绘学 | 176篇 |
大气科学 | 111篇 |
地球物理 | 766篇 |
地质学 | 1200篇 |
海洋学 | 168篇 |
天文学 | 8篇 |
综合类 | 152篇 |
自然地理 | 96篇 |
出版年
2024年 | 8篇 |
2023年 | 40篇 |
2022年 | 56篇 |
2021年 | 73篇 |
2020年 | 60篇 |
2019年 | 85篇 |
2018年 | 70篇 |
2017年 | 94篇 |
2016年 | 102篇 |
2015年 | 115篇 |
2014年 | 150篇 |
2013年 | 120篇 |
2012年 | 96篇 |
2011年 | 137篇 |
2010年 | 99篇 |
2009年 | 112篇 |
2008年 | 113篇 |
2007年 | 128篇 |
2006年 | 115篇 |
2005年 | 87篇 |
2004年 | 93篇 |
2003年 | 89篇 |
2002年 | 53篇 |
2001年 | 62篇 |
2000年 | 62篇 |
1999年 | 59篇 |
1998年 | 46篇 |
1997年 | 47篇 |
1996年 | 61篇 |
1995年 | 35篇 |
1994年 | 36篇 |
1993年 | 30篇 |
1992年 | 17篇 |
1991年 | 19篇 |
1990年 | 25篇 |
1989年 | 20篇 |
1988年 | 8篇 |
1987年 | 14篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 17篇 |
1974年 | 1篇 |
1954年 | 3篇 |
排序方式: 共有2677条查询结果,搜索用时 15 毫秒
281.
282.
本文首先通过Snoke发展的利用P波、 SV波和SH波的初动和振幅比方法计算秦岭—大别东段2008年以来83次ML≥2.5地震的震源机制解, 然后通过FMSI方法反演得到平均应力场的方向特征: 近东西向的水平挤压和近南北向的水平拉张作用, 与参数统计结果一致; 进而通过震源机制的一致性参数分析应力场方向的变化信息。 分析表明震源机制解呈现了由变化紊乱、 偏离平均应力场到变化一致、 趋于平均应力场再到变化紊乱、 偏离平均应力场的变化过程, 在显著地震前, 研究区整体的震源机制一致性参数处于持续低值状态; 计算了2008年以来93次ML≥2.5地震的视应力, 拟合视应力与震级的关系, 分析扣除震级影响的差视应力随时间变化特征, 近似反映研究区应力大小的变化。 分析表明显著地震前研究区的差视应力值呈现出明显的先上升后下降的变化过程, 在下降过程中发生地震; 最后利用震源机制一致性参数和差视应力综合分析研究区的应力状态, 均表明显著地震前震源区的构造应力场增强。 目前研究区的震源机制一致性参数较高、 差视应力较低, 研究区的应力水平较低。 相似文献
283.
通过建立三维计算模型,对隐伏正断层在均匀错动、倾斜错动和翘倾错动方式下地表土体的应力路径、破裂和变形特征进行了研究。根据地表破裂临界值,分析了工程建设“避让带”的宽度和起始位置的变化特征。根据行业规范,提出工程建设“关注带”的确定方法,分析了“关注带”的宽度和起始位置的变化特征,得到以下主要结论:①在断层错动过程中,位于两侧的地表土体应力路径变化明显不同,下盘一侧和上盘一侧分别以三轴拉伸和三轴压缩为主;②地表强变形带与地表破裂带的分布并不一致,需要综合考虑等效塑性应变和总位移比2个指标来评价同震地表错动对建筑物的影响;③当隐伏断层错动的垂直位移达到3m时,工程建设“避让带”的宽度在10—90m范围内变化,受上覆土体厚度和断层倾角的影响最大,而工程建设“关注带”的宽度在150—400m范围内变化,受上覆土体的性质影响最大。 相似文献
284.
基于CAP方法,使用地震波形资料,计算得到了2009年1月~2017年8月期间南北地震带及周边区域466个3.5级以上地震震源机制解。在补充收集1976年1月~2017年8月GCMT公布的259个4.5级以上地震震源机制解的基础上,分析了南北地震带地震震源机制解和应力特征。震源机制空间分布显示,不同断裂带、块体间表现出不同的震源机制空间分布特征,该特征与南北地震带不同段落活动构造性质基本吻合。作为青藏高原东边界的南北地震带,由于动力环境复杂,其内部P轴方向具有明显的差异性。这种差异主要表现为:南北地震带北段P轴呈NE向分布;龙门山断裂带及周边除NE段P轴取向为NW—NNW向外,其他地段P轴近EW向;川滇菱形块体内部P轴呈NNW向,而其西边界以西呈NNE向,东边界以东呈NW向,应力方向转换带的与川滇菱形块体边界基本一致。整体而言,南北地震带及近邻P轴方向由北到南发生了顺时针转动。 相似文献
285.
以一座主跨为820 m的双塔非对称单侧混合梁斜拉桥为研究对象,对其进行动力特性分析。运用相对运动法进行多点激励地震响应计算,分析行波效应对塔顶、主梁、塔底等关键位置动力响应的影响,同时对比分析入射角方向(即斜拉桥两侧相对方向)对其动力响应的影响。结果表明:与一致激励分析结果相比,多点激励使得主塔内力结果偏小,而使得主梁内力结果偏大;不同入射角地震激励对非对称大跨度斜拉桥的内力也会产生较大影响,内力变化可达20%。因此,在进行该类型非对称混合梁斜拉桥设计中,应考虑非一致激励效应和地震动输入方向的影响因素。 相似文献
286.
287.
塔克拉玛干沙漠腹地边界层风场特征 总被引:2,自引:0,他引:2
利用CFL-03边界层风廓线雷达在塔中开展了边界层风场探测研究,根据2010年4月1日至2010年11月30日连续的边界层风场探测资料,分析了塔克拉玛干沙漠腹地大气边界层风场时空分布特征,得到了4月至11月各月的高空风廓线。结果表明,塔中春秋季风场结构类似,中高层多为西风,风速较大,低层多为东风,风速较小,整层风速与高度成正比;夏季高层风速变小,低层风速增大,东风层增厚,夏季塔中地区上空100~200 m高度层存在明显风速切变带。 相似文献
288.
Geomorphic river design strives for natural resilience by encouraging geomorphic form complexity and morphological processes linked to greater habitat diversity. Increasing availability of high-resolution topographic data and spatial feature mapping methods provide advantages for morphological analysis and river restoration planning. We propose and evaluate an approach to quantifying topographic variability of geomorphic form and pixel-level surface roughness resulting from channel planform geometry differences using spatially continuous variety computation applied to component metrics including flow direction, aspect and planform curvature. We define this as the geomorphic form variation (GFV) approach and found it scalable, repeatable and a multi-stage analytical metric for quantifying physical aspects of river-bed topographic variability. GFV may complement process-based morphological feature mapping applications, hydraulic assessment indices and spatial habitat heterogeneity metrics commonly used for ecological quality evaluation and river restoration. The GFV was tested on controlled synthetic channels derived from River Builder software and quasi-controlled sinuous planform flume experiment channels. Component variety metrics respond independently to specific geometric surface changes and are sensitive to multi-scaled morphology change, including coarser-grained sediment distributions of pixel-level surface roughness. GFV showed systematic patterns of change related to the effects of channel geometry, vertical bed feature (pool-bar) frequency and amplitude, and bar size, shape and orientation. Hotspot analysis found that bar margins were major components of topographic complexity, whereas grain-scale variety class maps further supported the multi-stage analytical capability and scalability of the GFV approach. The GFV can provide an overall variety value that may support river restoration decision-making and planning, particularly when geomorphic complexity enhancement is a design objective. Analysing metric variety values with statistically significant hotspot cluster maps and complementary process-based software and mapping applications allows variety correspondence to systematic feature changes to be assessed, providing an analytical approach for river morphology change comparison, channel design and geomorphic process restoration. 相似文献
289.
Two large oil fields (QHD32-6 and QHD33-1), located in the middle part of the Shijiutuo Uplift, have generally suffered mild biodegradation. Based on multivariate statistical analysis of the biomarker parameters, this study discussed the origin and charging directions for these two oil fields.In contrast to Ed3-derived oil, all available oil samples from these two large oil fields displayed low C19/C23, C24/C26 and high G/H and 4-MSI, which are attributed to the mixtures of oils derived from the Shahejie (Es1 and Es3) source rocks. Oils in QHD32-6, which contain relatively more Es3-derived oil, are called Group I oils, and most oils in QHD33-1, which share relatively more Es1-derived oil, are called Group II oils. Our mixed oil experiments reveal the predominant Es3- and Es1-derived oil contribution for Group I and Group II oil groups, respectively; however, the selection of end member oils warrants further research.Based on comparisons of biomarker parameters, the QHD32-6 oil field was mainly charged in the north by oil generated from Shahejie formation source rocks in the Bozhong depression. However, oils from the north of QHD32-6 field display a remarkable difference to the oils in the south of this field, which may indicate that a charging pathway exists from the QHD33-1 field. Considering the variations in biomarker compositions in the west to -east and northwest to -southeast sections across the QHD33-1 and QHD32-6 oil fields, it can be deduced that Es3-sourced oil migrated westward to the QHD32-6 traps, and then charging by Es1 oil from the Bozhong Sag resulted in the QHD33-1 oil field being characterized by the mixture of Es3- and Es1-sourced oil. Moreover, migration of Es1-derived oil from the Qinnan Sag was not identified, implying that the QHD33-1 oil field is mainly charged from the northeast of the Bozhong Sag. 相似文献
290.
Influence of lunar phases,winds and seasonality on the stranding of mesopelagic fish in the Strait of Messina (Central Mediterranean Sea) 下载免费PDF全文
Pietro Battaglia Giovanni Ammendolia Mauro Cavallaro Pierpaolo Consoli Valentina Esposito Danilo Malara Ignazio Rao Teresa Romeo Franco Andaloro 《Marine Ecology》2017,38(5)
The shore stranding of mesopelagic fauna is a recurrent phenomenon in the Strait of Messina (Central Mediterranean Sea). The aim of this paper is to test the influence of lunar phases, winds and seasons upon the frequency of occurrence of strandings of mesopelagic fish. Species abundance in relation to these factors was quantified for the first time. Specimens were collected stranded on the shore along the Sicilian coast of the Strait of Messina between 2008 and 2016. Overall 32 species belonging to seven families (Gonostomatidae, Microstomatidae, Myctophidae, Paralepididae, Phosichthyidae, Sternoptychidae, Stomiidae) were found stranded. Myctophidae was the family including the highest number of species (16), whereas Gonostomatidae was the most abundant in terms of total number of individuals (47.2%), mainly thanks to the species Cyclothone braueri. The moon, which influences the strength of currents (highest during full and new moon phases) and irradiance (higher in some lunar phases, such as the full moon), affected the abundance of stranded mesopelagic fish in the study area. The highest number of stranding events was recorded during the new moon: 34.6% of the total relative abundance of stranded mesopelagic fish. Wind blowing from the sea towards the coastline (southeasterly and easterly winds) created the best conditions for strandings. The highest abundance of stranded specimens was recorded during the winter season. 相似文献