首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4068篇
  免费   856篇
  国内免费   873篇
测绘学   393篇
大气科学   349篇
地球物理   1169篇
地质学   2600篇
海洋学   409篇
天文学   19篇
综合类   257篇
自然地理   601篇
  2024年   5篇
  2023年   57篇
  2022年   134篇
  2021年   179篇
  2020年   157篇
  2019年   201篇
  2018年   182篇
  2017年   188篇
  2016年   183篇
  2015年   204篇
  2014年   220篇
  2013年   258篇
  2012年   294篇
  2011年   269篇
  2010年   277篇
  2009年   289篇
  2008年   259篇
  2007年   302篇
  2006年   281篇
  2005年   225篇
  2004年   256篇
  2003年   197篇
  2002年   158篇
  2001年   151篇
  2000年   108篇
  1999年   124篇
  1998年   119篇
  1997年   88篇
  1996年   79篇
  1995年   61篇
  1994年   61篇
  1993年   63篇
  1992年   46篇
  1991年   26篇
  1990年   26篇
  1989年   16篇
  1988年   7篇
  1987年   13篇
  1986年   11篇
  1985年   4篇
  1984年   3篇
  1983年   1篇
  1982年   2篇
  1981年   3篇
  1980年   1篇
  1979年   3篇
  1978年   1篇
  1977年   1篇
  1973年   1篇
  1954年   3篇
排序方式: 共有5797条查询结果,搜索用时 31 毫秒
1.
米氏凯伦藻(Karenia mikimotoi)近年在我国福建、浙江和广东沿海经常形成赤潮,其赤潮不仅影响到海洋生态系统的稳定,也严重威胁到水产养殖以及人类生命健康安全。本论文以米氏凯伦藻为研究对象,建立了米氏凯伦藻细胞表面膜蛋白质荧光标记技术和细胞膜蛋白质提取方法,运用荧光差异凝胶电泳技术(2-DDIGE)对膜蛋白质进行了分析,并研究了米氏凯伦藻的膜蛋白质组及其对环境温度变动的响应。实验共鉴定到44个细胞表面膜蛋白,其中有效注释27个,主要为转运蛋白、HSP70蛋白家族和捕光蛋白等。米氏凯伦藻在20°C条件下的细胞生长和光合作用要明显好于16°C和12°C,但16°C和12°C条件下的差别不大,表明低温限制了米氏凯伦藻的生长。当米氏凯伦藻从12°C快速转移至16°C和20°C时,藻细胞密度和光合作用效率短时间迅速降低,但细胞很快即适应温度变化。细胞膜上的转运蛋白和光合作用蛋白在其适应温度变化中起着重要作用。  相似文献   
2.
东海盆地中、新生代盆架结构与构造演化   总被引:6,自引:0,他引:6  
基于地貌、钻井、岩石测年和地震等资料,分析盆地地层分布、盆架结构、构造单元划分和裂陷迁移规律,结果表明东海盆地由台北坳陷、舟山隆起、浙东坳陷、钓鱼岛隆褶带和冲绳坳陷构成,是以新生代沉积为主、中生代沉积为辅的大型中、新生代叠合含油气盆地;古元古代变质岩系构成了盆地的基底。该盆地不仅是印度-太平洋前后相继的动力体系作用下形成的西太平洋沟-弧-盆构造体系域一部分,而且也是古亚洲洋动力体系作用下形成的古亚洲洋构造域和特提斯洋动力体系作用下形成的特提斯洋构造域一部分,晚侏罗世至早白垩世经历了构造体制转换,盆地格局发生重大变革,早白垩世以前主要受古亚洲-特提斯洋构造体制影响的强烈挤压造山和地壳增厚作用演变为早白垩世以来主要受太平洋构造体制控制的陆缘伸展裂陷和岩石圈减薄作用,经历侏罗纪古亚洲-特提斯构造体制大陆边缘拗陷和白垩纪以来太平洋构造体制弧后裂陷两大演化阶段。白垩纪以来太平洋构造体制的弧后裂陷演化阶段可细分为早白垩世至始新世裂陷期、渐新世至晚中新世拗陷期和中新世末至全新世裂陷期。  相似文献   
3.
Reactivation of pre‐existing intra‐basement structures can influence the evolution of rift basins, yet the detailed kinematic relationship between these structures and overlying rift‐related faults remains poorly understood. Understanding the kinematic as well as geometric relationship between intra‐basement structures and rift‐related fault networks is important, with the extension direction in many rifted provinces typically thought to lie normal to fault strike. We here investigate this problem using a borehole‐constrained, 3D seismic reflection dataset from the Taranaki Basin, offshore New Zealand. Excellent imaging of intra‐basement structures and a relatively weakly deformed, stratigraphically simple sedimentary cover allow us to: (a) identify a range of interaction styles between intra‐basement structures and overlying, Plio‐Pleistocene rift‐related normal faults; and (b) examine the cover fault kinematics associated with each interaction style. Some of the normal faults parallel and are physically connected to intra‐basement reflections, which are interpreted as mylonitic reverse faults formed during Mesozoic subduction and basement terrane accretion. These geometric relationships indicate pre‐existing intra‐basement structures locally controlled the position and attitude of Plio‐Pleistocene rift‐related normal faults. However, through detailed 3D kinematic analysis of selected normal faults, we show that: (a) normal faults only nucleated above intra‐basement structures that experienced late Miocene compressional reactivation, (b) despite playing an important role during subsequent rifting, intra‐basement structures have not been significantly extensionally reactivated, and (c) preferential nucleation and propagation of normal faults within late Miocene reverse faults and folds appears to be the key genetic relationship between contractionally reactivated intra‐basement structures and rift‐related normal faults. Our analysis shows that km‐scale, intra‐basement structures can control the nucleation and development of newly formed, rift‐related normal faults, most likely due to a local perturbation of the regional stress field. Because of this, simply inverting fault strike for causal extension direction may be incorrect, especially in provinces where pre‐existing, intra‐basement structures occur. We also show that a detailed kinematic analysis is key to deciphering the temporal as well as simply the spatial or geometric relationship between structures developed at multiple structural levels.  相似文献   
4.
为满足油田中后期精细开发及后续调整挖潜的需要,亟需进行更精细的储量研究,其中最重要的一环就是纵向细分计算单元.对于多层构造油藏,纵向细分主要根据油层组内隔夹层分布特征、小层物性特征,细分到以小层或者分布特征和物性相近且纵向上连续分布的小层组合为计算单元.根据纵向细分计算单元前后储量参数选值的理论推导,结合储层的沉积展布特征,认为含油面积范围内无储层尖灭时,仅平面油层厚度差异较大的油藏平均有效厚度变小.含油面积范围内储层尖灭时,正常三角洲沉积储层的油藏各参数一般都变小;而辫状河三角洲沉积储层的油藏一般平均有效厚度变小,平均有效孔隙度和平均含油饱和度变大.这一结论可以有效指导储量评估过程中纵向细分计算单元方案划分,为同类油田的精细储量研究提供技术支撑.  相似文献   
5.
Many glaciers in alpine regions are currently rapidly receding and thinning at historically unobserved rates causing changes in the velocity field and in normal and shear stresses affecting the surface expression of structures within the ice. We studied the distribution of brittle and ductile structures at the surface of Pasterze Glacier during a 14-year period by analysing orthophotos and digital elevation models of five stages (1998, 2003, 2006, 2009 and 2012). A structural glaciological mapping key was applied. Normal faults, strike-slip faults, en échelon structures (systematic stepping of fractures), thrust faults, and band ogives were distinguished. Results indicate substantial deceleration and glacier thinning in 1998–2012. Glacier thinning was not homogenous over time related to the uneven distribution of supraglacial debris causing differential ablation or the selective ablation effects of subglacial water channels. Peculiar supraglacial features observed are circular collapse structures with concentric crevasses which form when the ice between the surface and the roof of water channels decreases. The total length of brittle structures increased from 38.4 km to 56.9 km whereas the extent of the glacier tongue decreased by 25%. The fracture density doubled from 0.009 to 0.018 m/m2. Areas of the glacier tongue which were up to 100 m away from the nearest brittle structure increased by 16%. The visual appearance of thrust faults shifted upglacier due to decreasing glacier velocity causing horizontal shortening or due to exhumation of faults that did not previously extend to the surface. A large number of brittle structures are progressively independent from glacier motion. Our study suggests that glacier tongues which are in a state of rapid decay and thinning are prone to fracturing due to normal fault formation and glacier disintegration. Water further increases ablation rates substantially if rather large amounts drain through supra-, en- or subglacial water channels. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.  相似文献   
6.
一种优化的频率域三维声波有限差分模拟方法   总被引:1,自引:1,他引:0       下载免费PDF全文
为提高频率域有限差分(FD,finite-difference)正演模拟技术的计算精度和效率,基于旋转坐标系统的优化差分格式被广泛应用,但是只应用于正方形网格的情况.基于平均导数法(ADM)的优化差分格式,应用于正方形和长方形网格模拟.这些频率域有限差分算子,各自具有不同的差分格式和对应的优化系数求解表达式.本文基于三维声波方程发展了一种新的优化方法,只要给定FD模板形式,可直接构造频散方程,求取FD模板上各节点的优化系数.此方法的优点在于频率域FD算子的优化系数对应各个节点,可扩展优化其他格式.运用此优化方法,计算得到了不同空间采样间距比情况下27点和7点格式的优化系数.数值实验表明,优化27点格式与ADM 27点格式具有相同的精度,优化7点格式比经典的7点格式具有更小的数值频散.  相似文献   
7.
在存在强衰减介质的区域进行逆时偏移成像,需要既能补偿地震波传播过程中的振幅衰减,又能保持其相位不变的高效算法.为此,本文提出了一种利用有限差分模拟进行声波衰减补偿的方法来实现考虑衰减的逆时偏移.这种方法以线性黏弹性体模型为基础,保持其复模量的实部不变,并采用多项式多级优化方法修正复模量的虚部,从而实现在保持相位不变的同时补偿振幅,进而有效提高偏移成像的清晰程度,尤其是对于介质对地震波衰减极为强烈的情形.这一方法在声波方程中只增加了整数阶微分算子进行修正,能够保证利用有限差分法进行数值求解,有利于进行高效的大规模细粒度并行计算和GPU加速计算.数值实验结果验证了这一方法的可行性和有效性,能够很好地提高强衰减介质区域的偏移成像质量.  相似文献   
8.
为了研究近代海相沉积软土的变形特征,本文对珠江三角洲河口地区的全新世海相沉积的软土进行了室内单轴压缩与不同围压下作用下的三轴固结不排水(CU)、不固结不排水(UU)的剪切试验。试验结果表明:原状土和重塑土样的孔隙比随压力的增大而下降,且与加压方式有密切关系,加压幅度越小,土的扰动破坏越小;原状土样固结系数随着压力增加而递减,整体近似符合指数函数分布,重塑土样固结系数随着压力增加逐渐增大,整体近似符合线性函数分布。在相同的固结压力下,原状土样的强度低于重塑土样的强度。土样的应力应变曲线为应变硬化型,且围压对重塑土的影响小于原状土。  相似文献   
9.
传统方法主要通过导热系数低的材料和增加墙体厚度来达到抗震效果。该方法不仅浪费能源而且抗震性能较差,因此对建筑用玻化微珠保温混凝土剪力墙抗震结构进行设计,构建玻化微珠保温混凝土本构模型,获得与抗震结构相关的理论数据。根据所获取的数据,选择板壳单元SHELL63作为玻化微珠保温混凝土剪力墙抗震结构单元。依据抗震结构材料参数以及抗震结构单元,采用ANSYS软件模拟构建玻化微珠保温混凝土剪力墙抗震结构。分析实验结果可知,所设计建筑节能抗震结构的抗震性能较好,实际运用价值高。  相似文献   
10.
Using the 7 100 absolute first arrivals of P waves and 91 513 relative P arrival times of 726 events at the northeastern margin of the Ordos block since 2009, the 3D fine structure of P wave velocity within the depth of 15km in the crust was inverted by the double difference seismic tomography method. The results show that there exist obvious high-speed continuous bodies in the northwest of the study area, and their lateral areas increase gradually with depth, while the velocity of east and south is relatively low. The velocity inhomogeneity exists and differs at different depths. The lateral differences of velocity are related to seismicity and faults. The 5~15km depth profile shows that earthquakes tend to occur in the area with relatively high velocity or high speed transition zones, which to some extent reflects the fragility of regional crustal media and the strong differential movement of faults in vertical and horizontal directions where the crust body is easy to absorb and store strain energy and generate major earthquakes. A "Y"-shape low-velocity channel is present in the lower crust around Liangcheng, corresponding to the NW-trending Heilaoyao-Shahukou fault set, which may reveal the migration path of the Late Tertiary-Quaternary basalt eruption. The Helingeer M6.2 earthquake in 1976 was related to the formation of the locking section of the thermal welding in this area. The three-dimensional fine structure of P wave velocity presented in this paper provides intuitive seismological evidence for physical and chemical properties of crustal media and the deep tectonic environment of earthquake preparation.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号