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   117篇
  免费   15篇
  国内免费   16篇
大气科学   2篇
地球物理   33篇
地质学   41篇
海洋学   29篇
天文学   19篇
综合类   6篇
自然地理   18篇
  2021年   1篇
  2020年   1篇
  2019年   5篇
  2018年   4篇
  2017年   5篇
  2016年   6篇
  2015年   9篇
  2014年   4篇
  2013年   15篇
  2012年   9篇
  2011年   5篇
  2010年   7篇
  2009年   4篇
  2008年   10篇
  2007年   5篇
  2006年   4篇
  2005年   4篇
  2004年   5篇
  2003年   8篇
  2002年   3篇
  2001年   6篇
  2000年   3篇
  1999年   6篇
  1998年   5篇
  1996年   2篇
  1995年   5篇
  1994年   3篇
  1993年   1篇
  1988年   1篇
  1987年   1篇
  1985年   1篇
排序方式: 共有148条查询结果,搜索用时 31 毫秒
91.
对克隆的 WSSV基因组 Xba 1.8kb片段进行序列测定 ,根据所测定的核酸序列 ,通过计算机软件分析 ,设计出一对 PCR引物。所设计的 PCR引物能从纯化 WSSV及患白斑症的对虾组织中扩增出长为 92 1bp的目的 DNA片段 ,并且能检测出 0 .2~ 0 .4μg病虾肌肉组织中的 WSSV。健康对虾、感染MBV的斑节对虾仔虾及 SINPV的扩增结果均为阴性。表明所建立的 WSSV PCR检测法灵敏而且特异。  相似文献   
92.
南盘江盆地晚古生代盆地充填序列特征及生储盖组合划分   总被引:27,自引:9,他引:27  
以米级旋回层序为基本工作单元 ,米级旋回层序在长周期三级层序中的有序叠加形式为基础 ,在南盘江盆地晚古生代地层中共识别出 2 5个三级层序 (沉积层序 ) ;以构造不整合面为准 ,又可进一步归为 4个二级层序 (构造层序 )。根据地层、沉积特征及构造地质涵义 ,把层序界面归纳为 4种类型 :构造不整合面、沉积不整合面、淹没不整合面以及它们的相关面 ,其中构造不整合面类似于类型 界面 ,沉积不整合面类似于类型 界面。晚古生代南盘江盆地存在两个明显的造礁期。于三级海平面上升阶段主要发育礁滩相灰岩构成的储层 ;而在与三级海平面下降相关的强迫型海退过程中则发育白云岩构成的储层。因此 ,可把南盘江盆地区域性的潜在生储盖组合拟定为 3个 :以孤立台地上泥盆系生物礁和加里东运动不整合面为主要勘探对象的下组合 ;以石炭系大埔组白云岩为主要勘探对象的中组合 ;以茅口组上部和长兴组生物礁及礁顶相白云岩、东吴运动不整合面为主要勘探对象的上组合  相似文献   
93.
通过总结前人研究结果,利用数据整理、薄片观察等方法对古风化壳中孔隙和裂缝的发育规律进行了分析,并探讨了古风化壳结构的油气地质意义。研究结果表明,发育完全的古风化壳具有风化黏土层和半风化岩石的分层结构;半风化岩石中部孔隙和裂缝较发育,向上向下规模逐渐减小,至风化黏土层大孔隙和裂缝不发育。通过实例汇总认为,影响半风化岩石孔缝系统发育程度的主要因素包括风化基岩的岩性、风化的时间和强度、风化壳形成时期的剥蚀厚度和古风化壳的埋藏深度以及裂缝孔隙的充填特征。风化黏土层岩性致密,对油气的保存具有建设性作用;半风化岩层中部的孔缝连通体,既为油气运移的良好通道,也可作为油气聚集的有效储层。  相似文献   
94.
山东半岛白沙滩及邻近海域沉积物分布与控制因素   总被引:1,自引:0,他引:1  
对山东半岛白沙滩及邻近海域表层沉积物进行了粒度分析,结合相关的地质水文资料,探讨了海岸形态对沉积物粒度的影响。研究区底质类型以黏土质粉砂和砂质粉砂为主,由东北经古龙咀岬角向西南形成S形黏土质粉砂分布带,险岛湾地区沉积物呈带状由砂逐渐过渡到粉砂,潮汐和波浪是本区沉积物输运的主要水动力,岬角的存在影响了沉积物粒度的分布,粒度趋势分析表明沉积物整体有向岬角汇聚的输运趋势。  相似文献   
95.
《自然地理学》2013,34(1):64-78
Research efforts studying dune fields as spatial units are limited and the historical dynamics associated with dune field-scale pattern attributes are currently not well documented. Historical repeat aerial photography was used to quantify temporal changes in the crescentic dune pattern at the White Sands Dune Field, New Mexico. A Geographic Information System (GIS) was used to digitize ~ 14,000 dune crests from the combined photoset. Crest length, sinuosity, spacing, orientation, transport vector, and defect density were calculated from these measurements and statistically analyzed. The pattern analysis revealed that crest length, sinuosity, and defect density were highly variable pattern elements over a 60-year period. These dune pattern elements also exhibited temporal relationships, such as the inverse relationship between crest length and sinuosity over time. Defect density calculations revealed that the dune field is trending to a possible state of disorganization. This trend was temporarily interrupted in 1985 by a massive period of organization in which crest length was inversely related to defect density and defect density was proportionally related to sinuosity. The temporal variations in these pattern elements and the episodic organizational event in 1985 were likely climatically induced.  相似文献   
96.
We investigated the potential of ALOS/PALSAR for estimating the above-ground biomass (AGB) and other biophysical parameters (tree height, diameter at breast height (DBH), and tree stand density) in the boreal forest of Alaska. In July 2007, forest surveys were conducted along a south–north transect (150°W) to profile the ecotone from boreal forest to tundra in Alaska. In situ parameters were measured in 29 forests by a combination of the Bitterlich angle-count sampling method and the sampled-tree measuring method. These in situ values were compared with the backscatter intensity of ALOS/PALSAR. A strong positive logarithmic correlation was found between the backscatter intensity and the forest AGB, with the correlation being stronger in the HV than in the HH polarization mode. No obvious saturation was found in the sensitivity of the HV mode backscatter intensity to the forest AGB up to 120.7 Mg ha?1. Similarly, a robust sensitivity was found in the HV backscatter intensity to both tree height and DBH, but weak sensitivity was observed for tree density. The regression curve of HV backscatter intensity to the forest AGB appeared to be intensified by the uneven forest floor, particularly for forests with small AGB. The geographical distribution of the forest AGB was mapped, demonstrating a generally south-rich and north-poor forest AGB gradient.  相似文献   
97.
Short-wave infrared (SWIR) spectral reflectance of hydrothermally altered volcanic rocks in the footwall of the Hellyer massive sulfide deposit was measured with a portable PIMA-II infrared spectrometer. The Al–OH band was used to derive information on the octahedral Al content and the abundance of white mica (sericite) in the hydrothermal alteration and mineralization system. The range of the Al–OH band wavelength from 2192 nm to 2222 nm corresponds to the number of octahedral Al (Alvi) in white mica approximately from 3.9 to 3.0 (based on 4 octahedral cations per formula). This Alvi range represents a significant compositional variation, covering most of the compositional region between muscovite (Alvi = 4.0) and phengite (Alvi = 3.0). Furthermore, the spectral reflectance data show that the compositional variation of white mica is spatially related to hydrothermal alteration zoning, such that phengitic white mica tends to occur in 1) main upflow fluid channel, 2) intensely altered volcanic rocks, and 3) Pb–Zn mineralization, whereas muscovitic white mica was formed preferentially distal to massive sulfide mineralization on the margin of the footwall alteration system. The results suggest that the Al–OH band wavelength, and therefore the octahedral Al content, of white mica can be used as vectors to mineralization to map the hydrothermal system at Hellyer.  相似文献   
98.
研究盐度渐变和突变对WSSV在凡纳滨对虾(Litopenaeus vannamei)体内增殖的影响。结果表明,盐度由起始盐度(23±1)往高盐度(32±1)和低盐度(14±1)突变在整个实验中对凡纳滨对虾的累积死亡率和对虾体内病毒增殖的影响显著(P0.05),先感染WSSV实验,高盐度、起始盐度、低盐度累积死亡率分别为(61.1±10.7)%、(48.9±1.9)%、(63.3±12.0)%,病毒含量分别为6.0×107、3.7×106、5.3×107 copy/g;后感染WSSV实验,分别为(75.6±3.8)%、(52.2±13.4)%、(63.3±3.3)%;病毒含量分别为5.7×107、2.3×106、2.8×107 copy/g。盐度渐变实验各组至24h对虾死亡率低于18.9%,48h出现死亡高峰,72—96h存在显著差异(P0.05),先感染WSSV实验,高盐度、起始盐度、低盐度最大死亡率分别为(75.6±5.1)%、(38.9±5.1)%、(69.3±6.9)%,病毒含量分别为7.6×107、5.3×106、2.4×106copy/g;后感染WSSV实验分别为(46.7±10)%、(45.6±18.9)%、(74.4±13.9)%;病毒含量分别为5.1×106、4.8×105、1.0×107 copy/g。因此盐度变化会影响对虾的抗病能力,可造成对虾体内WSSV快速增殖;对携带WSSV对虾,盐度变化会大大提高WSSV从潜伏感染转为急性感染的可能,盐度是引起WSSV从潜伏感染转为急性感染的关键影响因子之一。  相似文献   
99.
Ashraf Youssef 《Icarus》2003,162(1):74-93
In early 1998 two of the three, long-lived anticyclonic, jovian white ovals merged. In 2000 the two remaining white ovals merged into one. Here we examine that behavior, as well as the dynamics of three earlier epochs: the Formation Epoch (1939-1941), during which a nearly axisymmetric band broke apart to form the vortices; the Kármán Vortex Street Epoch (1941-1994), during which the white ovals made up the southern half of two rows of vortices, and their locations oscillated in longitude such that the white ovals often closely approached each other but did not merge; and the Pre-merger Epoch (1994-1997), during which the three white ovals traveled together with intervening cyclones from the northern row of the Kármán vortex street in a closely spaced group with little longitudinal oscillation. We use a quasi-geostrophic model and large-scale numerical simulation to explain the dynamics. Our models and simulations are consistent with the observations, but none of the observed behavior is even qualitatively possible without assuming that there are long-lived, coherent cyclones longitudinally interspersed with the white ovals. Without them, the white ovals approach each other and merge on a fast, advective timescale (4 months). A necessary ingredient that allows the vortices to travel together in a small packet without spreading apart is that the strong, eastward-flowing jetstream south of the white ovals is coincident with a sharp gradient in background potential vorticity. The jet forms a Rossby wave and a trough of the wave traps the white ovals. In our simulations, the three white ovals were trapped before they merged. Without being trapped, the amount of energy needed to perturb two white ovals so that they merge exceeds the atmosphere’s turbulent energy (which corresponds to velocities of ∼1 m s−1) by a factor of ∼100. The mergers of the white ovals BC and DE were not observed directly, so there is ambiguity in labeling the surviving vortices and identifying which vortices might have exchanged locations. The simulation and modeling make the identifications clear. They also predict the fate of the surviving white oval and of the other prominent jovian vortex chains.  相似文献   
100.
胡菊 Hiei  E 《天文学报》1994,35(4):396-405
本文详细分析了1974年9月10日白光耀斑的光谱资料,并利用Non-LTE理论计算了它的半经验大气模型和辐射损失。由光谱测量得到:(1)连续辐射强度在巴耳末系限(3647)附近发生跳跃。在极大时刻,巴耳末跳跃可达约11%;(2)该耀斑有强而宽的巴耳末谱线发射,其高项巴耳末谱线轮廓的全半宽随主量子数n而变化,在8<n<9达到极小;(3)连续辐射极大时刻同微波爆发极大相一致,比Ha耀斑极大时刻提前几分钟。这三个特征很可能是Ⅰ类白光耀斑的典型特征。  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