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   1135篇
  免费   69篇
  国内免费   328篇
测绘学   41篇
大气科学   114篇
地球物理   539篇
地质学   505篇
海洋学   155篇
天文学   78篇
综合类   30篇
自然地理   70篇
  2024年   4篇
  2023年   13篇
  2022年   9篇
  2021年   16篇
  2020年   9篇
  2019年   13篇
  2018年   19篇
  2017年   12篇
  2016年   15篇
  2015年   19篇
  2014年   25篇
  2013年   24篇
  2012年   42篇
  2011年   88篇
  2010年   33篇
  2009年   31篇
  2008年   74篇
  2007年   62篇
  2006年   62篇
  2005年   78篇
  2004年   77篇
  2003年   63篇
  2002年   50篇
  2001年   38篇
  2000年   67篇
  1999年   34篇
  1998年   32篇
  1997年   38篇
  1996年   27篇
  1995年   34篇
  1994年   27篇
  1993年   48篇
  1992年   29篇
  1991年   41篇
  1990年   33篇
  1989年   27篇
  1988年   23篇
  1987年   34篇
  1986年   16篇
  1985年   23篇
  1984年   14篇
  1983年   14篇
  1982年   7篇
  1981年   7篇
  1980年   17篇
  1979年   4篇
  1937年   5篇
  1935年   5篇
  1934年   4篇
  1923年   4篇
排序方式: 共有1532条查询结果,搜索用时 31 毫秒
951.
CO(2) ocean storage by which liquefied CO(2) is injected into the deep-sea to mitigate the climate change would increase the CO(2) concentrations of the surrounding seawater. The biological impacts of such dynamic CO(2) environments are, however, unknown. We examined the acute toxicity of temporally changing seawater CO(2) concentrations on juveniles of Sillago japonica. Step-wise increases in ambient CO(2) to fCO(2) (fractional CO(2) concentration of the gas mixture bubbled into seawater) levels of 7% and 9% resulted in mortalities of 0.15 and 0.40-0.67 after 18 h, respectively. In contrast, one-step increases to these CO(2) levels killed all fish within 15 min. Further, a sudden drop of fCO(2) from 9-10% CO(2) to normocapnia (0.038%) killed all the surviving fish within a few minutes. These results demonstrate that impacts of ocean CO(2) storage need to be examined under conditions mimicking the dynamic changes in CO(2) levels expected to occur by the CO(2) injection procedure.  相似文献   
952.
对浙江省花岗岩的岩相学特征及力学性质进行了研究以确定它们之间的关系.矿物学研究显示出花岗岩的交代蚀变特征,这可以反映在岩石的力学性质上.岩石材料的物理性质决定其工程学性质,在相关定量参数确定的情况下,可以预测岩石材料的工程学性质.镜下观察、硬度、密度、孔隙度及点负荷强度均可作为所研究花岗岩的定量参数.上述每个因素都可提供岩石工程学的可靠预测.岩石的物理性质受岩石成分及蚀变状态控制,而岩石的工程学性质和力学特征是其物理性质的函数.  相似文献   
953.
A model for the petrogenetic affinity and original geotectonic setting of ortho-amphibolites from the Obudu Plateau was tested using the distribution patterns of trace and rare-earth elements from the geochemical analyses of twelve representative amphibolite samples. Discrimination plots, normalized patterns of the incompatible trace elements against average MORB, low ratios of Ba/Nb (9–23) and Ba/Ta (130–327) and other geochemical characteristics suggest that the protoliths were dominantly of tholeiitic MO...  相似文献   
954.
A gabbro-diorite plutonic complex from the Southeast Obudu Plateau, representing limited volumes of magma, was studied for its trace and rare-earth element characteristics, in an attempt to document its genetic and geodynamic history. Geochemical studies indicate that the gabbro samples are characterized by variable concentrations and low averages of such index elements as Cr (40×10-6–200×10-6; av. 80×10-6), Ni (40×10-6–170×10-6; 53.33×10-6) and Zr (110×10-6–240×10-6; 116.67×10-6); variable and high average...  相似文献   
955.
2001年12月13—14日,一个强劲的气旋风暴影响了西北太平洋地区,在俄勒冈州喀斯喀特山区产生了强地形降水。这个风暴是“改进微物理参数的观测验证试验(Improvement of Microphysical Parameterization through Observational Verification Experiment,IMPROVE)”第二阶段外场研究的气旋风暴之一。在风暴移过俄勒冈州喀斯喀特山区期间,获得了大量的实测和遥感探测数据,形成了由气象状态参数(温、压、湿、风向风速和垂直气流速度)、极化多普勒雷达探测数据和云微物理参数(云液态水、粒子浓度、粒子谱及图像)组成的完整数据库。12月13—14日的过程具有对流层底层前倾锋演变的特点,锋面向前上方伸展到了强烈发展的高空冷锋雨带中,云发展到了8~9km的高度。与以前华盛顿喀斯喀特山区分析的风暴重要差别是,这个风暴在俄勒冈州喀斯喀特山上空的锋前低层气流是与弱东风气流相反的强西风爬山气流。结果当高空冷锋雨带过境时,在高空产生了大量冰晶的同时在低层地形抬升地区又产生了丰富的液态水。机载实测、地基微波辐射计观测以及雪晶观测证实,同时存在高冰晶浓度和高空相对大的云水含量,地面冰晶凇附也严重。分析表明,锋面和地形的相互促进作用使降水得到了增强。  相似文献   
956.
三水盆地古近系莘庄组顶部至土布心组红岗段的全岩碳酸盐稳定同位素分析结果表明其形成期间经历了多次环境变迁。根据碳酸盐氧、碳同位素比值及其相互关系的变化,可识别三次海水入侵期。其时δ18O值大幅度向正值漂移。而由于受有机质降解的影响,相应时期的δ13 C均表现为低值。在不直接受海洋影响的湖相沉积阶段,δ18O与δ13 C的相关程度虽然未达到典型的封闭型湖泊水平,但仍呈现一定的正相关变化 (r =0.6 5 ),表明其湖水滞留时间较长。而频繁出现的石膏薄层沉积也指示湖盆的封闭性较好。这些均表明这一时期的三水盆地可能是一周期性封闭型湖泊。其稳定同位素组成主要受制于蒸发量/降雨量平衡的变化。而δ13 C比值往往还受有机质活动的控制,更多的是反映有机质生产力、埋藏与降解率。  相似文献   
957.
本文以网上地理数据维护为主线,分析了目前WebGIS应用的优缺点,阐述了基于B/S结构的跨平台网上地理数据维护系统的开发思路和具体应用模式。  相似文献   
958.
959.
The main aim of this research is to highlight the environment change indicators during the last 20 years in a representa-tive area of the southern part of Iraq(Basrah Province was taken as a case) to understand the main causes which led to widespread environment degradation phenomena using a 1:250000 mapping scale.Remote sensing and GIS’s software were used to classify Landsat TM in 1990 and Landsat ETM+ in 2003 imagery into five land use and land cover(LULC) classes:vegetation land,sand land,urban area,unused land,and water bodies.Supervised classification and Normalized Difference Vegetation Index(NDVI),Normalized Difference Build-up Index(NDBI),Normalized Difference Water Index(NDWI),Normalized Difference Salinity In-dex(NDSI),and Topsoil Grain Size Index(GSI) were adopted in this research and used respectively to retrieve its class boundary.The results showed a clear deterioration in vegetative cover(514.9 km2) and an increase of sand dune accumulations(438.6 km2),accounting for 10.1,and 10.6 percent,respectively,of the total study area.In addition,a decrease in the water bodies’ area was de-tected(228.9 km2).Sand area accumulations had increased in the total study area,with an annual increasing expansion rate of(33.7 km2·yr·1) during the thirteen years covered by the study.It is therefore imperative that Iraqi government undertake a series of pru-dent actions now that will enable to be in the best possible position when the current environmental crisis ultimately passes.  相似文献   
960.
Objectives of this work are to investigate effects of pH and ionic strength (IS) on virus transport in saturated soil and to develop a quantitative relationship for these effects. A series of 50-cm column experiments with clean quartz sand under saturated conditions and with pH values of 5, 6, 7, 8, and IS values of 1, 10, and 20 mM were conducted. Bacteriophage PRD1 was used as a model virus. Applying a one-site kinetic model, attachment, detachment, and inactivation rate coefficients were determined from fitting breakthrough curves using the software package Hydrus-1D. Attachment rate coefficients increased with decreasing pH and increasing IS, in agreement with DLVO theory. Sticking efficiencies were calculated from the attachment rate coefficients and used to develop an empirical formula for sticking efficiency as a function of pH and IS. This relationship is applicable under unfavorable conditions for virus attachment. We compared sticking efficiencies predicted by the empirical formula with those from field and column experiments. Within the calibrated range of pH and IS, the predicted and observed sticking efficiencies are in reasonable agreement for bacteriophages PRD1 and MS2. However, the formula significantly overestimates sticking efficiencies for IS higher than 100 mM. In addition, it performs less well for viruses with different surface reactivity than PRD1 and MS2. Effects of pH and IS on detachment and inactivation rate coefficients were also investigated but the experimental results do not allow constraining these parameters with sufficient certainty.  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