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   3089篇
  免费   516篇
  国内免费   1142篇
测绘学   74篇
大气科学   154篇
地球物理   684篇
地质学   2725篇
海洋学   518篇
天文学   64篇
综合类   122篇
自然地理   406篇
  2024年   20篇
  2023年   96篇
  2022年   115篇
  2021年   116篇
  2020年   156篇
  2019年   197篇
  2018年   163篇
  2017年   139篇
  2016年   147篇
  2015年   157篇
  2014年   216篇
  2013年   246篇
  2012年   188篇
  2011年   226篇
  2010年   184篇
  2009年   219篇
  2008年   202篇
  2007年   212篇
  2006年   210篇
  2005年   179篇
  2004年   173篇
  2003年   140篇
  2002年   121篇
  2001年   144篇
  2000年   99篇
  1999年   96篇
  1998年   97篇
  1997年   88篇
  1996年   73篇
  1995年   71篇
  1994年   61篇
  1993年   39篇
  1992年   33篇
  1991年   31篇
  1990年   27篇
  1989年   11篇
  1988年   5篇
  1987年   10篇
  1986年   13篇
  1985年   7篇
  1984年   2篇
  1983年   3篇
  1982年   2篇
  1981年   1篇
  1980年   2篇
  1979年   2篇
  1978年   2篇
  1977年   2篇
  1976年   1篇
  1954年   3篇
排序方式: 共有4747条查询结果,搜索用时 15 毫秒
81.
台湾海峡表层沉积物中重矿物特征及其物质来源   总被引:15,自引:0,他引:15  
陈华胃 《台湾海峡》1993,12(2):136-144
台湾海峡重矿物平均分布0.95%,其分布由西岸向东岸递减;由北部经台湾浅滩朝粤东方向缓慢递减。碎屑矿物46种,以角闪石、绿帘石、磁铁矿、钛铁矿、褐铁矿和片状矿物为主,它们占重矿物含量的67%。据主要重矿物含量将海峡分成8个矿物组合区。碎屑矿物来自海峡两岸基岩,通过河流搬运入海,有部分片状矿物为海峡以北的物质随浙闽沿岸流进入海峡而沉积。其海绿石是台湾西岸、澎湖岛第三系或第四系含海绿石岩层剥蚀后搬运入海沉积而成的。台湾浅滩的残留沉积物为晚更新世低海面时期的河口滨海相沉积物。由于处在开放的海洋环境,至今没有接受现代沉积。残留沉积物中的磁铁矿、钛铁矿、锆石等重矿物相对富集,含量高于近源海域,有可能赋存浅海砂矿。  相似文献   
82.
针对富水破碎地层注浆治理过程中传统水泥类材料难以实现注浆加固和堵水同步进行的难题,以硫铝酸盐水泥熟料和钢渣微粉为主要原材,成功制备了一种水泥基复合注浆材料(CGM)。通过与传统水泥材料进行性能对比试验,采用扫描电镜和X射线测试手段,分析了CGM材料制备工艺、组分含量和浆液制备条件对材料性能的作用规律,并检验了CGM材料的工程适用性。结果表明:CGM材料宜采用混合粉磨制备工艺,可显著提高其水化活性,且粉磨时间应不超过45 min。钢渣微粉含量越高,水灰比越大,结石体强度越低,凝结时间越长,水灰比超过1.2:1时,结石体后期会出现干缩。与传统水泥材料相比,CGM材料浆液凝结时间与黏度的环境敏感度低,具有显著的工程适用性和性能优越性。  相似文献   
83.
对直接溴化法合成六溴苯作了系统的研究,并对文献方法进行了一系列的改进。包括选择适当的摧化剂及其用量,使反应基本上在室温下进行;利用水汽蒸馏法驱除残余溴,可直接得到无色或近乎无色的纯净产物,而勿需进行还原脱溴,且得率较高。提出一种以溴化物形式回收副产物的方法。  相似文献   
84.
A calculation formula on spherical pattern of Qinghai-Tibet plateau moving model is established. Tibet massif moves norward by east in speed of 28 mm/a, Ganshu-Qinghai massif moves to northeast in speed of 15 mm/a, Qomolangma Feng moves northward by a few east in speed of 35 ~42 mm/a. The low latitude perimeter is longer than the nigh latitude perimeter. When the Tibet massif moves northward, its latitude perimeter must be contracted and the Tibet massif must move eastward by Coriolis. Coriolis force is inertial in earth rotation. It makes the fall body turning to east and the rising block turning westward. In the Northern Hemisphere, it makes the northward body turning to east and the southward block turning to west.This is the reason why the tectonic zones of western Pacific are different from those of eastern Pacific.  相似文献   
85.
为探究成岩作用过程中所形成的次生黏土矿物对储层物性的影响,借助偏光显微镜及扫描电镜的镜下分析,并结合储层物性等数据,对渤中凹陷附近沙垒田凸起东南部CFD18-2油田东三段高岭石的特征、类型、成因及其对储层物性的影响进行了研究。研究结果表明:1)研究区高岭石为有机酸溶蚀长石形成的自生高岭石,呈现典型的"手风琴状"及"蠕虫状",常充填于粒间孔、长石次生溶孔及碳酸盐胶结物溶孔中并形成良好的晶间孔隙。2)研究区主要发育两期高岭石,第一期主要充填于早期碳酸盐胶结物溶孔中,呈微小"蠕虫状";第二期充填于铁方解石与铁白云石胶结物溶孔中,部分高岭石具碱性溶蚀特征,且第二期高岭石与晚期伊利石共生。3)研究区东三段储层具备一定的渗流能力,使长石溶蚀形成的"副产物"被流体带走;但在破坏性成岩作用下,储层渗流能力会逐渐变差,最终使得"副产物"堆积于孔隙,降低储层物性。4)高岭石及同期硅质胶结物对储层物性的影响是一把"双刃剑",当高岭石及硅质的体积分数低于6.81%时,长石溶蚀有利于改善储层物性;当高岭石及硅质的体积分数高于6.81%时,则不利于储层物性的改善。  相似文献   
86.
Andean grasslands ecosystems are fragile environments with rigorous climatologic conditions and low and variable food for the grazing. The Apolobamba area is located in the Bolivian Andean Mountains. Its high grasslands provide a natural habitat for wild and domestic camelids such as vicuna(Vicugna vicugna) and alpaca(Lama pacos). The botanical diversity plays an essential role in maintaining vital ecosystem functions. The objectives of this research were to determine the seasonal changes in soil properties, to study the vegetation changes during the wet and dry seasons and the influence of soil properties and camelid densities on the vegetation in the Apolobamba grasslands. Four zones with different vicuna populations were selected to be studied. The following soil parameters were determined: total organic carbon, total nitrogen, available phosphorous, cation exchange capacity, exchangeable cations, pH and texture. The vegetation season changes were studied through botanical identification, above-ground biomass, plant cover and species richness. Results showed that some soil properties such as C/N ratio, CEC, silt and clay percentages kept stable against the seasonal changes. Generally, soil nutrients were relatively higher during the dry season in the surface and subsurface. The results did not point out the predominant vegetation growth during the wet season. The seasonal vegetation growth depended on each species. Thegood soil fertility corresponded to the highest plant cover. Soil fertility presented no influence on the above-ground biomass of the collected species. The negative influence of camelid grazing on soil properties could not be assessed. However, overgrazing could affect some plant species. Therefore, protection is needed in order to preserve the biodiversity in the Andean mountain grasslands.  相似文献   
87.
This work deals with the impacts of dams on large gravel -bed rivers in terms of altering coarse transport regimes and the relationship with river morphodynamics. Using data collected by a tracer -based monitoring programme carried out in a 4 -km -long study sector of the Parma River (Italy), located downstream from a relatively recently established dam, we applied a virtual velocity approach to estimate the coarse bed material load at four river cross -sections. Monitoring and calculation results provided new insights into the impacts of the dam on streambed material mobility and the sediment regime over the 17 -month calculation period. A longitudinal gradient of effects was observed along the study sector. Sections located closer to the dam are characterized by more evident impacts due to deficits in coarse sediment input from upstream. Sediment mobility here is strongly altered, especially in the highly armoured main channel, and the overall bed material load is extremely low. A partial recovery of sediment dynamics was observed at the sections located further from the dam, where estimates indicate higher sediment yield. The observed longitudinal trend in the coarse sediment transport regime matches the morphology, as the river shifts downstream from a sinuous configuration with alternate bars to a wandering one. The novel insights into alteration of coarse sediment dynamics and the relationship with river morphodynamics are potentially applicable to many other fluvial contexts affected by similar impoundments. © 2019 John Wiley & Sons, Ltd.  相似文献   
88.
从描述波流相互作用的Holton-Dunkerton简称H-D)模型出发,应用延拓方法求解常微分方程的分岔问题,研究冬季平流层波动模型的分岔特性.给出了大气行星波2与流相互作用的底部边界强迫波、底部边界平均纬向风场、风切变等参数的分岔特性,同时给出了波1与流相互作用的底部边界强迫波的分岔特性的结果.  相似文献   
89.
This research aims to understand how insurance, rainfall, land cover and urban flooding are related and how these variables influenced the material damage in the Lisbon Metropolitan Area (LMA) during the 2000–2011 period. Correlation coefficients show strong relationships between built-up areas and claims (0.94) and payouts (0.88). Despite no significant relationships being found between rainfall and the amount of material damage per event, three likelihood levels of flooding were determined for hourly rainfall. Unlike the studied period, the number of claims and their spatial distribution during the 2008 extreme rainfall event were strongly dependent on rainfall. Flooding related to the old watercourses assumed greater importance during this extreme event, recovering a more natural/ancient hydrological behaviour. In the LMA, the greatest material damage was the result of high-magnitude/low-probability rainfall events. Lower magnitude events can trigger numerous claims in heavily built-up areas, but they are hardly capable of producing large material damage.  相似文献   
90.
长沙、株洲、湘潭三市土壤中重金属元素的来源   总被引:13,自引:0,他引:13  
为追踪长沙、株洲、湘潭3个城市表层土壤中Cd、As、Pb等重金属元素的来源,分析了土壤、基岩、大气干湿沉降、水、悬浮物等介质之间重金属元素的质量平衡和土壤自身重金属元素含量随时间变化的特点。结果显示,除As外,区内基岩中的重金属元素含量低于克拉克值;残积物中Cd相对基岩贫化,As、Pb、Cu、Hg等重金属元素相对基岩的富集小于3倍;大气干湿沉降重金属元素相对土壤富集了数倍至数十倍;湘江水体主要向沿江潮土提供As、Cd物源:近50年内土壤重金属元素有较高的增加速率。不同介质间重金属元素的质量变化特点支持长沙、株洲、湘潭三市土壤重金属元素富集的主要物源为大气沉降,地表水及悬浮物是沿江湖土重金属元素的主要物源,基岩对土壤提供的物源有限的结论。  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