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   610篇
  免费   115篇
  国内免费   115篇
测绘学   40篇
大气科学   49篇
地球物理   181篇
地质学   348篇
海洋学   53篇
天文学   20篇
综合类   89篇
自然地理   60篇
  2023年   13篇
  2022年   25篇
  2021年   41篇
  2020年   24篇
  2019年   32篇
  2018年   33篇
  2017年   32篇
  2016年   40篇
  2015年   37篇
  2014年   35篇
  2013年   31篇
  2012年   35篇
  2011年   34篇
  2010年   25篇
  2009年   22篇
  2008年   29篇
  2007年   25篇
  2006年   19篇
  2005年   10篇
  2004年   18篇
  2003年   10篇
  2002年   15篇
  2001年   16篇
  2000年   20篇
  1999年   36篇
  1998年   24篇
  1997年   32篇
  1996年   21篇
  1995年   15篇
  1994年   16篇
  1993年   15篇
  1992年   4篇
  1991年   8篇
  1990年   9篇
  1989年   10篇
  1988年   8篇
  1986年   6篇
  1985年   3篇
  1984年   6篇
  1979年   1篇
  1977年   1篇
  1976年   1篇
  1958年   3篇
排序方式: 共有840条查询结果,搜索用时 171 毫秒
711.
高频旋回分析有利于揭示浅水碳酸盐岩台地海平面升降、沉积物响应、古地理单元演化及对盆地内沉积分异影响等诸多沉积学信息。依据鄂尔多斯盆地横山地区丰富的钻井取心资料,通过岩石类型、典型五级旋回结构等分析,对中奥陶统马家沟组马五6亚段开展了高频旋回研究。研究表明: (1)基于研究区内马五6亚段碳酸盐岩与蒸发岩共生组合的9种岩石类型,识别出5种典型T-R单旋回,其中旋回Ⅰ、Ⅱ、Ⅲ为云质潟湖—颗粒滩—灰泥丘沉积,Ⅳ、Ⅴ为膏盐质潟湖沉积,并以5种单旋回为基本单元搭建骨架井五级高频旋回结构剖面;(2)依据五级高频旋回结构剖面建立的Fischer图解,认为马五6亚段处于三级旋回晚期,内部发育“2个”四级旋回,具有初次海退—快速海侵—再次海退的相对海平面变化趋势;(3)结合五级旋回厚度统计,认为第1套四级旋回沉积期处于隆拗格局发育阶段,该时期横山凸起作为水下高地,对东西两侧海水封隔明显;至第2套四级旋回沉积阶段,填平补齐作用导致横山凸起影响减弱,海水循环得到改善,凸起东西两侧盐度明显下降。以马五6亚段为例的高频旋回研究,在横山地区马五6亚段地层学、沉积学研究中取得了较好的效果,以期在鄂尔多斯盆地马家沟组未来研究中得到进一步推广和实践。  相似文献   
712.
应用钻井、测井和地震等资料对川中龙岗地区下侏罗统大安寨段的沉积相类型及其展布特征进行了研究,将大安寨段的湖泊相沉积划分为4个亚相及若干微相。大安寨段沉积时期的沉积展布及演化主要受湖平面升降控制,经历过一次较大规模的水进水退旋回:大三沉积时期发生水进,主体为浅湖—半深湖沉积;大一三沉积时期水体最深,主体为半深湖沉积;大一三沉积晚期发生水退,到大一沉积晚期湖平面已降至较低水平,主体为滨浅湖沉积,介壳灰岩发育。沉积相带在平面上具有明显的环带状展布特征,据此建立了大安寨段沉积模式。大一段主要发育5个介壳滩,是最有利于储集层发育的沉积相带。储集岩类型主要为介壳灰岩和泥质介壳灰岩,结合裂缝发育分析,预测了3个有利储集层区带。  相似文献   
713.
The new century has witnessed a strategic breakthrough in unconventional oil gas.Hydrocarbon accumulated in micro-/nano-scale pore throat shale systems has become an important domain that could replace current oil gas resources.Unconventional oil gas plays an increasingly important role in our energy demand.Tight gas,CBM,heavy oil and asphaltic sand have served as a key domain of exploration development,with tight oil becoming a 'bright spot' domain and shale gas becoming a 'hotspot' domain.China has made great breakthroughs in unconventional oil gas resources,such as tight gas,shale gas,tight oil and CBM,and great progress in oil shale,gas hydrate,heavy oil and oil sand.China has an estimated(223-263)×10~8t of unconventional oil resources and(890-1260)×l0~(12)m~3 of gas resources.China has made a breakthrough for progress in unconventional oil gas study.New progress achieved in fine-grained sedimentary studies related to continental open lacustrine basin large-scale shallow-water delta sand bodies,lacustrine basin central sandy clastic flow sediments and marine-continental fine-grained sediments provide a theoretical basis for the formation and distribution of basin central reservoir bodies.Great breakthroughs have been made in unconventional reservoir geology in respect of research methodology technology,multi-scale data merging and physical simulation of formation conditions.Overall characterization of unconventional reservoirs via multi-method and multi-scale becomes increasingly popular and facilitates the rapid development of unconventional oil gas geological theory,method and technology.The formation of innovative,continuous hydrocarbon accumulation theory,the establishment of the framework of the unconventional oil gas geological theory system,and the determination of the implications,geological feature,formation mechanism,distribution rule and core technology of unconventional oil gas geological study lays a theoretical foundation for extensive unconventional oil gas exploration and development.Theories and technologies of unconventional oil gas exploration and development developed rapidly,including some key evaluation techniques such as 'sweet spot zone' integrated evaluation and a six-property evaluation technique that uses hydrocarbon source,lithology,physical property,brittleness,hydrocarbon potential and stress anisotropy,and some key development engineering technologies including micro-seismic monitoring,horizontal drilling completion and "factory-like" operation pattern, "man-made reservoir" development,which have facilitated the innovative development of unconventional oil gas.These breakthroughs define a new understanding in four aspects:①theoretical innovation;② key technologies;③ complete market mechanism and national policy support;and ④ well-developed ground infrastructure,which are significant for prolonging the life cycle of petroleum industry,accelerating the upgrade and development of theories and technologies and altering the global traditional energy structure.  相似文献   
714.
姜琪  代晶晶  田淑芳 《测绘科学》2021,46(8):141-147
针对云雾天获取的影像对比度低、地物细节模糊,严重影响影像后期处理的问题,该文基于WorldView-3(WV-3)遥感影像,通过薄云最优化变换(HOT)与点云算法相结合、暗通道两种方法去除或削减图像中云层信息,并采用相关系数、光谱扭曲度等参数定量评价两种方法的去云效果.结果显示,HOT与点云算法结合与暗通道两种方法均能对WV-3影像中云层信息进行去除,其中后者去云结果各波段的相关系数比前者去云结果平均高出6.9%,后者的光谱扭曲度比前者平均低出1.67%.因此暗通道去云方法要优于HOT与点云算法结合的去云方法.  相似文献   
715.
There is agreement that ultrafine maghemite grains (<100 nm) are responsible for the magnetic enhancement of Chinese loess. Recent studies show that grain size distribution of ultrafine pedogenic maghemite grains within the aeolian sequences on the central Chinese Loess Plateau deposited in the last 8 Ma is consistent. However, whether the observed grain size distribution of ultrafine pedogenic maghemite grains can be observed in the western Chinese Loess Plateau and modern soils is not entirely clear. Here we find that young surface soil samples across the Chinese Loess Plateau and early Neogene loessic soils from the western Chinese Loess Plateau also show consistent grain size distribution with that of ultrafine pedogenic maghemite grains on the central Chinese Loess Plateau. The fact that young surface soil samples show apparent magnetic enhancement suggests that the ultrafine pedogenic maghemite grains derive from oxidation of ultrafine magnetite grains. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   
716.
1:25万、1:100万基础数据是江苏省地理空间信息基础框架的重要组成部分,本文根据江苏省地理空间信息基础框架的要求,结合全省现有的基础地理数据库建设的现状,叙述了对全省范围内的17幅1:25万DLG、DOM,4幅1:100万DLG数据如何进行修测、更新和入库,并建立相应的元数据库的过程和方法.  相似文献   
717.
This paper deals with the proportional distribution, niche breadth and niche overlap of two metazoan parasites (copepods species,Ergasilus anchoratus Markewitsch, 1946 andPseudergasilus parasiluri Yamaguti, 1936) and monogeneans (in the genusSilurodiscoides Gussev, 1976) found on gills of the fish,Silurus asotus L. from Jiangkou Reservoir in Jiangxi Province, China.E. anchoratus was the common and dominant metazoan parasite on gills of the fish in this locality as indicated by the higher infection levels, and distributed almost evenly on gill arches without any observed gill-arch preference, as shown by the wide proportional distribution and broad niche breadth.P. parasiluri and monogeneans inSilurodiscoides were found also without any significant gill-arch preference although they had a rather narrow niche breadth. The observed significant overlap betweenP. parasiluri andSilurodiscoides spp. may be simply due to the lower infection levels of the parasites. The observed pattern of unrestricted distributions of the parasitic copepods on gills of the fish may be accounted for, at least in part, by the moving ability of the copepods, and may also indicate that the parasite does not exhibit any feature of gill-arch preference. However, further experimental research is needed to verify the microhabitat distribution of the parasites. Project 39370122 supported by NSFC.  相似文献   
718.
Variation of pH in atmospheric precipitation in cities of Jilin province   总被引:1,自引:0,他引:1  
Acidrainhasbeenaglobalenvironmentalprob-lem.SouthChinaandthesouthwestpartarethetwomainacidrainareasinChinawiththerangeof'pHvariationfrom4.5to4.O,andseientificstudiesonacidrainfocusontheseareas(WANGetal.,l997).Inre-centyears,however,inthenorthpartofChinaacidrainoccurred'notunusuallyandevenmoreseriouslysuchasincitiesofTumenandHunchuninJilinProvince.Itisessentialtodelveintothetemporalandspatialvariationsofatmosphericprecipitationacidityandthevariationscausesintheseareastoprovidesci-entificba…  相似文献   
719.
Hu  Peng  Liu  Bin  Ma  Qian  Liu  Shufang  Liu  Xinfu  Zhuang  Zhimeng 《中国海洋湖沼学报》2019,37(3):1113-1124
Estradiol treatment during early life stages of tiger puffer Takifugu rubripes induces feminization in genetic males. However, the ovaries in genetic males may revert to testes once estradiol treatment is halted.Therefore studies should investigate molecular mechanisms underlying ovary-to-testis recovery in genetic males after treatment. In the present study, tiger puffer were exposed to 10, and 100 μg/L 17 p-estradiol(E_2)from 15 to 100 days post-hatching(dph), then gonad phenotypes and expression profiles of six sex-related genes(cyp19a,foxl2, dmrtl, amh, sox9a, and sox9b) were characterized after the exposure. Results showed that both 10 and 100 μg/L E_2 induced ovarian development in genetic males at 100 dph. However, all ovaries induced by 10 μg/L E_2 first developed into intersexual gonads and subsequently reverted to testes after the exposure. As for treatment of 100 pg/L E_2, while the rest of the ovaries maintained morphological stability,percentages of intersexual gonads reached 38%-57%, and none were reverted to testes. Increased mRNA levels of cyp19 a,foxl2 and sox9b and decreased mRNA levels of dmrtl, amh, and sox9a were observed during the ovarian development in genetic males. While contrary gene expression profiles were detected during ovary-to-testis transformation. The mRNA levels of all the six genes were increased during the development of intersexual gonads. These results indicated that up-regulation of dmrtl, amh and sox9a is associated with initial ovary-to-intersexual transformation, and suppression of foxl2, cypl9a and sox9b is essential for complete ovary-to-testis recovery in genetic males. This research will help to trace the molecular processes underlying gonadal transformation in teleosts.  相似文献   
720.
基于我国首颗全极化雷达卫星高分三号(GF-3)和Landsat8数据,研究浓密植被覆盖地表土壤水分反演方法。为了提高浓密植被覆盖地表土壤水分反演精度,首先利用PROSAIL模型、实测植被参数及Landsat8光学数据分析了8种植被指数与植被冠层含水量的相关性,从中优选出归一化差异水指数(NDWI5)用于反演植被冠层含水量,并通过分析植被含水量和植被冠层含水量的关系,构建植被含水量模型;然后结合植被含水量反演模型和简化MIMICS模型校正了植被对雷达后向散射系数的影响,最后基于AIEM建立裸土后向散射系数模拟数据集,发展一种主动微波和光学数据协同反演浓密植被覆盖地表土壤水分模型,并以山东省禹城市为研究区,实现了玉米覆盖下HH、VV和HH+VV 3种模式土壤水分反演。实验结果表明: ① NDWI5为最佳植被指数,对于去除植被影响有较好效果;② 基于此方法,利用GF-3和Landsat8卫星数据反演得到的土壤水分具有较高的精度;③ 相比HH和VV两种极化模式,HH+VV双通道模式对土壤水分反演结果更好,决定系数(R2)为0.4037,均方根误差(RMSE)为0.0667 m 3m -3。  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