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   3405篇
  免费   565篇
  国内免费   715篇
测绘学   142篇
大气科学   720篇
地球物理   739篇
地质学   1814篇
海洋学   395篇
天文学   166篇
综合类   367篇
自然地理   342篇
  2024年   13篇
  2023年   52篇
  2022年   166篇
  2021年   172篇
  2020年   117篇
  2019年   132篇
  2018年   170篇
  2017年   154篇
  2016年   185篇
  2015年   156篇
  2014年   199篇
  2013年   177篇
  2012年   166篇
  2011年   180篇
  2010年   206篇
  2009年   192篇
  2008年   167篇
  2007年   156篇
  2006年   108篇
  2005年   93篇
  2004年   60篇
  2003年   89篇
  2002年   79篇
  2001年   96篇
  2000年   106篇
  1999年   162篇
  1998年   124篇
  1997年   146篇
  1996年   135篇
  1995年   121篇
  1994年   106篇
  1993年   89篇
  1992年   82篇
  1991年   64篇
  1990年   55篇
  1989年   48篇
  1988年   42篇
  1987年   15篇
  1986年   25篇
  1985年   17篇
  1984年   11篇
  1983年   16篇
  1982年   8篇
  1981年   10篇
  1980年   7篇
  1978年   3篇
  1977年   2篇
  1975年   2篇
  1969年   1篇
  1958年   1篇
排序方式: 共有4685条查询结果,搜索用时 15 毫秒
51.
锌对虾夷扇贝和刺参幼体的毒性效应   总被引:11,自引:1,他引:11  
以虾夷扇贝和刺参幼体为材料,采用急性毒性试验和亚急性毒性试验的方法,研究了锌对虾夷扇贝稚贝及幼贝和刺参幼虫的毒性效应。结果显示,锌对虾夷扇贝稚贝的24hLC50 6.40mg/L,48hLC50为3.31mg/L,72hLC50为1.39mg/L,96hLC50为0.61mg/L;而对幼贝的24hLC50为3.32mg/L,48hLC50为2.64mg/L。72hLC50为1.76mg/L,96h  相似文献   
52.
黄海、东海底栖生物的生态特点   总被引:12,自引:0,他引:12  
黄海和东海,连同渤海,是面积广阔的陆缘海,绝大部分区域的深度不超过160m,超过200m的深水区范围很小,东海平均深度370m,黄海平均深度44m ,渤海只有18m。黄海、东海的海底地形、海流系统和水团结构都相当复杂。强大的高温高盐黑潮暖流水和以长江为主体的低盐冲淡水占主导地位。北部的黄海冷水则是在特殊的地理和气候条件下形成的夏季低温水团,而台湾暖流和黄海暖流对局部海域水文状况也有一定的影响。它们的存在及其相互作用决定了黄海和东海海域的底栖动物区系性质。此外,海底沉积物结构类型及其理化性质对底栖动物的组成和分布也有相当大的影响,不同种的分布式样明显地反映了粗、细、软、硬底质的分布轮廓;而有些底内动物的活动又对沉积物的物理纹层和化学特点产生一定的影响。 黄海、东海底栖动物种类很多,有的种数量很大,不少种(虾类)是具有经济价值的渔业捕捞对象,或是经济鱼虾的天然饵料,有些又可作为水团、海流的指示种,与生物资源的开发利用以及海洋生物科学的发展有密切关系。本文根据多年调查资料,系统反映中国东部近海大型底栖动物的概貌和生态特点,内容包括种类和群落组成、分布及其与水文和沉积环境的关系。  相似文献   
53.
Through field geological survey,the authors found that abundant thrust faults developed in the Longmen (龙门) Mountain thrust belt.These faults can be divided into thrust faults and strike-slip faults according to their formation mechanisms and characteristics.Furthermore,these faults can be graded into primary fault,secondary fault,third-level fault,and fourth-level fault according to their scale and role in the tectonic evolution of Longmen Mountain thrust belt.Each thrust fault is composed of several secondary faults,such as Qingchuan (青川)-Maowen (茂汶) fault zone is composed of Qiaozhuang (乔庄) fault,Qingxi (青溪) fault,Maowen fault,Ganyanggou (赶羊沟) fault,etc..The Longmen Mountain thrust belt experienced early Indosinian movement,Anxian (安县) movement,Yanshan (燕山)movement,and Himalayan movement,and the faults formed gradually from north to south.  相似文献   
54.
The significance of crop yield estimation is well known in agricultural management and policy development at regional and national levels. The primary objective of this study was to test the suitability of the method, depending on predicted crop production, to estimate crop yield with a MODIS-NDVI-based model on a regional scale. In this paper, MODIS-NDVI data, with a 250 m resolution, was used to estimate the winter wheat (Triticum aestivum L.) yield in one of the main winter-wheat-growing regions. Our study region is located in Jining, Shandong Province. In order to improve the quality of remote sensing data and the accuracy of yield prediction, especially to eliminate the cloud-contaminated data and abnormal data in the MODIS-NDVI series, the Savitzky–Golay filter was applied to smooth the 10-day NDVI data. The spatial accumulation of NDVI at the county level was used to test its relationship with winter wheat production in the study area. A linear regressive relationship between the spatial accumulation of NDVI and the production of winter wheat was established using a stepwise regression method. The average yield was derived from predicted production divided by the growing acreage of winter wheat on a county level. Finally, the results were validated by the ground survey data, and the errors were compared with the errors of agro-climate models. The results showed that the relative errors of the predicted yield using MODIS-NDVI are between −4.62% and 5.40% and that whole RMSE was 214.16 kg ha−1 lower than the RMSE (233.35 kg ha−1) of agro-climate models in this study region. A good predicted yield data of winter wheat could be got about 40 days ahead of harvest time, i.e. at the booting-heading stage of winter wheat. The method suggested in this paper was good for predicting regional winter wheat production and yield estimation.  相似文献   
55.
三峡库区消落区土壤磷释放的环境影响因子   总被引:2,自引:0,他引:2  
以三峡库区消落区万州段为试验基地,选取释磷能力较强的紫色冲积土,根据三峡水库消落带的"干湿交替"空间和时间特征,进行万州断面土壤磷释放的影响因子的实验室模拟试验和万州江面淹没的对照试验。研究发现,随着淹水时间的变化,各种形态的磷在前5~10d各形态磷变化有相当的差异,随后变化趋势趋于稳定。TP有降低的趋势,土壤磷有一定的释放。野外研究表明Olsen-P在淹水10周左右到达最大值,以后缓慢降低。淹水时土壤Olsen-P增加,干燥后降低。多次淹水时每次淹水后土壤的有效磷水平都略有增加,落干后相较上次落干后有效磷水平降低,最后一次淹水后相较与初始时Olsen-P水平低16.7%。随着淹水深度增加,土壤的Olsen-P水平在淹水时由淹水前20.53mg/kg提高到43.23mg/kg,增加110.6%。当上覆水磷浓度较低时(<2mg/L),磷吸附到达平衡的时间较短,约需要6周。当上覆水磷浓度较高(>2mg/L)时,磷吸附到达平衡的时间较长。微生物活动对淹水土壤的磷释放有一定影响,有微生物时磷的释放高于无微生物者0.048mg/L。种植植物的土壤在淹水后Olsen-P含量大于土壤直接淹没时的释放量,种植狗牙根(Cynodondactylon)和野地瓜藤(Ficustikoua)的土壤中Olsen-P分别较未种植物土壤释放量高出21.5%和12.7%。  相似文献   
56.
利用井田勘探资料及野外地质资料,对井田构造基本形态、断层发育特征及其分布规律进行分析,总结出该井田的构造模式、构造成因、构造组合特征及其演化规律。研究认为井田的构造格局是多期构造运动综合作用、相互叠加的结果。井田构造形态为一略有起伏的平缓单斜,区内NE向的高角度正断层,多属张扭性,EW向构造早于NE向构造,构造复合部位较复杂,其余区段较简单;伸展构造是井田构造的主体样式,这一结论对下一步继续勘探及煤矿开采具有指导意义。  相似文献   
57.
倒Y形竖井联系测量   总被引:1,自引:0,他引:1  
介绍在小直径竖井联系测量中取消联系三角形,改用单根钢丝定点,全站仪引测高程的施测方法。  相似文献   
58.
The Junggar Basin in NW China contains lacustrine hydrocarbon source rocks which are among the highest quality of hydrocarbon potential in the world. Oil reservoirs in the basin are very substantial: target reservoirs span Carboniferous to Tertiary strata and include Permo-Triassic lacustrine and fluvial sandstones. The Junggar Basin was a foreland basin during the late Permian to Cenozoic, possibly with strike-slip tectonics at the southern margin during Mesozoic time. The Cangfanggou Group, as one of the major reservoirs, is well-exposed in the eastern part of the southern Junggar Basin. A measured outcrop section and a number of borehole logs coupled with resistivity logs were used to attempt sequence stratigraphic analysis. Detailed sedimentological studies on the outcrops and borehole cores have demonstrated that the Cangfanggou Group is characterized by alternating lacustrine and fluvial deposits. Four depositional sequences have been recognized. For each sequence, the basal boundary is marked by erosional truncation of fluvial channel conglomeratic sandstones in sharp contact with underlying lacustrine or floodplain mudstones. The top of each lowstand systems tract is normally overlain by the transition to lacustrine or maximum flooding surface. The transgressive systems tract is normally not identifiable at the basin margin, but was developed in the basinward area and characterized by interbedded fining-upward distal fluvial and shallow lacustrine deposits. The highstand systems tract at the basin margin is characterized by very thick floodplain mudstones or shallow lacustrine deposits, and by typical coarsening-upward parasequences of shallow lacustrine deposits in more basinward areas. Sediment input to the basin was controlled by tectonics and climate. Depositional sequences were probably controlled by fluctuating change of lake level: this was in turn controlled by climate (runoff), modified by tectonics in specific areas.The sandstones studied are exclusively volcanic litharenites. Diagenetic studies suggest that the calcite cementation, pore-filling clay minerals and zeolites occluded substantial porosity in the sandstones examined because they are compositionally immature. However, notable secondary porosity in varying proportions is present in the sandstones of the Cangfanggou Group, resulting from the dissolution of unstable detrital grains. The lowstand fluvial/distal fluvial sandstones recorded the highest average porosity and highest permeability, in which some primary porosity may remain because early formed clay coatings inhibited further compaction. The combination of residual primary porosity and significant amount of secondary porosity in the sandstones of the Cangfanggou Group may constitute moderate to good reservoirs. In contrast, the lacustrine fine-grained sandstones is characterized by clay authigenesis and zeolitization, in which the porosity was obliterated by the zeolites and extensive illitization; the lowstand fluvial channel sandstones in the basin margin areas are characterized by extensive calcite cementation which greatly reduced the porosity and permeability.This is the fifth paper in a series of papers published in this issue on Climatic and Tectonic Rhythms in Lake Deposits.  相似文献   
59.
镁铁- 超镁铁岩是揭示地幔物质组成和壳幔相互作用的重要窗口,也是Ni- Cu- PGE- Cr等金属矿产资源的重要载体。不同的镁铁- 超镁铁岩体赋矿特征明显不同:蛇绿岩以产出铬铁矿床为特征,阿拉斯加型岩体主要赋含铂族元素(PGE)矿床,大型层状岩体则可同时产出铬铁矿床、PGE矿床和Cu- Ni硫化物矿床。这种成矿差异显然与赋矿岩体形成的构造背景、母岩浆经历的岩浆演化过程有关,但缺少关键控制因素的研究。前人对上述不同种类矿床的研究工作主要集中于地幔源区的部分熔融、上升过程中或岩浆房内的围岩混染和结晶分异等岩浆过程,而极少关注流体作用。近年来,实验岩石学和岩石地球化学的研究均表明幔源岩浆演化过程中的流体活动可能对成矿元素的富集迁移起到至关重要的作用,同时这些成矿元素的赋存状态和分配系数也在不断更新。厘清Cr和PGE在熔体演化——尤其是流体出溶过程中的地球化学行为,刻画并揭示其迁移富集、分离和再富集的成矿过程及控制因素,已成为当前岩浆矿床研究的热点。本文围绕富水流体与铬铁矿和PGE成矿关系的科学问题,总结了不同镁铁- 超镁铁岩体的成矿差异以及铬铁矿和PGE矿床成矿过程中的流体活动记录,提出流体性质和组分对铬铁矿和PGE迁移富集的控制作用,强调有必要开展蛇绿岩、大型层状镁铁- 超镁铁岩体和阿拉斯加型岩体的对比研究。  相似文献   
60.
By use of the dispersion equation given by Song, Wu, and Dryer (1987) for a cylinder plasma with mass motion and gravity included, we investigate the linear current instabilities developed in loop prominences. The results indicate that the mode of linear instability depends mainly on whetherv s 2 > or not, wherev s is the sonic velocity at heightz, =GM/(R +z) is the gravity potential,G the gravitational constant,M andR the mass and the radius of the Sun respectively. Ifv s 2 > , then the sausage instability will be dominant. Otherwise, the kink instability will be more important. A possible explanation of knot structure, which appears sometimes in solar loop prominences has been given.  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