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   4600篇
  免费   901篇
  国内免费   1282篇
测绘学   307篇
大气科学   806篇
地球物理   1171篇
地质学   2224篇
海洋学   947篇
天文学   243篇
综合类   604篇
自然地理   481篇
  2024年   48篇
  2023年   109篇
  2022年   223篇
  2021年   259篇
  2020年   238篇
  2019年   260篇
  2018年   316篇
  2017年   277篇
  2016年   295篇
  2015年   220篇
  2014年   277篇
  2013年   267篇
  2012年   355篇
  2011年   312篇
  2010年   299篇
  2009年   279篇
  2008年   226篇
  2007年   254篇
  2006年   206篇
  2005年   191篇
  2004年   131篇
  2003年   127篇
  2002年   96篇
  2001年   111篇
  2000年   158篇
  1999年   214篇
  1998年   146篇
  1997年   169篇
  1996年   119篇
  1995年   109篇
  1994年   102篇
  1993年   98篇
  1992年   68篇
  1991年   53篇
  1990年   45篇
  1989年   30篇
  1988年   20篇
  1987年   19篇
  1986年   8篇
  1985年   6篇
  1984年   6篇
  1983年   12篇
  1982年   6篇
  1981年   7篇
  1980年   2篇
  1979年   3篇
  1977年   1篇
  1958年   2篇
  1957年   1篇
  1954年   2篇
排序方式: 共有6783条查询结果,搜索用时 15 毫秒
121.
Terrigenous clastic reservoir rocks are widespread in China,and nearly all the industrial oil and gas accumulations in eastern China occur in the clastic rocks.The study shows that organic inclusions are mostly distributed in the secondary fissures and pores which were ormed in the process of oil-rock interaction,rather than in the cements or secondary enlargements. The organic inclusions are dominantly organic gas-rich or are composed of pure hydrocarbons.Homogenization temperatures range mainly from 120℃ to 130℃,which shows a relatively high maturity of organic matter.Vertical and horizontal temperature changes provide the grounds for the investigation of basin evolution and thermal fluid-kinetics-model.Fluorescence spectral characteristics o the organic inclusions indicate that oils and gases in the area studied probably have experienced two-stage or two-time migration.Micro-fluorescence rescearch is one of the effective approaches to oil/source correlation and oil migration-stage determination.The abundance and occurrence of organic inclusions is one of the indicators of oil and gas abundance and accumulation in rock layers.With the help of other informatio,organic inclusions can provide the basis for the prospective assessment of oil and gas in clastic reservoir rocks.  相似文献   
122.
褐潮是一种由微微型藻引发的藻华,已连续4年在渤海海域暴发,给当地带来巨大的经济损失。目前,在褐潮尚不能有效预防、预控情况下,如何控制和减少其对生态环境和沿海经济带来的危害,成为一个急需解决的问题。利用改性黏土消除藻华是一种快速有效、并得以多次现场成功应用的方法。考察比较了多种不同产地、不同类型的黏土及其改性后对褐潮生物种Aureococcus anophagef ferens(抑食金球藻)的去除效果,选取5种典型的黏土作为主要的实验材料,开展了不同条件下黏土及其改性后对A. anophagef ferens去除效率的影响研究。实验发现,因A. anophagef ferens个体微小、藻华时密度大,相同用量的黏土或改性黏土对其去除效率低于常见的硅藻、甲藻生物;不同类型黏土对其去除效率有影响,高岭土的去除效率高于蒙脱土;而黏土改性后对A. anophagef ferens的去除效率提高3倍以上。在此基础上,还考察分析了黏土粒径、分散介质等因素对A. anophagef ferens去除效率的影响,为进一步研发安全、高效的褐潮应急消除材料做了有益探索。  相似文献   
123.
近年来,东海频现对大黄鱼(Larimichthys crocea)大网捕获(吨级规模以上),引起了对衰退中的野生大黄鱼资源保护的担忧。为查明东海野生大黄鱼洄游规律,本文根据2020−2022年在浙江近海产卵场和外海越冬场采集的不同批次野生大黄鱼样本,利用激光剥蚀电感耦合等离子体质谱法(LA-ICP-MS)测定耳石微化学元素,结合聚类分析和主成分分析(PCA),对不同大黄鱼群体进行了划分,结合耳石的年轮特征,研究大黄鱼群体的栖息生境,推测大黄鱼洄游路线。结果表明,Ba/Ca $\leqslant$0.004 2时可以判定为海洋栖息生境,Ba/Ca $\geqslant$ 0 .008 1时可以判定为河口栖息生境,0.004 2 < Ba/Ca< 0.008 1时可以判定为混合栖息生境。根据该阈值范围,判定本次采样的大黄鱼的5种生境类型:(1)近海产卵,近海混合水域短暂栖息类型(占22.2%);(2)近海产卵,阶段性往复混合生境栖息类型(占15.6%);(3)近海产卵,河口水域育幼索饵,外海越冬栖息类型(31.1%);(4)河口产卵、混合水域育幼,外海越冬栖息类型(6.7%);(5)近海混合水域短暂产卵,绝大部分时间海洋生境栖息类型(24.4%)。对所有样本的Ba/Ca核心、峰值和边缘值进行PCA,5种栖息类型的样本均匀分布在第一轴的两侧,大部分时间都在海洋生境中的G1、G2和G5,以及受陆源影响较大的G3和G4。同时,PCA结果同时也显示,5种类型样本均会近同步地在近海岛礁处出现,这表明外海、近海和河口的群体由于洄游等因素的影响会出现混合,说明了舟山岛礁水域对大黄鱼产卵育幼的重要性。本研究为东海大黄鱼近海产卵场、索饵场和外海越冬场之间洄游履历,以及大黄鱼洄游路线推测提供一定依据。  相似文献   
124.
秦松  王希华  童顺  曾呈奎 《海洋与湖沼》1995,26(S1):126-129
于1993年1月 - 1993年4月,为了制备遗传性纯一的基因工程受体,用酶法从海带雌配子体单性生殖系分离雌性单倍体原生质体。从皱纹盘鲍消化腺及消化器官提取出鲍酶,在25°C下、1%(WV)的鲍酶与2%(WV)的纤维素酶(Onozuka R-10)合用能有效解离单细胞和原生质体。海带雌性原生质体经2d培养再生了细胞壁。  相似文献   
125.
We reported recently some rapid changes of sunspot structure in white-light(WL) associated with major flares.We extend the study to smaller events and present here results of a statistical study of this phenomenon.In total,we investigate 403 events from 1998 May 9 to 2004 July 17,including 40 X-class,174 M-class,and 189 C-class flares.By monitoring the structure of the flaring active regions using the WL observations from the Transition Region and Coronal Explorer(TRACE),we find that segments in the outer sunspot structure decayed rapidly right after many flares;and that,on the other hand,the central part of sunspots near the flare-associated magnetic neutral line became darkened.These rapid and permanent changes are evidenced in the time profiles of WL mean intensity and are not likely resulted from the flare emissions.Our study further shows that the outer sunspot structure decay as well as the central structure darkening are more likely to be detected in larger solar flares.For X-class flares,over 40% events show distinct sunspot structure change.For M-and C-class flares,this percentage drops to 17% and 10%,respectively.The results of this statistical study support our previously proposed reconnection picture,i.e.,the flare-related magnetic fields evolve from a highly inclined to a more vertical configuration.  相似文献   
126.
Using NSO/Kitt Peak synoptic charts from 1975 to 2003, we group the main solar magnetic fields into two categories: one for active regions (ARs) and the other for extended bipolar regions (EBRs). Comparing them, we find that there exist three typical characteristics in the variability of EBRs: First, there exists a correlation between ARs and EBRs. The phase of EBR flux has a delay nearly two CRs. Second, we find that the EBR flux has two prominent periods at 1.79 years and 3.21 years. The 1.79-year period seems to only belong to large-scale magnetic features. Lastly, the North – South asymmetry of EBR flux is not very significant on a time scale of one solar cycle. However, during solar maxima, its dominance is found to shift from one hemisphere to the other.  相似文献   
127.
介绍海洋要素时-空变化大数据量主成分分析方法,讨论并澄清主成分分析在海洋大数据序列应用中的一些问题和易混淆的概念。根据美国国家海洋与大气局(NOAA)卫星观测的海表面温度红外资料,对加拿大温哥华岛西部大陆架区8个夏季海表面温度的空间变化进行主成分分析,以促进主成分方法在海洋大数据序列分析中的应用。  相似文献   
128.
The curvature of the γ-ray spectrum in blazars may reflect the intrinsic distribution of emitting electrons, which will further give some information on the possible acceleration and cooling processes in the emitting region. The γ-ray spectra of Fermi blazars are normally fitted either by a single power-law(PL) or a log-normal(call Logarithmic Parabola, LP) form. The possible reason for this difference is not clear. We statistically explore this issue based on the different observational properties of 1419 Fermi blazars in the 3 LAC Clean Sample. We find that the γ-ray flux(100 Me V–100 Ge V) and variability index follow bimodal distributions for PL and LP blazars, where the γ-ray flux and variability index show a positive correlation. However, the distributions of γ-ray luminosity and redshift follow a unimodal distribution. Our results suggest that the bimodal distribution of γ-ray fluxes for LP and PL blazars may not be intrinsic and all blazars may have an intrinsically curved γ-ray spectrum, and the PL spectrum is just caused by the fitting effect due to less photons.  相似文献   
129.
2015年以来,秦皇岛近岸海域暴发了绿潮,对北戴河旅游区的环境和生态系统造成了严重影响。绿藻微观繁殖体在绿潮的形成过程中起到重要作用,主要包括孢子、配子、幼苗和营养片段。绿藻微观繁殖体作为绿潮的“种源”,其分布规律可以反映绿潮的“藻源”位置。本研究于2016年4-9月和2017年1月对秦皇岛近岸海域绿藻微观繁殖体的调查,探究了其分布规律以及生物量变化。结果显示,绿藻微观繁殖体主要分布在近岸海域,由近岸向远岸海域逐渐降低。绿藻微观繁殖体的数量在7、8月份最高,在冬季最低。受绿潮影响严重的海域微观繁殖体数量高于其它海域。秦皇岛近岸海域的绿藻微观繁殖体为该海域绿潮的种源,其分布规律表明秦皇岛近岸海域绿潮起源于本地。  相似文献   
130.
Abstract

The use of soft clay and dredged marine clays as the construction material is challenging. This is because the high water content, high compressibility and low permeability of the clay causing the instability of ground and structure. This detrimental effect of soft clay can be improved by the cement solidification process, which is relatively cheap and efficient. This paper mainly focuses on the study of improvement on the mechanical behavior of cement mixed marine clay. The soil is reconstituted by using ordinary Portland cement of 5%, 10%, 15% and 20% by its mass. The study reveals that cementation of clay significantly improves the peak and residual strength of soil. Similarly, the primary yield stress of the soil is also improved from 16 to 275?kPa as cement content increases from 5% to 20%, respectively. By using statistical tools, the relationships between various parameters are established, which are very important to define the mechanical behavior of the clay. This study reveals that the yield surface of the solidified marine clay is not a smooth elliptical surface. Rather it is composed of two linear surfaces followed by a log-linear surface which can be modeled by using simple parameters obtained from triaxial tests.  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