全文获取类型
收费全文 | 6841篇 |
免费 | 1392篇 |
国内免费 | 3243篇 |
专业分类
测绘学 | 321篇 |
大气科学 | 991篇 |
地球物理 | 1663篇 |
地质学 | 6049篇 |
海洋学 | 835篇 |
天文学 | 118篇 |
综合类 | 428篇 |
自然地理 | 1071篇 |
出版年
2024年 | 81篇 |
2023年 | 184篇 |
2022年 | 326篇 |
2021年 | 350篇 |
2020年 | 421篇 |
2019年 | 423篇 |
2018年 | 379篇 |
2017年 | 416篇 |
2016年 | 415篇 |
2015年 | 475篇 |
2014年 | 544篇 |
2013年 | 500篇 |
2012年 | 570篇 |
2011年 | 532篇 |
2010年 | 440篇 |
2009年 | 492篇 |
2008年 | 433篇 |
2007年 | 502篇 |
2006年 | 530篇 |
2005年 | 448篇 |
2004年 | 396篇 |
2003年 | 362篇 |
2002年 | 297篇 |
2001年 | 239篇 |
2000年 | 249篇 |
1999年 | 207篇 |
1998年 | 199篇 |
1997年 | 183篇 |
1996年 | 160篇 |
1995年 | 135篇 |
1994年 | 131篇 |
1993年 | 112篇 |
1992年 | 92篇 |
1991年 | 68篇 |
1990年 | 38篇 |
1989年 | 52篇 |
1988年 | 27篇 |
1987年 | 32篇 |
1986年 | 8篇 |
1985年 | 8篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1981年 | 5篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 3篇 |
1954年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
This study makes use of the concept of wave age in estimating ocean wave period from space borne altimeter measurements of backscattering coefficient and significant wave height. Introduction of wave age allowed better accounting of the difference between swells and wind waves. Using two years (1998 and 1999) data of TOPEX/Poseidon altimeter and ocean data buoy observations in the Indian Ocean, coefficients were generated for wave period, which were subsequently tested against data for the years 2000 and 2001. The results showed the wave period accuracy to be of the order of 0.6 sec (against 1.3 sec obtained with the semiempirical approach, reported earlier). 相似文献
182.
183.
Laser line scan imaging and chirp sub-bottom profiling were used to detail the morphology of a submarine mud volcano and brine-filled crater at 652 m water depth in the northern Gulf of Mexico. The mud volcano has a relief of 6 m and a basal diameter of about 80 m. The feature comprises a central, brine-filled crater (253 m2) surrounded by a continuous bed of methanotrophic mussels (Bathymodiolus childressi) covering 434 m2 and a patchy bed covering an additional 214 m2 of the periphery. The brine pool was mostly <2 m deep, but there were two holes of >28 m and 12 m deep, respectively at the northern end of the pool which emitted continual streams of small clear bubbles. Sub-bottom profiles indicated three distinct strata beneath the present surface of the mud volcano. Integration of 17 profiles shows that the mud volcano has been built in at least three successive stages: the lowest stage deposited 35,400 m3, while the middle and upper stages deposited 7700 and 20,400 m3, respectively. Piston cores were taken at the northern edge of the mussel bed and a site ∼100 m southwest of the pool. Mussel and lucinid shells were recovered from the closer core, lucinid shells from the distant core. A mussel shell from 3.4 m sub-bottom had a Δ14C age of 16.2 ka. Mixture of modern carbon with “carbon dead” reservoir material would produce actual ages ∼2 ka less than the radiocarbon ages. 相似文献
184.
Models for petroleum generation used by the industry are often limited by (a) sub-optimal laboratory pyrolysis methods for studying hydrocarbon generation, (b) over-simple models of petroleum generation, (c) inappropriate mathematical methods to derive kinetic parameters by fitting laboratory data, (d) primitive models of primary migration/expulsion and its coupling with petroleum generation, and (e) insufficient use of subsurface data to constrain the models. Problems (a), (b) and (c) lead to forced compensation effects between the activation energies and frequency factors of reaction kinetics that are wholly artificial, and which yield poor extrapolations to geological conditions. Simple switch or adsorption models of expulsion are insufficient to describe the residence time of species in source rocks. Yet, the residence time controls the thermal stresses to which the species are subjected for cracking to lighter species. 相似文献
185.
The difference-frequency (DF) ultrasound generated by using parametric effect promises to improve detection depth owing to its low attenuation, which is beneficial for deep tissue imaging. With ultrasound contrast agents infusion, the harmonic components scattered from the microbubbles, including DF, can be generated due to the nonlinear vibration. A theoretical study on the DF generation from microbubbles under the dual-frequency excitation is proposed in formula based on the solution of the RPNNP equation. The optimisation of the DF generation is discussed associated with the applied acoustic pressure, frequency, and the microbubble size. Experiments are performed to validate the theoretical predictions by using a dual-frequency signal to excite microbubbles. Both the numerical and experimental results demonstrate that the optimised DF ultrasound can be achieved as the difference frequency is close to the resonance frequency of the microbubble and improve the contrast-to-tissue ratio in imaging. 相似文献
186.
Dhananjai Pandey Martin Sinha Lucy MacGregor Satish Singh 《Marine Geophysical Researches》2008,29(3):185-193
The effects of the ocean coast on magnetotelluric (MT) data have been studied and results are described in this article. MT
soundings from various sites along coastal plains of Kachchh were acquired where volcanic rocks overlie Mesozoic sediments
having potential resource prospects. The region of our study lies in the proximity of Gulf of Kachchh and Arabian Sea. This
article describes the effects the presence of a conductive body on the acquired MT data. With the help of synthetic MT modelling
and induction arrows we demonstrate the influence of shallow conductive body on the MT data. The modelling results when compared
to the field observations show that much of the high frequencies are unaffected by the coast, however low frequencies of the
data do seem to be distorted by onshore–offshore resistivity contrasts. The least affect on high frequencies can be attributed
to very shallow water depth as well as to the fact that the onshore–offshore resistivity contrasts are not large enough to
influence the high frequency data badly. The results presented here report for the first time the effect of the coast on the
analysis on MT data from Kachchh. Due to several academic as well as industrial onshore–offshore MT experiments presently
being carried out to explore its hydrocarbon potential, our results have an important bearing on designing and acquisition
of future MT surveys in this region. 相似文献
187.
台湾海峡西岸闽江口和九龙江口沉积物中碎屑锆石铀-铅定年及物源意义 总被引:2,自引:1,他引:2
碎屑锆石能较好保存源区岩石组成的信息,其铀-铅年龄是沉积物源区示踪有效的方法。利用激光剥蚀等离子体质谱和多接收器等离子体质谱联机(LA-MC-ICPMS)技术,对台湾海峡西部闽江口和九龙江口沉积物中的碎屑锆石进行原位铀-铅测年,同时获得锆石的微量元素。研究表明大部分碎屑锆石的钍与铀含量之比值大于0.1,稀土元素球粒陨石标准化配分曲线呈左倾型,并具有明显的铈含量正异常和铕含量负异常,结合阴极发光图像表明这些锆石绝大部分为岩浆成因。闽江口和九龙江口的碎屑锆石具有相同的印支期(峰值为236 Ma)、燕山早期(峰值为155 Ma)和燕山晚期(峰值为110 Ma)等3个峰值,这三期碎屑锆石占主要部分,反映闽江口和九龙江口碎屑沉积物主要来自各流域上游印支期和中下游燕山期岩体,而闽江口碎屑锆石具有明显的加里东期峰(峰值为465 Ma)和前寒武纪(553~2 765 Ma)较多的碎屑锆石,这些碎屑物质可能来自上游武夷山地区加里东期和前寒武纪基底的物质。对比闽江口碎屑锆石铀-铅年龄与台湾岛西部海滨沙滩(苗栗-新竹、嘉义-台南)中的独居石年龄具有相似的谱峰特征,表明台湾岛西部海滨沙滩碎屑物质可能有部分来自闽江。闽江口碎屑物质明显区别于福建的九龙江和浙江的瓯江,这为判别台湾海峡沉积物的物质来源提供可靠的依据。 相似文献
188.
非线性效应对浅水水波变形的影响 总被引:3,自引:0,他引:3
本文采用波数矢量无旋和波能守恒方程建立了一个考虑非线性作用的浅水水波变形数值模型,模型中采用Battjes关系与波数矢量无旋,波能守恒方程一起来求解波浪在浅水中变形的波浪要素,在波能守恒方程中考虑了底摩擦的影响。利用本文提出的数值模型对一个斜坡浅滩水域波浪折射绕射现象进行了验证,验证计算中用一个非线性经验弥散关系近似浅水水波变形的非线性效应并与用线性弥散关系的计算结果进行了比较,结果说明使用非线性 相似文献
189.
190.
Alexander Korobkin 《Ocean Engineering》1998,25(8):687-714
The impact of an elastic plate onto the compressible fluid without free surface deformation is considered. The ability of the liquid volume to be deformed is geometrically limited which leads to severe impact conditions. The present analysis is focused on the stresses in the plate and the hydrodynamic loads under the impact. The motivation for this research comes from ship hydrodynamics, where the hulls of a catamaran restrict the liquid outflow and the water impacts onto the wetdeck. The influence of the air on the impact process is investigated. The analysis did not reveal any great advantage of utilizing the air-cushion effect or ejection of air into the water near the impact region to prevent high stresses in the elastic plate. It was found that in the problem considered, the stress peaks far from the plate centre and the one-mode approximation does not provide correct information about the stress level. 相似文献