全文获取类型
收费全文 | 1668篇 |
免费 | 227篇 |
国内免费 | 432篇 |
专业分类
测绘学 | 351篇 |
大气科学 | 285篇 |
地球物理 | 168篇 |
地质学 | 968篇 |
海洋学 | 208篇 |
天文学 | 127篇 |
综合类 | 149篇 |
自然地理 | 71篇 |
出版年
2024年 | 8篇 |
2023年 | 11篇 |
2022年 | 49篇 |
2021年 | 57篇 |
2020年 | 90篇 |
2019年 | 62篇 |
2018年 | 80篇 |
2017年 | 83篇 |
2016年 | 82篇 |
2015年 | 78篇 |
2014年 | 103篇 |
2013年 | 111篇 |
2012年 | 124篇 |
2011年 | 127篇 |
2010年 | 106篇 |
2009年 | 113篇 |
2008年 | 96篇 |
2007年 | 105篇 |
2006年 | 96篇 |
2005年 | 84篇 |
2004年 | 86篇 |
2003年 | 65篇 |
2002年 | 64篇 |
2001年 | 69篇 |
2000年 | 70篇 |
1999年 | 54篇 |
1998年 | 33篇 |
1997年 | 44篇 |
1996年 | 28篇 |
1995年 | 28篇 |
1994年 | 26篇 |
1993年 | 21篇 |
1992年 | 14篇 |
1991年 | 13篇 |
1990年 | 13篇 |
1989年 | 7篇 |
1988年 | 8篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有2327条查询结果,搜索用时 218 毫秒
1.
光学遥感图像船舶检测主要面临两个挑战:光学遥感图像背景复杂,船舶检测易受海浪、云雾及陆地建筑等多方面干扰;遥感图像分辨率低,船舶目标小,对于其分类与定位带来很大困难;针对上述问题,在FPN的基础上,提出一种融入显著性特征的卷积神经网络模型A-FPN (Attention-Based Feature Pyramid Networks)。首先,利用卷积提取图像特征金字塔;然后,利用顶层金字塔逐级构建显著特征层,抑制背景信息,通过金字塔顶层的细粒度特征提高浅层特征的表达能力,构建自上而下的多级显著特征映射结构;最后利用Softmax分类器进行多层级船舶检测。A-FPN模型利用显著性机制引导不同感受下的特征进行融合,提高了模型的分辨能力,对遥感图像处理领域具有重要应用价值。实验阶段,利用公开的遥感目标检测数据集NWPU VHR-10中的船舶样本进行测试,准确率为92.8%,表明A-FPN模型适用于遥感图像船舶检测。 相似文献
2.
Implementation of a coupled plastic damage distinct lattice spring model for dynamic crack propagation in geomaterials 下载免费PDF全文
This paper studies dynamic crack propagation by employing the distinct lattice spring model (DLSM) and 3‐dimensional (3D) printing technique. A damage‐plasticity model was developed and implemented in a 2D DLSM. Applicability of the damage‐plasticity DLSM was verified against analytical elastic solutions and experimental results for crack propagation. As a physical analogy, dynamic fracturing tests were conducted on 3D printed specimens using the split Hopkinson pressure bar. The dynamic stress intensity factors were recorded, and crack paths were captured by a high‐speed camera. A parametric study was conducted to find the influences of the parameters on cracking behaviors, including initial and peak fracture toughness, crack speed, and crack patterns. Finally, selection of parameters for the damage‐plasticity model was determined through the comparison of numerical predictions and the experimentally observed cracking features. 相似文献
3.
V. J. Mclntyre A. C. Gilmore J. B. Hearnshaw 《Journal of Astrophysics and Astronomy》1990,11(1):81-123
Photographic spectra of SN1987A in the LMC have been obtained from 1987 February 25 to 1988 June 30. Microdensitometer tracings
of these have been reduced to intensity and corrections for instrumental response have been applied to the spectra. This paper
presents these data in an atlas format, discusses the reduction procedures in detail, and presents radial velocity measurements
of selected lines in the spectra 相似文献
4.
The problem of clock synchronization: A relativistic approach 总被引:2,自引:0,他引:2
Sergei A Klioner 《Celestial Mechanics and Dynamical Astronomy》1992,53(1):81-109
The problem of synchronization of the Earth-based clocks has been discussed in the framework of General Relativity Theory. The synchronization is considered as the transformation of the observers' proper time scales to the coordinate time scale of local inertial geocentric reference system, which is single for all the observers. The formulas for the relativistic corrections occurring in some methods of Earth-based clock synchronization (transported clock, duplex communication via geostationary satellite and meteor-burst link, LASSO experiments) have been derived enabling one to attain the accuracy of 0.1 ns. 相似文献
5.
Roberto F. Viotti Rosario González-Riestra Takashi Iijima Stefano Bernabei Riccardo Claudi Jochen Greiner Michael Friedjung Vito Francesco Polcaro Corinne Rossi 《Astrophysics and Space Science》2005,296(1-4):435-439
We review the main characteristics of the symbiotic system AG Draconis, with special emphasis on its optical and X-ray variations.
We also discuss the X-ray to visual energy distribution during quiescence and outburst and describe our spectroscopic and
X-ray observations during the 2003 outburst.
Based on X-ray observations collected with the XMM–Newton Observatory, on INES data from the IUE satellite, and on optical
spectra collected with the Asiago–Cima Ekar, Bologna–Loiano and La Palma–Galileo Italian telescopes. 相似文献
6.
7.
笔者曾对广西芒场锡多金属矿田的稳定同位素进行研究。本文根据硫、铅、氢、氧、碳等稳定同位素组成和锶的初始值提供的信息.探讨了矿床成因。并结合矿田矿床地质特征、控矿条件及有关统计参数,参考前人对矿床认识的基础上,修正提出了该矿田混合热液成矿模式,可供类似矿床研究和找矿的参考。 相似文献
8.
Radio observations of some active regions (ARs) obtained with the Nobeyama radioheliograph at λ=1.76cm are used for estimating the magnetic field strength in the upper chromosphere, based on thermal bremsstrahlung. The results are compared with the magnetic field strength in the photosphere from observations with the Solar Magnetic Field Telescope (SMFT) at Huairou Solar Observing Station of Beijing Astronomical Observatory. The difference in the magnetic field strength between the two layers seems reasonable. The solar radio maps of active regions obtained with the Nobeyama radioheliograph, both in total intensity (I-map) and in circular polarizations (V-map), are compared with the optical magnetograms obtained with the SMFT. The comparison between the radio map in circular polarization and the longitudinal photospheric magnetogram of a plage region suggest that the radio map in circular polarization is a kind of magnetogram of the upper chromosphere. The comparison of the radio map in total intensity with the photospheric vector magnetogram of an AR shows that the radio map in total intensity gives indications of magnetic loops in the corona, thus we have a method of defining the coronal magnetic structure from the radio I-maps at λ=1.76 cm. Analysing the I-maps, we identified three components: (a) a compact bright source; (b) a narrow elongated structure connecting two main magnetic islands of opposite polarities (observed in both the optical and radio magnetograms); (c) a wide, diffuse, weak component that corresponds to a wide structure in the solar active region which shows in most cases an S or a reversed S contour, which is probably due to the differential rotation of the Sun. The last two components suggest coronal loops on different spatial scales above the neutral line of the longitudinal photospheric magnetic field. 相似文献
9.
The sand–loess transition zone in north China is sensitive to climate change, and is an ideal place to investigate past environmental changes. However, past climate change at millennial–centennial timescales in this region has not been well reconstructed because of limited numerical dating. Alternations of sandy loam soils with aeolian sand layers in the Mu Us and Otindag sand fields, which lie along the sand–loess transition zone, indicate multiple intervals of dune activity and stability. This change is probably a response to variations of the East Asian monsoon climate during the late Quaternary. The single aliquot regeneration (SAR) optically stimulated luminescence (OSL) dating protocol, which has been successfully applied to aeolian deposits worldwide, is applied to these two sand fields in this study. The OSL ages provide reliable constraints for reconstruction of past climate changes at suborbital timescale. Sections in both sand fields contain aeolian sand beds recording millennial‐scale episodes of dry climate and widespread dune activation, including episodes at about the same time as Heinrich Event 5 and the Younger Dryas in the North Atlantic region. These results demonstrate the potential of aeolian sediments in semi‐arid north China to record millennial‐scale climatic events, and also suggest that dry–wet climate variation at the desert margin in China may be linked to climatic change elsewhere in the Northern Hemisphere, through atmospheric circulation. This article was published online on 27 November 2008. An error was subsequently identified. This notice is included in the online and print versions to indicate that both have been corrected (16 December 2008). Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
10.
We present observations of a sample of Herbig AeBe stars, as well as the FU Orionis object V1057 Cygni. Our K-band (2.2μm)
observations from the Palomar Testbed Interferometer (PTI) used baselines of 110 m and 85 m, resulting in fringe spacings
of ∼4 mas and 5 mas, respectively. Fringes were obtained for the first time on V1057 Cygnias well as V594 Cas. Additional
measurements were made of MWC147, while upper limits to visibility-squared are obtained for MWC297, HD190073, and MWC614.
These measurements are sensitive to the distribution of warm, circumstellar dust in these sources. If the circumstellar infrared
emission comes from warm dust in a disk, the inclination of the disk to the line of sight implies that the observed interferometric
visibilities should depend upon hour angle. Surprisingly, the observations of Millan-Gabet, Schloerb and Traub (2001)(hereafter
MST) did not show significant variation with hour angle. However, limited sampling of angular frequencies on the sky was possible
with the IOTA interferometer, motivating us to study a subset of their objects to further constrain these systems.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献