全文获取类型
收费全文 | 535篇 |
免费 | 16篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 26篇 |
大气科学 | 36篇 |
地球物理 | 141篇 |
地质学 | 145篇 |
海洋学 | 23篇 |
天文学 | 145篇 |
综合类 | 10篇 |
自然地理 | 28篇 |
出版年
2023年 | 2篇 |
2022年 | 8篇 |
2021年 | 15篇 |
2020年 | 8篇 |
2019年 | 7篇 |
2018年 | 22篇 |
2017年 | 28篇 |
2016年 | 22篇 |
2015年 | 27篇 |
2014年 | 26篇 |
2013年 | 50篇 |
2012年 | 19篇 |
2011年 | 22篇 |
2010年 | 14篇 |
2009年 | 23篇 |
2008年 | 21篇 |
2007年 | 12篇 |
2006年 | 13篇 |
2005年 | 12篇 |
2004年 | 13篇 |
2003年 | 9篇 |
2002年 | 9篇 |
2001年 | 8篇 |
2000年 | 5篇 |
1999年 | 5篇 |
1998年 | 7篇 |
1997年 | 13篇 |
1996年 | 7篇 |
1995年 | 6篇 |
1993年 | 4篇 |
1992年 | 6篇 |
1991年 | 7篇 |
1990年 | 8篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 8篇 |
1984年 | 15篇 |
1983年 | 11篇 |
1982年 | 5篇 |
1981年 | 8篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1972年 | 3篇 |
1971年 | 3篇 |
1957年 | 1篇 |
排序方式: 共有554条查询结果,搜索用时 15 毫秒
11.
Deforestation has been a major cause of climate change and other environmental problems. An accurate estimation of the volume of deforested area is needed for United Nations Reducing Emissions from Deforestation and Forest Degradation (UN-REDD+) policies implementation and global carbon accounting. Accurate information about three-dimensional (3-D) structure of forests is required to quantify forest carbon stock. This study demonstrates the use of different digital elevation models (DEMs) to monitor changes in height due to deforestation in Cambodia to support climate change mitigation policies of UN-REDD+. The Shuttle Radar Topographic Mission-DEM (SRTM-DEM), Advanced Spaceborne Thermal Emission and Reflection Radiometer Global DEM (ASTER-GDEM) and Panchromatic Remote sensing Instrument for Stereo Mapping-Digital Surface Model (PRISM-DSM) data were calibrated using Ice Cloud and land Elevation Satellite Geoscience Laser Altimeter System (ICESat–GLAS) data. The results obtained from this study clearly indicate the changes in the height of forests due to deforestation activity. The height of cutover forest generated from the PRISM-DSM and SRTM-DEM is more reliable than that from the PRISM-DSM and ASTER-GDEM data. Field data has also been used to validate the height of the cutover forests, which shows ±5 m uncertainties in the estimation. 相似文献
12.
Juan Porta Jorge Parapar Paula García Gracia Fernández Juan Touriño Ramón Doallo Francisco Ónega Inés Santé Pablo Díaz David Miranda Rafael Crecente 《Earth Science Informatics》2013,6(4):209-226
Land abandonment and stagnation of rural markets in the last few years have become one of the main concerns of rural administrations. The use of Web and GIS (Geographic Information System) technologies can help to mitigate the effects of these problems. This paper pro-poses a novel Web-GIS tool with spatial capabilities for the dynamization of rural land markets by encouraging the transfer of land from owners to farmers through the leasing of plots. The system, based on open source software, offers information about the properties, their environment and their owners. It uses standards for handling the geographic information and for communicating with external data sources. This system was used as the basis for the development of SITEGAL, the tool for the management of the Land Bank of Galicia (www.bantegal.com/sitegal). SITEGAL has been operational since 2007 obtaining benefits for both administration and users (farmers and land owners), and promoting the e-Government. 相似文献
13.
A.?Solai M.?Suresh?GandhiEmail author K.?Chandrasekaran V.?Ram?Mohan 《Arabian Journal of Geosciences》2013,6(7):2419-2446
The sediments of the estuaries and offshore Tuticorin along the southern coast of Tamil Nadu were studied for their textural variation. Ninety sediment samples were collected from three sectors (river, estuary, offshore) on the basis of prevailing energy conditions and oceanographic parameters. Frequency curves show unimodal to bimodal in nature. Offshore samples also fall in bimodal with medium and fine sand where sediment is additionally deposited by the Tamiraparani River. The mean values ranging from 1.69 to 2.01 φ with medium to fine sand. The fine sand in the riverine region indicates the depositional nature of the sediments. In pre-monsoon, the standard deviation ranges from 0.29 to 0.81 φ, which falls in the very well sorted to moderately well sorted. Estuary ranges from 0.63 to 1.30 φ, with moderately sorted, whereas in the marine sediments ranges from 0.4 to 1.01 φ, with moderately to moderately well- sorted and some patches of poorly sorted is observed. In both monsoons, the grain size variations are mostly influenced in river and offshore samples compared to the estuaries. The CM pattern indicates the deposition of sediments in graded suspension. In estuary, the sediments are medium sand with moderately to poorly sorted, fine skewed which are indicative of denudation processes taking place there. In marine, the sediments are medium-to-fine, moderately-to-well sorted, and fine skewed to very fine skewed sediments, probably as a result of the influence of palaeo-sediments deposited by rivers from inland as well as by waves and currents from offshore. Further, the marine samples also highlight depositional processes. 相似文献
14.
S. N. Charan E. V. S. S. K. Babu S. M. Naqvi J. G. Rana Prathap M. Ram Mohan D. Srinivasa Sarma 《Journal of the Geological Society of India》2009,73(3):371-378
The major part of the Peninsular Gneiss in Dharwar craton is made up of Trondjhemite-Tonalite-Granodiorite (TTG) emplaced
at different periods ranging from 3.60 to 2.50 Ga. The sodic-silicic magma precursors of these rocks have geochemical features
characteristic of partial melting of hydrated basalt. In these TTGs, enclaves of amphibolites (± garnet) are abundant. These
restites are considered to be the residue of a basaltic crust after its partial melting. A detailed study of these (residue)
enclaves reveals textures formed due to the process of partial melting. Major, trace and REE analysis of these residue enclaves
and the melt TTGs and microprobe analysis of the coexisting minerals show partitioning of REE and HFSE between the precursor
melt of TTGs and the upper amphibolite facies residues. Formation of garnetiferous amphibolites with biotite, Cpx and plagioclase
consequent to melting, has squeezed the original MORB type of basaltic crust and given rise to the TTGs, depleted in Y, Yb,
K2O, MgO, FeO, TiO2 and enriched in La, Th, U, Zr and Hf. Coevally during the process of melting, the hydrated basalt was depleted in Na2O, Al2O3, LREE, Th, U and enriched in K2O, MgO, Nb, Ti, Yb, Y, Sc, Ni, Cr and Co. Mineral chemistry of co-existing garnet-biotite and amphibole-plagioclase in these
amphibolitic (restite) enclaves indicates an average temperature of 700 ± 50° C and pressure of 5 ± 1 Kbar. These data are
inferred to indicate that during the garnet stability field metamorphism, effective fractionation of HREE and HFSE has taken
place between the restites having Fe-Mg silicates, ilmenites and the extracted melt generated from the MORB type of hydrated
basalt. These results are strongly substantiated by the reported melting experiments on hydrated basalts. 相似文献
15.
A procedure to generate new exact solutions to Einstein equations for perfect fluids is applied to LRS Bianchi type I line-element. Starting from some known solutions a class of new perfect fluid solutions of Bianchi type I are presented. The physical and kinematical properties of spatially homogeneous and anisotropic cosmological models are studied. 相似文献
16.
Employing a realistic ionospheric model and a suitable energetic electron spectra, detailed power calculations are carried out to confirm the generation mechanism of low-latitude VLF emissions observed both in the satellites and on the ground. Raypaths of the radiated waves are also calculated to account for the attenuation and spreading losses. It is shown that 100 eV?1 keV electrons radiating incoherently in the Cerenkov mode are the main sources of these emissions. 相似文献
17.
Effects of Hall current on free convection and mass transfer flow through a porous medium bounded by a vertical surface has been analysed. The problem is solved analytically. The velocity profiles are shown on graphs. Effects ofm (Hall parameter).K
* (permeability parameter), and Sc (Schmidt number) on velocity are discussed. 相似文献
18.
19.
20.
Kamaljit Singh Rajkumar Megha Maheshwari Jayaprasad Pallipad Dhani Ram Rajak Raj Kumar 《Marine Geodesy》2015,38(3):497-509
Radar altimetry provides an important geophysical parameter, backscatter coefficient (σ0), which is useful in studying target surface characteristics. Ku-band (Oceansat-2 scatterometer- OSCAT) and Ka-band (SARAL-AltiKa altimeter) data are concurrently used to characterize polar surface features over the Antarctic region. Maximum-likelihood classification has been employed to classify combined data set (AltiKa and OSCAT) for discrimination among sea ice, open water, and ice sheet (interior and exterior). The sea ice region obtained using the current approach has been compared with sea ice boundary derived from passive microwave data. 相似文献