全文获取类型
收费全文 | 2578篇 |
免费 | 45篇 |
国内免费 | 27篇 |
专业分类
测绘学 | 245篇 |
大气科学 | 163篇 |
地球物理 | 523篇 |
地质学 | 911篇 |
海洋学 | 59篇 |
天文学 | 627篇 |
综合类 | 21篇 |
自然地理 | 101篇 |
出版年
2022年 | 23篇 |
2021年 | 29篇 |
2020年 | 34篇 |
2019年 | 29篇 |
2018年 | 138篇 |
2017年 | 111篇 |
2016年 | 151篇 |
2015年 | 76篇 |
2014年 | 130篇 |
2013年 | 150篇 |
2012年 | 105篇 |
2011年 | 115篇 |
2010年 | 98篇 |
2009年 | 110篇 |
2008年 | 88篇 |
2007年 | 62篇 |
2006年 | 59篇 |
2005年 | 56篇 |
2004年 | 32篇 |
2003年 | 41篇 |
2002年 | 54篇 |
2001年 | 36篇 |
2000年 | 39篇 |
1999年 | 44篇 |
1998年 | 34篇 |
1997年 | 37篇 |
1996年 | 28篇 |
1995年 | 27篇 |
1994年 | 34篇 |
1993年 | 29篇 |
1992年 | 31篇 |
1991年 | 40篇 |
1990年 | 28篇 |
1989年 | 41篇 |
1988年 | 48篇 |
1987年 | 52篇 |
1986年 | 36篇 |
1985年 | 35篇 |
1984年 | 61篇 |
1983年 | 58篇 |
1982年 | 34篇 |
1981年 | 21篇 |
1980年 | 27篇 |
1979年 | 16篇 |
1978年 | 19篇 |
1976年 | 12篇 |
1974年 | 11篇 |
1973年 | 12篇 |
1972年 | 14篇 |
1971年 | 10篇 |
排序方式: 共有2650条查询结果,搜索用时 15 毫秒
141.
Natural Resources Research - Investigations on the Paleogene lignites were carried out through geochemical, mineralogical and petrographic analyses to assess their paleodepositional environment,... 相似文献
142.
Monopod caisson foundation is a viable alternative for supporting offshore wind turbines located at shallow water depths. This foundation system has to resist overturning moment generated due to resultant lateral load, arising from wind and water wave action, that can act at any loading height above the seabed. This paper presents results of a numerical investigation performed to determine the influence of loading height, caisson geometry and superstructure load on the ultimate lateral capacity, initial stiffness, and soil failure zone of the foundation, when installed in very dense sand. Both the ultimate and serviceable states of the caisson foundation obtained from the analyses are represented in terms of envelopes plotted between lateral load and overturning moment. Simplified expressions, which take into account the influence of caisson geometry, loading height, and soil properties, are also presented to serve as a preliminary base for design of the monopod caisson foundation. 相似文献
143.
We applied fast Fourier transform techniques and Morlet wavelet transform on the time series data of coronal index, solar flare index, and galactic cosmic ray, for the period 1986–2008, in order to investigate the long- and mid-term periodicities including the Rieger (\({\sim }130\) to \({\sim }190\) days), quasi-period (\({\sim }200\) to \({\sim }374\) days), and quasi-biennial periodicities (\({\sim }1.20\) to \({\sim }3.27\) years) during the combined solar cycles 22–23. We emphasize the fact that a lesser number of periodicities are found in the range of low frequencies, while the higher frequencies show a greater number of periodicities. The rotation rates at the base of convection zone have periods for coronal index of \({\sim }1.43\) years and for solar flare index of \({\sim }1.41\) year, and galactic cosmic ray, \({\sim }1.35\) year, during combined solar cycles 22–23. In relation to these two solar parameters (coronal index and solar flare index), for the solar cycles 22–23, we found that galactic cosmic ray modulation at mid cut-off rigidity (\(\hbox {Rc} = 2.43\hbox {GV}\)) is anti-correlated with time-lag of few months. 相似文献
144.
In this paper, we have studied the magnetized quark matter (QM) and strange quark matter (SQM) distributions in the presence of \(f(R,T)\) gravity in the background of Friedmann-Lemaître-Robertson-Walker (FLRW) metric. To get exact solutions of modified field equations we have used \(f(R,T ) = R + 2 f(T)\) model given by Harko et al. with two different parametrization of geometrical parameters i.e. the parametrization of the deceleration parameter \(q \), and the scale factor \(a \) in hybrid expansion form. Also, we have obtained Einstein Static Universe (ESU) solutions for QM and SQM distributions in \(f(R,T)\) gravity and General Relativity (GR). All models in \(f(R,T)\) gravity and GR for FRW and ESU Universes with QM also SQM distributions, we get zero magnetic field. These results agree with the solutions of Akta? and Aygün in \(f(R,T)\) gravity. However, we have also discussed the physical consequences of our obtained models. 相似文献
145.
M. C. Ramadevi B. T. Ravishankar N. Sitaramamurthy G. Meena Brajpal Singh Anand Jain Reena Yadav Anil Agarwal V. Chandra Babu Kumar Ankur Kushwaha S. Vaishali Nirmal Kumar Iyer Anuj Nandi Girish V. Vivek Kumar Agarwal S. Seetha Dipankar Bhattacharya K. Balaji Manoj Kumar Prashanth Kulshresta 《Journal of Astrophysics and Astronomy》2017,38(2):32
We report the in-orbit performance of Scanning Sky Monitor (SSM) onboard AstroSat. The SSM operates in the energy range 2.5 to 10 keV and scans the sky to detect and locate transient X-ray sources. This information of any interesting phenomenon in the X-ray sky as observed by SSM is provided to the astronomical community for follow-up observations. Following the launch of AstroSat on 28th September, 2015, SSM was commissioned on October 12th, 2015. The first power ON of the instrument was with the standard X-ray source, Crab in the field-of-view. The first orbit data revealed the basic expected performance of one of the detectors of SSM, SSM1. Following this in the subsequent orbits, the other detectors were also powered ON to find them perform in good health. Quick checks of the data from the first few orbits revealed that the instrument performed with the expected angular resolution of 12’ \(\times \) 2.5\(^\circ \) and effective area in the energy range of interest. This paper discusses the instrument aspects along with few on-board results immediately after power ON. 相似文献
146.
Manish P. Kale Shirish A. Ravan P. S. Roy Sarnam Singh 《Journal of the Indian Society of Remote Sensing》2009,37(3):457-471
Satellite remote sensing is a proven tool for mapping landuse patterns and estimating vegetation biomass/carbon. Present study
aims at estimating the potential of forests of Radhanagari WLS (Western Ghats, India) to sequester the atmospheric carbon-di-oxide,
using ground based observations coupled with satellite remote sensing. The study area was stratified for dominant forest types
based on the structure and composition of vegetation and elevation variations. Permanent sample plots were laid down in these
homogeneous vegetation strata (HVS) to make different observations during time 1 and time 2. Carbon sequestration by plantations
was also studied and compared with natural forests. Species and area-specific biomass equations were used for estimating carbon
pool and sequestration. Among natural forests ‘mixed moist deciduous’ forests exhibited highest sequestration rate (8%), whereas,
plantation as obvious had a comparatively higher sequestration rate than natural forests (20.27%). Total carbon sequestration
by forests of the Radhanagari WLS between 2004 and 2006 is 78742.09 tons. Eligible land for reforestation activity under clean
development mechanism (CDM) of Kyoto Protocol was identified using satellite remote sensing using 1989 and 2005 datasets and
it was observed that the potential land that can be used for reforestation activity is 10080 ha. 相似文献
147.
Assessment of groundwater potential zones using GIS technique 总被引:1,自引:0,他引:1
A case study was conducted to find out the groundwater potential zones in Kattakulathur block, Tamil Nadu, India with an aerial
extent of 360.60 km2. The thematic maps such as geology, geomorphology, soil hydrological group, land use / land cover and drainage map were prepared
for the study area. The Digital Elevation Model (DEM) has been generated from the 10 m interval contour lines (which is derived
from SOI, Toposheet 1:25000 scale) and obtained the slope (%) of the study area. The groundwater potential zones were obtained
by overlaying all the thematic maps in terms of weighted overlay methods using the spatial analysis tool in ArcGIS 9.2. During
weighted overlay analysis, the ranking has been given for each individual parameter of each thematic map and weights were
assigned according to the influence such as soil −25%, geomorphology − 25%, land use/land cover −25%, slope − 15%, lineament
− 5% and drainage / streams − 5% and find out the potential zones in terms of good, moderate and poor zones with the area
of 49.70 km2, 261.61 km2 and 46.04 km2 respectively. The potential zone wise study area was overlaid with village boundary map and the village wise groundwater
potential zones with three categories such as good, moderate and poor zones were obtained. This GIS based output result was
validated by conducting field survey by randomly selecting wells in different villages using GPS instruments. The coordinates
of each well location were obtained by GPS and plotted in the GIS platform and it was clearly shown that the well coordinates
were exactly seated with the classified zones. 相似文献
148.
S. Patro C. Chatterjee S. Mohanty R. Singh N. S. Raghuwanshi 《Journal of the Indian Society of Remote Sensing》2009,37(1):107-118
A coupled 1D-2D hydrodynamic model, MIKE FLOOD was used to simulate the flood inundation extent and flooding depth in the
delta region of Mahanadi River basin in India. Initially, the 1D model MIKE 11 was calibrated using river water level and
discharge data of various gauging sites for the monsoon period (June to October) of the year 2002. Subsequently, the calibrated
set up was validated using both discharge and water level data for the same period of the year 2001. The performance of calibration
and validation results of MIKE 11 were evaluated using different performance indices. A bathymetry of the study area with
a spatial resolution of 90m was prepared from SRTM DEM and provided as an input to the 2D model, MIKE 21. MIKE 11 and MIKE
21 models were then coupled using lateral links to form the MIKE FLOOD model set up for simulating the two dimensional flood
inundations in the study area. Flood inundation is simulated for the year 2001 and the maximum flood inundation extent simulated
by the model was compared with the corresponding actual inundated area obtained from IRS-1D WiFS image. 相似文献
149.
Chamundeeswari V.V. Singh D. Singh K. 《Geoscience and Remote Sensing Letters, IEEE》2009,6(2):214-218
In single-band single-polarized SAR images, intensity and texture are the information source available for unsupervised land cover classification. Every textural feature measure identifies texture patterns by different approaches. For efficient land cover classification, textural measures have to be chosen suitably. Therefore, in this letter, the role of various intensity and textural measures is analyzed for their discriminative ability for unsupervised SAR image classification into various land cover types like water, urban, and vegetation areas. To make the algorithm adaptable, these textural features are fused using principal component analysis (PCA), and principal components are used for classification purposes. To highlight the effectiveness of PCA, the difference between PCA- and non-PCA-based classifications is also analyzed. Analysis of the role of texture measures for unsupervised classification of real-world SAR data with application of PCA is presented in this letter. The analysis of how every individual feature measure contributes for classification process is presented, and then, textural measures for a feature set are chosen according to their role in improving classification accuracy. By analysis, it is observed that the feature set comprising mean, variance, wavelet components, semivariogram, lacunarity, and weighted rank fill ratio provides good classification accuracy of up to 90.4% than by using individual textural measures, and this increased accuracy justifies the complexity involved in the process. 相似文献
150.
In river bank filtration, impurities present in the river water travel with the bank filtrate towards the pumping well. During this passage, certain types of impurities, such as turbidity, total coliform, and so forth, may get attenuated; however, it is interesting to note that some of the instant raw river water quality parameters, such as alkalinity and electrical conductivity, increase after the passage of water through the porous medium. This occurs because water, when passing through the soil pores, absorbs many of the solutes that cause an increase in alkalinity and electrical conductivity. Measurements at a river bank filtration site for a year showed that alkalinity of 116–32 mg l?1 in river water increased to 222.4–159.9 mg l?1 in the river bank filtered water. Likewise, the electrical conductivity increased from 280–131 μS cm?1 to 462–409.6 μS cm?1. This study uses a probabilistic approach for investigating the variation of alkalinity and electrical conductivity of source water that varies with the natural logarithm of the concentration of influent water. The probabilistic approach has the potential of being used in simulating the variation of alkalinity and electrical conductivity in river bank filtrate. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献