全文获取类型
收费全文 | 1630篇 |
免费 | 53篇 |
国内免费 | 52篇 |
专业分类
测绘学 | 155篇 |
大气科学 | 162篇 |
地球物理 | 302篇 |
地质学 | 695篇 |
海洋学 | 143篇 |
天文学 | 217篇 |
综合类 | 17篇 |
自然地理 | 44篇 |
出版年
2022年 | 17篇 |
2021年 | 21篇 |
2020年 | 17篇 |
2019年 | 15篇 |
2018年 | 39篇 |
2017年 | 53篇 |
2016年 | 73篇 |
2015年 | 33篇 |
2014年 | 59篇 |
2013年 | 118篇 |
2012年 | 55篇 |
2011年 | 63篇 |
2010年 | 46篇 |
2009年 | 63篇 |
2008年 | 48篇 |
2007年 | 46篇 |
2006年 | 60篇 |
2005年 | 43篇 |
2004年 | 38篇 |
2003年 | 30篇 |
2002年 | 40篇 |
2001年 | 42篇 |
2000年 | 37篇 |
1999年 | 44篇 |
1998年 | 30篇 |
1997年 | 29篇 |
1996年 | 29篇 |
1995年 | 17篇 |
1994年 | 23篇 |
1993年 | 20篇 |
1992年 | 19篇 |
1991年 | 18篇 |
1990年 | 27篇 |
1989年 | 18篇 |
1988年 | 22篇 |
1987年 | 27篇 |
1986年 | 24篇 |
1985年 | 27篇 |
1984年 | 29篇 |
1983年 | 26篇 |
1982年 | 24篇 |
1981年 | 22篇 |
1980年 | 22篇 |
1979年 | 22篇 |
1978年 | 16篇 |
1975年 | 16篇 |
1974年 | 14篇 |
1973年 | 23篇 |
1972年 | 16篇 |
1971年 | 22篇 |
排序方式: 共有1735条查询结果,搜索用时 15 毫秒
141.
D. P. sharma A. K. Jain S. C. Chakravarty K. Kasturirangan K. R. Ramanathan U. R. Rao 《Astrophysics and Space Science》1972,17(2):409-425
The electron production rates in the night-time D-region arising from the transit of strong celestial X-ray sources Sco X-1,
Tau X-1 and Galactic Center are estimated and compared with the ambient electron production rates resulting from other known
stable agencies. Using the experimentally measured values of the night-time electron densities, the number of additional electrons/cc
expected from the passage of these sources is computed. For the 164 kHz transmission from Tashkent, received at Ahmedabad,
the associated enhancement in the attenuation is calculated using the full wave admittance technique of Barron and Budden.
Reasonable agreement is shown to exist between the calculated values of the attenuation and those of direct observations. 相似文献
142.
A. K. Jain U. B. Jayanthi K. Kasturirangan U. R. Rao 《Astrophysics and Space Science》1976,45(2):433-438
The paper presents experimental evidence for the existence of fast intensity fluctuations with time scales of the order of a minute in the X-ray emission from Cyg X-1 at energies greater than 29 keV. Spectral variations over time intervals of 20–25 min are also observed in the same energy range. Whereas, similar intensity and spectral fluctuations have been reported earlier at lower energies the observations presented here is the first evidence for the existence of similar fluctuations at high energies. 相似文献
143.
M. N. Rao 《Astrophysics and Space Science》1970,6(2):315-320
Analysis of abundance patterns of rare gases Ne22, Ar36, Kr84 and Xe130 on Earth and in ordinary and carbonaceous chondrites is presented. A mechanism of chemical adsorption of rare gases at the planetesimal stage during their accretion is proposed to generate the abundance pattern of the heavy rare gases on the Earth. The calculated values for Xe and Kr agree well with the observed values whereas for Ar, the agreement is poor. 相似文献
144.
Summary. We present palaeomagnetic results from the Durgapipal and Rudraprayag formations, which are basic volcanic formations in the Lesser Himalayas of Uttar Pradesh State. NRM measurements and AF demagnetization stability tests were made on specimens cored from oriented block samples collected at representative sites. Mean stable remanent magnetic directions were used for calculating the Virtual Geomagnetic Pole (VGP) positions; where necessary tectonic corrections were applied.
The virtual geomagnetic north poles were found to be located at:
A new, continuous Phanerozoic apparent polar wandering curve for the Indian subcontinent has been plotted from the available palaeomagnetic data and the VGP positions reported in this paper. As a result, the gap in the Indian palaeomagnetic data from the Lower Carboniferous to the Cambrian has been partially filled. The locations of the pole positions for the two formations on the Phanerozoic polar wandering curve for the Indian subcontinent, have been found to coincide with the stratigraphic ages assigned to them on the basis of rather limited geological and palaeontological evidence.
The Cambrian and Permian poles for the Salt Range in the NW Himalayas and the Permian pole for the Kumaon Himalayas are grouped along with the pole positions of contemporaneous formations of the Peninsular Shield. The palaeomagnetic data thus suggests that the two formations are autochthonous in nature. 相似文献
The virtual geomagnetic north poles were found to be located at:
- (a).
Durgapipal (Permian): λ
p = 10° S, Lp = 42° W; - (b).
Rudraprayag (Silurian-Devonian): λ
p = 30° S, Lp = 12° W.
A new, continuous Phanerozoic apparent polar wandering curve for the Indian subcontinent has been plotted from the available palaeomagnetic data and the VGP positions reported in this paper. As a result, the gap in the Indian palaeomagnetic data from the Lower Carboniferous to the Cambrian has been partially filled. The locations of the pole positions for the two formations on the Phanerozoic polar wandering curve for the Indian subcontinent, have been found to coincide with the stratigraphic ages assigned to them on the basis of rather limited geological and palaeontological evidence.
The Cambrian and Permian poles for the Salt Range in the NW Himalayas and the Permian pole for the Kumaon Himalayas are grouped along with the pole positions of contemporaneous formations of the Peninsular Shield. The palaeomagnetic data thus suggests that the two formations are autochthonous in nature. 相似文献
145.
A. S. Prakasarao D. P. Sharma U. B. Jayanthi U. R. Rao 《Astrophysics and Space Science》1971,10(1):150-155
Rocket measurements, of the diffuse X-ray background in the energy range 2–18 keV, conducted from Thumba Equatorial Rocket Launching Station (TERLS), India, are presented. The estimates of the cosmic background are derived by the method which employs the Earth and its atmosphere as a shutter to intercept the celestial X-rays. The results are shown to be consistent with a power law photon spectrum.13.6
–3.3
+4.3
E
–1.73±0.15 photons/cm2-sec-keV-ster the spectrum being much flatter than that observed at higher energies. 相似文献
146.
中西部地区高温炎热 台风引发致洪暴雨 总被引:1,自引:0,他引:1
7月份,黄淮南部、江淮、江南南部、华南、西北中东部降水偏多,皖、苏、湘、赣、闽、桂、粤等省(区)出现暴雨洪涝灾害;东北中南部、华北东南部、西南地区大部降水偏少,四川、重庆等地出现严重伏旱。全国大部地区月平均气温接近常年或偏高,江南大部、华南北部、四川东部、重庆、新疆东南部、内蒙古西部等地出现多日高温酷热天气。西北太平洋及南海有3个热带气旋活动,其中0604号强热带风暴碧利斯和0605号台风格美先后在福建省登陆西行,使江南、华南暴雨成灾。 相似文献
147.
N. Rama Rao P. K. Garg S. K. Ghosh 《Journal of the Indian Society of Remote Sensing》2006,34(1):69-78
Motivated by the increasingly availability and importance of hyperspectral remote sensing data, this study aims to determine
whether current generation narrowband hyperspectral remote sensing data could be used to estimate vegetation Leaf Area Index
(LAI) accurately than the traditional broadband multispectral data. A comparative study has been carried out to evaluate the
performance of the narrowband Normalized Difference Vegetation Index (NDV1) derived from Hyperion hyperspectral sensor with
that of derived from IRS LISS-III for the estimation of LAI of some major agricultural crops (e.g. cotton, sugarcane and rice)
in part of Guntur district, India. It has been found that the narrowband NDVI derived from Hyperion has shown better results
over its counterpart derived from broadband LISS-III. Linear regression models have been used which with selected subsets
of individual Hyperion bands performed better to predict LAI than those based on the broadband datasets, although the potential
to overfit models using the large number of available Hyperion bands is a concern for further research. 相似文献
148.
P. L. S. Rao 《Theoretical and Applied Climatology》2006,84(4):191-205
Summary The kinetic energy budget of the Asian summer monsoon is examined making use of the daily averaged (0000 and 1200 UTC) reanalysis
data for forty-year (1960–99) period produced by the National Centres for Environmental Prediction-National Centre for Atmospheric
Research (NCEP-NCAR). Especially, the features associated with evolution and established phases of the monsoon are elucidated.
Some features are typical during both phases. The lower tropospheric balance is governed by adiabatic generation and frictional
dissipation. On the other hand, the upper tropospheric balance is regulated by adiabatic generation and flux divergence. The
adiabatic generation of kinetic energy within the boundary layer is mostly due to the meridional component.
Contrasting characteristics during both phases are summarized as follows. Subtropical westerly jet controls the balance of
kinetic energy during the evolution period. On the other hand, Tropical easterly jet and Somali jet modulate the balance during
the established period. The adiabatic generation of kinetic energy is predominantly driven by the zonal component during the
evolution phase and by the meridional component during the established phase of monsoon, respectively. This aspect essentially
delineates the roles of zonal and meridional regimes during the evolution and established phases of the monsoon. 相似文献
149.
150.
A. V. Suresh Babu V. Venkateshwar Rao I. V. Muralikrishna 《Journal of the Indian Society of Remote Sensing》2007,35(1):81-91
Water Utilisation Index (WUI) defined as area irrigated per unit volume is a measure of water delivery performance and constitutes
one of the important spatial performance indicators of an irrigation system. WUI also forms basis for evaluating the adequacy
of seasonal irrigation supplies in an irrigation system (inverse of WUI is delta, i.e. depth of water supplied to a given
irrigation unit). In the present study WUI and adequacy indicators were used in benchmarking the performance of Nagarjunasagar
Left Canal Command (NSLC) in Andhra Pradesh. Optimised temporal satellite data of rabi season during the years 1990–91 and
1998–99 was used in deriving irrigated crop areas adopting hierarchical classification approach. Paddy is the predominant
crop grown and cotton, chillies, sugarcane etc. are the other crops grown in the study area. Equivalent wet area (paddy crop
area) was estimated using the operationally used project specific conversion factors. WUI was estimated at disaggregated level
viz., distributary, irrigation block, irrigation zone level using the canal discharge data. At project level, WUI estimated
to be 65 ha/MCM and 92 ha/MCM during rabi season of 1990–91 and 1998–99 years respectively. A comparison of total irrigated
area and discharges corresponding to both the years indicate that irrigation service is extensive and sub optimal during 1998–99
and it is intensive and optimal in 1990–91. It was also observed that WUI is lesser in blocks of with higher Culturable Command
Area (CCA) compared to the blocks of lower CCA. All the disaggregated units were ranked into various groups of different levels
of water distribution performance. The study demonstrates the utility of WUI as spatial performance indicator and thus useful
for benchmarking studies of irrigation command areas. The WUI together with satellite data derived spatial irrigation intensity,
crop productivity constitutes important benchmarking indices in irrigation command areas. 相似文献