全文获取类型
收费全文 | 820篇 |
免费 | 8篇 |
国内免费 | 9篇 |
专业分类
测绘学 | 49篇 |
大气科学 | 28篇 |
地球物理 | 213篇 |
地质学 | 289篇 |
海洋学 | 32篇 |
天文学 | 193篇 |
综合类 | 3篇 |
自然地理 | 30篇 |
出版年
2022年 | 7篇 |
2021年 | 12篇 |
2020年 | 9篇 |
2019年 | 13篇 |
2018年 | 30篇 |
2017年 | 30篇 |
2016年 | 38篇 |
2015年 | 20篇 |
2014年 | 30篇 |
2013年 | 33篇 |
2012年 | 22篇 |
2011年 | 31篇 |
2010年 | 27篇 |
2009年 | 32篇 |
2008年 | 37篇 |
2007年 | 31篇 |
2006年 | 25篇 |
2005年 | 20篇 |
2004年 | 11篇 |
2003年 | 14篇 |
2002年 | 14篇 |
2001年 | 17篇 |
2000年 | 16篇 |
1999年 | 15篇 |
1998年 | 19篇 |
1997年 | 11篇 |
1996年 | 10篇 |
1995年 | 14篇 |
1994年 | 15篇 |
1993年 | 11篇 |
1992年 | 7篇 |
1991年 | 18篇 |
1990年 | 7篇 |
1989年 | 10篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 7篇 |
1985年 | 11篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 19篇 |
1981年 | 8篇 |
1980年 | 12篇 |
1979年 | 11篇 |
1978年 | 11篇 |
1977年 | 7篇 |
1974年 | 9篇 |
1973年 | 9篇 |
1972年 | 12篇 |
1971年 | 10篇 |
排序方式: 共有837条查询结果,搜索用时 15 毫秒
741.
Gupta HS Baghel A 《Population geography : a journal of the Association of Population Geographers of India》1994,16(1-2):49-58
"The present paper explains the fertility differentials among socio-cultural groups and regions in the central Indian state of Madhya Pradesh and examines the role of such factors as general and female literacy, age at marriage and infant mortality in determining the fertility levels. The study indicates that no single factor is of overwhelming importance. These factors in combination, as revealed by multivariate analysis, account for about 29 percent inter-district variation in fertility in the state." 相似文献
742.
We constructed a procurement portfolio for the Indian power sector using two variants of the dynamic conditional correlation GARCH model to derive time-varying correlations between major coal indices. We used prices and qualities of observed cargos to adjust indices for quality gaps as well as for freight costs and power plant efficiency factors. Using the relative homogeneity of the energy content of imports from Australia, South Africa, and Indonesia, we found that the regional seaborne market is highly correlated during normal economic conditions, while suffering brief departures in correlation during demand and supply shocks. Our results show that the buying behavior of power producers is aligned with the mean-variance efficient portfolio of delivered prices using time-varying correlation estimates, but not free-on-board coal index prices. This study challenges the notion that thermal coal importers only source material with a freight price advantage and highlights the importance of coal quality gaps in power production. 相似文献
743.
S. K. Nath P. Sengupta S. K. Srivastav S. N. Bhattacharya R. S. Dattatrayam R. Prakash H. V. Gupta 《Journal of Earth System Science》2003,112(3):441-462
Site response in and around Delhi is studied using digital seismograms recorded by a thirteen-station VSAT-based 24-bit digital
Delhi telemetry network of the India Meteorological Department. Nine local (M
l ≥ 2.3) and nine regional (M
l ≥ 3.9) earthquakes are selected for the estimation of site amplification factor using the classical standard spectral ratio
for regional events (Ridge Delhi Observatory being the reference station), normalized standard spectral ratio for local events,
horizontal-to-vertical spectral ratio or receiver function and the generalized inversion techniques in the frequency range
of 0.5 to 7.5 Hz. Site response curves at all the thirteen stations exhibit station to station variation of the site amplification
factor reflecting the changes in geologic/geotectonic/soil conditions. A comparison of the site response values obtained by
the generalized inversion with those computed using receiver function technique shows a large scatter even though the pattern
of the curves remain more or less similar. However, the site effects computed by generalized inversion and standard spectral
ratio exhibit a good 1:1 correspondence. The peaks yielded by all the methods have been observed to occur at the same frequencies.
It is evident that the softer fluvial deposits of the newer alluvium of the east Yamuna sector show steeper site amplification
gradient at lower frequencies, while the greater Delhi experiences moderate site amplification. The variation of site response
corroborates the abrupt changes in intensity from one location to another due to local site condition. 相似文献
744.
Fresh water availability and demand are unevenly distributed both temporally and geographically. Furthermore, the availability
of fresh water has remained more or less constant, while the demand for clean water is steadily increasing. With demand surpassing
supply, an integrated water resource management approach is required to ensure even distribution of potable water to all levels
of society while protecting the environment. Inter-basin water transfer (IBWT) is an approach being applied in various countries
around the world, with varying environmental and social implications. The Interlinking of Rivers (ILR) scheme is an example
of such a project being planned in India. The research described in this paper was based on the ILR project and includes an
assessment of the IBWT programme in some of the tributaries of the Brahmaputra and Ganga Rivers in the eastern part of India,
covering the district of Jalpaiguri, West Bengal, India. Geoinformation has been used in association with physical and socio-economic
factors to identify potential dam and reservoir sites and to delineate the optimal route for canals to transfer water from
the Brahmaputra basin to the Ganga basin for further transportation to the water-deficient regions of India. 相似文献
745.
S. K. Gupta J. Christiansen Y. Hayashi A. Jain P. K. Mohanty K. C. Ravindran B. Satyanarayana 《Experimental Astronomy》2013,35(3):507-526
Arrival time of particles in an extensive air shower (EAS) is a key physical parameter to determine its direction. EAS direction is useful for studies of anisotropy and composition of cosmic rays, and search for multi-TeV γ-rays sources. Accurate timing may be used to search exotic phenomena such as production of new particles at extremely high energies available during early stages of development of EAS and also for detecting sub-relativistic hadrons in EAS. Time to digital converters (TDCs) are used to perform this task. Traditional TDCs operate in the START-STOP mode with limited dynamic range and single-hit capability. With the advent of high luminosity collider LHC, need for TDCs with large dynamic range, multi-hit capability and TRIGGERED mode of operation became necessary. A 32 channel TDC was designed for the GRAPES-3 experiment on a CAMAC platform around TDC32, an ASIC developed by micro-electronics group at CERN, Geneva. Four modules were operated in the GRAPES-3 experiment. Here, we present details of the circuit design and their performance over several years. The multi-hit feature of this device was used to study the time structure of particles in the EAS on time scale of ~1 μs. The distribution of time intervals in the multi-hit data shows an exponential profile with a time constant of ~370 ns. These delayed particles are likely to be neutrons produced in the EAS core that were recorded in the scintillator detectors following the relativistic EAS front. 相似文献
746.
Neal S. Gupta Hong Yang Qin Leng Derek E.G. Briggs George D. Cody Roger E. Summons 《Organic Geochemistry》2009,40(7):802-809
Analysis of modern Metasequoia leaves revealed the presence of the structural polyester cutin, guaiacyl lignin units and polysaccharides. Analysis of environmentally decayed Metasequoia leaves revealed that guaiacyl lignin units and cellulose were degraded more than vinyl phenol (the last being the primary pyrolysis product of cutin and plant cuticles) suggesting that cutin is more stable than lignin and cellulose during degradation, contrary to some previous studies. This observation is supported by electron microscopy showing changes in the cellular structure and cuticle of modern, decayed and fossil Metasequoia leaves. Metasequoia fossils from the Eocene of Republic (Washington State) showed a significant aliphatic component, but biopolymeric lignin and polysaccharides were not detected. Fossils from the Eocene of Axel Heiberg revealed the presence of lignin and an aliphatic polymer up to C29 with cellulose, and fossils from the Miocene Clarkia deposit (Idaho) revealed lignin and an aliphatic polymer up to C27 without any polysaccharides. Modern Metasequoia needles heated experimentally in confined conditions generated a macromolecular composition with an aliphatic polymer up to C32 and additional phenolic compounds similar to those present in the fossils. Experimental heating of cutin is known to generate an aliphatic polymer with carbon chain length units <C20. Thus, the n-alkyl component with chain length units >C20 in the heated Metasequoia needles is a product of incorporation of longer chain plant waxes, indicated by the odd/even predominance of the >C27 n-alkanes. The resistant nature of cutin compared to lignin and polysaccharides explains the presence of an n-alkyl component (<C20) in fossil leaves even when polysaccharides are absent and lignin has decayed; cutin and its diagenetically altered products contribute significantly to the presence of aliphatic components in terrestrially derived sedimentary organic matter. 相似文献
747.
S. R. Das Gupta Chitta Ranjan Nayak Santanu Das Gupta 《Astrophysics and Space Science》1984,103(2):271-284
The present paper contains extensive tables of the values of theH-functionH(z,<0) and of the moments ofQ(x) (in terms of which the moments ofH(z, ) can be determined) appropriate for transfer of radiation in active amplifying media in which<0. These values have been computed correct to the 7th decimal place for values of in the range (–10–12)-(–1030) and for values of z[0,1] with the aid of a 48-point gaussian quadrature formula. 相似文献
748.
Tushar Gupta Rajesh Rai Ashok Jaiswal B. K. Shrivastva 《Geotechnical and Geological Engineering》2014,32(3):705-712
The present paper discusses the impact of the geometrical parameters of the coal rib and the mine dump on the stability of the coal rib. The geometrical parameters such as the slope angle, the height of dragline dump, the height of main dump, the gradient of seam and the thickness of coal rib have been considered as input to the numerical model for the stability analysis of the coal rib. Sensitivity analysis has been performed based on the results of the analysis in term of factor of safety of the coal rib. The input parameters have been classified in terms of significance (i.e. very high significance, high significance medium significance and low significance). The factor of safety is more influenced by highly significant parameters. The height and the slope angle of dragline dump and the thickness of the coal rib are highly significant parameters for the stability of the coal rib. The gradient of the seam is a medium significant parameter whereas, height of main dump and the number of dragline cut dump are low significant parameters for stability of coal rib. 相似文献
749.
Rima Chatterjee Saurabh Datta Gupta Partha Pratim Mandal 《Journal of the Geological Society of India》2017,89(5):573-580
The Deccan trap basalt, laid down by multiple lava flows during upper Cretaceous to Paleocene times forms the basement of current study in Cambay basin. As such, there is great interest and value in fracture detection and evaluation of fractured basement reservoirs in the Cambay basin. The procedure for identification and evaluation of natural as well as induced fractures in basaltic basement of the Cambay basin is presented in this work. In this study formation micro-imager (FMI) and extended range micro-imager (XRMI) log data for fracture identification is used. The Deccan trap basaltic basement of the study area, comprising five wells in the Tarapur-Cambay block, has potential for holding commercial hydrocarbon due to the presence of fractures and weathered basement. Both image logs (FMI, XRMI) identify three types of fracture including open (conductive), partially open and closed (resistive) fractures, of which open and partially open fractures are important for hydrocarbon accumulation. Fracture dip ranges from 10° to 80°. Image logs have also identified washout, breakout and drilling-induced fracture zones. The strike direction of the open natural fractures for four wells varies from N60°E to N30°E whereas the strike direction of most natural fracture in the fifth well is oriented towards N20°W. The orientations of drilling-induced fractures and breakouts may be interpreted for the in-situ stress direction over the logged interval. Drilling-induced tensile fractures, identified over the depth interval of 1969–1972 m, and borehole breakouts over the interval of 1953–1955 m in one well, suggest an orientation of maximum in-situ horizontal compressive stress (SH) lies in the north-south direction. The azimuths of open natural fractures in the same well vary from north-south to N30°E. It is expected that the direction of fluid flow will be controlled by open natural fractures and therefore would be in a direction parallel to the SH direction, which is orthogonal to the minimum horizontal stress (Sh) direction. The orientations observed are consistent with the present day SH direction in the study area of Cambay basin. 相似文献
750.
Surajit Misra Sukanta Roy Vikrant Bartakke Gaurav Athavale Harsh Gupta 《Journal of the Geological Society of India》2017,89(2):131-132
Mn-rich ilmenites (up to 7 % MnO) have been identified in dykes cutting the Malanjkhand porphyry copper mining area in Madhya Pradesh. The dykes are hydrothermally altered and are of tholeiitic affinities. Lamprophyres have been reported from nearby areas. The presence of Mn-rich Ilmenites in the Malanjkhand copper mine dykes and the occurrence of lamprophyres and the pervasive potassium metasomatism in the area strongly suggests a possibility of finding diamondiferous rocks in the area. Such high-Mn bearing ilmenites are associated with diamondiferous deposits in other parts of the world, e.g. Juina kimberlites, Brazil. Mn-bearing ilmenite is considered as an indicator mineral for kimberlite/diamond occurrences. The presence of kimberlite pipes in Raipur district and the association of Mn-rich ilmenite with kimberlites is a fortuitous coincidence for venture-some mining companies. A probable explanation for the origin of manganese layers in the context of ‘rift’ tectonic environment is offered. Also a possible link between the dykes, quartz veins in the Malanjkhand granitic rocks and the copper mineralization is proposed. 相似文献