全文获取类型
收费全文 | 179篇 |
免费 | 8篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 30篇 |
大气科学 | 9篇 |
地球物理 | 44篇 |
地质学 | 57篇 |
海洋学 | 3篇 |
天文学 | 41篇 |
综合类 | 3篇 |
自然地理 | 2篇 |
出版年
2024年 | 1篇 |
2022年 | 2篇 |
2021年 | 1篇 |
2020年 | 5篇 |
2019年 | 4篇 |
2018年 | 10篇 |
2017年 | 18篇 |
2016年 | 8篇 |
2015年 | 13篇 |
2014年 | 22篇 |
2013年 | 9篇 |
2012年 | 10篇 |
2011年 | 16篇 |
2010年 | 8篇 |
2009年 | 6篇 |
2008年 | 6篇 |
2007年 | 4篇 |
2006年 | 9篇 |
2005年 | 4篇 |
2004年 | 2篇 |
2003年 | 9篇 |
2002年 | 5篇 |
2001年 | 1篇 |
2000年 | 3篇 |
1999年 | 1篇 |
1998年 | 4篇 |
1997年 | 1篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1984年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有189条查询结果,搜索用时 15 毫秒
141.
Sanjay Jain 《Journal of Astrophysics and Astronomy》1997,18(4):363-379
In a universe whose elementary constituents are point particles there does not seem to be any obvious mechanism for avoiding
the initial singularities in physical quantities in the standard model of cosmology. In contrast in string theory these singularities
can be absent even at the level where spacetime is treated classically. This is a consequence of the basic degrees of freedom
of strings in compact spaces, which necessitate a reinterpretation of what one means by a very small universe. We discuss
the basic degrees of freedom of a string at the classical and quantum level, the minimum size of strings (string uncertainty
principle), the t-duality symmetry, and string thermodynamics at high energy densities, and then describe how these considerations
suggest a resolution of the initial singularity problem. An effort has been made to keep this writeup self-contained and accessible
to non-string theorists. 相似文献
142.
The present paper analyzes the dual frequency signals from GPS satellites recorded at Varanasi (Geographic latitude 25°, 16′ N,
longitude 82°, 59′ E) near the equatorial ionization anomaly (EIA) crest in India, to study the effect of geomagnetic storm
on the variation of TEC, during the low solar active period of May 2007 to April 2008. Three most intense—but still moderate
class—storms having a rapid decrease of Dst-index observed during the GPS recorded data have been analyzed, which occurred on 20 November 2007, 9 March 2008 and 11 October
2008 were selected and storm induced features in the vertical TEC (VTEC) have been studied considering the mean VTEC value
of quiet days as reference level. The possible reasons for storm time effects on VTEC have been discussed in terms of local
time dependence, storm wind effect as well as dawn-dusk component of interplanetary electric field (IEF) Ey intensity dependence. 相似文献
143.
Natural Hazards - The precise focal depth of earthquakes is a crucial parameter for better characterising of complex tectonic zones, especially where seismogenic faults occur at different depths.... 相似文献
144.
ABSTRACTMaintaining a positive cost–benefit ratio for hydrologic data collection is challenging for public water management and regulatory agencies given budgetary constraints. We examine and discuss the evolution of a government agency in the last 20 years to identify the best practices for collecting, managing, and disseminating hydrologic data. The best practices include (1) developing in-house field data collection systems and integrating them with commercial, off-the-shelf software for the enterprise database, (2) standardizing hardware and data acquisition software, (3) collaborating with other agencies and tracking their data collection efforts to avoid redundancy, (4) utilizing open-source programming languages for data quality assurance, (5) rationalizing data collection network, and (6) taking a proactive approach with data collection to better prepare for hydroclimatic variability. Transforming data into easily understandable information and its near-real-time dissemination can add value to the data (e.g. providing inundation instead of water level above mean sea level) in the future. 相似文献
145.
Stream temperature response to variable glacier coverage in coastal watersheds of Southeast Alaska 总被引:1,自引:0,他引:1 下载免费PDF全文
Jason B. Fellman Sonia Nagorski Sanjay Pyare Andrew W. Vermilyea Durelle Scott Eran Hood 《水文研究》2014,28(4):2062-2073
We measured stream temperature continuously during the 2011 summer run‐off season (May through October) in nine watersheds of Southeast Alaska that provide spawning habitat for Pacific salmon. The nine watersheds have glacier coverage ranging from 0% to 63%. Our goal was to determine how air temperature and watershed land cover, particularly glacier coverage, influence stream temperature across the seasonal glacial meltwater hydrograph. Multiple linear regression models identified mean watershed elevation (related to glacier extent) and watershed lake coverage (%) as the strongest landscape controls on mean monthly stream temperature, with the weakest (May) and strongest (July) models explaining 86% and 97% of the temperature variability, respectively. Mean weekly stream temperature was significantly correlated with mean weekly air temperature in seven streams; however, the relationships were weak to non‐significant in the streams influenced by glacial run‐off. Streams with >30% glacier coverage showed decreasing stream temperatures with rising summer air temperatures, whereas those with <30% glacier coverage exhibited summertime warming. Glaciers also had a cooling effect on monthly mean stream temperature during the summer (July through September) equivalent to a decrease of 1.1 °C for each 10% increase in glacier coverage. The maximum weekly average temperature (an index of thermal suitability for salmon) in the six glacial streams was substantially below the lower threshold for optimum salmon growth. This finding suggests that although glaciers are important for moderating summer stream temperatures, future reductions in glacier run‐off may actually improve the thermal suitability of some glacially dominated streams in Southeast Alaska for salmon. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
146.
Modeling of groundwater draft based on satellite-derived crop acreage estimation over an arid region of northwest India 总被引:1,自引:0,他引:1
Bidyut Kumar Bhadra Sanjay Kumar Rakesh Paliwal A. T. Jeyaseelan 《Hydrogeology Journal》2016,24(7):1681-1698
Over-exploitation of groundwater for agricultural crops puts stress on the sustainability of natural resources in the arid region of Rajasthan state, India. Hydrogeological study of groundwater levels of the study area during the pre-monsoon (May to June), post-monsoon (October to November) and post-irrigation (February to March) seasons of 2004–2005 to 2011–2012 shows a steady decline of groundwater levels at the rate of 1.28–1.68 m/year, mainly due to excessive groundwater draft for irrigation. Due to the low density of the groundwater observation-well network in the study area, assessment of groundwater draft, and thus groundwater resource management, becomes a difficult task. To overcome the situation, a linear groundwater draft model (LGDM) has been developed based on the empirical relationship between satellite-derived crop acreage and the observed groundwater draft for the year 2003–2004. The model has been validated for a decade, during three year-long intervals (2005–2006, 2008–2009 and 2011–2012) using groundwater draft, estimated through a discharge factor method. Further, the estimated draft was validated through observed pumping data from random sampled villages (2011–2012). The results suggest that the developed LGDM model provides a good alternative to the estimation of groundwater draft based on satellite-based crop area in the absence of groundwater observation wells in arid regions of northwest India. 相似文献
147.
148.
Yamuna Singh P. S. C. Pandit Sanjay Bagora P. K. Jain 《Journal of the Geological Society of India》2017,89(2):115-130
Co-genetic pegmatites associated with the granite of the Kawadgaon area in the Bastar craton, Central India, contain a wide range of ore minerals of Nb, Ta, Be, Sn, Zr, Ti, and REE, including columbite-tantalite, ixiolite, pseudo-ixiolite, wodginite, tapiolite, microlite, fersmite, euxenite, aeschynite, beryl, cassiterite, monazite, xenotime, zircon, ilmenite, triplite, and magnetite. There is a distinct vertical zonation between the rare metal and tin pegmatites in apical parts of the host granite. Geochemically, these are LCT-S type, beryl-columbite-phosphate pegmatites that have notably high contents of SiO2 (av. 73.80%), Rb (av. 381 ppm), and Nb (av. 132 ppm). The investigated granites probably were derived from the melting of older crustal rocks, as indicated by a high initial 87Sr/86Sr isotopic ratio, and the major-element geochemistry of the granites and pegmatites. Plots of mol. CaO/(MgO+FeOt) vs. mol. Al2O3/(MgO+FeOt) suggest that the source rock was pelitic metasediments. Based on the available data, it is postulated that the derivation of pegmatites from the parent granite occurred shortly after granite emplacement in the late Archaean-early Proterozoic (~2500 Ma). The K/Rb, Ba/Rb, and Rb/Sr ratios of the felsic bodies reveal that a substantial part of the granite formed from evolved melts, and further fractionation produced the co-genetic pegmatites and associated rare metal and rare earth deposits. 相似文献
149.
Thomas E. McHugh Poonam R. Kulkarni Charles J. Newell John A. Connor Sanjay Garg 《Ground water》2014,52(6):898-907
Quantifying the overall progress in remediation of contaminated groundwater has been a significant challenge. We utilized the GeoTracker database to evaluate the progress in groundwater remediation from 2001 to 2011 at over 12,000 sites in California with contaminated groundwater. This paper presents an analysis of analytical results from over 2.1 million groundwater samples representing at least $100 million in laboratory analytical costs. Overall, the evaluation of monitoring data shows a large decrease in groundwater concentrations of gasoline constituents. For benzene, half of the sites showed a decrease in concentration of 85% or more. For methyl tert‐butyl ether (MTBE), this decrease was 96% and for TBE, 87%. At remediation sites in California, the median source attenuation rate was 0.18/year for benzene and 0.36/year for MTBE, corresponding to half‐lives of 3.9 and 1.9 years, respectively. Attenuation rates were positive (i.e., decreasing concentration) for benzene at 76% of sites and for MTBE at 85% of sites. An evaluation of sites with active remediation technologies suggests differences in technology effectiveness. The median attenuation rates for benzene and MTBE are higher at sites with soil vapor extraction or air sparging compared with sites without these technologies. In contrast, there was little difference in attenuation rates at sites with or without soil excavation, dual phase extraction, or in situ enhanced biodegradation. The evaluation of remediation technologies, however, did not evaluate whether specific systems were well designed or implemented and did not control for potential differences in other site factors, such as soil type. 相似文献
150.