全文获取类型
收费全文 | 319篇 |
免费 | 16篇 |
国内免费 | 7篇 |
专业分类
测绘学 | 8篇 |
大气科学 | 18篇 |
地球物理 | 60篇 |
地质学 | 176篇 |
海洋学 | 21篇 |
天文学 | 28篇 |
综合类 | 8篇 |
自然地理 | 23篇 |
出版年
2023年 | 3篇 |
2022年 | 15篇 |
2021年 | 15篇 |
2020年 | 10篇 |
2019年 | 12篇 |
2018年 | 20篇 |
2017年 | 16篇 |
2016年 | 22篇 |
2015年 | 13篇 |
2014年 | 27篇 |
2013年 | 35篇 |
2012年 | 16篇 |
2011年 | 24篇 |
2010年 | 8篇 |
2009年 | 14篇 |
2008年 | 11篇 |
2007年 | 8篇 |
2006年 | 9篇 |
2005年 | 1篇 |
2004年 | 6篇 |
2003年 | 3篇 |
2002年 | 5篇 |
2001年 | 9篇 |
2000年 | 3篇 |
1999年 | 3篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1995年 | 4篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1987年 | 4篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1971年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有342条查询结果,搜索用时 46 毫秒
191.
Vernon Cooray Mahbubur Rahman Vladimir Rakov 《Journal of Atmospheric and Solar》2009,71(17-18):1877-1889
Different approaches are used in estimating the global production of NOx by lightning flashes, including field measurements carried out during thunderstorm conditions, theoretical studies combining the physics and chemistry of the electrical discharges, and measurements of NOx yield in laboratory sparks with subsequent extrapolation to lightning. In the latter procedure, laboratory data are extrapolated to lightning using the energy as the scaling quantity. Further, in these studies only the return strokes are considered assuming that contributions from other processes such as leaders, continuing currents, M components, and K processes are negligible. In this paper, we argue that the use of energy as the scaling quantity and omission of all lightning processes other than return strokes are not justified. First, a theory which can be used to evaluate the NOx production by electrical discharges, if the current flowing in the discharge is known, is presented. The results obtained from theory are compared with the available experimental data and a reasonable agreement is found. Numerical experiments suggest that the NOx production efficiency of electrical discharges depends not only on the energy dissipated in the discharge, but also on the shape of current waveform. Thus, the current signature, can influence extrapolation of laboratory data to lightning flashes. Second, an estimation of the NOx yield per lightning flash is made by treating the lightning flash as a composite event consisting of several discharge processes. We show that the NOx production takes place mainly in slow discharge processes such as leaders, M components, and continuing currents, with return strokes contributing only a small fraction of the total NOx. The results also show that cloud flashes are as efficient as ground flashes in NOx generation. In estimating the global NOx production by lightning flashes the most influencing parameter is the length of the lightning discharge channel inside the cloud. For the total length of channels inside the cloud of a typical ground flash of about 45 km, we estimate that the global annual production of NOx is about 4 Tg(N). 相似文献
192.
The Lebanese crustal segment is part of a much larger carbonate platform deposited along the northwestern margin of the Arabian Plate, in the eastern Mediterranean region. It is made up mainly of Jurassic–Cretaceous carbonate rocks. Most of this stratigraphic sequence is exposed in the Nahr Ibrahim canyon and surrounding areas in central Lebanon. The various formations, from the oldest unit (the Lower Jurassic Kesrouane Formation) to the Upper Cretaceous Chekka Formation, are made up of different types of carbonate rocks including micritic limestone, medium‐ to coarse‐grained dolostone, biomicritic (chalk), biosparrudite limestone, micritic dolostone, pelmicrite, marl and marly limestone. Results of this first chemical investigation on the Lebanese carbonate platform show that the micritic limestone of the Kesrouane Formation is relatively enriched in Ca, Na and Sc, and has low rare‐earth element (REE) contents. The marl units of the Hammana Formation are enriched in Al, Fe, K, Ti, Rb, Ga, Nb, U, Th and REE. The chalk of the Chekka Formation shows the highest phosphorus content. A significant increase in P and Sr contents with time (from the Lower Jurassic to the Upper Cretaceous carbonate units) characterizes the Lebanese sequence; this is interpreted to be related to a possible increase in continental weathering rates during the Mesozoic. Enrichment in Ni, Ti and Nb in some formations is interpreted to be linked to Mesozoic volcanism in central Lebanon. The Cretaceous formations are subdivided according to their REE patterns into two distinct groups: limestones (Mdairej, Sannine and Maameltain formations) which are depleted in REE; and marl/chalk (Hammana and Chekka formations) which are significantly enriched in REE. On several geochemical variation diagrams, such as the K–Ti–P triangular plot, the Lebanese Mesozoic carbonate formations are found to occupy distinct compositional fields. Thus, carbonate geochemistry could prove to be a powerful tool (especially when combined with petrographic data) in characterizing and correlating carbonate formations (chemical stratigraphy), particularly in regions where field evidence may be limited. Results of this study have significant implications for the entire carbonate platform that covers a large part of the eastern Mediterranean region. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
193.
Syed Wajid Ali Shah Mujaddad ur Rehman Samina Anwar Azam Hayat Ghulam Shabir Razia Tahseen Moazur Rahman Ejazul Islam Samina Iqbal Muhammad Afzal 《洁净——土壤、空气、水》2023,51(2):2100435
The presence of trace antibiotics in domestic and industrial effluents poses a risk of toxicity to fauna and flora. The application of floating treatment wetlands (FTWs) is a low-cost and sustainable approach to remediate wastewater. In this study, the performance of different macrophytes vegetated in FTWs for the remediation of ciprofloxacin (CIP)-contaminated water is investigated. Six macrophytes, Brachia mutica, Typha domingensis, Phragmites australis, Canna indica, Cyperus laevigatus, and Leptochloa fusca, are vegetated in FTWs for the removal of CIP (50 mg L−1) from water. The FTWs show the potential to remove 30–43.58 mg L−1 CIP from water in 28 days. They also reduce the chemical oxygen demand (118–138 mg L−1) and biochemical oxygen demand (35–45 mg L−1) of water. Among the macrophytes, C. indica removes maximum (43.58 mg L−1) and T. domingensis minimum (30 mg L−1) CIP. Canna indica and T. domingensis exhibit the maximum and minimum increase (30% and 12% of dry biomass) in growth, respectively. This study reveals that the FTWs vegetated with different plant species exhibit varying performances in removing CIP from water. This investigation is a step forward toward sustainable bioremediation of water contaminated with antibiotics. 相似文献
194.
Abdul Razzaq Khan Adil Aslam Mir Sharjil Saeed Muhammad Rafique Khawaja M. Asim Talat Iqbal Abdul Jabbar Saeed Ur Rahman 《Acta Geophysica》2018,66(5):1073-1079
The aim of this study is to assess the performance of linear discriminate analysis, support vector machines (SVMs) with linear and radial basis, classification and regression trees and random forest (RF) in the classification of radionuclide data obtained from three different types of rocks. Radionuclide data were obtained for metamorphic, sedimentary and igneous rocks using gamma spectroscopic method. A P-type high-purity germanium detector was used for the radiometric study. For analysis purpose, we have determined activity concentrations of 232Th, 226Ra and 40K radionuclides, published elsewhere (Rafique et al. in Russ Geol Geophys 55:1073–1082, 2014), in different rock samples and built the classification model after pre-processing the data using three times tenfold cross-validation. Using this model, we have classified the new samples into known categories of sedimentary, igneous and metamorphic. The statistics depicts that RF and SVM with radial kernel outperform as compared to other classification methods in terms of error rate, area under the curve and with respect to other performance measures. 相似文献
195.
Natural Resources Research - Water saturation, organic matter content, and fractures are important factors influencing the physical properties of shale. In this study, shale samples were subjected... 相似文献
196.
The evaluation of seismic response of soil sites constitutes an important problem with respect to groundmotion amplification and soil instability because of liquefaction. The base motion generated during earthquake is a random process. In addition, the soil sites are usually homogenous with randomly varying characteristics. The uncertainties associated with the input motion and site characteristics may lead to a wide range of variability of the site response. In this paper, a Monte-Carlo based stochastic finite element method is used to study the variability of seismic response. 相似文献
197.
In this study, a set of generalized analytical solutions are developed for the wave-induced response of a saturated porous seabed under plane strain condition. When considering the water waves originating in deep water and travelling towards the shore, their velocities, lengths and heights vary. Depending on the characteristics of the wave and the properties of the seabed, different formulations (fully dynamic, partly dynamic, quasi-static) for the wave-induced response of the seabed are possible. The solutions for the response with these formulations are established in terms of non-dimensional parameters. The results are presented in terms of pore pressure, shear stress and vertical effective stress distributions within the seabed. For typical values of wave period and seabed permeability, the regions of applicability of the three formulations are identified and plotted in parametric spaces. With given wave and seabed characteristics, these regions provide quick identification of the appropriate formulation for an adequate evaluation of the wave-induced seabed response. 相似文献
198.
Steven J. Jaret Brian L. Phillips David T. King Jr Tim D. Glotch Zia Rahman Shawn P. Wright 《Meteoritics & planetary science》2017,52(1):147-163
Coesite has been identified within ejected blocks of shocked basalt at Lonar crater, India. This is the first report of coesite from the Lonar crater. Coesite occurs within SiO2 glass as distinct ~30 μm spherical aggregates of “granular coesite” identifiable both with optical petrography and with micro‐Raman spectroscopy. The coesite+glass occurs only within former silica amygdules, which is also the first report of high‐pressure polymorphs forming from a shocked secondary mineral. Detailed petrography and NMR spectroscopy suggest that the coesite crystallized directly from a localized SiO2 melt, as the result of complex interactions between the shock wave and these vesicle fillings. 相似文献
199.
We present an objective optimization procedure to determine the roughness parameters for very rough boundary-layer flow over model urban canopies. For neutral stratification the mean velocity profile above a model urban canopy is described by the logarithmic law together with the set of roughness parameters of displacement height d, roughness length \(z_0\), and friction velocity \(u_*\). Traditionally, values of these roughness parameters are obtained by fitting the logarithmic law through (all) the data points comprising the velocity profile. The new procedure generates unique velocity profiles from subsets or combinations of the data points of the original velocity profile, after which all possible profiles are examined. Each of the generated profiles is fitted to the logarithmic law for a sequence of values of d, with the representative value of d obtained from the minima of the summed least-squares errors for all the generated profiles. The representative values of \(z_0\) and \(u_*\) are identified by the peak in the bivariate histogram of \(z_0\) and \(u_*\). The methodology has been verified against laboratory datasets of flow above model urban canopies. 相似文献
200.
Shahida Waheed Asma Rahman Naila Siddique Shujaat Ahmad 《Geostandards and Geoanalytical Research》2007,31(2):133-141
The National Research Council (NRC), Ottawa, Canada sandy marine sediment reference material HISS-1 was characterised for thirty-seven trace elements by neutron activation optimised irradiation, cooling and counting protocols using the low power Miniature Neutron Source Reactor (MNSR) as a neutron source. This INAA methodology quantified twenty additional elements including ten rare earth (Ce, Dy, Eu, Ho, La, Lu, Nd, Sm, Tb and Yb) and ten other elements (Ba, Br, Cs, Ga, Hf, Rb, Sc, Ta, Th and Zr) missing in the final NRCC certification. A large number of values produced by different irradiation schemes together with the use of certified reference materials in the quantification step that showed good precision, provided confidence in the results. The reliability of the REE data was checked by plotting chondrite-normalised graphs. 相似文献