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排序方式: 共有188条查询结果,搜索用时 31 毫秒
131.
Ekaterina V. KOROCHANTSEVA Mario TRIELOFF Cyrill A. LORENZ Alexey I. BUYKIN Marina A. IVANOVA Winfried H. SCHWARZ Jens HOPP Elmar K. JESSBERGER 《Meteoritics & planetary science》2007,42(1):113-130
Abstract— Radiochronometry of L chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 Myr ago. Fossil meteorites from Sweden indicate a highly increased influx of extraterrestrial matter in the Middle Ordovician ~480 Myr ago. An association with the L‐chondrite parent body event was suggested, but a definite link is precluded by the lack of more precise radiometric ages. Suggested ages range between 450 ± 30 Myr and 520 ± 60 Myr, and can neither convincingly prove a single breakup event, nor constrain the delivery times of meteorites from the asteroid belt to Earth. Here we report the discovery of multiple 40Ar‐39Ar isochrons in shocked L chondrites, particularly the regolith breccia Ghubara, that allow the separation of radiogenic argon from multiple excess argon components. This approach, applied to several L chondrites, yields an improved age value that indicates a single asteroid breakup event at 470 ± 6 Myr, fully consistent with a refined age estimate of the Middle Ordovician meteorite shower at 467.3 ± 1.6 Myr (according to A Geologic Time Scale 2004). Our results link these fossil meteorites directly to the L‐chondrite asteroid destruction, rapidly transferred from the asteroid belt. The increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate, which implies severe environmental stress. 相似文献
132.
Previous simulations of martian global dust storms with a simple low-order model showed the desired interannual variability of storms if one of the model parameters—the threshold wind speed for starting saltation and lifting dust from the surface—was finely tuned. In this paper we show that the fine-tuning of this parameter could be the result of negative feedback in which processes associated with global dust storms raise the threshold and small-scale processes like dust devils, which are active in years between the storms, lower the threshold. 相似文献
133.
Valeri Gitis Alexander Derendyaev Pavel Metrikov Alexey Shogin 《Natural Hazards》2012,62(3):1021-1036
In this paper, we present two network geoinformation technologies. Their common major features are (1) network access to geographical information, which may be distributed over network servers or reside on the user PC, (2) modern methods of knowledge acquisition from spatial and spatio-temporal data and (3) high interactivity and clarity of visualization boosting the decision-making efficiency. The technologies are incorporated into web-GIS GeoProcessor 2.0 and GeoTime 2.0. By means of analyzing several case studies, we argue that the proposed applications are effective tools for seismic hazard research. 相似文献
134.
This paper presents the results of field and laboratory tests of railway ballast.Field tests were aimed to study vibrational acceleration of ballast particles and ballast layer stressed state in terms of train traffic with heavy axle loads.The test results are vibrational acceleration and stress values distribution in ballast layer and experimental relationships of vertical and horizontal vibroaccelerations damping in terms of train operation with axle load up to 300 kN.Laboratory stabilometer tests were directed to study the change of ballast strength properties due to vibrodynamic impact and shows that for dynamic loading with 10,25,55 Hz frequencies vibrodynamic impact influences strength properties insignificantly and coincides with the accuracy of test equipment.Stated test results provide references for calculation of ballast and sub-ballast bearing capacity. 相似文献
135.
Alexey Lyubushin 《GPS Solutions》2018,22(4):116
The traditional approach of GPS investigations is determining trends which are connected with the motion of tectonic plates. At the same time, a global GPS network provides the possibility of investigating statistical properties of high-frequency earth surface tremor in different parts of the world. Based on the results of coherence and correlation analysis of noise components of daily three-component GPS time series, representing measurements of earth surface displacements at 1097 stations, we have found that, during 2010–2011, there was a significant increase in the average level of noise coherence or correlation with dominant periods 7–9 days of surface tremor in nine regions of the earth, and in some of these regions, the average level of coherence or correlation is still high and does not return to the previous level. The increase of the average level of coherence and correlation could be detected on the graphs purely visually, while the middle time point of the time interval in which the ascending occurred is detected more precisely by a formal method based on the use of the Fisher’s ratio. 相似文献
136.
137.
Alexander?G.?SokolEmail authorView authors OrcID profile Anatoly?A.?Tomilenko Taras?A.?Bul’bak Alexey?N.?Kruk Pavel?A.?Zaikin Ivan?A.?Sokol Yurii?V.?Seryotkin Yury?N.?Palyanov 《Contributions to Mineralogy and Petrology》2018,173(6):47
Interactions in a Fe–C–O–H–N system that controls the mobility of siderophile nitrogen and carbon in the Fe0-saturated upper mantle are investigated in experiments at 6.3–7.8 GPa and 1200–1400 °C. The results show that the γ-Fe and metal melt phases equilibrated with the fluid in a system unsaturated with carbon and nitrogen are stable at 1300 °C. The interactions of Fe3C with an N-rich fluid in a graphite-saturated system produce the ε-Fe3N phase (space group P63/mmc or P6322) at subsolidus conditions of 1200–1300 °C, while N-rich melts form at 1400 °C. At IW- and MMO-buffered hydrogen fugacity (fH2), fluids vary from NH3- to H2O-rich compositions (NH3/N2?>?1 in all cases) with relatively high contents of alkanes. The fluid derived from N-poor samples contains less H2O and more carbon which mainly reside in oxygenated hydrocarbons, i.e., alcohols and esters at MMO-buffered fH2 and carboxylic acids at unbuffered fH2 conditions. In unbuffered conditions, N2 is the principal nitrogen host (NH3/N2?≤?0.1) in the fluid equilibrated with the metal phase. Relatively C- and N-rich fluids in equilibrium with the metal phase (γ-Fe, melt, or Fe3N) are stable at the upper mantle pressures and temperatures. According to our estimates, the metal/fluid partition coefficient of nitrogen is higher than that of carbon. Thus, nitrogen has a greater affinity for iron than carbon. The general inference is that reduced fluids can successfully transport volatiles from the metal-saturated mantle to metal-free shallow mantle domains. However, nitrogen has a higher affinity for iron and selectively accumulates in the metal phase, while highly mobile carbon resides in the fluid phase. This may be a controlling mechanism of the deep carbon and nitrogen cycles. 相似文献
138.
139.
跨西伯利亚大铁路的主要作用在于实现欧亚两洲西部和东部的交流。沿西伯利亚大铁路及其分支的交通经济带的形成意义重大。本文揭示了西伯利亚大铁路及其分支以及"一体化的欧亚基础设施系统"这一宏伟工程长远发展的主要特征,描述了西伯利亚大铁路和中国新丝绸之路影响区域的比较特征,提出了欧亚跨大陆板块交通经济带发展中面临的环境问题。研究认为,尽管在欧亚交通领域西伯利亚大铁路会面临一些竞争,但通过促进沿交通走廊区域的可持续发展,新丝绸之路建设将会为参与的国家带来显著的社会经济效益和环境效益,也必然会促进欧亚更密切的区域经济一体化和区域安全。 相似文献
140.
A new computer-aided design (CAD) program called Falcon 3.0 has been created to simulate and evaluate receiver designs. This
CAD tool aims to solve tasks at the design stage of navigation receivers that operate with signals from Navigational Satellite
Time and Ranging (NAVSTAR) and Global Navigational Satellite System (GLONASS). Specific tasks include:
· Defining the receiver frequency plan (frequencies of oscillators and filters that guarantee accuracy of the receiver)
· Specifying parameters for the digital processing of received signal
· Computing interpath and interchannel biases
· Computing statistical characteristics of correlation signals I, Q, dl, and dQ
· Calculating noise and multipath errors of measurements
· Determeining energy losses
This CAD tool has been used in the design of receivers such as Javad Positioning Systems (JPS) Legacy, Regency, Odyssey, and
Eurocard and in the prediction of their accuracy. It has been shown that experimental and predicted data agree well. ? 2000
John Wiley & Sons, Inc. 相似文献