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51.
J. Hakkila V. Vo C. Meegan J. Horack G. Fishman D. Hartmann G. Pendleton M. Briggs W. Paciesas 《Astrophysics and Space Science》1995,231(1-2):23-26
A repeater statistic (derived from the two-point angular correlation function and accounting for location errors) is used to identify constraints on the rate of burst repetition from the BATSE 1B catalog. Because of the anisotropic sky exposure, repeating sources should create additional effects in the large-scale distribution of close burst pairs.National Research Council Fellow at NASA/MSFC 相似文献
52.
53.
K. Kh. Truong T. S. Vo Yu. M. Mikhailov A. L. Kharitonov O. V. Kapustina 《Geomagnetism and Aeronomy》2010,50(5):690-694
The preliminary results of observations of VLF electromagnetic signals (atmospherics) in the mountains of North Vietnam (Dien
Bien Phu, 21°23′50″ N, 103°0′28″ E) are presented. Primarily, signals of typical atmospherics with a maximum in the frequency
range of 4–9 kHz were observed; sometimes they were accompanied by so-called “tails” at frequencies less than 1 kHz, and also
tweeks of usually short duration (about 10 ms) were observed. Several parameters of the ionosphere, as well as the distance
to sources of atmospherics (in accordance to the data of the World Wide Lightning Location Network (TOGA)), were estimated
by spectral-temporal characteristics of tweeks. 相似文献
54.
An energy approach is proposed as a complement to the stress approach commonly considered for investigating soil desiccation cracking. The elastic strain energies before and after crack initiation are estimated by both numerical and analytical solutions. The energy released by cracking is then compared with the fracture energy to discuss crack initiation conditions. This leads to combined energy and stress conditions for crack initiation following Leguillon's theory. An approximate analytical solution is derived from a variational formulation of the porous elastic body equations. A cohesive zone model and finite element code are used to simulate crack propagation in an unsaturated porous body. This analysis shows that the energy criterion is reached before the stress criterion, and this can explain unstable crack propagation at the beginning. The approximate analytical solution allows predicting correctly the crack depth and opening in its initiation stage. 相似文献
55.
Hsiung Bin-Chen Benson Likitlersuang Suched Phan Khac Hai Pisitsopon Piroon 《Acta Geotechnica》2021,16(6):1923-1938
Acta Geotechnica - Due to tool, time and budget limitations, 2-dimensional analyses of deep excavation are often adopted for engineering practice; thus, the 3-dimensional behaviour is ignored.... 相似文献
56.
Overhangs are frequently observed in riverbanks, coastal headlands, and rock formations. The geometry of an overhang is an input into most slope stability analyses and is often idealised or back-calculated from empirical data. This study investigates the geometries of overhang slopes, which exist while in limiting conditions satisfying static equilibrium with soil strength governed by the Mohr-Coulomb failure criterion. The overhanging contour is formulated as the unknown in a boundary value problem and solved for using the slip line theory. Analyses consider nonhomogeneous soils, where cohesion and, if unsaturated, the contribution of suction to the effective stress vary linearly with depth. The solutions are presented in general dimensionless charts. Applications of the charts are illustrated via examples. It has been observed that soil with varying amounts of friction and cohesion could develop overhanging arches of different sizes and shapes. This study shows that the curvature of an overhang becomes more pronounced for small values of φ′ . It is also demonstrated that changing the contribution of suction to the effective stress has a direct impact on the size of an unsaturated soil overhang. Overhanging slope shapes observed in reality may be different from the idealised subset studied in this paper since real slopes are not necessarily at impending failure. The real shapes may be influenced by various physical processes such as weathering, stress variations caused by cycles of wetting-drying. Even so, the results presented in the paper indicated how key soil properties influence slope shapes, albeit it while in limiting conditions. 相似文献
57.
The Lateglacial and Holocene in Central Europe: a multi‐proxy environmental record from the Bohemian Forest,Czech Republic
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Klára Vočadlová Libor Petr Pavla Žáčková Marek Křížek Lenka Křížová Simon M. Hutchinson Miroslav Šobr 《Boreas: An International Journal of Quaternary Research》2015,44(4):769-784
The Hercynian mountain ranges were islands of mountain glaciation and alpine tundra in a Central European ice‐free corridor during the Late Pleistocene. Today they are notable areas of glacial landforms, alpine‐forest free areas, peatlands and woodlands. However, our knowledge of the Lateglacial and early Holocene environmental changes in this region is limited. We present a new multi‐proxy reconstruction of a mid‐altitude environment in the Bohemian Forest spanning this period. A core (5.2 m length) in the ?erné Lake cirque (1028 m a.s.l.) was subjected to lithological, geochemical, pollen and macrofossil analysis supplemented by two optically stimulated luminescence (OSL) and 10 AMS radiocarbon dates. We determined the impact of regional and supraregional climate changes on the environment. The two most significant changes in sedimentation during the Lateglacial (17.6 and 15.8–15.5 cal. ka BP) were synchronous with regional glacial chronostratigraphy. Unlike Central European mountain ranges, in the Bohemian Forest the Younger Dryas was not coincident with glacier re‐advance, but was a dry, cold episode with low lake levels, which prevailed until the early Preboreal. Plant macrofossils indicate local establishment of Betula nana and Betula pendula/pubescens at 15.4–13.4 cal. ka BP. Comparison with Holocene records from Central Europe shows a similar immigration history of vegetation at mid and higher altitudes. The tree line exceeded an altitude of ~1000 m a.s.l. around 10.5 cal. ka BP and coincided with rapid geochemical changes in the sediment. The 8.2 ka BP event did not have any response in the sedimentary record, but corresponded to stabilization of the Picea abies population and expansion of Fagus. Fagus colonized the Bohemian Forest earlier than other Hercynian mid‐mountains, but never predominated in the composition of the forest at higher elevations. Abies alba was the last tree species that immigrated to the study area. 相似文献
58.
利用GIS能将蜂窝网络数据库中空间与非空间数据相结合的优势,来提供必要的参数,以更准确地预测蜂窝网络的覆盖强度,研究设计了一种基于COST-231-Walfisch-Ikegami的GIS模型,用来预测越南胡志明市移动蜂窝网络的信号强度,该模型可以帮助确定无线电的视距和非视距情况。该模型对路径方向损失、多屏衍射损耗和阴影增益进行了更精确的预测。实验中采用的地图比例尺是1∶2 000,采用的平均楼层高度为3 m。统计结果表明,该GIS模型预测的路径损耗克服了每个基站的实测路径损耗,在市区可以用来预测微蜂窝覆盖范围。 相似文献
59.
Landslide susceptibility assessment using SVM machine learning algorithm 总被引:10,自引:0,他引:10
This paper introduces the current machine learning approach to solving spatial modeling problems in the domain of landslide susceptibility assessment. The latter is introduced as a classification problem, having multiple (geological, morphological, environmental etc.) attributes and one referent landslide inventory map from which to devise the classification rules. Three different machine learning algorithms were compared: Support Vector Machines, Decision Trees and Logistic Regression. A specific area of the Fruška Gora Mountain (Serbia) was selected to perform the entire modeling procedure, from attribute and referent data preparation/processing, through the classifiers' implementation to the evaluation, carried out in terms of the model's performance and agreement with the referent data. The experiments showed that Support Vector Machines outperformed the other proposed methods, and hence this algorithm was selected as the model of choice to be compared with a common knowledge-driven method – the Analytical Hierarchy Process – to create a landslide susceptibility map of the relevant area. The SVM classifier outperformed the AHP approach in all evaluation metrics (κ index, area under ROC curve and false positive rate in stable ground class). 相似文献
60.
Vít Voženílek 《GeoJournal》1994,32(3):241-245
The territory of the Czech Republic has been investigated by geographers for many decades. The Moravian Gate situated in the eastern part of the Czech Republic, became a place of high human concentration due to its geographical location. Geomorphological research in the Czech republic has reached an impressive level in the field of both the morphostructure of the whole territory and the extent of glaciation. The article treats the relationship between the morphostructure and the extent of glaciation in a geographically complicated region. Both of these have been investigated separately. The synthesis of all results brings new knowledge and suggestions for future research in the region close to the main European watershed. The paper is completed on the basis of the author's research including the complex geomorphological analysis, especially morphostructural and morfosculptural analysis, the investigation of the features and genesis of georelief, the digital elevation model of studied area, the detailed geomorphological mapping in 1:10.000 scale and the compilation of denudation chronology. 相似文献