排序方式： 共有36条查询结果，搜索用时 31 毫秒
1.
R. J-M. Grognard 《Solar physics》1982,81(1):173-180
The electron distribution functions measured at 1 AU in an electron stream passing the ISEE-3 spacecraft (Lin et al., 1981) are used as input data to a programme which simulates in a one-dimensional model the interaction between fast electrons and plasma waves (quasi-linear relaxation) together with the plasma wave scattering off the background ions. While the computed spectral energy density of the plasma waves excited in resonance with the streaming of electrons is below Zakharov's threshold of strong turbulence, it is sufficiently high to undergo a fast induced scattering off the background ions. The resulting spectrum is concentrated around the wave vector k = 0. Some simple analytic considerations show that this stage leads necessarily to the crossing of Zakharov's threshold and therefore this indirect excitation of strong turbulence seems to play an essential role in understanding the Langmuir turbulence generated by the motion of fast electron beams in the interplanetary plasma. 相似文献
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Prediction of ground displacements and velocities from groundwater level changes at the Vallcebre landslide (Eastern Pyrenees, Spain) 总被引：2，自引：5，他引：2
In active landslides, the prediction of acceleration of movement is a crucial issue for the design and performance of warning systems. The landslide of Vallcebre in the Eastern Pyreenes, Spain, has been monitored since 1996 and data on rainfall, groundwater levels and ground displacements are measured on a regular basis. Displacements observed in borehole wire extensometers have shown an immediate response of the landslide to rainfall episodes. This rapid response is likely due to the presence of preferential drainage ways. The occurrence of nearly constant rates of displacement in coincidence with steady groundwater levels suggests the presence of viscous forces developed during the movement. An attempt to predict both landslide displacements and velocities was performed at Vallcebre by solving the momentum equation in which a viscous term (Bingham and power law) was added. Results show that, using similar rheological parameters for the entire landslide, computed displacements reproduce quite accurately the displacements observed at three selected wire extensometers. These results indicate that prediction of displacements from groundwater level changes is feasible. 相似文献
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Stability conditions in an area located NW of Barcelona (Spain) are discussed. Here, several mass movements were observed, mainly affecting weathered Paleozoic slates. Many of these failures involved slopes cut along recent infrastructures: debris flows, wedge and plane failures, generally surficial, occurred more frequently. After a detailed geological and geomorphologic survey, geomechanic characterization was carried out, according to RMR and SMR classifications. This rating gave a prediction of slope behaviour, in fairly good agreement with the real observed one.
Stability numerical analysis was carried out for the main cut slopes, based upon the Limit Equilibrium Method. First of all, the deterministic factor of safety was computed using the mean values of parameters. After that, a simulation technique based upon the Monte Carlo Method was applied in order to obtain factor of safety distributions. The probability of failure was estimated as P(F<1).
Finally, results from deterministic and probabilistic approaches were compared. The effectiveness of different possible remedial measures was highlighted by means of a sensitivity analysis, which showed that the more important parameters in the study area are the geometrical ones (height, slope and failure plane angles). The final technical solutions adopted are briefly outlined. 相似文献
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Underwater photogrammetry provides an efficient nondestructive means for measurement in environments with limited accessibility. With the growing use of consumer cameras, its application is becoming easier, thus benefiting a wide variety of disciplines. However, utilizing cameras for underwater photogrammetry poses some nontrivial modeling problems due to refraction effect and the extension of the imaging system into a unit of both the camera and the protecting housing device. This paper studies the effect that the underwater environment has on the photogrammetric process, and proposes a model for describing the geometric distortions and for estimating the additional parameters involved. The proposed model accounts not only for the multimedia effect, but also for inaccuracies related to the setting of the camera and housing device. The paper shows that only a small number of additional parameters is needed to model both elements and to preserve the collinearity relation. The results show that no unique setup is needed for estimating the additional parameters and that the estimation is insensitive to noise or first approximations. Experiments show that high levels of accuracy can be achieved. 相似文献
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Aubry Vanderstraeten Steeve Bonneville Stefania Gili Jeroen de Jong Wendy Debouge Philippe Claeys Nadine Mattielli 《Geostandards and Geoanalytical Research》2020,44(2):307-329
Atmospheric dust is an integral component of the Earth system with major implications for the climate, biosphere and public health. In this context, identifying and quantifying the provenance and the processes generating the various types of dust found in the atmosphere is paramount. Isotopic signatures of Pb, Nd, Sr, Zn, Cu and Fe are commonly used as sensitive geochemical tracers. However, their combined use is limited by the lack of (a) a dedicated chromatographic protocol to separate the six elements of interest for low‐mass samples and (b) specific reference materials for dust. Indeed, our work shows that USGS rock reference materials BHVO‐2, AGV‐2 and G‐2 are not applicable as substitute reference materials for dust. We characterised the isotopic signatures of these six elements in dust reference materials ATD and BCR‐723, representatives of natural and urban environments, respectively. To achieve this, we developed a specific procedure for dust, applicable in the 4–25 mg mass range, to separate the six elements using a multi‐column ion‐exchange chromatographic method and MC‐ICP‐MS measurements. 相似文献
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G. Matas N. Lantada J. Corominas J. A. Gili R. Ruiz-Carulla A. Prades 《Landslides》2017,14(5):1565-1578
A rockfall is a mass instability event frequently observed in road cuts, open pit mines and quarries, steep slopes and cliffs. After its detachment, the rock mass may disaggregate and break due to the impact with the ground surface, thus producing new rock fragments. The consideration of the fragmentation of the rockfall mass is critical for the calculation of the trajectories of the blocks and the impact energies and for the assessment of the potential damage and the design of protective structures. In this paper, we present RockGIS, a GIS-based tool that simulates stochastically the fragmentation of the rockfall, based on a lumped mass approach. In RockGIS, the fragmentation is triggered by the disaggregation of the detached rock mass through the pre-existing discontinuities just before the impact with the ground. An energy threshold is defined in order to determine whether the impacting blocks break or not. The distribution of the initial mass between a set of newly generated rock fragments is carried out stochastically following a power law. The trajectories of the new rock fragments are distributed within a cone. The fragmentation model has been calibrated and tested with a 10,000 m3 rockfall that took place in 2011 near Vilanova de Banat, Eastern Pyrenees, Spain. 相似文献
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Doñana National Park is an area of approximately 500 km2 located on the SW coast of Spain that shows one of the greatest geoid gradients on the entire Iberian Peninsula, due to its peculiar tectonic characteristics. So, it is necessary to elaborate an accurate geoid model that can be used with GPS for precise surveying, since the existing ones are insufficient, due to their poor resolution and their limited adaptation to a small area with such a strong gradient. The least squares prediction method was tested in order to obtain the undulation from GPS/orthometric points. The results obtained were unsatisfactory because of the strong geoid gradient. In order to improve accuracy the remove-restore technique was used. Global geopotential model EIGEN-CG01C and a Digital Elevation Model (DEM) with a 25 × 25 m resolution and an accuracy better than 3 m were used. Thus, the final geometrical geoid obtained reaches the precision required by other disciplines (3 cm in any point within the Park). Particularly, the geoid model has allowed for the acquisition of a precision DEM that is essential to formulate a hydrodynamic model for the Doñana marsh functions. 相似文献
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通过在东昆仑活动断裂带西大滩段进行断层气测试,首次获取了该断裂带中Rn和CO2的释放量。在2004年开挖的2～3m深的探槽内,氡浓度可达20732Bq.m-3,氡发射率可达433mBq.m-2.s-1,远高于在地表的氡浓度505～2380Bq.m-3与氡发射率7～28.19mBq.m-2.s-1(地表氡发射率均值为14.7mBq.m-2.s-1,与世界平均值相当)。从而我们推断该断裂具有从上部第四系覆盖物到深部花岗岩之间的良好连通性。在地表CO2的析出率平均值为18.9g.m-2.d-1,与通常的背景值相当,在探槽中和距离断层1km的地方没有明显的空间变化,但是在断层北侧3km处的一个近乎直立的千枚岩小山上,CO2的析出率却很高,为421g.m-2.d-1,同时该处氡的发射率也高,达503mBq.m-2.s-1,因此,有必要在该断裂附近进行长期监测 相似文献