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611.
Jules M. Blais Jacob Kalff R. Jack Cornett R. Douglas Evans 《Journal of Paleolimnology》1995,13(2):169-178
We used sediment chronology data from fourteen published studies of lake cores across much of North America and Scandinavia in order to make a large scale comparison of the different dating techniques. The uncertainty of210Pb derived dates was determined using common sediment event markers: the stable Pb rise, the137Cs rise, and theAmbrosia pollen rise. For all data combined, the 95% confidence intervals for the stable Pb rise and theAmbrosia rise, were approximately 30 years. These 95% confidence intervals are slightly higher than those derived by First-Order Error analysis performed by others on210Pb derived dates. When comparing the concordance of two210Pb models (CRS and CIC) against markers of known history, we found that the CRS model dates (constant rate of supply) had consistently better agreement than the CIC model dates (constant initial concentration). Major discrepancies between137Cs and210Pb were common, but were consistently more severe in sediments of soft water lakes pointing to an inability of sediments with low mineral content to immobilize Cs.This is the ninth of a series of papers to be published by this journal following the 20
th
anniversary of the first application of210Pb dating of lake sediments, Dr. P. G. Appleby is guest editing this series. 相似文献
612.
613.
激光扫描仪测量是获取三维空间信息的最有效方法之一。本文通过设计针对性的试验分析了SICK-LMS291激光扫描仪的测角和测距精度,建立数学模型分别求解了其距离和角度改正参数,结果表明改正后的激光扫描仪距离和角度测量精确度都得到了明显的提高,验证了本文采用的检校方法的可行性。 相似文献
614.
《Geomechanics and Geoengineering》2013,8(4):215-228
As is well known, numerically handling, by means of finite element codes, localisation problems involving softening materials is still quite delicate. As soon as strain localisation occurs, mesh dependence and serious problems of convergence take place. This paper deals with this type of problem in the case where localisation occurs in an ideal naturally cemented granular specimen tested under plane strain conditions. Different versions (a local elasto-plastic, a local viscoplastic and a non-local viscoplastic) of the same strain softening model are taken into consideration and the relative numerical results are critically discussed and compared. The snap-back problem is numerically taken into account and it has been demonstrated to be affected not only by the softening parameters but also by the viscous nucleus definition. To highlight the relationship between viscosity and non-locality, the results of a numerical parametric analysis are finally discussed. 相似文献
615.
Rock joint modeling using a visco-plastic multilaminate model at constant normal load condition 总被引:2,自引:0,他引:2
Reza Mahin Roosta Mohammad Hossein Sadaghiani Ali Pak Yaser Saleh 《Geotechnical and Geological Engineering》2006,24(5):1449-1468
Rock joints play an important role in the behavior of rock masses under normal and shear loading conditions. Numerical simulation
of the behavior of jointed rock masses is not an easy task due to complexities involved in the problem such as joint roughness,
joint shear strength, hardening and softening phenomenon and mesh dependency. In this study for modeling purposes, a visco-plastic
multilaminate model considering hardening and softening effects has been employed. For providing the necessary data for numerical
simulation, a series of laboratory experiments have been carried out on regular tooth-shape asperities made by gypsum, under
constant normal load conditions. Shear stress–shear displacement and normal displacement–shear displacement of artificial
joint specimens are simulated using the proposed numerical model at constant normal load condition (CNL). The results indicate
the capability of the model for simulating rock joints behavior in both strength and deformation field. Although the numerical
model has been developed for simulating the behavior of artificial joints, the concept of the method can also be used for
natural rock joints. 相似文献
616.
Assessment of different topographic corrections in AWiFS satellite imagery of Himalaya terrain 总被引:2,自引:0,他引:2
V. D. Mishra J. K. Sharma K. K. Singh N. K. Thakur M. Kumar 《Journal of Earth System Science》2009,118(1):11-26
The influence of topographic effects in optical satellite imagery is not investigated very extensively in the Himalayan terrain.
The topographic variability causes a problem of differential illumination due to steep and varying slopes in rugged Himalayan
terrain. Therefore, topographic corrections are essential for qualitative and quantitative analysis of snow cover applications.
The present paper discusses the implementation of different topographic correction models on AWiFS sensor onboard IRS P6 satellite
images and the qualitative and quantitative comparative analysis in detail. Both the Lambertian and non-Lambertian assumptions
have been considered in the present analysis with the aim to explore best suitable empirical model for rugged terrain. The
main topographic methods implemented are:
Lambertian assumptions are found to be very unrealistic over Himalayan terrain as these lead to either underestimation or
overestimation of physical parameters significantly both on sunlit slopes as well as the slopes away from the Sun. This problem
is overcome by considering non-Lambertian assumption. Minneart constant and C-correction coefficients for all AWiFS satellite
bands are estimated using regression analysis. All the results due to topographic effects are investigated qualitatively and
quantitatively using four criteria namely visual analysis, validation with field measurements (in-situ observations), spectral reflectance of training samples of snow on the south and north aspects and graphically. The visual
analysis confirms the minimization of three dimensional relief effects in two-stage normalization and slope matching methods
and retrieves some of the information under mountain shadow. Due to the very bright surface of snow fields there is likely
to be more diffuse reflected light in these areas than over darker vegetated surfaces. The qualitative analysis in other methods
does not extract any information on shady slopes. The quantitative validation of topographic results in satellite imagery
with in-situ observations shows underestimation of spectral reflectance of snow significantly except for slope matching technique. It
is also apparent that although all the topographic methods correct the reflectance of training snow samples on the south and
north aspects but most acceptable values are achieved using slope matching. The results obtained from graphical analysis reveal
that mean reflectance after all topographic corrections are independent of illumination. This study also suggests that the
suitability of topographic models can not be concluded as successful based on single criterion. Slope matching technique is
the only technique which satisfies all the four criteria successfully and produces the best result for Himalayan terrain. 相似文献
• | C-correction |
• | Minneart corrections |
• | Civco’s modified version of cosine correction |
• | two-stage normalization and |
• | slope matching technique. |
617.
Possibledynamicsofnormal-faultearthquakesintheuppercrustofthesouthpartoftheQinghai-XizangPlateau张东宁,许忠淮Dong-NingZHANGandZhong... 相似文献
618.
CHEN Yongping 《中国海洋工程》2001,(2):241-252
Based on the fluid motion equations, the physical meaning of eddy viscosity coefficient and the rationality of the Boussinesq hypothesis are discussed in this paper. The effect of the coefficient on numerical stability is analyzed briefly. A semi-enclosed rectangular sea area, with an orthogonal spur dike, is applied in a 2-D numerical model to study the effect of horizontal eddy viscosity coefficient (AH). The computed result shows that AH has little influence on the tidal level and averaged flow velocity, but has obvious influence on the intensity and the range of return flow around near the spur dike. Correspondingly, a wind-driven current pool and an annular current are applied in a 3-D numerical model respectively to study the effect of vertical eddy viscosity coefficient (Av). The computed result shows that the absolute value of Av is inversely proportional to that of horizontal velocity, and the vertical gradient value of Av determines the vertical distribution of horizontal velocity. The distrib 相似文献
619.
裂变径迹定年法Zeta常数的测定 总被引:4,自引:0,他引:4
用第6届国际裂变径迹测定地质年代学术讨论会推荐的三种锆石、两种磷灰石和一种榍石国际年龄标样对Zeta值进行了系统刻度,对NBS-SRM612铀标准玻璃测定的锆石Zeta值为(335.6±8.2)acm ̄2,磷灰石(343.3±14.6)acm ̄2,榍石为(347.7±15.8)acm ̄2,三种矿物之间的Zeta值无很大差异,统一的加权平均Zeta值为(339.2±6.6)acm ̄2。 相似文献
620.
新元古代(650MaB.P.)是地球演化历史上最重大的转折时期之一,这一时期地球气候的模拟研究,对于了解气候变迁、现代气候的形成、自然地理环境的演变有重要意义; 在地质学上,对于地层划分和对比,地壳演化研究以及矿产资源成因和探测都有重要意义。本文在借鉴国内外已有研究的基础上,应用MPM2——一个中等复杂程度的地球系统模式(EMIC),通过太阳常数和大气二氧化碳浓度的敏感性试验,对新元古代地球气候进行了模拟研究,结果表明在650Ma前,地球全球平均温度一直低于零度,即地球一直是“雪球”,直到650Ma前,全球平均温度大于零,“雪球”消融,雪球时代才结束。 相似文献