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1.
重要矿产资源尤其是战略性矿产的安全保障历来是世界各国关注的重点之一。关于战略性矿产的研究,欧美等发达国家和地区已经建立了一套比较成熟的理论和方法,并不定期更新战略性矿产目录。而我国对于战略性矿产的研究相对较晚,尚未形成统一的内涵意义、评价体系和方法。为此,本文在重新界定战略性矿产内涵基础上构建供应风险-经济价值二维评价体系,运用定性和定量相结合的评价方法综合评价我国主要矿产资源的战略性。 相似文献
2.
In most limit state design codes, the serviceability limit checks for drilled shafts still use deterministic approaches. Moreover, different limit states are usually considered separately. This paper develops a probabilistic framework to assess the serviceability performance with the consideration of soil spatial variability in reliability analysis. Specifically, the performance of a drilled shaft is defined in terms of the vertical settlement, lateral deflection, and angular distortion at the top of the shaft, corresponding to three limit states in the reliability analysis. Failure is defined as the event that the displacements exceed the corresponding tolerable displacements. The spatial variability of soil properties is considered using random field modeling. To illustrate the proposed framework, this study assesses the reliability of each limit state and the system reliability of a numerical example of a drilled shaft. The results show the system reliability should be considered for the serviceability performance. The importance measures of the random variables indicate that the external loads, the performance criteria, the model errors of load transfer curves and soil strength parameter are the most important factors in reliability analysis. Moreover, it is shown that the correlation length and coefficient of variation of soil strength can exert significant impacts on the calculated failure probability. 相似文献
3.
In reliability analysis, the crude Monte Carlo method is known to be computationally demanding. To improve computational efficiency, this paper presents an importance sampling based algorithm that can be applied to conduct efficient reliability evaluation for axially loaded piles. The spatial variability of soil properties along the pile length is considered by random field modeling, in which a mean, a variance, and a correlation length are used to statistically characterize a random field. The local averaging subdivision technique is employed to generate random fields. In each realization, the random fields are used as inputs to the well-established load transfer method to evaluate the load–displacement behavior of an axially loaded pile. Failure is defined as the event where the vertical movement at the pile top exceeds the allowable displacement. By sampling more heavily from the region of interest and then scaling the indicator function back by a ratio of probability densities, a faster rate of convergence can be achieved in the proposed importance sampling algorithm while maintaining the same accuracy as in the crude Monte Carlo method. Two examples are given to demonstrate the accuracy and the efficiency of the proposed method. It is shown that the estimate based on the proposed importance sampling method is unbiased. Furthermore, the size of samples can be greatly reduced in the developed method. 相似文献
4.
莱州湾西部地区环境条件的联合概率分析 总被引：2，自引：0，他引：2
在海洋环境条件联合概率分析理论的基础上，对莱州湾西部海的羊角沟地区进行了综合环境条件的联合概率分析，以模拟技术中的重点抽样法为理论根据峄该地区为水位ζ、风速Ｕ及波高Ｈ的三维随机变量作了统计分析，得出了多年一遇的各环境条件联合出现的数值，模拟结果令人满意。 相似文献
5.
集合卡曼滤波由于易于使用而被广泛地应用到陆面数据同化研究中,它是建立在模型为线性、误差为正态分布的假设上,而实际土壤湿度方程是高度非线性的,并且当土壤过干或过湿时会发生样本偏斜.为了全面评估它在同化表层土壤湿度观测来反演土壤湿度廓线的性能,特引入不需要上述假设的采样重要性重采样粒子滤波,比较非线性和偏斜性对同化算法的影响.结果显示:不管是小样本还是大样本,集合卡曼滤波都能快速、准确地逼近样本均值,而粒子滤波只有在大样本时才能缓慢地趋近;此外,集合卡曼滤波的粒子边缘概率密度及其偏度和峰度与粒子滤波完全不同,前者粒子虽不完全满足正态分布,但始终为单峰状态,而后者粒子随同化推进经历了单峰到双峰再到单峰的变化. 相似文献
6.
In this paper, we discuss several possible approaches to improving the performance of the ensemble Kalman filter (EnKF) through improved sampling of the initial ensemble. Each of the approaches addresses a different limitation of the standard method. All methods, however, attempt to make the results from a small ensemble as reliable as possible. The validity and usefulness of each method for creating the initial ensemble is based on three criteria: (1) does the sampling result in unbiased Monte Carlo estimates for nonlinear flow problems, (2) does the sampling reduce the variability of estimates compared to ensembles of realizations from the prior, and (3) does the sampling improve the performance of the EnKF? In general, we conclude that the use of dominant eigenvectors ensures the orthogonality of the generated realizations, but results in biased forecasts of the fractional flow of water. We show that the addition of high frequencies from remaining eigenvectors can be used to remove the bias without affecting the orthogonality of the realizations, but the method did not perform significantly better than standard Monte Carlo sampling. It was possible to identify an appropriate importance weighting to reduce the variance in estimates of the fractional flow of water, but it does not appear to be possible to use the importance weighted realizations in standard EnKF when the data relationship is nonlinear. The biggest improvement came from use of the pseudo-data with corrections to the variance of the actual observations. 相似文献
7.
王朗月 《测绘与空间地理信息》1999,22(4):19-20
结合作者在生产过程中开发数字摄影测量辅助系统的经验,对开发数字摄影测量辅助系统的目标、原则、注意事项以及目前该系统的功能等进行了参考性介绍,并根据生产应用的经验,对开发数字产品生产辅助系统的重要性提出了自己的观点。 相似文献
8.
Role of competition in restoring resource poor arid systems dominated by invasive grasses 总被引：2，自引：1，他引：1
Understanding the role competition intensity and importance play in directing vegetation dynamics is central to developing restoration strategies, especially in resource poor environments. We hypothesized 1) competition would be intense among invasive and native species, but 2) competition would be unimportant in explaining variation in target plant biomass and survivorship relative to other factors driving these variables. We performed a two year addition series field experiment to quantify competition intensity and importance. Densities of two invasive (cheatgrass and medusahead) and two native (Sandberg’s bluegrass and bluebunch wheatgrass) species were arranged in monocultures and mixtures of two, three and four species, producing varying total densities and species proportions. Multiple linear regression models predicting individual plant biomass and survivorship were developed. Based on biomass, competition intensity coefficients ranged from −0.38 to 0.63 with R2 < 0.06. All survivorship data produced poor fitting regression models (R2 < 0.05). Our results suggest neither competition intensity nor importance influenced plant dominance in resource poor environments during the two years of establishment. Land managers may be more successful at restoration of resource poor ecosystems by overcoming abiotic barriers to plant establishment rather than focusing on plant-plant interactions. 相似文献
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