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211.
This paper deals with ground-hugging, gas–pyroclast currents from explosive volcanic eruptions and their deposits. Key field observations and laboratory determinations are proposed to relate specific deposit types with flow regimes and particle concentration in the transport and depositional systems. Three relevant flow scenarios and corresponding deposit types have been recognized from a survey of pyroclastic successions of the Vulsini Volcanic District (central Italy): (1) dilute, turbulent, pyroclastic currents producing normally or multiply graded beds by direct suspension sedimentation; (2) concentrated bedload regions beneath suspension currents, depositing inversely graded beds by traction carpet sedimentation; (3) self-sustained, high particle concentration, laminar, mass flows developing massive, poorly sorted bodies, with opposite grading of coarse lithic and pumice clasts, overlying fine-grained, inversely graded, basal layers. Main distinguishing criteria include the occurrence and pattern of clast grading, clast–thickness relationships, grain size, ash matrix componentry and pyroclast size–density relationships. Downcurrent and temporal transitions among identified flow scenarios are likely to occur for changing energy conditions and gas–pyroclast ratio both on regional and local scales. The nature and efficiency of magma fragmentation, volatile content, conduit geometry (which determine the characteristics of the erupted mixture and possible lateral blast component at the vent), and the angle of incidence of the column collapse, are suggested as the main factors controlling the generation of one type over the other at flow inception. Dilute, fine-grained, overpressured eruption clouds are thought to favor the formation of low particle concentration turbulent currents. Column collapse over slightly inclined volcano slopes, causing a high degree of compression of the collapsing mixture and of gas expulsion, would favor the generation of high particle concentration pyroclastic currents. 相似文献
212.
挡块对斜拉桥抗震性能的影响 总被引:1,自引:0,他引:1
本文基于现有永和斜拉桥挡块设施的严重破损,引入非线性挡块元,采用有限元计算分析挡块对斜拉桥抗震性能的影响。结果表明,斜拉桥塔根挡块的调协对减震效果影响不大,但挡块具有减小塔顶残余变形的有利作用。鉴于此,对永和斜拉桥挡块进行修复是必要的。 相似文献
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The environmental impact of small-scale gold mining in Ghana: identifying problems and possible solutions 总被引:2,自引:0,他引:2
Gavin Hilson 《The Geographical journal》2002,168(1):57-72
This paper examines the environmental impacts of small-scale gold mining in Ghana, and prescribes a series of recommendations for improving environmental performance in the industry. Since the enactment of the Small Scale Gold Mining Law in 1989, which effectively legalized small-scale gold mining in the country, industrial operations, collectively, have made important contributions to national gold output, foreign exchange earnings and employment. Accompanying this pattern of socio-economic growth, however, have been increased environmental complications – namely, mercury pollution and land degradation. The Ghanaian Minerals Commission has been burdened with most of the jurisdictional responsibilities related to small-scale mining, but with a staff of only 35–40 people working with a pool of highly obsolete research resources, it is clearly incapable of facilitating sufficient environmental improvement on its own. It is concluded that marked environmental improvements can only be achieved if: (1) assistance is provided to the Minerals Commission from local governmental bodies and academic units; (2) industry-specific environmental management tools and strategies are designed and implemented; (3) concerted effort is made to prospect for deposits suitable for small-scale gold mining, a key to preventing unnecessary exploration; and (4) a nation-wide industrial mercury study is commissioned, and a mercury retorting programme is implemented. 相似文献
216.
One of the main factors that affects the performance of MLP neural networks trained using the backpropagation algorithm in mineral-potential mapping isthe paucity of deposit relative to barren training patterns. To overcome this problem, random noise is added to the original training patterns in order to create additional synthetic deposit training data. Experiments on the effect of the number of deposits available for training in the Kalgoorlie Terrane orogenic gold province show that both the classification performance of a trained network and the quality of the resultant prospectivity map increasesignificantly with increased numbers of deposit patterns. Experiments are conducted to determine the optimum amount of noise using both uniform and normally distributed random noise. Through the addition of noise to the original deposit training data, the number of deposit training patterns is increased from approximately 50 to 1000. The percentage of correct classifications significantly improves for the independent test set as well as for deposit patterns in the test set. For example, using ±40% uniform random noise, the test-set classification performance increases from 67.9% and 68.0% to 72.8% and 77.1% (for test-set overall and test-set deposit patterns, respectively). Indices for the quality of the resultant prospectivity map, (i.e. D/A, D × (D/A), where D is the percentage of deposits and A is the percentage of the total area for the highest prospectivity map-class, and area under an ROC curve) also increase from 8.2, 105, 0.79 to 17.9, 226, 0.87, respectively. Increasing the size of the training-stop data set results in a further increase in classification performance to 73.5%, 77.4%, 14.7, 296, 0.87 for test-set overall and test-set deposit patterns, D/A, D × (D/A), and area under the ROC curve, respectively. 相似文献
217.
Use of GIS layers, in which the cell values represent fuzzy membership variables, is an effective method of combining subjective geological knowledge with empirical data in a neural network approach to mineral-prospectivity mapping. In this study, multilayer perceptron (MLP), neural networks are used to combine up to 17 regional exploration variables to predict the potential for orogenic gold deposits in the form of prospectivity maps in the Archean Kalgoorlie Terrane of Western Australia. Two types of fuzzy membership layers are used. In the first type of layer, the statistical relationships between known gold deposits and variables in the GIS thematic layer are used to determine fuzzy membership values. For example, GIS layers depicting solid geology and rock-type combinations of categorical data at the nearest lithological boundary for each cell are converted to fuzzy membership layers representing favorable lithologies and favorable lithological boundaries, respectively. This type of fuzzy-membership input is a useful alternative to the 1-of-N coding used for categorical inputs, particularly if there are a large number of classes. Rheological contrast at lithological boundaries is modeled using a second type of fuzzy membership layer, in which the assignment of fuzzy membership value, although based on geological field data, is subjective. The methods used here could be applied to a large range of subjective data (e.g., favorability of tectonic environment, host stratigraphy, or reactivation along major faults) currently used in regional exploration programs, but which normally would not be included as inputs in an empirical neural network approach. 相似文献
218.
试论少数民族地区的生态化旅游开发 总被引:6,自引:7,他引:6
生态化旅游开发是有别于传统的旅游开发的一种可持续开发与发展的理念,它追求的是生态、社会和经济综合效益的最大化,应该成为旅游开发与旅游业发展的主要方向。民族地区的旅游开发具有特定的区域生态特征。其生态化旅游开发的核心问题是旅游环境容量的确定与旅游环境的保护、民族特色与民族文化的弘扬以及注重旅游经济发展过程中旅游业的拉动效应及其依托性之间的关系处理等。 相似文献
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