Effect of Initial Stress Conditions on the Threshold Shear Strain of Nanjing＇s Saturated Fine Sand   总被引：2，自引：0，他引：2  

Chen Guoxing  Pan Hua 《中国地震研究》2010,24(1):128-136

By using GDS dynamic hollow cylinder torsional apparatus,a series of cyclic torsional triaxial tests under complex initial consolidation condition are performed on Nanjing saturated fine sand. The effects of the initial principal stress direction α0,the initial ratio of deviatoric stress η0,the initial average effective principal stress p0 and the initial intermediate principal stress parameter b0 on the threshold shear strain γt of Nanjing saturated fine sand are then systematically investigated. The results show that γt increases as η0,p0 and b0 increase respectively,while the other three parameters remain constant. α0 has a great influence on γt,which is reduced when α0 increases from 0° to 45° and increased when α0 increases from 45° to 90°. The effect of α0 on γt plays a leading role and the effect of η0 will weaken when α0 is approximately 45°.  相似文献   

Do 2-amino-3,8-dimethylimidazo[4,5-f] quinoxaline data support the conclusion of threshold carcinogenic effects?     

Chao Chen  Kate Z. Guyton 《Stochastic Environmental Research and Risk Assessment (SERRA)》2008,22(4):487-494

The objectives of this paper are to (1) reexamine the data that were used to support the conclusion of a threshold effect for 2-amino-3,8-dimethylimidazo[4,5-f] quinoxaline (MeIQx)-induced initiation and carcinogenicity at low doses in the rat liver, and (2) discuss issues and uncertainties about assessing cancer risk at low doses. Our analysis is part of an effort to understand proper interpretation and modeling of data related to cancer mechanisms and is not an effort to develop a risk assessment for this compound. The data reanalysis presented herein shows that the low-dose initiation activity of MeIQx, which can be found in cooked meat, cannot be dismissed. It is argued that the threshold effect for carcinogenic agents cannot be determined by statistical non-significance alone; more relevant biological information is required. A biologically motivated procedure is proposed for data analyses. The concept and procedure that are appropriate for analyzing MeIQx data are equally applicable to other compounds with comparable data. The ideas and approaches in this presentation are those of the authors and do not necessarily represent the positions or policies of the USEPA.  相似文献   

音频大地电磁法强干扰压制试验研究   总被引：9，自引：6，他引：3       下载免费PDF全文

汤井田  刘子杰  刘峰屹  肖晓  李晋  张林成  谢勇 《地球物理学报》2015,58(12):4636-4647

压制非相关噪声已经有多种比较成熟的技术方法,如多次叠加、远参考、Robust阻抗估计等,但这些方法对压制矿集区存在的强电磁噪声(通常属于相关噪声)基本没有效果.本文在前期提出的针对矿集区大地电磁强噪声压制的数学形态滤波基础上,开展了音频大地电磁法强干扰压制的试验研究.在四川西昌某AMT测点附近,布置了大功率电磁发送站,通过接地导线向地下供20A的方波电流,同时在供电期和间歇期用凤凰公司MTU-5A仪器观测4个水平分量的电磁场.在室内采用数学形态滤波和阈值法相结合的手段在时间域对含有人工源强干扰的电磁场数据进行处理,以视电阻率对比、频谱和极化方向等参数对处理效果进行评价.结果表明:结构元素恰当的数学形态滤波可以对人工源强干扰进行识别、去除,阈值法则可以消除形态滤波后的脉冲干扰,二者结合可以有效地压制AMT中的强干扰噪声,提高数据可靠性.本文结果对AMT实际资料处理具有重要的实用和参考价值.  相似文献   

Costly stakeholder participation creates inertia in marine ecosystems     

《Marine Policy》2017

Ecosystems often shift abruptly and dramatically between different regimes in response to human or natural disturbances. When ecosystems tip from one regime to another, the suite of available ecosystem benefits changes, impacting the stakeholders who rely on these benefits. These changes often create some groups who stand to incur large losses if an ecosystem returns to a previous regime. When the participation cost in the decision-making process is extremely high, this can “lock in” ecosystem regimes, making it harder for policy and management to shift ecosystems out of what the majority of society views as the undesirable regime. Public stakeholder meetings often have high costs of participation, thus economic theory predicts they will be dominated by extreme views and often lead to decisions that do not represent the majority viewpoint. Such extreme viewpoints can create strong inertia even when there is broad consensus to manage an ecosystem towards a different regime. In the same manner that reinforcing ecological feedback loops make it harder to exit an ecosystem regime, there are decision-making feedback loops that contribute additional inertia.  相似文献   

Spatial and temporal variations in sediment accumulation and their impacts on coral communities in the Sanya Coral Reef Reserve,Hainan, China     

《Deep Sea Research Part II: Topical Studies in Oceanography》2013

This study investigated the spatial and temporal variations of sediment accumulation and their impacts on coral communities in four sites at two or three depths (3 m, 6 m and 9 m) at the Sanya Coral Reef Reserve by deploying sediment traps on the sea floor during 2007–2009. Rainfall and typhoon events, which appeared to control sediment accumulation in the sea floor of the coral reef, were positively correlated with total sediment and sand-sized (i.e. 63–2000 µm) sediment accumulation. Sediment accumulation rate significantly decreased with the distance far away from the coast in Sanya. The mean sediment accumulation rates in Ximaozhou, Luhuitou and Xiaodonghai during 2007 to 2009 were close to 20 mg cm⁻² d⁻¹, and they were significantly higher than that in Yalongwan, probably as a result of terrestrial soil erosion caused by strong coast human activities (e.g. coastal construction, dredging and hillside clearing). Correlation analysis revealed that silt-clay-sized sediment accumulation rate was highly negatively correlated with total live coral cover and coral cover in some taxa, such as Montipora and branching Porites. whereas, Diploastrea heliopora was positively correlated with silt-clay-sized sediment accumulation. Correlation analysis also suggested that silt-clay-sized sediment accumulation had a higher efficiency in predicting the spatial variation of total live coral cover in Sanya than did the total sediment accumulation. Based on this investigation, we conclude that high rates of sediment accumulation pose a severe threat to the Sanya Coral Reef Reserve, highlighting the importance of integrated watershed management practices in the Sanya Coral Reef Reserve.  相似文献   

中国西北不同沙源地区起沙阈值的对比分析与研究   总被引：8，自引：1，他引：7  

朱好  张宏升 《气象学报》2010,68(6):977-984

沙尘粒子进入大气所受到的空气动力因子和阴力因子分别用摩擦速度u,和临界摩擦速度u(?)来定量表示。临界摩擦速度是影响沙尘释放和输送的重要参数。文中利用2004年春季浑善达克沙地和2006年春季黄土高原与戈壁3种不同沙源地区开展的微气象学和沙尘暴观测资料,分析了沙尘天气过程中沙尘浓度随摩擦速度的变化关系,估算了临界起沙摩擦速度u(?),并与不同学者给出的结果进行对比。结果表明:沙尘天气过程中,临近起沙阶段,沙尘浓度较低,摩擦速度增加快,为起沙阶段提供了利的动力条件;起沙阶段,动力和热力湍流混合使得沙尘浓度迅速升高;沙尘天气减弱阶段,由于重力沉隆作用,沙尘浓度随摩擦速度近似呈线性递减关系。以空气中的沙尘浓度大于200μg/m~3作为起沙的判据条件,得到浑善达克沙地和戈壁地区的临界起沙摩擦速度分别为0.6和0.45 m/s。由于受地形及非均匀下垫面的影响,黄土高原的临界起沙摩擦速度与风向有关。西北风反映了来自西北方向毛乌素沙地的影响,u(?)为0.35 m/s,偏南风时对应观测站点南部黄土丘陵沟壑区的起沙特征,u(?)为0.7 m/s。黄土丘陵沟壑区的地表粗糙度值和土壤粒子间的相互作用力较大,从而导致了较大的临界起沙摩擦速度。不同的起沙阈值条件和风速频率分布与沙尘释放能力的不同相对应。  相似文献   

无锡极端气温事件的气候分析   总被引：1，自引：1，他引：0  

阮蔚琳  钱永甫  马志强  章志芹  李琼川 《气象科学》2008,28(Z1)

为了解气候增暖情况下,无锡地区极端最高和最低气温的气候特征,用阈值检测方法对无锡市1959-2007年的日最高气温、日最低气温进行了研究,分析了极端事件的发生规律,得出主要结论:(1)从年代际看,不论是日最高气温或日最低气温,2001-2007年偏高事件最多,偏低事件最少.(2)最低气温从1980s初开始稳定上升.(3)在3、5、7、12月比较容易出现异常的气温.月内异常气温出现次数多少主要与季节更替和天气系统的转换有关.(4)日最低气温不论冬春夏秋,都是明显升高.日最高气温冬、春、秋季呈升高趋势,而夏季震荡加大.即在夏季,日最高气温偏高的事件增多,日最高气温偏低的事件也增多.  相似文献   

夏季极端高温对武汉市人口超额死亡率的定量评估   总被引：4，自引：0，他引：4  

杨宏青  陈正洪  谢森  叶殿秀  龚洁 《气象与环境学报》2013,29(5):140-143

利用1998-2008年夏季（6-8月）武汉市居民逐日死亡人数（率）及同期逐日气象要素,分析极端高温对超额死亡率的影响程度及其阈值,采用逐步回归法建立定量评估模型,并进行预报效果检验和典型年回代检验,以利于开展高温对健康影响的评估及医疗气象预报。结果表明：武汉夏季人群超额死亡率可以定量表述;极端高温对超额死亡率影响最大,湿度、气压、风速几乎没有影响;超额死亡率随日最高气温升高呈指数规律增加,高温致超额死亡的阈值为35.0 ℃,“热日”比“非热日”平均死亡率高出50.7 %;采用日最高气温≥35 ℃的有效累积温度及当日平均气温建立的超额死亡率评估模型对2003年夏季一次高温热浪过程的回代试验、2007年和2008年夏季的评估试验效果较好,表明该模型可用于实际评估业务中。  相似文献   

Characterizing bi-temporal patterns of land surface temperature using landscape metrics based on sub-pixel classifications from Landsat TM/ETM+     

《International Journal of Applied Earth Observation and Geoinformation》2015

Landscape patterns in a region have different sizes, shapes and spatial arrangements, which contribute to the spatial heterogeneity of the landscape and are linked to the distinct behavior of thermal environments. There is a lack of research generating landscape metrics from discretized percent impervious surface area data (ISA), which can be used as an indicator of urban spatial structure and level of development, and quantitatively characterizing the spatial patterns of landscapes and land surface temperatures (LST). In this study, linear spectral mixture analysis (LSMA) is used to derive sub-pixel ISA. Continuous fractional cover thresholds are used to discretize percent ISA into different categories related to urban land cover patterns. Landscape metrics are calculated based on different ISA categories and used to quantify urban landscape patterns and LST configurations. The characteristics of LST and percent ISA are quantified by landscape metrics such as indices of patch density, aggregation, connectedness, shape and shape complexity. The urban thermal intensity is also analyzed based on percent ISA. The results indicate that landscape metrics are sensitive to the variation of pixel values of fractional ISA, and the integration of LST, LSMA. Landscape metrics provide a quantitative method for describing the spatial distribution and seasonal variation in urban thermal patterns in response to associated urban land cover patterns.  相似文献   

沙尘天气过程临界起沙因子的研究进展   总被引：6，自引：1，他引：5  

朱好  张宏升 《地球科学进展》2011,26(1):30-38

临界起沙摩阻风速u*t或临界起沙风速ut是土壤可蚀性的度量指标之一,是表征沙尘颗粒进入大气的重要因子,也是沙尘暴预报模块中最重要的参数之一.野外实验、模式参数化和统计理论是近年来确定临界起沙摩阻风速u*t或临界起沙风速ut的3种主要方法.利用不同方法获取u*t或ut的结果可以相互校验.野外实验的实测结果可为获取适用的起...  相似文献