~(87)Sr原子束的塞曼减速器磁场的研究   总被引：1，自引：0，他引：1  

王心亮  阮军  马杰  郭文阁  任兆玉  张首刚 《时间频率学报》2008,31(2)

应用塞曼效应和多普勒冷却的理论,结合实验条件计算了87Sr原子束塞曼减速器的磁场.数值模拟给出了磁场线圈的层数、圈数、供电电流和其他参数,并据此制作了纺锤状线圈.测量结果表明,研制的纺锤状线圈的轴向磁场满足87Sr光晶格钟实验的要求.  相似文献   

非稳态相移法叠前深度偏移   总被引：5，自引：3，他引：2  

刘超颖  王成祥 《地球物理学进展》2004,19(3):543-546

介绍一种能够适应介质速度横向变化的非稳态相移算子及其叠前深度偏移方法．为了克服常规相移偏移算法中要求速度横向不变的缺点,出现了基于非稳态滤波器理论的非稳态相移算子,即PSPI算子、NSPS算子和SNPS算子,其中SNPS算子是将前二者结合起来的一种对称的非稳态相移算子,它比前二者具有更高的精度和稳定性．为了提高运算速度,基于非稳态相移算子的叠前深度偏移算法采取了分片均匀近似的策略,Marmousi模型的叠前深度偏移结果证明了该算法的可行性和有效性。  相似文献   

中国工业产业空间转移及中部地区发展对策研究   总被引：11，自引：0，他引：11  

樊新生  李小建 《地理与地理信息科学》2004,20(2):64-68

定量研究表明,中国大多数工业产业重心并未按人们的预期发生相应的空间转移。东部地区所有产业都占有较大份额,其中相当大一部分仍存在继续向东部地区转移的趋势。通过产业转移的影响因素分析,笔者认为产业转移是多种因素综合作用的结果,短时期内沿海的传统产业向中西部大规模转移的条件还不成熟。中部地区应该从多方面重新审视自己的比较优势,制定相应的经济发展对策。  相似文献   

Modeling river dune development and dune transition to upper stage plane bed          下载免费PDF全文

Suleyman Naqshband  Olav van Duin  Jan Ribberink  Suzanne Hulscher 《地球表面变化过程与地形》2016,41(3):323-335

Large asymmetric bedforms known as dunes commonly dominate the bed of sand rivers. Due to the turbulence generation over their stoss and lee sides, dunes are of central importance in predicting hydraulic roughness and water levels. During floods in steep alluvial rivers, dunes are observed to grow rapidly as flow strength increases, undergoing an unstable transition regime, after which they are washed out in what is called upper stage plane bed. This transition of dunes to upper stage plane bed is associated with high transport of bed sediment in suspension and large decrease in bedform roughness. In the present study, we aim to improve the prediction of dune development and dune transition to upper stage plane bed by introducing the transport of suspended sediment in an existing dune evolution model. In addition, flume experiments are carried out to investigate dune development under bed load and suspended load dominated transport regimes, and to get insight in the time scales related to the transition of dunes to upper stage plane bed. Simulations with the extended model including the transport of suspended sediment show significant improvement in the prediction of equilibrium dune parameters (e.g. dune height, dune length, dune steepness, dune migration rate, dune lee side slope) both under bed load dominant and suspended load dominant transport regimes. The chosen modeling approach also allows us to model the transition of dunes to upper stage plane bed which was not possible with the original dune evolution model. The extended model predicts change in the dune shapes as was observed in the flume experiments with decreasing dune heights and dune lee slopes. Furthermore, the time scale of dune transition to upper stage plane bed was quite well predicted by the extended model. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Seismic response of deep Quaternary sediments in historical center of L’Aquila City (central Italy)     

《Soil Dynamics and Earthquake Engineering》2016

On 6 April 2009 a M_w=6.1 earthquake produced severe destruction and damage over the historic center of L’Aquila City (central Italy), in which the accelerometer stations AQK and AQU recorded a large amount of near-fault ground motion data. This paper analyzes the recorded ground motions and compares the observed peak accelerations and the horizontal to vertical response spectral ratios with those revealed from numerical simulations. The finite element method is considered herein to perform dynamic modeling on the soil profile underlying the seismic station AQU. The subsurface model, which is based on the reviewed surveys that were carried out in previous studies, consists of 200–400 m of Quaternary sediments overlying a Meso-Cenozoic carbonate bedrock. The Martin-Finn-Seed's pore-water pressure model is used in the simulations. The horizontal to vertical response spectral ratio that is observed during the weak seismic events shows three predominant frequencies at about 14 Hz, 3 Hz and 0.6 Hz, which may be related to the computed seismic motion amplification occurring at the shallow colluvium, at the top and base of the fluvial-lacustrine sequence, respectively. During the 2009 L’Aquila main shock the predominant frequency of 14 Hz shifts to lower values probably due to a peculiar wave-field incidence angle. The predominant frequency of 3 Hz shifts to lower values when the earthquake magnitude increases, which may be associated to the progressive softening of soil due to the excess pore-water pressure generation that reaches a maximum value of about 350 kPa in the top of fluvial-lacustrine sequence. The computed vertical peak acceleration underestimates the experimental value and the horizontal to vertical peak acceleration ratio that is observed at station AQU decreases when the earthquake magnitude increases, which reveals amplification of the vertical component of ground motion probably due to near-source effects.  相似文献   

化学位移 MRI 对初诊2型糖尿病患者肝脏脂肪沉积的量化研究?          下载免费PDF全文

高翔  袁雁雯  靳二虎  张洁  洪旭  杨正汉  马大庆 《CT理论与应用研究》2016,(4):385-392

目的：探讨初诊2型糖尿病（T2DM）患者肝脏脂肪含量变化及相关因素。方法：收集初诊的 T2DM 患者（观察组）69例（女性16人,男性53人,平均年龄为（44.4±12.9）岁）及身体健康者（对照组）32例（女性15人,男性17人,平均年龄为（42.7±9.6）岁）。采用化学位移磁共振同、反相位成像技术分别测量肝脏各段的脂肪含量,比较两组间肝脏脂肪含量及 T2DM组肝脏各段的脂肪含量差异;分析 T2DM 组肝脏脂肪含量与体重、身体质量指数、腰围、臀围、腰臀比、腹壁皮下脂肪含量、腹腔脂肪含量、空腹血糖、空腹胰岛素、空腹 C 肽、血中总胆固醇、甘油三酯、高密度脂蛋白及低密度脂蛋白的相关性。结果：T2DM 组及健康对照组肝脏平均脂肪分数分别为19.55％±3.95％及15.74％±2.94％,差异有统计学意义（t =5.402,P <0.05）。T2DM 组肝脏Ⅰ至Ⅷ段的脂肪分数分别为18.67％±5.28％、19.52％±4.22％、19.37％±4.81％、19.63％±3.67％、19.39％±4.15％、19.44％±4.18％、19.67％±4.50％和20.43％±4.38％,各段的脂肪分数差异没有统计学意义（F =0.800,P >0.05）;T2DM 组肝脏平均脂肪分数与体重（r =0.316）、身体质量指数（r =0.302）、腰围（r =0.291）、臀围（r =0.279）、腹壁皮下脂肪含量（r =0.310）、空腹胰岛素（r =0.265）及空腹 C 肽（r =0.268）呈正相关（P <0.05）,而与腰臀比（r =0.195）、腹腔脂肪含量（r =0.117）、空腹血糖（r =-0.039）、血中总胆固醇（r =-0.146）、甘油三酯（r =0.240）、高密度脂蛋白（r =0.012）及低密度脂蛋白（r =-0.137）没有相关性（P >0.05）。结论：初诊 T2DM 组的肝脏脂肪含量高于健康对照组,并且与体重、身体质量指数、腰围、臀围、腹壁皮下脂肪含量、空腹胰岛素及空腹 C 肽呈正相关。  相似文献   

海岸沙丘形态对台风响应的初步观测     

董玉祥  黄德全  张雪琴 《地理研究》2016,35(10):1925-1934

基于2014年第10号强台风“麦德姆”登陆前后福建平潭岛海岸前丘、爬坡沙丘、横向沙丘和沙席等海岸沙丘形态的高精度测量数据,分析了典型海岸沙丘在台风登陆前后的形态变化,初步总结了华南海岸风沙地貌形态的台风响应模式。结果表明：4类典型海岸沙丘在台风“麦德姆”登陆前后的形态变化较大且主要是以体积减小和沙丘高度降低为主,测点高程的平均变化值为0.22 m,样区海岸沙丘体积平均减小0.59%,测点高程变化的平均值为高度降低0.05 m,但不同类型海岸沙丘及其不同部位的响应变化并非一致且存在较大差异,此与台风“麦德姆”的强侵蚀力及不同海岸沙丘的形态及其下伏地形、表面植被分布等的差异有关。  相似文献   

揭西石肚溪河谷壶穴的形态与成因     

曾兰华  蔡锦城  欧先交 《热带地理》2016,36(4):619-625

揭西石肚溪壶穴的成因此前曾引起广东地学界的争鸣。为了对该问题提供更多新的证据材料,详细测量了该河流河谷232 处壶穴的形态特征并进行统计分析。结果表明：壶穴的大小、深度和形态等在河床纵剖面上的分布没有明显规律;椭圆形和长条形壶穴较为多见;壶穴基本沿节理发育,且长轴走向与主节理即河谷的走向一致;部分壶穴内壁和底部分别可见擦痕和碎屑。壶穴的形成发育受河床基岩岩性、地质构造、河床比降、流水作用等多种因素影响,尤其是节理的控制作用非常明显。壶穴为山区河床上流水侵蚀的产物,与冰川作用无关。壶穴在河床具有一定比降、河水流速较快、适量节理发育的山区石质河床中出现是很正常的现象。  相似文献   

考虑电流型畸变的大地电磁三维反演          下载免费PDF全文

李鑫  白登海  闫永利  马晓冰  孔祥儒 《地球物理学报》2016,59(6):2302-2315

大地电磁测深(MT)的观测数据易受到由近地表小尺度非均匀体或地形起伏引起的电流型畸变干扰,消除或压制这种干扰对获取可靠的深部电性结构至关重要.当区域结构为二维时,电流型畸变可采用张量分解等方法予以消除或压制.当区域结构为三维时,畸变问题更加复杂和严重,传统张量分解方法往往效果不佳或无效,严重地制约了MT三维反演技术的实用性.对此,本文提出一种考虑电流型畸变的MT三维反演算法,将完整的电流型畸变参数引入到目标函数,并采用非线性共轭梯度法与电阻率参数同时反演,从而达到压制畸变的目的.该算法有两个关键点:一是通过分析实测数据所遭受畸变的分布特征,在目标函数中对其进行有效约束;二是在迭代过程中,通过自适应地调整双正则化因子保障算法的稳定和效率.理论模型测试结果显示,常规三维反演算法不能合理解释数据中的畸变成分,而只能通过引入虚假异常体强制地拟合受畸变数据,从而造成电阻率模型严重失真.与之相比,本文算法能够在反演中自动求解各测点所受到的畸变,获得更接近真实的电阻率模型.  相似文献   

近50年来中国经济重心移动路径分析   总被引：58，自引：3，他引：55  

乔家君  李小建 《地域研究与开发》2005,24(1):12-16

中国经济重心主要分布在豫皖交界处,1978年前大体位于安徽省西部,1978年后几乎全部分布于河南省东南部;1952～2000年间经济重心移动轨迹呈"士"字型态势发展;近50年来总移动趋势为由北向南,总移动距离为173.14km;每年移动距离和方向与中国区域经济发展政策有密切关系。  相似文献