Magnetic fabrics and rock magnetism of Proterozoic dike swarm from the southern São Francisco Craton, Minas Gerais State, Brazil     

Maria Irene Bartolomeu Raposo  Alexandre O. Chaves  Paulo Lojkasek-Lima  Manoel Souza D'Agrella-Filho  Wilson Teixeira   《Tectonophysics》2004,378(1-2):43-63

Magnetic fabric and rock magnetism studies were performed on 32 mafic dikes of a Proterozoic dike swarm from the southern São Francisco Craton (SFC; Minas Gerais State, SE Brazil). Magnetic anisotropies were determined by applying anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of remanent magnetization (ARM). The latter was performed imposing both anhysteretic (total (AAR) and partial pAAR)) and isothermal remanence magnetizations (AIRM). Partial anhysteretic remanence anisotropy was performed based on remanent coercivity spectra from a pilot specimen of each site. In most sites, AMS is dominantly carried by ferromagnetic minerals, however, in some sites, the paramagnetic contribution exceeds 70% of bulk susceptibility. Rock magnetism and thin section analysis allow classifying the dikes as non-hydrothermalized and hydrothermalized. Magnetic measurement shows that the mean magnetic susceptibility is usually lower than 5×10⁻³ (SI). Ti-poor titanomagnetites up to pure magnetite pseudo-single-domain (PSD) grain sizes carry the majority of magnetic fabrics for non-hydrothermalized dikes whereas coarse to fine grained Ti-poor titanomagnetites carry the majority of magnetic fabrics for hydrothermalized dikes.Three primary AMS fabrics are recognized which are coaxial with ARM fabric, except for two dikes, from both non-hydrothermalized and hydrothermalized dikes. Normal AMS fabric surprisingly is not dominant (31%). The parallelism between AMS, pAAR_0–30, pAAR_30–60 and pAAR_60–90 fabrics in the hydrothermalized dikes indicates that magnetic grains formed due to late-stage crystallization or to remobilization of iron oxides due to hydrothermal alteration after dike emplacement have acquired a mimetic fabric coaxial with the primary fabric given by coarse-grained early crystallized Ti-poor titanomagnetites. This fabric is interpreted as magma flow in which the analysis of K_max inclination permitted the inference that the dikes were fed by horizontal or subhorizontal fluxes (K_max<30°). Intermediate AMS fabric is the most important (41%) in the investigated swarm. It is interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike strike. ARM determinations for these sites also remained intermediate except for two dikes. In one of them, AIRM fabric resulted in normal AMS fabric while for the other AAR fabric resulted in inverse AMS fabric. A combination of AMS and ARM fabrics suggest that magmatic fabric for both dikes were overprinted by some late local event, probably related to Brasiliano orogenic processes after dike emplacement. InverseInverse AMS fabric is a minority (four dikes). ARM determinations also remained inverse suggesting a primary origin for inverse AMS fabric.  相似文献   

GPS广播星历参数拟合算法   总被引：25，自引：2，他引：25  

崔先强焦文海  贾小林尹豫疆 《测绘学院学报》2004,21(4):244-246

介绍了GPS广播星历参数的最小二乘估计方法。推导了相应的计算公式。计算结果表明。文中给出的公式是正确的，其拟合精度以用户距离误差(URE)示时，对预报轨道的损失小于1cm。  相似文献   

郑州强对流天气成因分析   总被引：1，自引：0，他引：1  

赵培娟 吴蓁郑世林  席世平 《河南气象》2005,(4):11-13

对2004年郑州出现的7次强对流天气过程的天气形势、影响系统及稳定度的分析结果表明：华北低涡和NW气流形势下存在着低层升温、高层降温机制,使大气层结趋于不稳定,当测站高低空温差或温度平流差达到一定量值,且近地层存在辐合系统时,易出现强对流; SW气流或高压控制时,大气高温高湿,具有较强不稳定能量,若850 hPa或地面出现辐合系统时,易产生强对流;地面湿度连续数天加大或保持在某一值域,其上空温湿24 h变化呈上趋冷下趋暖或上趋干下趋湿并达到一定量值,预示强对流的发生;700～500 hPa湿度明显减小,24 h温度露点差加大4 ℃以上,或近地层θse≥350K,中低层Δθse≥26 K,θse小值位于700 hPa或500 hPa,其厚度≥2000 m,易出现强雷雨大风; 700 hPa以下t-td≤4.3 ℃,或连续4天850 hPa t-td≤7 ℃、700 hPa t-td≤5 ℃、500 hPa t-td≤9 ℃,PW≥12,可预示短时暴雨的出现.  相似文献   

内蒙古中西部地区震源参数和场地响应反演   总被引：15，自引：0，他引：15       下载免费PDF全文

高立新  郑斯华  丁风和 《地震工程学报》2005,27(2):109-114

采用Atkinson方法，基于遗传算法，利用呼和浩特数字地震台网7个子台记录的9次质量较高的地震事件资料，计算了内蒙古中西部地区的品质因子Q值和非弹性衰减系数c（f）。在所研究的频率范围内，前者和频率．厂有很好的线形关系：Q（f）=116．8f^1.000后者与频率．厂的变化关系不明显。在此基础上采用Moya方法反演了7个地震台的场地响应和地震的震源参数，并对它们之间的相互关系进行了讨论。结果显示在所研究的频率域内7个基岩台站的场地均不同程度的存在放大作用，其次震源参数间具有较好的线性关系。本文还详细介绍了Atkinson方法和Moya方法的原理和计算步骤。  相似文献   

Origin of layering in cumulate gabbros in the Oman ophiolite: Insights from magnetic susceptibility measurements in the Wadi Sadm area     

Kiichiro  Kawamura  Takahiro  Hosono  Huda Mohamed  Allawati  Yujiro  Ogawa  Hidetsugu  Taniguchi 《Island Arc》2005,14(4):564-570

Abstract   Magnetic susceptibility and the anisotropy of magnetic susceptibility were measured on an 800-cm-thick succession of cumulate gabbro in the Sadm area of the Oman ophiolite. The section contained three distinct cumulate units. The susceptibility tends to decrease upward in each from a melanocratic layer (several tens of centimeters thick) to a leucocratic layer (a few meters thick). The susceptibility decreases in accordance with the decreasing number of magnetite grains, which are the alteration product mainly of olivine minerals. This suggests the cyclic downward accumulation of olivine in the cumulate gabbro. The apparent strain deduced from the patterns of magnetic and grain fabrics was the result mostly of simple shear, so that the layering of gabbro is understood to be formed primarily by a crystal cumulus process followed by simple shear deformation.  相似文献   

应用遥感方法研究黄河三角洲地表蒸发及其与下垫面关系   总被引：10，自引：1，他引：10  

崔亚莉  徐映雪  邵景力  张长春 《地学前缘》2005,12(Z1):159-165

文中主要应用遥感方法计算了黄河三角洲地表蒸发量及其地表特征参数。地表特征参数及其合理组合揭示出黄河三角洲下垫面的基本特征:农田植被指数和天然植被的植被指数有不同的变化规律,下垫面覆盖度低,裸地较多,地表较湿润,蒸发量较大。蒸发量时空分布主要受下垫面条件控制,滨海裸地和受人类活动影响较大的农田等地蒸发量较大,年际平均蒸发量在570~860 mm之间。  相似文献   

上扬子地壳区域性地层岩石物性力学参数与滑动层位关系研究   总被引：5，自引：2，他引：5       下载免费PDF全文

孙岩  刘德良  朱文斌  郭继春  李本亮  郭建  王锋  赵忠岩 《地质科学》2005,40(4):532-538

在中下扬子地区地壳区域性滑动层位岩石物性力学参数研究的基础上,对上扬子地区地壳的相应岩层进行了系统的定量测试。其滑动层位各种参量在塑性、韧性和粘性上所表现出特征的规律,在我国南方型数字地层中,具有一定的普遍性和可比性。同时,也揭示了地壳的构造分层性,并可作为上扬子板块浅层地质—地球物理特征分析的重要依据和建库信息。  相似文献   

Détermination expérimentale des paramètres équivalents d'un milieu poreux hétérogène en écoulements uniforme ou radial     

Charles Danquigny  Philippe Ackerer 《Comptes Rendus Geoscience》2005,337(6):563-570

Tracer tests are carried out in a heterogeneous porous medium that has a 3D correlated random distribution of the permeabilities. The fitting of numerical models provides the values of equivalent permeability and macrodispersivity characterizing a 2D homogeneous horizontal medium. Different flow configurations are studied: uniform, radial and pump and treat (doublet). The fitted parameter sets are independent of the flow type, except for the doublet. They are greater than the values predicted by stochastic theories, due to the small number of correlation lengths explored by the tracer and the limited extension of the experimental set-up. To cite this article: C. Danquigny, P. Ackerer, C. R. Geoscience 337 (2005).  相似文献   

沉降预测的非等步长灰色时变参数模型     

柳治国 《探矿工程》2005,32(3):1-4

针对经典灰色系统模型的不足，根据灰色系统理论的信息处理原则，在模型中引入遗忘因子，建立了灰色系统沉降预测的非等步长灰色时变参数模型，并在求解过程中引入遗忘因子以修正预测结果。模型充分考虑了预测系统的时变性和灰色性，从而降低对预测系统状态的预测误差。实例预测表明，灰色时变参数模型可以将工后总沉降量的预测误差控制在23%以内。误差检验结果显示，预测结果的精度等级较引入遗忘因子修正前有显著的提高。  相似文献   

基于Stacking集成学习的恒星/星系分类研究          下载免费PDF全文

李超  张文辉  李然  王俊义  林基明 《天文学报》2020,61(2):21-111

机器学习在当今诸多领域已经取得了巨大的成功,但是机器学习的预测效果往往依赖于具体问题.集成学习通过综合多个基分类器来预测结果,因此,其适应各种场景的能力较强,分类准确率较高.基于斯隆数字巡天(Sloan Digital Sky Survey,SDSS)计划恒星/星系中最暗源星等集分类正确率低的问题,提出一种基于Stacking集成学习的恒星/星系分类算法.从SDSS-DR7(SDSS Data Release 7)中获取完整的测光数据集,并根据星等值划分为亮源星等集、暗源星等集和最暗源星等集.仅针对分类较为复杂且困难的最暗源星等集展开分类研究.首先,对最暗源星等集使用10折嵌套交叉验证,然后使用支持向量机(Support Vector Machine,SVM)、随机森林(Random Forest,RF)、XGBoost(eXtreme Gradient Boosting)等算法建立基分类器模型;使用梯度提升树(Gradient Boosting Decision Tree,GBDT)作为元分类器模型.最后,使用基于星系的分类正确率等指标,与功能树(Function Tree,FT)、SVM、RF、GBDT、XGBoost、堆叠降噪自编码(Stacked Denoising AutoEncoders,SDAE)、深度置信网络(Deep Belief Network,DBN)、深度感知决策树(Deep Perception Decision Tree,DPDT)等模型进行分类结果对比分析.实验结果表明,Stacking集成学习模型在最暗源星等集分类中要比FT算法的星系分类正确率提高了将近10%.同其他传统的机器学习算法、较强的提升算法、深度学习算法相比,Stacking集成学习模型也有较大的提升.  相似文献