Fractal analysis of mingled/mixed magmas: an example from the Upper Pollara eruption (Salina Island, southern Tyrrhenian Sea, Italy)     

Rosanna De Rosa  Paola Donato  Guido Ventura   《Lithos》2002,65(3-4):299-311

Upper Pollara eruption products (13 ka, Salina Island, Italy) include both homogeneous and heterogeneous pumices resulting from mixing/mingling processes between an HK andesite and a high-SiO₂ rhyolite. Representative samples of heterogeneous pumices are collected and analyzed in order to check the correspondence between glass composition and morphological features of the mingling/mixing structures. Image analysis techniques are applied and eight grey color ranges (classes) are extracted from high-resolution scans of pumice. Class 1 (lighter colors) and class 8 (darker colors) show end-member glass compositions, i.e. HK andesite and high-SiO₂ rhyolite, respectively. These two classes show spot- to cluster-like morphological structures. Intermediate classes show an HK dacitic to rhyolitic composition and a banding- to fold-like morphology. Fractal analysis by box-counting of the boundary pattern of eight grey classified images is performed over a length scale of 0.028–1.8 cm. Fractal dimension D is between 1.01 and 1.84. Coupled fractal analysis and geochemical data reveal that D increases as the degree of magma interaction (homogenization) increases. This feature well fits the results from numerical models on the convective mixing of fluids driven by thermal convection. We conclude that the increase of D observed in the Upper Pollara samples reflects the transition from fractal mixing to homogenization. End-member magmas (HK andesite and high-SiO₂ rhyolite) represent isolated mixing regions, while homogenized magmas represent active mixing regions. In the analyzed pumices, isolated and active mixing regions coexist at scales between 10⁻⁴ and 10⁻² m. Morphological and compositional features of the Upper Pollara pumices result from turbulence.  相似文献   

有限弹性变形地壳中的地震波     

范家参 《地震研究》2002,25(1):48-52

地壳由半无限大的基岩上一层厚度为H^-的表土层组成，入射地震波为垂直的SH波，产生水平地面运动。当浅源大地震发生时，在极震区以外行波传播产生地面运动将使地壳介质有非线性的有限弹性变形。用小参数摄动法使非线性控制方程为线性化的小参数各阶控制方程，得出头两阶线性控制方程的解析解。  相似文献   

剥蚀及地幔作用下青藏高原隆升过程的数值模拟   总被引：7，自引：2，他引：7       下载免费PDF全文

李祖宁  傅容珊  黄建华 《地球物理学报》2002,45(4):516-524

修改了England和Mckenzie的黏性薄层流变模型中控制大陆形变的连续性方程,将剥蚀作用对高原隆升演化的影响直接引入该方程,并考虑下伏地幔小尺度对流对增厚岩石层的搬离作用对高原隆升演化后期的影响,用有限差分法直接模拟青藏高原隆升过程. 数值模拟结果所显示的高原隆升演化过程与实际观测资料吻合较好,揭示了高原隆升演化过程的非平稳和多阶段的特性;同时还表明上地幔小尺度对流对岩石层底部的搬离作用可能是最近8Ma以来高原快速隆升的主导机制.  相似文献   

基于波动方程的广义屏叠前深度偏移   总被引：15，自引：7，他引：15       下载免费PDF全文

金胜汶  许士勇  吴如山 《地球物理学报》2002,45(5):684-690

地震波传播算子的计算效率和精度是制约三维叠前深度偏移的关键因素. 广义屏传播算子(GSP, Generalized Screen Propagator)是一种在双域中实现的广角单程波传播算子. 这一方法略去了在非均匀体之间发生的交混回响,但它可以正确处理包括聚焦、衍射、折射和干涉在内的各种多次前向散射现象. 通过背景速度下的相移和扰动速度下的陡倾角校正,广义屏算子能够适应地层速度的强烈横向变化. 这种算子可以直接应用于炮集叠前偏移,通过将广义屏算子作用于双平方根方程,还可以获得一种高效率、高精度的炮检距域叠前深度偏移方法,用于二维共炮检距道集和三维共方位角道集的深度域成像. 本文首先简述了炮检距域广义屏传播算子的理论,进而讨论了共照射角成像(CAI, Common Angle Imaging)条件,由此给出各个不同照射角(炮检距射线参数)下的成像结果,进而得到共照射角像集. 由于照射角和炮检距的对应关系,共照射角像集又为偏移速度分析和AVO(振幅随炮检距变化)分析等提供了有力工具.  相似文献   

Efficient methods for solving water flow in variably saturated soils under prescribed flux infiltration     

Xiaoxian Zhang  A. Glyn Bengough  John W. Crawford  Iain M. Young 《Journal of Hydrology》2002,260(1-4):75-87

The non-linear solvers in numerical solutions of water flow in variably saturated soils are prone to convergence difficulties. Many aspects can give rise to such difficulties and in this paper we address the gravity term and the prescribed-flux boundary in the Picard iteration. The problem of the gravity term in the Picard iteration is iteration-to-iteration oscillation as the gravity term is treated, by analogy with the time-step advance technique, ‘explicitly’ in the iteration. The proposed method for the gravity term is an improvement of the ‘implicit’ approach of Zhang and Ewen [Water Resour. Res. 36 (2000) 2777] by extending it to heterogeneous soil and approximating the inter-nodal hydraulic conductivity in the diffusive term and the gravity term with the same scheme. The prescribed-flux boundary in traditional methods also gives rise to iteration-to-iteration oscillation because there is no feedback to the flux in the solution at the new iteration. To reduce such oscillation, a new method is proposed to provide such a feedback to the flux. Comparison with traditional Picard and Newton iteration methods for a wide range of problems show that a combination of these two proposed methods greatly improves the stability and consequently the computational efficiency, making the use of small time step and/or under-relaxation solely for convergence unnecessary.  相似文献   

Isotope variations in a Sierra Nevada snowpack and their relation to meltwater     

Padinare V. Unnikrishna  Jeffrey J. McDonnell  Carol Kendall   《Journal of Hydrology》2002,260(1-4):38-57

Isotopic variations in melting snow are poorly understood. We made weekly measurements at the Central Sierra Snow Laboratory, California, of snow temperature, density, water equivalent and liquid water volume to examine how physical changes within the snowpack govern meltwater δ¹⁸O. Snowpack samples were extracted at 0.1 m intervals from ground level to the top of the snowpack profile between December 1991 and April 1992. Approximately 800 mm of precipitation fell during the study period with δ¹⁸O values between −21.35 and −4.25‰. Corresponding snowpack δ¹⁸O ranged from −22.25 to −6.25‰. The coefficient of variation of δ¹⁸O in snowpack levels decreased from −0.37 to −0.07 from winter to spring, indicating isotopic snowpack homogenization. Meltwater δ¹⁸O ranged from −15.30 to −8.05‰, with variations of up to 2.95‰ observed within a single snowmelt episode, highlighting the need for frequent sampling. Early snowmelt originated in the lower snowpack with higher δ¹⁸O through ground heat flux and rainfall. After the snowpack became isothermal, infiltrating snowmelt displaced the higher δ¹⁸O liquid in the lower snowpack through a piston flow process. Fractionation analysis using a two-component mixing model on the isothermal snowpack indicated that δ¹⁸O in the initial and final half of major snowmelt was 1.30‰ lower and 1.45‰ higher, respectively, than the value from simple mixing. Mean snowpack δ¹⁸O on individual profiling days showed a steady increase from −15.15 to −12.05‰ due to removal of lower δ¹⁸O snowmelt and addition of higher δ¹⁸O rainfall. Results suggest that direct sampling of snowmelt and snow cores should be undertaken to quantify tracer input compositions adequately. The snowmelt sequence also suggests that regimes of early lower δ¹⁸O and later higher δ¹⁸O melt may be modeled and used in catchment tracing studies.  相似文献   

贡嘎山东坡亚高山森林区蒸散力的估算   总被引：3，自引：1，他引：3  

周杨明  程根伟  杨清伟 《山地学报》2002,20(2):135-140

以海螺沟 30 0 0m气象站的观测资料为基础 ,运用Penman公式法　空气饱和差法和桑斯维特公式法 ,计算了贡嘎山东坡亚高山林林的年平均蒸散力 ,其结果为分别为 431 81mm、171 4mm和 44 6 4mm。通过分析蒸散力的影响因素 ,对这三种计算蒸散力的方法作了比较 ,对计算结果存在的差异作了比较合理的解释 ,认为用Penman公式法可以估计出本研究区的蒸散力。同时 ,对蒸散力及其影响因自进行了相关分析 ,指出湿度和风不是速制约研究区蒸散力的主导因子 ,并分析了蒸散力与水面蒸发的关系 ,由此推导出估算蒸散力的简便方程 :PE =6 6 77 0 6 91E60 1 0 75 1ITm 0 0 396Pm。  相似文献   

云南省金沙江流域土壤流失方程研究   总被引：17，自引：2，他引：17  

杨子生 《山地学报》2002,(Z1)

云南金沙江流域是长江中上游水土流失最严重的区域。本项研究以“通用土壤流失方程”(USLE)为蓝本,运用小区实验等手段,综合分析了各个侵蚀因子,建立了云南金沙江流域土壤流失方程A=R·K·LS·c·P,并确定了方程中诸因子的求算方法和数值,以及该流域土壤允许流失量,为方程的应用提供了基本的技术数据。同时,还进行了方程的检验,方程计算值与小区实测值的相对误差在6.3％以下,表明该方程在实际应用中是可靠的。该方程的建立,可为云南金沙江流域预测预报土壤侵蚀,制定土地合理利用规划方案、水土保持措施和土地生态安全格局提供了一套可靠的科学方法和依据。  相似文献   

云南省金沙江流域水土流失基本特征分析     

杨子生  梁洛辉  王云鹏 《山地学报》2002,(Z1)

在应用新近建立的云南金沙江流域土壤流失方程测算该流域各县(市、区)年均土壤流失总量和各地类年均 土壤流失量基础上,分析了该流域水土流失的总体特征、各地类水土流失特征和不同坡度级坡耕地的水土流失特 征,为因地制宜地防治水土流失提供了依据。  相似文献   

Determining the magnetic field in the core–mantle boundary zone by non-harmonic downward continuation     

L. Ballani  H. Greiner-Mai  D. Stromeyer 《Geophysical Journal International》2002,149(2):374-389

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