Distribution of uranium in intrusive bodies due to combined migration and diffusion

Uranium is enriched in the border zones of magmatic bodies and the enrichment is believed to be caused by the migration of hydrous solutions which carry that element along intergranular paths towards the contact zone with the wall rock. We propose that the contact zone is a geochemical barrier at which the uranium, present in the solution, would be deposited if it were not for diffusion away from the increased concentration at the margins.The two particle flows, the one caused by migration and the other caused by diffusion, can be described by a differential equation, whose solution is the concentration of uranium as a function of time, diffusion coefficient and velocity of migration.The distribution of uranium in two intrusive bodies, the Mont Blanc granite (Swiss Alps) and a pluton in the Dshetui-Oguz massif (U.S.S.R.), gives the following parameters: duration of process 0.3–1 m.y., diffusion coefficient 4 × 10^?4 to 5 × 10^?4 cm²/s, and velocity of migration 0.1–0.3 cm/year.The combined process of migration and diffusion is assumed to be an important mechanism for controlling the distribution of uranium throughout the earth's crust and for its change in geological time.     

Nonlinear diffusion: Self-similarity and traveling-waves

The presence of fluids in earth materials causes the propagation of fractures, which is observed on large scales to be regulated by nonlinear diffusion processes. In contrast with linear diffusion processes, nonlinear diffusion can result in flows with a pulse-like character (i.e., the fluid is confined to a finite space) whose profiles remain unchanged in time and which possess either self-similar or traveling-wave characters. Entropy-like (Lyapunov functional) arguments are developed here to demonstrate the asymptotic trend to self-similarity or traveling-waves in a broad class of nonlinear diffusion problems.     

Seismic fault preserving diffusion

This paper focuses on the denoising and enhancing of 3-D reflection seismic data. We propose a pre-processing step based on a non-linear diffusion filtering leading to a better detection of seismic faults. The non-linear diffusion approaches are based on the definition of a partial differential equation that allows us to simplify the images without blurring relevant details or discontinuities. Computing the structure tensor which provides information on the local orientation of the geological layers, we propose to drive the diffusion along these layers using a new approach called SFPD (Seismic Fault Preserving Diffusion). In SFPD, the eigenvalues of the tensor are fixed according to a confidence measure that takes into account the regularity of the local seismic structure. Results on both synthesized and real 3-D blocks show the efficiency of the proposed approach.     

The generation and its sealing condition of natural gas in the Tadong area

Focusing on the two natural gas exploration geological problems with abundant source of oil cracking gas in the late stage and the sealing condition of the oil cracking gas reservoir, the kinetics of oil cracking gas and the evaluation parameters of gas cap rock are adopted to the study on the natural gas accumulation conditions in the Tadong area. Both the study on the kinetics of oil cracking gas and the statistical results of reservoir bitumen reveal that the geological formation of oil cracking gas in the Tadong area is located in the top of Cambrian. Two kinds of oil cracking gas geological models at least, namely well Mandong-1’s early rapid generation model (Middle Ordovician-end Silurian) and peak cracking model (with the natural gas conversion rate >90%), namely well Yingnan-2’s two-stage generation model of oil cracking gas, have been set up. The oil cracking gas of Yingnan-2 in the late stage is very significant in the evaluation of natural gas exploration in the Tadong area. The evaluation results of the cap rock show that the microscopic parameters of cap rock from the lower assemblage of Cambrian-Ordovician are better than those from the upper assemblage. The former has strong capillary sealing ability and higher cap rock breakthrough pressure than the upper assemblage, with strong sealing ability, so that natural gas dissipates mainly by diffusion. According to the above investigations, the lower assemblage Cambrian-Ordovician natural gas of Kongquehe slope, Tadong low uplift and Yingjisu depression in the Tadong area prospects well.

     

The generation and its sealing condition of natural gas in the Tadong area

Focusing on the two natural gas exploration geological problems with abundant source of oil cracking gas in the late stage and the sealing condition of the oil cracking gas reservoir, the kinetics of oil cracking gas and the evaluation parameters of gas cap rock are adopted to the study on the natural gas accumulation conditions in the Tadong area. Both the study on the kinetics of oil cracking gas and the statistical results of reservoir bitumen reveal that the geological formation of oil cracking gas in the Tadong area is located in the top of Cambrian. Two kinds of oil cracking gas geological models at least, namely well Mandong-1’s early rapid generation model (Middle Ordovician-end Silurian) and peak cracking model (with the natural gas conversion rate >90%), namely well Yingnan-2’s two-stage generation model of oil cracking gas, have been set up. The oil cracking gas of Yingnan-2 in the late stage is very significant in the evaluation of natural gas exploration in the Tadong area. The evaluation results of the cap rock show that the microscopic parameters of cap rock from the lower assemblage of Cambrian-Ordovician are better than those from the upper assemblage. The former has strong capillary sealing ability and higher cap rock breakthrough pressure than the upper assemblage, with strong sealing ability, so that natural gas dissipates mainly by diffusion. According to the above investigations, the lower assemblage Cambrian-Ordovician natural gas of Kongquehe slope, Tadong low uplift and Yingjisu depression in the Tadong area prospects well.     

Heat diffusion from a plane,periodic source

Summary The solution of the heat diffusion equation with a periodic, plane source is applied to problems of the diurnal and annual range of surface temperature, and its lag on insolation. The application of diffusion theory to practical problems is much simplified by the introduction of the concept of a heat capacity for a surface source. Heat capacity may be calculated from the thermal characteristics of the diffusing substance, and the nature of the periodic source. Heat capacities so determined give diurnal surface temperature ranges fo the order revealed in nature. It is possible to account for the way in which the annual temperature range changes with latitude. The annual temperature range computed on diffusion theory alone is about 250% in excess of observed values, but the discrepancy may be accounted for by the interlatitudinal exchange of heat. The indications are that in average circumstances the air absorbs 50%–100% more heat from a plane source than the soil.     

地震次生毒气泄漏与扩散的数值模拟与动态仿真

文章对破坏性地震中可能发生的次生毒气泄漏与扩散灾害进行了数值模拟与动态仿真研究，设定了几种有实际意义的数值分析模型，分别进行了结构地震破坏，有毒有害气体泄漏，在一定泄漏和气象条件下的向周围空间扩散的动态仿真研究，给出了典型毒气扩散过程中可致人员死亡，严重危险和有感区域的范围的动态变化结果，作者认为地震次生毒气泄漏与扩散研究对大地震应急、城市和大型企业防灾都有重要的意义和广阔的应用前景。     

时间域航空电磁扩散特征和成像深度研究

为了研究时间域航空电磁系统扩散特征,基于连续性边界条件将电磁场向下延拓得到地下介质中各点处的频率域响应,通过傅里叶变换将其变换到时间域.计算阶跃波形发射时垂直磁偶极子和水平磁偶极子在均匀半空间介质和两层介质中产生的电磁场,通过电流密度矢量图和随时间变化的电流密度等值线展示电磁场在地下介质中的扩散过程.电磁场在地下的扩散受电阻率结构的影响.在良导地区电磁场扩散慢、衰减快,而在高阻地区电磁场扩散快、衰减慢.垂直磁偶极子在地下产生的感应电流形成一个电流环随时间向下、向外扩散.水平磁偶极子在地下产生的感应电流形成两个互相叠加的电流环.时间域电磁场扩散代表着真正物理意义上的电磁扩散,因为它展示了地下电磁场空间分布随时间的变化和场强的衰减.基于对感应电流环的研究,我们探究了扩散深度和时间域航空电磁数据成像深度的关系.时间域电磁场扩散规律的研究不仅加深了对电磁扩散特征的理解,更有助于提高航空电磁数据解释水平.     

Effects of salinity on methane gas hydrate system

Using an approximately analytical formation, we extend the steady state model of the pure methane hydrate system to include the salinity based on the dynamic model of the methane hydrate system. The top and bottom boundaries of the methane hydrate stability zone (MHSZ) and the actual methane hy-drate zone (MHZ), and the top of free gas occurrence are determined by using numerical methods and the new steady state model developed in this paper. Numerical results show that the MHZ thickness becomes thinner with increasing the salinity, and the stability is lowered and the base of the MHSZ is shifted toward the seafloor in the presence of salts. As a result, the thickness of actual hydrate occur-rence becomes thinner compared with that of the pure water case. On the other hand, since lower solubility reduces the amount of gas needed to form methane hydrate, the existence of salts in sea-water can actually promote methane gas hydrate formation in the hydrate stability zone. Numerical modeling also demonstrates that for the salt-water case the presence of methane within the field of methane hydrate stability is not sufficient to ensure the occurrence of gas hydrate, which can only form when the methane concentration dissolved in solution with salts exceeds the local methane solubility in salt water and if the methane flux exceeds a critical value corresponding to the rate of diffusive methane transport. In order to maintain gas hydrate or to form methane gas hydrate in marine sedi-ments, a persistent supplied methane probably from biogenic or thermogenic processes, is required to overcome losses due to diffusion and advection.     

地震次生毒气泄漏与扩散数值模拟的参数分析

本文在破坏性地震中可能发生的次生毒气泄漏与扩散灾害的数值模拟与动态仿真研究的基础上，分析了影响毒气扩散的主要参数。在设定的几种工况下，给出相应的图表结果，直观地描述了各参数对扩散过程的影响。进一步验证了所采用毒气扩散简化模型的准确性，对大震应急和防御与减轻城市和大型企业的地震次生毒气泄漏与扩散灾害都有重要的意义。     

平衡剖面技术在国内外油气勘探中的最新应用

平衡剖面技术是油气勘探中的一项辅助解释技术,它提供正演与反演两种模拟过程.正演模拟过程可以迅速、有效地检验某种地质假设,为解释人员提供合理的解释模型,同时,正演过程可以动态显示构造变形历史,为研究油气藏的形成、演化提供依据;而反演模拟过程则可以迅速地对剖面解释结果进行合理性检验,避免解释中的随意性,提高剖面解释的质量和效率.本文对平衡剖面有关的概念、原理、应用条件及变形机制等进行了系统归纳与总结,并详细介绍了国内外平衡剖面技术在油气勘探中的最新应用实例.     

砂岩核磁共振响应模拟及受限扩散

本文运用随机游走方法模拟了砂岩储层中流体的核磁共振(NMR)响应及其受限扩散现象.通过改变数字岩心的分辨率模拟生成不同孔隙尺寸的砂岩,研究了不同孔隙尺寸砂岩饱含水时流体扩散系数随扩散时间的变化关系,同时模拟了砂岩饱和单相流体和两相流体的NMR响应;研究了流体的受限扩散系数与横向弛豫时间T2的关系,分析了表面弛豫率和胶结指数对润湿相流体受限扩散系数线位置的影响,并将其用于解释砂岩储层的D-T2分布.结果表明:孔隙流体的扩散系数会随扩散时间的增加而逐渐减小并趋于定值.随着岩石孔隙尺寸的减小,受限扩散现象越明显,受限扩散对岩石NMR响应的影响也越大.润湿相流体受限扩散系数线的位置受岩石胶结指数和表面弛豫率的影响较大.由于润湿相流体扩散系数减小,导致D-T2分布中润湿相流体信号偏离其自由扩散系数线,需要利用流体的受限扩散系数线准确识别D-T2分布中的润湿相流体.     

各向异性扩散在电磁测深曲线平滑中的应用

电磁类地球物理方法由于极易受到各类噪声的干扰,使得估计的视电阻率曲线或相位曲线发生畸变,严重地影响了反演解译精度,如何对这类观测曲线进行合理有效地平滑,是目前数据处理中的重点,也是难点.本文把各向异性扩散（anisotropic diffusion AD）引入到曲线平滑中,提出了各向异性扩散的曲线平滑方法,以平滑点梯度值的降函数作为扩散速度,在梯度较大的位置予以较小的扩散速度以保护特征,在梯度较小的位置予以较大的扩散速度平滑噪声引起的扰动.同时为了减小“飞点”(outliers)对平滑的干扰,采用局部单调扩散进行预处理.实验结果显示,本文提出的平滑方法有效地平滑了噪声与“飞点”的干扰,恢复了曲线的基本形态,有效地保持了曲线基本特征.     

A fracture network model and open fracture analysis of a tight sandstone gas reservoir in Dongpu Depression, Bohaiwan Basin, eastern China

Fracture systems can significantly influence productivity in tight sandstone reservoirs. Understanding the in situ fracture network is of importance in exploration and development of such reservoirs. This paper illustrates that stress-strain analysis, based on geological processes, is a credible prediction method for fracture networks and enables an estimation of which fractures are likely to contribute most to flow based on fracture orientation. This study shows the workflow for fracture modelling and its application in a sandstone reservoir using structural validation tools and interactive fracture simulations constrained with lithology, well log and core data. The fracture network model can help us understand not only what a fractured reservoir looks like but also how the existing fracture network behaves. An important consideration is that we need to understand and constrain the geological history to assess the fracture network properly.     

Slope flows governed by diffusion effects in seas, lakes, and reservoirs

The hydrodynamics of slope flows governed by heat and salt diffusion near a rigid sloping impermeable boundary in stratified water bodies is studied. The physics of this phenomenon consists in that, because of the impermeability of the shore slope, the fluxes of heat and salt normal to the slope surface must be zero. Since the background stratification does not meet this condition, diffusion boundary layers appear along the shore slope. It is shown that, depending on the ratio between diffusion coefficients of heat and salt and the parameters of stratification by temperature and salinity, of convection the slope flows can be either ascending or descending. Moreover, under some conditions, a finger-type regime can form in boundary layers, though the background stratification is stable. The appearing slope flows, though they are local, initiate global transverse ventilation of water in the entire water body.     

Pressure dependence of diffusion in ionic solids

A continuum model of point defects in ionic solids is developed and applied to the estimation of the pressure dependence of diffusion (in alkali halides). Activation enthalpies are expressed using macroscopic physical constants (lattice energy, elastic constants, dielectric constants, etc.) by introducing empirical parameters related to the microscopic nature of defects. Systematic relations are found between these parameters and other physical properties (ionic radius, volume fraction of anions in crystals, etc.). The nature of parameters for anions is shown to be significantly different from that for cations, probably due to the difference in “softness” between anions and cations. Such relations make it possible to estimate activation energy and volume of diffusion from the lattice energy, ionic radius, elastic constants, dielectric constants and their pressure derivatives. The calculated activation volumes agree well with experimental data. The effect of phase transition is also examined by comparing NaCl and CsCl structures. It is suggested that by phase transition from a less dense (NaCl) structure to a denser one (CsCl), the anion diffusion constant may not decrease as much as expected from the associated change in other physical properties. The validity of the models of van Liempt and of Keyes is examined. It is shown that van Liempt's model works well except for the migration process (especially anions), and that Keyes' model does not work so well for all processes.     

Solution of the linear diffusion equation for modelling erosion processes with a time varying diffusion coefficient

In the present paper the differential equation of the temporal development of a landform (mountain) with a time dependent diffusion coefficient is solved. It is shown that the shape and dimensions of the landform at time t are independent of the specific variation of the diffusion coefficient with time; they only depend on the mean value of the diffusion coefficient in the time interval where the erosion process takes place. Studying the behaviour of the solution of the differential equation in the wave number domain, it is concluded that Fourier analysis may help in estimating, in quantitative terms, the initial dimensions, the age or, alternatively, the value of the diffusion coefficient of the landform. The theoretical predictions are tested on a hill of the southern part of the Ural mountainous region, in order to show how the results of the mathematical analysis can be used in describing, in quantitative terms, the morphological development of landforms due to erosion processes. Copyright © 2007 John Wiley & Sons, Ltd.     

Factors controlling the noble gas abundance patterns of deep-sea basalts

A number of processes may modify the noble gas composition of silicate liquids so that the composition of noble gases observed in glassy margins of deep-sea basalts is not that of the upper mantle. Differential solubility enhances the light noble gases relative to the heavier gases; however, we demonstrate that the observed abundance pattern cannot be attributed to solubility of noble gases with atmospheric proportions. Partial melting and fractional crystallization increase the noble gas content of all species relative to mantle concentrations, but do not fractionate their relative abundances. Noble gases may be lost from an ascending magma in various ways, the most important, however, may be exclusion of gas from crystals forming at the time of solidification, which is shown to result in marked loss of gas from the basalt. Small amounts of low-temperature alteration of solidified basalt can produce dramatic changes in the noble gas abundance pattern, since the adsorption coefficients for the different noble gas favor uptake of heavy species relative to the light species. Atmospheric contamination can account for observed variations in the ⁴⁰Ar/³⁶Ar ratio of oceanic basalts. The degree of crystallinity of glassy margins of deep-sea basalts may control the helium abundance of these samples; however, the uniform ³He/⁴He values reported apparently reflect a relatively constant proportion of radiogenic and primordial helium in the mantle.     

He diffusion in olivine

Helium diffusion in olivine (dunite xenolith) has been measured in the temperature range 1180–1460°C; a linear Arrhenius function was obtained with an activation energy of 120₋₂₇⁺³² kcal/mole, and a pre-exponential factor (D₀) = 2.2 × 10⁸ cm²/s.

Diffusion mechanisms are not a viable means of degassing He from the mantle. Olivine phenocrysts can be expected to retain previously trapped He, during cooling in extrusive basalts, provided the flow units are thinner than 50 m; xenoliths will retain mantle He signatures only if magma transport times are less than 50 years, or if the He fugacity in the magma is high enough to prevent xenolith degassing. The lower oceanic crust is probably substantially degassed of He. Trapped He will be qualitatively retained in quenched submarine basalt glass only if the cooling rate is faster than 5 × 10¹⁴°C/m.y.; glass at several centimeters depth in a basalt flow (near the spherulite zone) will have cooling rates lower than this, so He loss may be significant in many basalt glass samples.     

龙滩水库诱发地震的孔隙压力扩散特征

利用孔隙压力扩散机制,分析了广西省龙滩水库与蓄水有关的小震精定位数据,获得了5个分区的流体孔隙压力扩散特征.龙滩水库与蓄水有关的小震活动表现出明显分区现象,使用多源孔隙压力扩散过程触发机理,计算出5个分区流体孔隙压扩散系数值为0.04～0.65 m2/s.根据5个分区流体孔隙压力扩散程度,将这5区划分为I、Ⅱ、Ⅲ三种类...