Introduction     

Alison Ord  Bruce E. Hobbs  Hans-Bernd Mühlhaus 《Pure and Applied Geophysics》1991,136(4):371-374

  相似文献   

Numerical modelling of deformation and fluid flow in the Shuikoushan district, Hunan Province, South China   总被引：3，自引：2，他引：3  

Yanhua Zhang  Ge Lin  Paul Roberts  Alison Ord 《Ore Geology Reviews》2007,31(1-4):261-278

The Shuikoushan district, in Hunan Province, South China, contains major Pb–Zn–Au–Ag polymetallic mineralisation. Two groups of numerical models have been constructed to study the interactions between deformation and fluid flow in the district during the Yanshanian compression event (180 to 90 Ma). The first group includes district-scale conceptual models of coupled deformation and fluid flow during folding. The models show that fluid flow patterns are controlled by deformation within the fold system inferred for the district. During regional shortening and folding, fluids are generally focused towards the fold hinge/core areas along higher permeability layers (in particular Permian limestone units), in preference to flowing across the low permeability seal units (Permian and Jurassic terrestrial sequences). The efficiency of this fluid focusing can only be significantly increased if these folded seal units are allowed to undergo permeability increase as a result of tensile failure. The modelling results show that permeability enhancement localises mostly at fold hinges, dominantly within the silicified zone on the top of the Permian limestone unit. This process results in increased flow velocities and facilitates fluid focusing towards fold hinge/core locations at this silicified rock horizon. The second group includes deposit scale models for the Kangjiawan deposit, which is one of the two major deposits in the Shuikoushan district. The models show patterns of tensile failure, permeability creation, fluid focusing and mixing, and fracture development along a selected exploration cross section through the deposit. These results are consistent with the observed brecciation and mineralisation features. Regions of maximum brecciation in the district are associated with: (1) a combination of fold hinge and fault intersection locations (structural); and (2) the silicified zone and Permian limestone unit (lithological). Such brecciation zones are associated with extensive fluid focusing and mixing, and therefore represent the most favourable locations for mineralisation in the district. On the basis of this work, ideas for future research work and mineral exploration in the district are proposed.  相似文献   

An interface-condition substitution strategy for theoretical study of dissolution-timescale reactive infiltration instability in fluid-saturated porous rocks     

Chongbin Zhao  Bruce E. Hobbs  Alison Ord 《国际地质力学数值与分析法杂志》2019,43(8):1576-1593

A theoretical study of reactive infiltration instability is conducted on the dissolution timescale. In the present theoretical study, the transient behavior of a dissolution-timescale reactive infiltration system needs to be considered, so that the upstream region of the chemical dissolution front should be finite. In addition, the chemical dissolution front of finite thickness should be considered on the dissolution timescale. Owing to these different considerations, it is very difficult, even in some special cases, to derive the first-order perturbation solutions of the reactive infiltration system on the dissolution timescale. To overcome this difficulty, an interface-condition substitution strategy is proposed in this paper. The basic idea behind the proposed strategy is that although the first-order perturbation equations in the downstream region cannot be directly solved in a purely mathematical manner, they should hold at the planar reference front, which is the interface between the upstream region and the downstream region. This can lead to two new equations at the interface. The main advantage of using the proposed interface-condition substitution strategy is that through using the original interface conditions as a bridge, the perturbation solutions for the dimensionless acid concentration, dimensionless Darcy velocity, and their derivatives involved in the two new equations at the interface can be evaluated just by using the obtained analytical solutions in the upstream region. The proposed strategy has been successfully used to derive the dimensionless growth rate, which is the key issue associated with the theoretical study of dissolution-timescale reactive infiltration instability in fluid-saturated porous rocks.  相似文献   

Finite element analysis of flow patterns near geological lenses in hydrodynamic and hydrothermal systems   总被引：2，自引：0，他引：2  

Chongbin Zhao  B. E. Hobbs  H. B. Mühlhaus  & A. Ord 《Geophysical Journal International》1999,138(1):146-158

We use theoretical and numerical methods to investigate the general pore-fluid flow patterns near geological lenses in hydrodynamic and hydrothermal systems respectively. Analytical solutions have been rigorously derived for the pore-fluid velocity, stream function and excess pore-fluid pressure near a circular lens in a hydrodynamic system. These analytical solutions provide not only a better understanding of the physics behind the problem, but also a valuable benchmark solution for validating any numerical method.
  Since a geological lens is surrounded by a medium of large extent in nature and the finite element method is efficient at modelling only media of finite size, the determination of the size of the computational domain of a finite element model, which is often overlooked by numerical analysts, is very important in order to ensure both the efficiency of the method and the accuracy of the numerical solution obtained. To highlight this issue, we use the derived analytical solutions to deduce a rigorous mathematical formula for designing the computational domain size of a finite element model. The proposed mathematical formula has indicated that, no matter how fine the mesh or how high the order of elements, the desired accuracy of a finite element solution for pore-fluid flow near a geological lens cannot be achieved unless the size of the finite element model is determined appropriately.
  Once the finite element computational model has been appropriately designed and validated in a hydrodynamic system, it is used to examine general pore-fluid flow patterns near geological lenses in hydrothermal systems. Some interesting conclusions on the behaviour of geological lenses in hydrodynamic and hydrothermal systems have been reached through the analytical and numerical analyses carried out in this paper.  相似文献   

Theoretical analyses of chemical dissolution‐front instability in fluid‐saturated porous media under non‐isothermal conditions          下载免费PDF全文

Chongbin Zhao  B. E. Hobbs  A. Ord 《国际地质力学数值与分析法杂志》2015,39(8):799-820

This paper mainly deals with the theoretical aspects of chemical dissolution‐front instability problems in two‐dimensional fluid‐saturated porous media under non‐isothermal conditions. In the case of the mineral dissolution ratio (that is defined as the ratio of the dissolved‐mineral equilibrium concentration in the pore fluid to the molar concentration of the dissolvable mineral in the solid matrix of the fluid‐saturated porous medium) approaching zero, the corresponding critical condition has been mathematically derived when temperature variation effects are considered. As a complementary tool, the computational simulation method is used to simulate the morphological evolution of chemical dissolution fronts in two‐dimensional fluid‐saturated porous media under non‐isothermal conditions. The related theoretical and numerical results have demonstrated that: (i) a temperature increase in a non‐isothermal chemical dissolution system can have some influence on the propagation speed of the planar chemical dissolution front in the system. Generally, the chemical dissolution front in the non‐isothermal chemical dissolution system propagates slower than that in the counterpart isothermal chemical dissolution system when the temperature of the non‐isothermal chemical dissolution system is higher than that of the counterpart isothermal chemical dissolution system; (ii) a temperature increase in the non‐isothermal chemical dissolution system can stabilize the chemical dissolution front propagating in the system, because it can cause a decrease in the Zhao number of the system but does not affect the critical Zhao number of the system; and (iii) the temperature gradient in the upstream direction of a chemical dissolution front is smaller than that in the downstream direction of the chemical dissolution front when the non‐isothermal chemical dissolution system is supercritical. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

Mary Kathleen metamorphic‐hydrothermal uranium — rare‐earth element deposit: Ore genesis and numerical model of coupled deformation and fluid flow     

N. H. S. Oliver  P. J. Pearson  R. J. Holcombe  A. Ord 《Australian Journal of Earth Sciences》2013,60(3):467-484

The Mary Kathleen U‐REE orebody of the Proterozoic Mt Isa Block was the product of chemical and physical interaction between regional metamorphic/hydrothermal fluids and preexisting calcic skarns. The deposit provides excellent examples of mechanical control on ore localisation and of the complexity of ores in rocks with protracted thermal histories. Host skarns were produced by contact metasomatism around the 1740 Ma Burstall Granite, whereas the allanite‐uraninite ore formed under amphibolite‐facies conditions, late during the D₂ phase of the ca 1550–1500 Ma Isan orogeny. Observations of ore geometry are consistent with previous geochronologic data demonstrating a large time gap between skarn formation and ore genesis. Numerical modelling of coupled deformation and fluid flow suggests that veins at the core of ore shoots may have formed as tensile or shear fractures during coupling of the competent skarn host with the late‐D₂ Mary Kathleen Shear Zone, allowing a change of orientation of ore shoots with distance from the shear zone. Mineral chemistry and petrographic observations suggest the possible role of a redox control on chemical localisation of ore by conversion of Fe²⁺‐rich clinopyroxene‐rich skarn host to Fe³⁺‐rich secondary garnet ‘skarn’ and uraninite‐allanite ore. Alternately, fluid pressure drops as a consequence of fracturing of the host skarn may have triggered fluid unmixing, or fluid mixing, leading to ore precipitation. Available data do not allow clear definition of the ultimate source of the U and REE, nor of the specific chemical ore‐forming mechanism. However, regional constraints, previous Sm–Nd modelling, and our numerical models suggest a combination from proximal skarn hosts and from distal sources accessed by flow of metamorphic and/or late tectonic igneous‐derived fluids. The deposit has some similarities with ironstone‐hosted Cu–Au ± U deposits found in the nearby Cloncurry Belt.  相似文献   

A neutral hydrogen distance limit to the relativistic binary PSR J1141–6545     

S. M. Ord  M. Bailes  W. van Straten 《Monthly notices of the Royal Astronomical Society》2002,337(2):409-412

  相似文献   

The Magnetic Field of the Solar Corona from Pulsar Observations     

S. M. Ord  S. Johnston  J. Sarkissian 《Solar physics》2007,245(1):109-120

We present a novel experiment with the capacity to independently measure both the electron density and the magnetic field of the solar corona. We achieve this through measurement of the excess Faraday rotation resulting from propagation of the polarised emission from a number of pulsars through the magnetic field of the solar corona. This method yields independent measures of the integrated electron density, via dispersion of the pulsed signal and the magnetic field, via the amount of Faraday rotation. In principle this allows the determination of the integrated magnetic field through the solar corona along many lines of sight without any assumptions regarding the electron density distribution. We present a detection of an increase in the rotation measure of the pulsar J1801-2304 of approximately 170 rad m² at an elongation of 0.96° from the centre of the solar disc. This corresponds to a lower limit of the magnetic field strength along this line of sight of >41.8 nT. The lack of precision in the integrated electron density measurement restricts this result to a limit, but application of coronal plasma models can further constrain this to approximately 0.5 μT, along a path passing 2.7 solar radii from the solar limb, which is consistent with predictions obtained using extensions to the source surface models published by the Wilcox Solar Observatory.  相似文献   

Theoretical analyses of nonaqueous phase liquid dissolution‐induced instability in two‐dimensional fluid‐saturated porous media     

Chongbin Zhao  B. E. Hobbs  A. Ord 《国际地质力学数值与分析法杂志》2010,34(17):1767-1796

This paper deals with the theoretical aspects of nonaqueous phase liquid (NAPL)‐dissolution‐induced instability in two‐dimensional fluid‐saturated porous media including solute dispersion effects.After some weaknesses associated with the previous work are analyzed and overcome, a comprehensive dimensionless number, known as the Zhao number, is proposed to represent the main driving force and three controlling mechanisms of an NAPL‐dissolution system that has a finite domain. The linear stability analysis is carried out to derive the critical value of the comprehensive dimensionless number of the NAPL‐dissolution system in a limit case as the ratio of the equilibrium concentration to the density of the NAPL approaches zero. As a result, a theoretical criterion that can be used to assess the instability of planar NAPL‐dissolution fronts in two‐dimensional fluid‐saturated porous media of finite domains has been established. Not only can the present theoretical results be used for the theoretical understanding of the effect of solute dispersion on the instability of an NAPL‐dissolution front in the fluid‐saturated porous medium of either a finite domain or an infinite domain, but also they can be used as benchmark solutions for verifying numerical methods employed to simulate detailed morphological evolution processes of NAPL‐dissolution fronts in two‐dimensional fluid‐saturated porous media. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Assessment of the instability hazard of a granite boulder     

L. R. Alejano  C. Ordóñez  J. Armesto  T. Rivas 《Natural Hazards》2010,53(1):77-95

This paper aims to assess the instability hazard of a granite boulder. The procedure was first to consider the geological setting and geomorphologic features of the boulder in relation to typical granite landscape forms. Climatic and seismic data were next obtained from different sources, and geomechanical parameters were measured in situ and in the laboratory, with terrestrial laser scanning techniques used to measure shape and volume and to conduct a detailed survey of the boulder. Different analytical approaches were then applied to the calculation of boulder safety factors against sliding and toppling. Since the boulder was considered to be unstable in the worst possible scenario, a particle code approach was used to determine its fall trajectory, calculate the final run out of the block, and assess the risk for houses located at the foot of the slope. Finally, conclusions were drawn regarding the instability hazard, and recommendations were made in regard to definitively stabilizing the granite boulder.  相似文献