Analytical and numerical models of hydrothermal fluid flow at fault intersections

Fault intersections are the locus of hot spring activity and Carlin‐type gold mineralization within the Basin and Range, USA. Analytical and numerical solutions to Stokes equation suggest that peak fluid velocities at fault intersections increase between 20% and 47% when fracture apertures have identical widths but increase by only about 1% and 8% when aperture widths vary by a factor of 2. This suggests that fault zone intersections must have enlarged apertures. Three‐dimensional finite element models that consider intersecting 10‐ to 20‐m wide fault planes resulted in hot spring activity being preferentially located at fault zone intersections when fault zones were assigned identical permeabilities. We found that the onset of convection at the intersections of the fault zones occurred in our hydrothermal model over a narrow permeability range between 5 × 10^?13 and 7 × 10^?13 m². Relatively high vertical fluid velocities (0.3–3 m year^?1) extended away from the fault intersections for about 0.5–1.5 km. For the boundary conditions and fault plane dimensions used, peak discharge temperatures of 112°C at the water table occurred with an intermediate fault zone permeability of 5 × 10^?13 m². When fault plane permeability differed by a factor of 2 or more, the locus of hot spring activity shifted away from the intersections. However, increasing the permeability at the core of the fault plane intersection by 40% shifted the discharge back to the intersections. When aquifer units were assigned a permeability value equal to those of the fault planes, convective rolls developed that extend about 3 km laterally along the fault plane and into the adjacent aquifer.     

Earth politics: Territory and the subterranean – Introduction to the special issue

The special issue Earth Politics: Territory and the Subterranean explores how and to what political and economic effects people have territorialized the underground. Through studies of a range of activities – from scientific exploration to 3-D geological modeling to laboratory analysis to recreational caving – authors in the issue challenge the idea that the subterranean is a world apart, detached from the sociopolitical worlds of the surface, and instead focus on the complicated relations and processes that remake and weave meaning into often unseen depths. In this introductory article, we situate the issue within expanding literatures on geological materiality, territorial politics, and vertical/volumetric space, and we discuss two overlapping themes running through the issue's articles: the politics of subterranean knowledge production and the politics of subterranean materialities. We conclude by reflecting on the meaning of ‘earth politics’, emphasizing the injustices that derive from – and are sedimented into – dominant modes of knowing and interacting with the matters of the subsurface.     

Hydrothermal convection in and around mineralized fault zones: insights from two- and three-dimensional numerical modeling applied to the Ashanti belt, Ghana

Among hydrogeological processes, free convection in faults has been cited as a possible cause of gold mineralization along major fault zones. Here, we investigate the effects of free convection to determine whether it can cause giant orogenic gold deposits and their regular spatial distribution along major fault/shear zones. The approach comprises: (i) coupled two- and three-dimensional numerical heat- and fluid-flow simulations of simplified geological models; and (ii) calculation of the rock alteration index (RAI) to delineate regions where precipitation/dissolution can occur. Then, comparing the deduced alteration patterns with temperature distribution, potential areas of gold mineralization, defined by T  > 200°C and RAI < 0, are predicted. The models are based on the orogenic Paleoproterozoic ore deposits of the Ashanti belt in western Africa. These deposits occur in the most permeable parts of the fault zone, where the lateral permeability contrast is the highest. For a simple geometry, with a fault zone adjacent to a sedimentary basin half as permeable, we note a transition from three-dimensional circulation within the fault to a two-dimensional convective pattern in the basin far from the fault. Moreover, whereas two-dimensional undulated isotherms dominate in the basin, three-dimensional corrugated isotherms result from the preferred convective pattern within the fault, thus enhancing a periodic distribution of thermal highs and lows. In our most elaborate three-dimensional model with an imposed lateral permeability gradient, the RAI distribution indicates that fluid circulation in fault zones gives rise to a spatial periodicity of alteration patterns consistent with field data.     

Phase‐field modeling of epitaxial growth of polycrystalline quartz veins in hydrothermal experiments

Mineral precipitation in an open fracture plays a crucial role in the evolution of fracture permeability in rocks, and the microstructural development and precipitation rates are closely linked to fluid composition, the kind of host rock as well as temperature and pressure. In this study, we develop a continuum thermodynamic model to understand polycrystalline growth of quartz aggregates from the rock surface. The adapted multiphase‐field model takes into consideration both the absolute growth rate as a function of the driving force of the reaction (free energy differences between solid and liquid phases), and the equilibrium crystal shape (Wulff shape). In addition, we realize the anisotropic shape of the quartz crystal by introducing relative growth rates of the facets. The missing parameters of the model, including surface energy and relative growth rates, are determined by detailed analysis of the crystal shapes and crystallographic orientation of polycrystalline quartz aggregates in veins synthesized in previous hydrothermal experiments. The growth simulations were carried out for a single crystal and for grain aggregates from a rock surface. The single crystal simulation reveals the importance of crystal facetting on the growth rate; for example, growth velocity in the c‐axis direction drops by a factor of ~9 when the faceting is complete. The textures produced by the polycrystal simulations are similar to those observed in the hydrothermal experiments, including the number of surviving grains and crystallographic preferred orientations as a function of the distance from the rock wall. Our model and the methods to define its parameters provide a basis for further investigation of fracture sealing under varying conditions.     

Hydrothermal,multiphase convection of H2O‐NaCl fluids from ambient to magmatic temperatures: a new numerical scheme and benchmarks for code comparison

Thermohaline convection of subsurface fluids strongly influences heat and mass fluxes within the Earth's crust. The most effective hydrothermal systems develop in the vicinity of magmatic activity and can be important for geothermal energy production and ore formation. As most parts of these systems are inaccessible to direct observations, numerical simulations are necessary to understand and characterize fluid flow. Here, we present a new numerical scheme for thermohaline convection based on the control volume finite element method (CVFEM), allowing for unstructured meshes, the representation of sharp thermal and solute fronts in advection‐dominated systems and phase separation of variably miscible, compressible fluids. The model is an implementation of the Complex Systems Modelling Platform CSMP++ and includes an accurate thermodynamic representation of strongly nonlinear fluid properties of salt water for magmatic‐hydrothermal conditions (up to 1000°C, 500 MPa and 100 wt% NaCl). The method ensures that all fluid properties are taken as calculated on the respective node using a fully upstream‐weighted approach, which greatly increases the stability of the numerical scheme. We compare results from our model with two well‐established codes, HYDROTHERM and TOUGH2, by conducting benchmarks of different complexity and find good to excellent agreement in the temporal and spatial evolution of the hydrothermal systems. In a simulation with high‐temperature, high‐salinity conditions currently outside of the range of both HYDROTHERM and TOUGH2, we show the significance of the formation of a solid halite phase, which introduces heterogeneity. Results suggest that salt added by magmatic degassing is not easily vented or accommodated within the crust and can result in dynamic, complex hydrologies.     

3D numerical modeling of hydrothermal processes during the lifetime of a deep geothermal reservoir

Understanding hydrothermal processes during production is critical to optimal geothermal reservoir management and sustainable utilization. This study addresses the hydrothermal (HT) processes in a geothermal research doublet consisting of the injection well E GrSk3/90 and production well Gt GrSk4/05 at the deep geothermal reservoir of Groß Schönebeck (north of Berlin, Germany) during geothermal power production. The reservoir is located between ?4050 to ?4250 m depth in the Lower Permian of the Northeast German Basin. Operational activities such as hydraulic stimulation, production (T = 150°C; Q = ?75 m³ h^?1; C = 265 g l^?1) and injection (T = 70°C; Q = 75 m³ h^?1; C = 265 g l^?1) change the HT conditions of the geothermal reservoir. The most significant changes affect temperature, mass concentration and pore pressure. These changes influence fluid density and viscosity as well as rock properties such as porosity, permeability, thermal conductivity and heat capacity. In addition, the geometry and hydraulic properties of hydraulically induced fractures vary during the lifetime of the reservoir. A three‐dimensional reservoir model was developed based on a structural geological model to simulate and understand the complex interaction of such processes. This model includes a full HT coupling of various petrophysical parameters. Specifically, temperature‐dependent thermal conductivity and heat capacity as well as the pressure‐, temperature‐ and mass concentration‐dependent fluid density and viscosity are considered. These parameters were determined by laboratory and field experiments. The effective pressure dependence of matrix permeability is less than 2.3% at our reservoir conditions and therefore can be neglected. The results of a three‐dimensional thermohaline finite‐element simulation of the life cycle performance of this geothermal well doublet indicate the beginning of thermal breakthrough after 3.6 years of utilization. This result is crucial for optimizing reservoir management. Geofluids (2010) 10 , 406–421     

Downward hydrocarbon migration predicted from numerical modeling of fluid overpressure in the Paleozoic Anticosti Basin,eastern Canada

The Anticosti Basin is a large Paleozoic basin in eastern Canada where potential source and reservoir rocks have been identified but no economic hydrocarbon reservoirs have been found. Potential source rocks of the Upper Ordovician Macasty Formation overlie carbonates of the Middle Ordovician Mingan Formation, which are underlain by dolostones of the Lower Ordovician Romaine Formation. These carbonates have been subjected to dissolution and dolomitization and are potential hydrocarbon reservoirs. Numerical simulations of fluid‐overpressure development related to sediment compaction and hydrocarbon generation were carried out to investigate whether hydrocarbons generated in the Macasty Formation could migrate downward into the underlying Mingan and Romaine formations. The modeling results indicate that, in the central part of the basin, maximum fluid overpressures developed above the Macasty Formation due to rapid sedimentation. This overpressured core dissipated gradually with time, but the overpressure pattern (i.e. maximum overpressure above source rock) was maintained during the generation of oil and gas. The downward impelling force associated with fluid‐overpressure gradients in the central part of the basin was stronger than the buoyancy force for oil, whereas the buoyancy force for gas and for oil generated in the later stage of the basin is stronger than the overpressure‐related force. Based on these results, it is proposed that oil generated from the Macasty Formation in the central part of the basin first moved downward into the Mingan and Romaine formations, and then migrated laterally up‐dip toward the basin margin, whereas gas throughout the basin and oil generated in the northern part of the basin generally moved upward. Consequently, gas reservoirs are predicted to occur in the upper part of the basin, whereas oil reservoirs are more likely to be found in the strata below the source rocks. Geofluids (2010) 10 , 334–350     

Gramsci and the issue of religion: Catholic modernism and the Italian Partito Popolare

ABSTRACT

Gramsci's interest in Italian politics led him to tackle a key issue in the present-day discourse: the relationship between the Holy See and the national State. Additionally, he paid close attention to internal issues of Christianity, from its origins to his own times and – similar to many other socialist thinkers – he believed that there were several echoes between the early Christian experiences and contemporary socialism. From this arose his concern with the religious crisis of the early twentieth century – so-called ‘Modernism’ – as well as the story of the Partito Popolare (Popular Party, PPI), the organization founded by the priest Luigi Sturzo after the First World War, which was marked – especially amongst its left-wing components – by its anti-fascist positions.     

论华中抗日根据地的优抚工作

华中抗日根据地十分注重对退伍、伤残军人和抗属的优抚工作:制定优抚法律、政策,设立社会保障机构,开展优抚政策的宣传教育,组织拜年和节日慰问活动,帮助抗属解决生产生活问题,减轻各种税费,抚恤烈士,安置伤残、退伍军人。优抚经费既包括政府拨款,也包括民间筹集。华中抗日根据地优抚工作的特点是:物质与精神保障相结合;适度保障,维持基本生活;对国民党军队一视同仁;把保障与鼓励自力更生相结合。优抚工作对于动员民众参军参战,密切党政军民关系,配合统一战线的开展等方面都具有重要的意义。     

Topography‐driven flow versus buoyancy‐driven flow in the U.S. midcontinent: implications for the residence time of brines

Topography‐driven flow is normally considered to be the dominant groundwater flow system in uplifted sedimentary basins. In the U.S. midcontinent region east of the Rocky Mountains, the presence of brines derived from dissolution of halite suggests that significant topography‐driven flushing has occurred to remove older brines that presumably formed concurrently with Permian evaporites in the basin. However, the presence of evaporites and brines in the modern basin suggests that buoyancy‐driven flow could limit topography‐driven flushing significantly. Here we used numerical models of variable‐density fluid flow, halite dissolution, solute transport, and heat transport to quantify flow patterns and brine migration. Results indicate the coexistence of large‐scale topography‐ and buoyancy‐driven flow. Buoyancy‐driven flow and low permeability evaporites act to isolate brines, and the residence time of the brines was found to be quite long, at least 50 Myr. The modern distribution of salinity appears to reflect near‐steady‐state conditions. Results suggest that flushing of original evaporatively‐concentrated brines occurred tens of millions of years ago, possibly concurrent with maximum uplift ca. 60 Ma. Simulations also suggest that buoyancy‐driven convection could drive chemical exchange with crystalline basement rocks, which could supply significant Ca²⁺, Sr²⁺, and metals to brines.     

邓承修与中法桂越段边界交涉——以台北“故宫博物院”典藏桂越段边界条约档案为中心

清中期以前,越南被视为藩属,清廷未曾与其划分明确界线。中法战争后清廷与法国签订《中法会订越南条约十款》,越南与清朝藩属关系断绝,划界之议遂启。光绪十一年,清廷派邓承修与法国官员进行划界事宜谈判,经过冗长的交涉辩论,最终双方签订界约、绘制界线舆图。台北“故宫博物院”藏邓承修与法国勘界官员所订条约、界图和相关档案,是还原这段桂越段边界交涉情形的重要资料。     

基于Remake的三维建模技术在摩崖石刻数字化重建中的应用

由于长期暴露于自然环境中,许多摩崖石刻在日晒雨淋、风沙磨蚀等多种因素作用下不断风化,前景堪忧。因此,当务之急是采取有效的措施保护和保存这类珍贵的文化遗产。数字化三维重建技术能够完整地保存石刻现状,为日后的维护、修复和学术研究提供真实、详尽的科学依据。基于Remake三维建模技术的方法与特点,本工作探讨该技术在摩崖石刻数字化重建中的应用。以广西桂林"桂海碑林"内的石刻为研究对象,运用Remake软件对数字相机获取的数字影像进行处理,对摩崖石刻进行快速精确的逆向建模。同时,将三维扫描仪获取的点云数据与本研究获取的模型数据进行同名点精度检测,分析Remake三维重建的精度。三维模型数据可以作为数字档案进行存档和展示,加强对摩崖石刻类文物的保护措施,具有重要的意义。     

Symbiotic modeling: Linguistic anthropology and the promise of chiasmus

Reflexive observations and observations of reflexivity: such agendas are by now standard practice in anthropology. Dynamic feedback loops between self and other, cause and effect, represented and representamen may no longer seem surprising; but, in spite of our enhanced awareness, little deliberate attention is devoted to modeling or grounding such phenomena. Attending to both linguistic and extra-linguistic modalities of chiasmus (the X figure), a group of anthropologists has recently embraced this challenge. Applied to contemporary problems in linguistic anthropology, chiasmus functions to highlight and enhance relationships of interdependence or symbiosis between contraries, including anthropology’s four fields, the nature of human being and facets of being human.