The forward problem of electromagnetic induction: accurate finite-difference approximations for two-dimensional discrete boundaries with arbitrary geometry

We describe finite-difference approximations to the equations of 2-D electromagnetic induction that permit discrete boundaries to have arbitrary geometrical relationships to the nodes. This allows finite-difference modelling with the flexibility normally ascribed to finite-element modelling. Accuracy is demonstrated by comparison with finite-element computations. We also show that related approximations lead to substantially improved accuracy in regions of steep, but not discontinuous, conductivity gradient.     

藏北高原地壳及上地幔导电性结构——超宽频带大地电磁测深研究结果

为了研究西藏中、北部壳、幔导电性结构,讨论高原中、北部岩石圈热状态,1998年和1999年（INDEPTH（Ⅲ） MT）在西藏中、北部完成了德庆—龙尾错（500线）和那曲—格尔木（600线）超宽频带大地电磁深探测剖面的研究.研究结果表明,西藏中、北部以昆仑山断裂为界,其南北壳、幔电性结构有很大差异.昆仑山断裂以北地壳和上地幔为高阻区.而昆仑山以南,地壳和上地幔的导电性有明显的分层结构：地壳上部以不连续的高阻体为主,夹有局部低阻异常体,沿南北方向上地壳的电性结构复杂,具有不连续、分块的特点;但中、下地壳为大范围的高导异常区,区内发育有大规模、不相连续、产状各异的高导体,其电阻率均小于4Ωm;在班公—怒江和金沙江缝合带之下,壳内高导体都具有向上地幔延伸的趋势,存在连通壳、幔的低阻通道.根据西藏高原中、北部壳、幔电性结构的研究推断：如同藏南一样,这里也普遍存在部分熔融体和热流体,它们的成因主要与班公—怒江和金沙江缝合带的壳-幔热交换、热活动有关,这是两期形成的壳-幔热交换通道.其中,班公—怒江缝合带的壳-幔热交换通道形成时间比金沙江缝合带早.因此,研究区壳、幔的热活动是从南边和西边开始,向北、向东扩展,导致现今西藏中、北部地壳和上地幔的热流分布由西向东、由南向北增大.     

Planform of convection with strongly temperature-dependent viscosity

Abstract

This paper experimentally investigates the convective planform near critical in a fluid layer whose temperature-dependent viscosity varies from top to bottom by up to a factor of 1500. Convection occurs in three different planforms: rolls, hexagons and squares. The square planform, which appears only for fluids with viscosity variation greater than about 50, replaces the hexagonal convection pattern as the Rayleigh number increases much above critical. The large amplitude of hexagonal convection with strong viscosity variation precludes studying the hexagon-square transition with perturbation methods of the type used to study the hexagon-roll transitions at smaller viscosity variations.     

Finite element analysis of excavation

The finite element method has often been used to simulate excavation. When the soil is linearly elastic, the results of excavation should be independent of the number of stages in the excavation process, and lack of such independence indicates an incorrect procedure. The simple direct method described in this paper provides the required independence in the case of linearly elastic materials, and hence can be used for multi-stage excavation in non-linear problems without excessive errors. However methods whose errors increase with the number of stages of excavation are quite unsuitable for non-linear problems. Alternative methods of analysis, errors arising from the inability of the elements to model adequately the stress gradients near the toe of the excavation and excavation adjacent to a diaphragm wall are discussed.     

Contaminant migration in a regular two- or three-dimensional fractured network: Reactive contaminants

A semi-analytic solution for the migration of dilute reactive contaminant species in a fractured medium having a one-, two- or three-dimensional fracture network is presented. The application of the technique to practical problems is discussed and illustrated by examples. The effects of sorption in the matrix and on the surface of the fractures are considered for a range of parameters. It is demonstrated that the synergistic interaction of the various parameters is such that the effect of changing a single variable may not be immediately obvious and a designer should perform computations for the range of expected parameters to determine the maximum impact. For example, it is shown that it is not necessarily conservative to consider only one-dimensional or only three-dimensional diffusion into the matrix; which of these cases is most critical will depend on the values of other parameters. It is also shown that an effective leachate collection system may significantly reduce the impact of those contaminants which do escape into the fractured media.     

What can ecological science tell us about opportunities for carbon sequestration on arid rangelands in the United States?

Scientific interest in carbon sequestration on rangelands is largely driven by their extent, while the interest of ranchers in the United States centers on opportunities to enhance revenue streams. Rangelands cover approximately 30% of the earth's ice-free land surface and hold an equivalent amount of the world's terrestrial carbon. Rangelands are grasslands, shrublands, and savannas and cover 312 million hectares in the United States. On the arid and semi-arid sites typical of rangelands annual fluxes are small and unpredictable over time and space, varying primarily with precipitation, but also with soils and vegetation. There is broad scientific consensus that non-equilibrium ecological models better explain the dynamics of such rangelands than equilibrium models, yet current and proposed carbon sequestration policies and associated grazing management recommendations in the United States often do not incorporate this developing scientific understanding of rangeland dynamics. Carbon uptake on arid and semi-arid rangelands is most often controlled by abiotic factors not easily changed by management of grazing or vegetation. Additionality may be impossible to achieve consistently through management on rangelands near the more xeric end of a rangeland climatic gradient. This point is illustrated by a preliminary examination of efforts to develop voluntary cap and trade markets for carbon credits in the United States, and options including payment for ecosystem services or avoided conversion, and carbon taxation. A preliminary analysis focusing on cap and trade and payment for avoided conversion or ecosystem services illustrates the misalignment between policies targeting vegetation management for enhanced carbon uptake and non-equilibrium carbon dynamics on arid United States rangelands. It is possible that current proposed carbon policy as exemplified by carbon credit exchange or offsets will result in a net increase in emissions, as well as investment in failed management. Rather than focusing on annual fluxes, policy and management initiatives should seek long-term protection of rangelands and rangeland soils to conserve carbon, and a broader range of environmental and social benefits.     

Analysis of a point sink embedded in a porous elastic half space

Closed-form solutions are presented for the steady-state distributions of displacement, pore pressure and stress around a point sink embedded in a homogeneous, isotropic elastic half space. These solutions have been evaluated for a typical case of a sink (pump) buried in sand and the magnitude of the settlement of the ground surface has been estimated.