The frequency response of the horizontal magnetic field for a conductive channel

Summary. Babour & Mosnier's results showing no frequency dependence between the anomalous horizontal magnetic field above a conductor and the difference between the horizontal magnetic fields above and below the conductor over a wide range of frequencies led them to conclude that this effect is due to current channelling. A two-dimensional numerical model of a conductive channel, with a uniform horizontal source field, shows the same effect over a wide range of frequencies. Thus local induction can show the same effect.     

大河三角洲河口海岸演化机理模型研究:(II)模型构建与实证

本文以z坐标下的三维斜压海洋动力学数值模式为基本模式原型,在整理渤海基本数据并诊断计算风生环流和热盐环流作为背景环流场基础上,初步建立了渤海海域动力环境数值模式。模式采用了经校正的Bagnold型方程来计算渤海底移质沉积物输运,悬移质计算则是取二维深度平均悬移质输运方程和河床变形方程,计算含沙量分布以及由悬移物引起的冲淤厚度。利用这种方法建立的沉积物输运模式,定量模拟了渤海沿岸和海底的沉积物输运方向和冲淤分布。模拟结果与通过多年实测水深估算获得的渤海海底沉积物的冲淤变化分布相比较,两者之间在基本结论上是比较一致的     

Uplift by thermal expansion of the lithosphere

Summary. If one can measure the anomalous horizontal magnetic field associated with a (locally bounded) two-dimensional conductivity anomaly, the transfer function which results from correlating the vertical with the anomalous horizontal magnetic field directly indicates the depth to an equivalent line-current. A. numerical model can be used to illustrate this. If three-dimensional effects (including current channelling) produce the current anomaly, interpretation in terms of conductive structure would be less clear. It has been claimed (Babour & Mosnier etc.) that such three-dimensional effects are experimentally observed in the highly coherent transfer functions determined from differential geomagnetic sounding experiments. These effects are, specifically, the 'linear polarization' of the anomalous fields, and the invariance of the phase of the measured anomalous field across the anomaly. It is suggested in this present paper that both these effects can be explained in terms of simple local induction models.
If the embedded two-dimensional anomaly is sufficiently close to the Earth's surface, the transfer function (between the vertical and the total horizontal field) contains more information than is usually interpreted. With this in mind, the magnetovariational data collected by Rooney & Hutton in the Kenyan Rift is re-examined.     

E-polarization induction in two thin half-sheets

Summary. An analytical solution is obtained for the E-polarization problem of electromagnetic induction in two adjacent half-sheets underlain by a uniform conducting half-space. In this mode the inducing magnetic field is assumed horizontal, uniform and perpendicular to the discontinuity. The same model was previously solved under B-polarization by Dawson & Weaver. The present solution then completes the study of two-dimensional induction in the described model. Further, it extends both the analytic E-polarization solution of Weidelt by the inclusion of an underlying conductor and that of Raval, Weaver & Dawson by the inclusion of arbitrary conductance values for the two surface sheets. The solution may be used as an idealized model of the coast effect and allows detailed study of the field behaviour near the discontinuity. The horizontal magnetic field on each side of the surface layer has a finite jump discontinuity at the interface and the vertical magnetic field exhibits a logarithmic singularity there. If the right-hand conductance (say) becomes infinite, the horizontal magnetic field exhibits an algebraic singularity as the coastline is approached from the right, while the vertical magnetic field does likewise from the left. Calculations are presented for the same two models as discussed in B-polarization by Dawson & Weaver and the results are compared to values obtained from a more general numerical scheme. The electric current distribution inside the conducting half-space is depicted for the second model.     

Modelling of electromagnetic fields in thin heterogeneous layers with application to field generation by volcanoes—theory and example

When interpreting electromagnetic fields observed at the Earth's surface in a realistic geophysical environment it is often necessary to pay special attention to the effects caused by inhomogeneities of the subsurface sedimentary and/or water layer and by inhomogeneities of the Earth's crust. The inhomogeneities of the Earth's crust are expected to be especially important when the electromagnetic field is generated by a source located in a magma chamber of a volcano. The simulation of such effects can be carried out using generalized thin-sheet models, which were independently introduced by Dmitriev (1969 ) and Ranganayaki & Madden (1980 ). In the first part of the paper, a system of integral equations is derived for the horizontal current that flows in the subsurface inhomogeneous conductive layer and for the vertical current crossing the inhomogeneous resistive layer representing the Earth's mantle. The terms relating to the finite thickness of the laterally inhomogeneous part of the model are retained in the equations. This only marginally complicates the equations, whilst allowing for a significant expansion of the approximation limits.
  The system of integral equations is solved using the iterative dissipative method developed by the authors in the period from 1978 to 1988. The method can be applied to the simulation of the electromagnetic field in an arbitrary inhomogeneous medium that dissipates the electromagnetic energy. When considered on a finite numerical grid, the integral equations are reduced to a system of linear equations that possess the same contraction properties as the original equations. As a result, the rate at which the iterative-perturbation sequence converges to the solution remains independent of the numerical grid used for the calculations. In contrast to previous publications on the method, aspects of the algorithm implementation that guarantee its effectiveness and robustness are discussed here.     

Solution of the H-polarization induction problem by a wavenumber integral equation approach

Summary. The H -polarization induction problem is solved in terms of an integral equation, which in the horizontal direction is transformed into the wavenumber domain. By this transformation the usual complicated integral expressions for the Green's tensor elements are removed. By extracting asymptotic features from the system of linear equations, we reduce the number of equations considerably independent of whether the horizontal variation in the conductivity is continuous or discontinuous. Likewise we reformulate the problem so that arbitrary conductivity contrasts may be studied. The method is finally tested by comparing with analytic solutions, and good agreement is achieved. Furthermore the numerical results indicate that a small amount of wavenumbers is required.     

Tee-SVX: Enhanced Oil Flow Rate in Solvent Vapor Extraction Process

The production rate in classical vapor extraction (VAPEX) is far too low for the process to be considered commercially viable. This is largely because the classical process utilizes forces of buoyancy and mass transfer by diffusion and dispersion to distribute the solvent and gravity to drain the diluted oil to the producer. This article presents a new well pattern Tee-SVX which may enhance the oil-flow rate by two to ten times over that of the classical approach. In the new well pattern, additional horizontal injectors, perpendicular to the injector and the producer in Classical VAPEX, are placed in the topmost region of a reservoir. The oil-rate-enhancement mechanism for this new well pattern involves two features. First, the operation pressure of the top injectors is set slightly higher than the bottom injector pressure. This facilitates a downward driving force to assist gravity drainage of diluted oil to the producer. Second, the supplementary injectors generate an additional diluted oil profile perpendicular to the diluted oil profile of the Classical VAPEX process. Therefore, in the new well pattern, the heavy oil is solvent contacted and diluted in both vertical planes (one plane perpendicular to and the other parallel to the horizontal producer), whereas in Classical VAPEX, the heavy oil is diluted in only one. A series of numerical simulations were conducted to evaluate this process. In order to obtain reliable evaluation results, the numerical dispersion was eliminated through extrapolating the simulation results at different grid sizes to an infinitesimal grid size (Δy → 0). The simulation results suggest that the oil-flow rate can be enhanced two to ten times greater than that with Classical VAPEX, depending on the well spacing of the top injectors. For example, for a well spacing of the top horizontal injectors of 120 m, the oil-flow rate from the original producers will be 5.5 times higher than that of Classical VAPEX. For thinner reservoirs, the Tee-SVX can enhance the oil-production rate higher compared with Classical VAPEX. For reservoirs with a small gas cap, Lateral-SVX with vertical injector is quite effective.     

Beach face and berm morphodynamics fronting a coastal lagoon

This study documents two different modes of berm development: (1) vertical growth at spring tides or following significant beach cut due to substantial swash overtopping, and (2) horizontal progradation at neap tides through the formation of a proto-berm located lower and further seaward of the principal berm. Concurrent high-frequency measurements of bed elevation and the associated wave runup distribution reveal the details of each of these berm growth modes. In mode 1 sediment is eroded from the inner surf and lower swash zone where swash interactions are prevalent. The net transport of this sediment is landward only, resulting in accretion onto the upper beach face and over the berm crest. The final outcome is a steepening of the beach face gradient, a change in the profile shape towards concave and rapid vertical and horizontal growth of the berm. In mode 2 sediment is eroded from the lower two-thirds of the active swash zone during the rising tide and is transported both landward and seaward. On the falling tide sediment is eroded from the inner surf and transported landward to backfill the zone eroded on the rising tide. The net result is relatively slow steepening of the beach face, a change of the profile shape towards convex, and horizontal progradation through the formation of a neap berm. The primary factor determining which mode of berm growth occurs is the presence or absence of swash overtopping at the time of sediment accumulation on the beach face. This depends on the current phase of the spring-neap tide cycle, the wave runup height (and indirectly offshore wave conditions) and the height of the pre-existing berm. A conceptual model for berm morphodynamics is presented, based on sediment transport shape functions measured during the two modes of berm growth.     

Present‐day stress orientation in the Clarence‐Moreton Basin of New South Wales,Australia: a new high density dataset reveals local stress rotations

Early phases of the Australian Stress Map project revealed that plate boundary forces acting on the Indo‐Australian Plate control the long wavelength of the maximum horizontal present‐day stress orientation in the Australian continent. However, all numerical models of the stress field to date are unable to predict the observed orientation of maximum horizontal stress in the northeast of New South Wales, Australia. Recent coal seam gas exploration in the Clarence‐Moreton Basin, eastern Australia, provides an opportunity to better evaluate the state of crustal stress in this part of the continent where only limited information was available prior to this study. Herein, we conduct the first analysis of the present‐day tectonic stress in the Clarence‐Moreton Basin, from drilling‐induced tensile fractures and borehole breakouts interpreted using 11.3 km of acoustic image logs in 27 vertical wells. A total of 2822 drilling‐induced stress indicators suggest a mean orientation of N069°E (±23°) for the maximum horizontal present‐day stress in the basin which is different from that predicted by published geomechanical‐numerical models. In addition, we find significant localised perturbations of borehole breakouts, both spatially and with depth, that are consistent with stress variations near faults, fractures and lithological contrasts, indicating that local structures are an important source of stress in the basin. The observation that structures can have a major control on the stresses in the basin suggests that, while gravity and plate boundary forces have the major role in the long wavelength (first‐order) stress pattern of the continent, local perturbations are significant and can lead to substantial changes in the orientation of the maximum horizontal present‐day stress, particularly at the basin scale. These local perturbations of stress as a result of faults and fractures have important implications in borehole stability and permeability of coal seam gas reservoirs for safe and sustainable extraction of methane in this area.     

Transient electromagnetic fields about an infinitesimally long grounded horizontal electric dipole on the surface of a uniform half-space

Summary. Laplace and Bessel Transforms are used to solve for the transient behaviour of the electromagnetic fields after switching off a steady current in a grounded infinitesimal horizontal dipole on the surface of a uniformly conducting half-space. Simple analytic expressions, which are valid for times sufficiently long after the switch that displacement terms can be ignored, are obtained on the surface of the half-space for the electric field and the time derivative of the magnetic field. At the instant of switching an infinitesimally long image becomes established directly under the source dipole. It is the diffusion of this image which gives the vertical magnetic field and horizontal electric fields their transient behaviour. During the transient, there is also a decaying charge distribution on the surface.     

Electromagnetic response of an arbitrarily shaped three-dimensional conductor in a layered earth—numerical results

Summary. In an earlier work, mathematical formulation on computing the electromagnetic response of an arbitrarily shaped three-dimensional inhomogeneity in a layered earth had been worked out using an integral equation technique. The method has been used to show its efficacy by computing numerical results. Introducing suitable changes of variables the secondary contributions to Green's dyadic are put in the form of convolution integrals and are computed using a digital linear filtering scheme. The matrix equation is solved for the unknown electric fields in the inhomogeneity. The scattered fields are then calculated at the surface of the Earth using the appropriate Green's dyadic. The performance of the computations has been shown by comparing the numerical results with those obtained by analogue modelling as well as by other numerical schemes. The use of digital linear filtering saves an enormous amount of computer time.
The effects of varying excitation-frequency, conductivity of the host medium and that of the overburden have been studied in detail for a horizontal loop system traversing over a two-layered earth with a prismatic inhomogeneity situated in the lower conducting half space.     

西藏古乡沟泥石流的数值模拟

泥石流运动数值模拟是确定泥石流泛滥范围、进行危险性分区、辅助减灾工程设计与检验的重要方法。以西藏波密古乡沟为实例,从泥石流的二维运动方程出发,使用一种新的数值方法对古乡沟50a一遇泥石流在堆积扇上的运动过程进行了数值模拟,获得了其最大流速、流深、动量和动能的空间分布,并据此确定了受泥石流危害较小的川藏公路穿越堆积扇方案。将泥石流数值模拟结果与游勇等1997年做的模型试验结果进行了比较,具有较好的相似性。     

A Model-Based Evaluation of Horizontal Wells for Improving the Functionality of an Urban Reservoir System

A method is proposed for utilizing the storage capacity of unconfined aquifers to increase the efficiency of existing water-supply reservoirs presently losing water to seasonal spillages. The approach proposed uses horizontal wells to control the watertable elevation in the reservoir catchments, permitting a degree of control over the discharge of the streams flowing to the reservoirs. This may provide an alternative to reservoir construction, thus avoiding loss of ecosystems or productive land. The use of horizontal wells provides a range of control options, including increasing stream discharge during summer and reducing and delaying peak discharge from rainfall events during winter. A numerical groundwater model is utilized to simulate the operation of hypothetical horizontal wells within a water supply catchment of Auckland City, New Zealand. Results suggest potential for a significant increase in summer stream flow, coupled with a reduction in winter peak discharges associated with present reservoir spillage.     

The mapping of induced currents around the Kenya Rift: a comparison of techniques

Summary. Single-station and inter-station transfer functions are derived from data recorded by a magnetometer array in and around the Kenya Rift Valley. Different methods of presentation of the transfer function estimates are compared in terms of their ability to define lateral variations in electrical conductivity. When, as in East Africa, the conductivity structure is complex and three-dimensional, by far the most useful method of presentation is to use the transfer functions to simulate the anomalous internal fields associated with regional current flow at a particular azimuth. The use of inter-station rather than single-station transfer functions is almost essential where the amplitudes of anomalous horizontal fields are of the same size or greater than the normal fields. Horizontal field transfer functions prove to be just as useful as those usually calculated for the vertical component, and provide strong additional constraints on the internal current system.
Maps of simulated vertical and horizontal fields of internal origin indicate the importance of current channelling around the Rift Valley. A conductor just to the east of Nairobi apparently funnels regionally induced currents into two conductors associated with the Rift and dome; one at shallow depths beneath the Rift floor, and a deeper body to the east.     

Signal contribution sections and their use in resistivity studies

Summary. The electrostatic analogue to direct current resistivity prospecting is employed to construct vertical sections through lines of four electrodes. These show the relative contributions made by individual volume elements of earth to the total potential difference measured between the two potential electrodes. Signal contribution sections enable a comparison of the response of various electrode configurations to subsurface conditions and help to explain, for example, why the dipole—dipole configuration is so suited to horizontal profiling and the Wenner and Schlumberger configurations to vertical sounding.
The spurious response of apparent resistivity curves to near surface lateral resistivity variations is also explained with particular attention being given to the Wenner tripotential configurations.     

Three-dimensional model for tides and surges with vertical eddy viscosity prescribed in two layers—II. Irish Sea with bed friction layer

Summary. Using the theory of Part I of this work, a three-dimensional numerical tidal model of the Irish Sea is formulated with eddy viscosity defined in two layers through the vertical. In the upper layer the viscosity is uniform through the depth; in the lower layer it reduces linearly with increasing depth to a small value at the sea-bed where a no-slip condition is imposed. After experiments with varying frictional levels, the model is verified against observations of tidal elevation throughout the Irish Sea and against observations of current at a position in Liverpool Bay. To a first approximation, the well-known square law of bottom friction (involving depth-mean current) is confirmed, but only when the bottom roughness length is particularly related to the thickness of the lower layer.     

Spatial and temporal variations in the thermodynamics since the Last Glacial Maximum at Jutulstraumen drainage basin,East Antarctica

A flow-line model is coupled to a 2D temperature model to simulate the thermodynamic changes of Jutulstraumen drainage basin due to grounding line retreat and increased surface temperature since the Last Glacial Maximum (LGM). The basin consists of a plateau drained by an outlet glacier, and the simulated ice volume reductions are 1% and 2% respectively of the current grounded volume. The mountain ranges H.U. Sverdrupfjella and Neumayerskarvet fringing the plateau were not overridden by ice at the LGM, while the Nashornet nunataks closer to the grounding line were. Today the glacier is almost in balance with the current climate, with the highest thinning rate?<?5.0×10^?3 ma^?1 at the plateau. The simulated present-day thermal regime of the outlet glacier shows a basal layer at the pressure melting point and negative temperature gradients with depth due to horizontal advection of cold ice from the plateau. Sensitivity studies show that strain heating and horizontal advection are important for the basal temperatures in the fast flowing outlet glacier and for about half of the wide basin at the polar plateau. Increased strain heating and horizontal advection since the LGM control the response time required to readjust to the new conditions, and it controls the present-day volume.     

植被带布局对局地流场的作用*

本文用一个二维数值模式,实施了六组试验,分析了植被带不同布局对局地温度场和垂直速度场的影响。结果指出,若植被带布局得当,植被—裸地之间的非均一性可望激发出更强的上升运动。这对于半干旱区对流性降水的形成是有利的。     

Two-dimensional induction in a thin sheet of variable integrated conductivity at the surface of a uniformly conducting earth

Summary. The forward solution of the general two-dimensional problem of induction in a model earth comprising a uniformly conducting half-space covered by a thin sheet of variable integrated conductivity is obtained. Unlike some previous treatments of similar problems, the method presented here does not require the field to be separated into its normal and anomalous parts. Both the E - and B -polarization modes of induction are considered and in each case the solution is expressed in terms of the horizontal component of the electric field satisfying, on the surface of the conductor, a singular integral equation whose kernel is a well-known analytic function. A recently published solution of the coast effect is included as a special case. The numerical procedure for solving the integral equations is described and some illustrative calculations are presented.     

山地、湖泊的三维大气－斜压水动力学耦合模式──在日本琵琶湖水－气系统中的应用

大气、湖泊的边界层是大气边界层和湖泊边界层两部分组成的，它们之间通过湍流交换连接在一起，形成一整体系统。经过分界面的动量通量是连续的，热量是守恒的。基于此，本文在近几年工作的基础上，建立了大气边界层与水动力学模式的耦合模型，并对日本琵琶湖进行了实验研究，获得了一些比正压和均匀风场模式更符合观测实际的结论：（１）无论夏季（ＳＳＷ）和冬季（ＮＮＥ）作用下，湖面均可形成正的风涡度场；（２）夏季时，风的正涡度场对形成琵琶湖中观测到的环流特征起着十分重要的作用，而且还构造出与环流相匹配的深水区低、浅水区高的温度场；（３）夏季时，初始温度场的水平分布特征对模式结果的影响很小；（４）冬季时，湖中无明显水平环流生成。