A calibrated,non-invasive method for measuring the internal interface height field at high resolution in the rotating,two-layer annulus

We describe a remote sensing method for measuring the internal interface height field in a rotating, two-layer annulus laboratory experiment. The method is non-invasive, avoiding the possibility of an interaction between the flow and the measurement device. The height fields retrieved are accurate and highly resolved in both space and time. The technique is based on a flow visualization method developed by previous workers, and relies upon the optical rotation properties of the working liquids. The previous methods returned only qualitative interface maps, however. In the present study, a technique is developed for deriving quantitative maps by calibrating height against the colour fields registered by a camera which views the flow from above. We use a layer-wise torque balance analysis to determine the equilibrium interface height field analytically, in order to derive the calibration curves. With the current system, viewing an annulus of outer radius 125?mm and depth 250?mm from a distance of 2?m, the inferred height fields have horizontal, vertical and temporal resolutions of up to 0.2?mm, 1?mm and 0.04?s, respectively.     

Les macrophytes du lac de Bienne

The study of the qualitative and quantitative distribution of macrophytic vegetation of Bienne lake is realized with the help of colour aerial photographies and verifications on the field. This distribution is discussed in relation to the trophic level of the lake and compared with that of the Geneva lake and Morat lake.     

La végétation macrophytique d'un lac hautement eutrophe du Plateau suisse: Le Baldegger See

The study of the qualitative and quantitative distribution of the Baldeggersee macrophytic vegetation is undertaken in 1980 with the help of colour aerial photographies and verification on the field. The comparison with old data allows then a short survey of the macrophytic evolution in relation to the eutrophication.      

Identification and investigation of sulphur plumes along the Namibian coast using the MERIS sensor

In the upwelling area along the Namibian coast of SW-Africa sulphur discolorations were investigated to study the impact of hydrogen sulphide on the ecosystem using satellite imagery. The formation of colloidal sulphur in the upper water layer results from the oxidation of hydrogen sulphide. The occurrence of sulphur plumes as well as their temporal and spatial development was investigated in relation to the driving meteorological and oceanographic conditions. Because of the sporadic occurrence of sulphur events and the limited number of ship-borne investigations in that area remote sensing of ocean colour is the only method to follow these phenomena continuously and synoptically. In the past the sulphur plumes were studied by true colour images derived from ocean colour satellite data like sea-viewing wide field of view sensor or moderate resolution imaging spectroradiometer and identified by their typical milky turquoise discoloration.     

转换函数和磁场振幅比的水平源场效应的数值计算研究

本文用三维有限差分法研究转换函数和磁场振幅比的某些源场效应,这些物理量常用于地震预报的研究工作。我们研究了几种水平均匀源场的情况,其中包括圆偏振的源场和极化方向不同的几种线性枞化源场。结果表明,即使在最简单的均匀水平源场的情况下,磁场振幅比肯定受源场变化的影响。然而,单台转换函数就不受均匀水平源场性质变化的影响。因此,转换函数随时间的变化适用于监测地震前兆,尤其适用于中低纬度地区,因该地区的源场垂直分量比较小。      

Magmatic structures in the Krkonoše–Jizera Plutonic Complex,Bohemian Massif: evidence for localized multiphase flow and small-scale thermal–mechanical instabilities in a granitic magma chamber

The present paper examines magmatic structures in the Jizera and Liberec granites of the Krkonoše–Jizera Plutonic Complex, Bohemian Massif. The magmatic structures are here interpreted to preserve direct field evidence for highly localized magma flow and other processes in crystal-rich mushes, and to capture the evolution of physical processes in an ancient granitic magma chamber. We propose that after chamber-wide mixing and hybridization, as suggested by recent petrological studies, laminar magma flow became highly localized to weaker channel-like domains within the higher-strength crystal framework. Mafic schlieren formed at flow rims, and their formation presumably involved gravitational settling and velocity gradient flow sorting coupled with interstitial melt escape. Local thermal or compositional convection may have resulted in the formation of vertical schlieren tubes and ladder dikes whereas subhorizontal tubes or channels formed during flow driven by lateral gradients in magma pressure. After the cessation or deceleration of channel flow, gravity-driven processes (settling of crystals and enclaves, gravitational differentiation, development of downward dripping instabilities), accompanied by compaction, filter pressing and melt segregation, dominated in the crystal mush within the flow channels. Subsequently, magmatic folds developed in schlieren layers and the magma chamber recorded complex, late magmatic strains at high magma crystallinities. Late-stage magma pulsing into localized submagmatic cracks represents the latest events of magmatic history of the chamber prior to its final crystallization. We emphasize that the most favorable environments for the formation and preservation of magmatic structures, such as those hosted in the Jizera and Liberec granites, are slowly cooling crystal-rich mushes. Therefore, where preserved in plutons, these structures may lend strong support for a “mush model” of magmatic systems.     

Tensional crack development in physical models with inhomogeneities under load

Summary The paper deals with a study of the mean stress field (_M) and its gradient (_M/x) in uniaxially loaded two-dimensional models of small thickness with respect to other dimensions, in which two inhomogeneities, e.g., a circular opening and a linear slit have been made. Particular attention was paid to the stress field development in the treated models durin gthe change of the mutual geometric configuration of both the inhomogeneities in question. The problem was analysed theoretically by means of Muskhelishvili's potentials of the two-dimensional theory of elasticity. To compare the results of calculations, the schlieren method was used to visualize and record the studied gradients of stress fields in physical models. The results of the calculations were calibrated by laboratory tests. In this way the individual model situations were classified from the point of view of their resistance to the applied load.     

An efficient multivariate random field generator using the fast Fourier transform

It is often convenient to use synthetically generated random fields to study the hydrologic effects of spatial heterogeneity. Although there are many ways to produce such fields, spectral techniques are particularly attractive because they are fast and conceptually straightforward. This paper describes a spectral algorithm for generating sets of random fields which are correlated with one another. The algorithm is based on a discrete version of the Fourier-Stieltjes representation for multidimensional random fields. The Fourier increment used in this representation depends on a random phase angle process and a complex-valued spectral factor matrix which can be readily derived from a specified set of cross-spectral densities (or cross-covariances). The inverse Fourier transform of the Fourier increment is a complex random field with real and imaginary parts which each have the desired coveriance structure. Our complex-valued spectral formulation provides an especially convenient way to generate a set of random fields which all depend on a single underlying (independent) field, provided that the fields in question can be related by space-invariant linear transformations. We illustrate this by generating multi-dimensional mass conservative groundwater velocity fields which can be used to simulate solute transport through heterogeneous anisotropic porous media.     

Dependence of borehole shear‐horizontal‐wave seismoelectric response on soil textures

The phenomenon of acoustic waves inducing electric fields in porous media is called the seismoelectric effect. Earlier investigators proposed the usage of seismoelectric effect for well logging. Soil texture has a strong influence on the coupled wave fields during shallow surface explorations. In this article, we study the borehole pure shear‐horizontal wave and the coupling transverse‐electric field (acoustic–electrical coupling wave fields) in the partially saturated soil. Combined with related theories, we expand the formation parameters to partially saturated forms and discuss the influence of soil texture conditions on the seismoelectric wave fields. The results show that the elastic and electrical properties of porous media are sensitive to water saturation. The compositions of the acoustic and electric fields for different soil textures do not change, but the waveforms differ. We also use the secant integral method to simulate the interface‐converted electromagnetic waves. The results show that interface response strength is greatly influenced by soil texture. In addition, considering the sensitivity of the inducing electric field to fluid salinity, we also simulate the time‐domain waveforms of electric field for different pore fluid salinity levels. The results show that as the salinity increases, the electric field amplitude decreases monotonically. The above conclusions have certain significance for the application of borehole shear wave and its coupled electric fields for resource exploration, saturation assessment and groundwater pollution monitoring.     

电离层-空气层-地球介质耦合下大尺度大功率可控源电磁波响应特征研究

电离层-空气层-地球介质（我们称为“地-电离层”模式）耦合下大功率可控源电磁波的研究在国内外较少.近年来,有学者做了相关的数值模拟,但没有考虑空气中位移电流的影响.我们根据研究对象和问题建立地质模型和相应的数学模型,采用R函数法进行“地-电离层”模式水平电缆接地偶极源的电磁波场强公式推导,同时考虑电离层和空气中位移电流的影响.在验证方法可靠的前提下,进行了数值计算.探讨了“地-电离层”模式大功率可控源电磁波的响应特征.数值计算结果表明,由于电离层和空气层中位移电流的影响,“地-电离层”模式大功率可控源电磁波的特征与地球半空间可控源电磁波的特征有很大的不同,体现在前者除了有近场、远场外,还存在由于电离层影响下场强衰减变小的波导场.并随着收发距增大和频率的增高,出现了电场和磁场方向图的变化.     

Gravity and magnetic field interpretation based on transformations of horizontal gradients in the VECTOR system

A method of potential field processing based on the transformation of vectors of the total horizontal gradient in windows of various sizes is considered. The gradients are calculated at the centers of triangles, whose vertices are points of observations, as a rule, of gravity and magnetic fields. Averaging of horizontal gradients of the field rather than initial values of the field is the main distinction of this approach from the known methods. This procedure, referred to in this paper as vector scanning of the field, makes it possible to obtain layer distributions of field sources in a 3-D diagram that is a quasi-density model of the study medium within the framework of certain model concepts. The paper presents a model example demonstrating the possibility of separating the fields produced by two sources located on a vertical line and an example illustrating the application of this method to the interpretation of the gravity field in the zone of the geodynamic influence of the Urals.     

Interpretation of MHD-sounding data from the Kola Peninsula by the electromagnetic migration method

The electromagnetic migration method, based on principles analogous to those of seismic migration, is developed.The concept of migrated fields is based on the Stratton-Chu type integrals, written in reverse time for the observed electromagnetic fields. Four types of migrated fields, which form a system of migration transformations of the transient electromagnetic field, are introduced.Study of the properties of the migrated fields by means of theoretical and model examples makes it possible to determine the optimal parameters of the migration procedure in which the anomalous field sources are localized by means of migration transformation.Information about the experiment using the MHD-generator (experiment ‘Chibini’) carried out on the Kola Peninsula to study the geoelectrical structure of the Earth's crust and upper mantle is included. The electromagnetic migration method for the interpretation of MHD-sounding data recorded on the Kola Peninsula, along a profile crossing the mineral-rich region of the Imandra-Varzuga structure, permits us to determine the location of a conducting zone at the depth of 10 km in the Earth's crust.     

中国境内各阶非偶极子磁场的时空变化

为了研究中国境内各阶非偶极子(ND)磁场,通过最新的地磁场模型CALS10K.1b计算了10000 BC—AD 1990年ND磁场在中国境内的时空及能量变化;22,23和24极子磁场零值线主要呈现出从我国中北部向东南部移动,从北部向东部移动,以及从东南部向西北部移动的趋势;除了26极子磁场,其余ND磁场在1500年以后均有“翘尾”现象,其所占主磁场的比值在10000 BC—AD 1500年呈震荡变化,从1500年开始快速上升;除了22极子磁场,其它各阶ND磁场能量随时间变化的趋势基本一致. 在5650 BC年左右,各阶ND磁场能量值均出现高值并随阶数的增大而衰减,反映了在该时间点通过地核发电机产生的磁场能量随径向距离而近似线性衰减,地磁场能量主要源自地球液态外核. 将CALS10K.1b模型结果与IGRF11模型对比后,得出CALS10K.1b模型所计算的ND磁场及能量值较为可靠,而两者差异主要源于数据和建模方法的不同.      

Micromechanics model of ice fields—II: Monte Carlo simulation

The graph model presented in Part I of this series provides the basis for development of a computer simulation of tightly packed ice fields taken as ensembles of square-shaped ice floes with random physical properties. A program based on an alternating-direction scheme is developed to model the time evolution of a field of ice floes in a rectangular domain. The simulation of a field in an Arctic channel shows that there is a strong tendency for an earlier onset of microscale plastic flows and formation of irregular clusters of ice floes and openings in a field with spatially random properties versus a field with deterministic spatially homogeneous properties. A special study is conducted of an elastic-plastic transition in a field of 101×101 floes. The transition to macroscopically plastic flow is possible only with a percolation of inelastic regions through the entire domain of the ice field. The fact that this percolation is characterized by a noninteger fractal dimension uncovers a (possibly principal) generation mechanism of ice field morphologies, and points to scale dependence in mechanics of ice fields for certain ranges of loads.     

The Variation of Stresses due to Aseismic Sliding and its Effect on Seismic Activity

— Numerical simulation of recurring large interplate earthquakes in a subduction zone is conducted to explore the effects of aseismic sliding on the variation of stresses and the activity of small earthquakes. The frictional force obeying a rate- and state-dependent friction law is assumed to act on the plate interface in a 2-D model of uniform elastic half-space. The simulation results show that large earthquakes repeatedly occur at a constant time interval on a shallow part of the plate interface and that aseismic sliding migrates from the upper aseismic zone as well as from the lower aseismic zone into the central part of the seismogenic zone before the occurrence of a large interplate earthquake. This spatiotemporal variation of aseismic sliding significantly perturbs the stresses in the overriding plate and in the subducting oceanic plate, leading to the precursory seismic quiescence in the overriding plate and the activation of the intermediate-depth earthquakes of down-dip tension type. After the occurrence of a large interplate earthquake, the activity of the intermediate-depth earthquakes of down-dip compression type in the subducting slab is expected to increase and migrate downward. This is because the downward propagation of postseismic sliding causes the downward migration of compressional-stress increase in the down-dip direction of the plate interface. The simulation result further indicates that episodic events of aseismic sliding may occur when the spatial distributions of friction parameters are significantly nonuniform. The variation of stresses due to episodic sliding is expected to cause seismicity changes.     

Near-surface geophysics: Complex investigations of the lithosphere-atmosphere interactions

The key trends and problems in the study of interactions between the geospheres in the near-surface areas of the continental crust of the Earth which are of most interest for the formation of living environment are formulated. The main periodicities in the variations of geophysical fields determining the natural environmental rhythms are determined. The interactions and transformations of geophysical fields (electric field in the ground and in the atmosphere, radon emanation field, and the field of microseismic vibrations) are considered. It is noted that the tectonic discontinuities and weak disturbances in the solid Earth such as lunar-solar tides and variations in the atmospheric pressure play an important role in the formation of the regimes of geophysical fields at the boundary of the Earth’s crust and the atmosphere. The synchronous instrumental observations reveal a mutual influence and significant correlation between the amplitude variations of geophysical fields of different nature.     

Midday reversal of equatorial ionospheric electric field

A comparative study of the geomagnetic and ionospheric data at equatorial and low-latitude stations in India over the 20 year period 1956–1975 is described. The reversal of the electric field in the ionosphere over the magnetic equator during the midday hours indicated by the disappearance of the equatorial sporadic E region echoes on the ionograms is a rare phenomenon occurring on about 1% of time. Most of these events are associated with geomagnetically active periods. By comparing the simultaneous geomagnetic H field at Kodaikanal and at Alibag during the geomagnetic storms it is shown that ring current decreases are observed at both stations. However, an additional westward electric field is superimposed in the ionosphere during the main phase of the storm which can be strong enough to temporarily reverse the normally eastward electric field in the dayside ionosphere. It is suggested that these electric fields associated with the V × Bz electric fields originate at the magnetopause due to the interaction of the solar wind and the interplanetary magnetic field.     

Erosion and terrace failure due to agricultural land abandonment in a semi‐arid environment

Agricultural land abandonment is currently widely spread in Mediterranean countries and a further increase is expected. Previous research has shown that abandoned fields in semi‐arid areas are more vulnerable to gully erosion. The absence of ploughing and slow vegetation recovery cause the formation of soil crusts with low infiltration rates, resulting in increased runoff and gully erosion risk. The objective of our study was to assess the extent and causes of erosion and terrace failure on abandoned fields and to discuss options for mitigation. The study was carried out in the Carcavo basin, a semi‐arid catchment in southeast Spain. At catchment scale all abandoned fields were surveyed and characteristics of each field were described. Additionally we surveyed abandoned and cultivated terraces and used statistical analyses to determine the factors that induce terrace failure. At field scale we constructed a detailed digital elevation model (DEM) for an abandoned terrace field in order to calculate sediment losses since time of abandonment. The results revealed that more than half the abandoned fields had moderate to severe erosion and the statistical analysis showed that these fields had significantly steeper slopes, were terraced and had cereals as previous land use. Factors that increase the risk of terrace failure were land abandonment, steeper terrace slope, loam texture, valley‐bottom position and shrubs on the terrace wall. The reconstructed erosion rate (87 ton ha^?1 year^?1) confirmed the importance of gully erosion on these abandoned terrace fields. Potential soil and water conservation practices to mitigate soil erosion after abandonment are: (1) maintenance of terrace walls, as a result more water is retained, which increases vegetation cover and consequently decreases erosion. (2) Revegetation with indigenous grass species on spots with concentrated flow, especially near terrace walls. Copyright © 2008 John Wiley & Sons, Ltd.     

An analogue model study of electromagnetic induction for island-continent ocean channels

The behaviour of time-varying electromagnetic fields near an island situated in a shallow ocean with a nearby continent is investigated using a scaled analogue model. To study the effect of the proximity of the continent, various island-continent distances are treated. The presence of the continent tends to augment enhancements of the field components at the island coastlines for all channel widths studied, while the island affects the enhancements of the fields over the continental coastline only for very narrow channel widths (half the island width or less), and does not affect the horizontal to vertical magnetic field ratio at the coastline at all. To examine the effect of the shape of the island, square and circular island models are used. For the frequencies studied, the island shape has little effect on the fields over the continental coastline, but over the island, the spatial variation of the fields is considerably less for the circular island than for the square island.     

Comparison of the time-domain electromagnetic field from an infinitesimal point charge and dipole source

An electromagnetic field is generated through the accelerating movement of two equal but opposite charges of a single dipole. An electromagnetic field can also be generated by a time-varying infinitesimal point charge. In this study, a comparison between the electromagnetic fields of an infinitesimal point charge and a dipole has been presented. First, the time-domain potential function of a point source in a 3D conductive medium is derived. Then the electric and magnetic fields in a 3D homogeneous lossless space are derived via the relation between the potential and field. The field differences between the infinitesimal point charge and the dipole in the step-off time, far-source, and near-source zones are analyzed, and the accuracy of the solutions from these sources is investigated. It is also shown that the field of the infinitesimal point charge in the near-source zone is different from that of the dipole, whereas the far-source zone fields of these two sources are identical. The comparison of real and simulated data shows that the infinitesimal point charge represents the real source better than the dipole source.