High-conductivity Layer Research Articles

PHYSICAL AND COMPUTER MODELING OF INDUCED POLARIZATION *

AbstractThe Laboratory of Geophysics of the University of Arizona was presented with an exploration problem by Falconbridge, Ltd. of Canada. Massive mineralized hemispherical “pods” are in the vicinity of a tuff layer of high conductivity and induced polarization response, covered by large thicknesses of resistive volcanics. The initial approach was to utilize electrolytic tank modeling. The extreme resistivity and IP contrasts proved to be difficult to recreate. Two dimensional modeling was attempted next with conductive paper, using copper and silver paint for anomalous masses. This method also proved inadequate. Finally, mathematical equations were solved which could model any arbitrary anomalous body in any steady state electrical field. Plane waves as well as point current sources producing non‐plane waves are possible. Finite difference equations were derived for the non‐linear partial differential equations under consideration. The equations were solved using a digital computer. Initially, the boundary conditions had to be satisfied at the boundaries of resistivity changes, severely restricting possible geometric shapes for anomalous bodies. The final and successful solution was to apply numerical techniques to obtain solutions of equations which require only that the relative resistivities through the area be specified. The Falconbridge problem and its solution are analyzed.

Layers of high thermal conductivity in the North Atlantic

Quartz-sand layers in the NW Atlantic basin have thermal conductivity values more than twice as high (up to 4.5 mcal/cm sec °C) as the values for normal sediment. The layers are thick enough to affect the measured temperature gradients; thus many thermal-conductivity measurements must be made to obtain an accurate heat-flow measurement in this part of the Atlantic. The conductivity of the layers cannot be estimated accurately from the water content of the sediments by using the empirical curve determined by Ratcliffe.

Electric conductivity anomaly in the Earth's crust and mantle

Much evidence that reflects anomalous distributions of electrical conductivity within the earth's crust and mantle has been put forward in recent years through geomagnetic and geoelectric observations. This anomaly is called the “CA” (conductivity anomaly), and the study of such heterogeneity has become one of the important topics of present-day geophysics. The existing CA's can be classified into two types. One is the “high-cut ΔZ type” which, as was first discovered at Mould Bay in the Canadian Arctic, is characterized by strong attenuation of the vertical magnetic variations of short period. The other is the “directional variation type” such as has been found in Japan. It seems likely that the former CA occurs over the surface of an unusually raised mantle layer of high conductivity and that the latter takes place over the areas where the top-surface of such a layer dips down into the deeper interior. Some of the undulations of the conducting layer are successfully correlated to those of high-temperature isotherms. It seems sometimes hard to ascribe the cause of a CA only to high underground temperatures. It is necessary in such cases, in order to be able to presume an extremely high conductivity, to assume partial melting, Fe-rich composition and hydration, although these points have not been worked out experimentally. The importance of electric currents, excited by a geomagnetic variation probably in a conductor such as an ocean, or in a water-saturated sedimentary layer has recently been pointed out. Such a “conduction effect type” mechanism may provide the third type of CA. The conduction effect seems to play an important role on the CA's found in north Germany, at Alert in the Canadian Arctic and at Eskdalemuir in Scotland. As has been the case in Japan, the western U.S.A. and Canada and so on, it is extremely useful to carry out multi-disciplinary geophysical studies over a CA area. Especially, studies of heat flow, seismic wave velocities and attenuation, and aeromagnetic and gravity surveys are useful for constructing a unified view of the underground structure.

Electrical Conductivity Anomaly and Terrestrial Heat Flow

There are two types in anomalies of short period geomagnetic variations: one (High cut ΔZ type) is characterised by high attenuation of high frequency components of ΔZ and the other (Directional variation type) by unusual directionality in the variation vector. “High-cut ΔZ type” anomaly is expected to be found in areas where the electrically conducting layer is raised and “Directional variation type” is expected to exist at boundaries between areas having different electrical properties. Reported anomalies of geomagnetic variations in various parts of the world are classified into the above two types and compared with the observations of terrestrial heat flow. Most of “High-cut ΔZ type” anomalies are associated with high heat flow, whereas most of “Directional variation type” anomalies are associated with the boundaries of high and low heat flow areas. Such correlations are interpreted in terms of the temperature distribution in the upper mantle. Typical of “Directional variation type” anomalies are found at continental margins, where the effect of sea water should not be overlooked. It is further shown that, from the view-point of both heat flow and the conductivity anomaly, there are two types in continental margins: normal (Atlantic type) and island arc type (Pacific type) continental margins. Finally, a model that appears reasonable is presented for Central Japan anomaly of which complex nature so far rejected theoretical interpretations: Central Japan anomaly is caused by the superposition of peninsula effect and undulation of high conductive layer under the Philippine Sea coastal area of southwestern Honshu where heat flow is higher in the sea than in the land. Unlike northeastern Honshu, southwestern Honshu is not a typical island arc.

Influence of Relaxation Effects in Nonequilibrium J × B Devices

The coupled distributions of current density, electron temperature, and plasma composition in nonequilibrium plasma flows depend strongly on relaxation effects arising from finite reaction rates and from electron-energy convection. This dependence is studied in multielectrode magnetohydrodynamic channels in conjunction with the influence of thermal and concentration diffusion, and of thermal and velocity boundary layers. Relaxation effects are shown to control the possibility of current shorting along the conductor walls and to influence the stability of the distributions. For slow reaction rates and large gas velocities, the current and plasma-property distributions tend to become uniform in the core of the flow, but in the electrode-boundary regions nonuniformities persist both in the transverse and in the flow direction, and are strongly dependent upon the gas temperature and velocity boundary layers. Consequently, although relaxation effects allow high conductivity layers to extend over the insulator segments of the electrode walls, the possibility of Hall potential shorting through this path depends critically on the length of insulator segment between electrodes. Converging solutions, describing stable operation, have been obtained for values of the local Hall parameter β/ε as high as 6, depending on the geometry. At high values of β/ε, the current density and plasma-property distributions become time dependent, resulting in overall electrical characteristics that oscillate by large factors.

Polar surfaces of zinc oxide crystals

Polar surfaces have been prepared by cleavage in ultrahigh vacuum. The electrical conductivity at these surfaces has been studied between 90 and 600 °K. Changes occur by annealing in vacuum and by adsorption of atomic hydrogen or of oxygen. Additional information is obtained from cleaving crystals under mercury and measuring the current versus voltage relation. Furthermore, changes of the reflectance of light have been observed which are induced by adsorption of atomic hydrogen or of oxygen. For clean polar surfaces the conductivity must be below an upper limit of 4 × 10 −12 A/V. Transient heating in vacuum produces a high surface conductivity of 10 −4 A/V on the Zn surface whereas the O surface does not change. Atomic hydrogen acts much faster on the O surface than on the Zn surface creating an accumulation layer of high conductivity. By contrast, the depletion layer caused by the adsorption of oxygen is more evident on the Zn surface. The results of the optical measurements support these data. An atomic model of the polar surfaces is discussed. Several arguments concerning the distribution of electrons at these surfaces are considered in connection with the idea of dangling bonds and with compensation of the internal dipole moment of the wurtzite structure.

Water saturation and high electrical conductivity in the lower continental crust

Recent geomagnetic depth sounding and magnetotelluric measurements indicate that in young tectonically active continental areas there is a layer of high electrical conductivity at depths of about 20 to 40 km. Comparison with published experimental results shows that the lower crust in these areas will have the required high conductivity if it is water saturated. In stable shield areas, the crust has become dehydrated through metamorphic processes and the high conductivity layer is absent. Geomagnetic depth sounding and magnetotelluric measurements should provide valuable information on the present stage of development, and on the conditions at depth, in the crust for different continental areas.

Transcontinental crustal sections of the U.S.S.R. and adjacent areas

Transcontinental sections of the earth's crust and Upper Mantle from the Kuril Islands to the Carpathians, and along other principal directions, in all about 15 000 km long, show the following features:(1) The depth of an asthenosphere horizon agrees well with the position of a high-conductivity layer in the Upper Mantle.(2) Hypocenters of earthquakes are located in fault zones. In the zone of the Kuril Island arc, they plunge into the Upper Mantle to depths of hundreds of kilometers, dipping towards the continent.(3) Boundary velocities along the Mohorovičić discontinuity are not dependent on its depth, for they are always in the range 8.0–8.2 km/s; velocities within the crust are correlated with the depths of layers; in the roots of high mountains a layer of 7.5–7.8 km/s has been distinguished near the base of the crust.(3) The crustal thickness in folded areas is in a certain accordance with the altitude of the surface of the earth's solid cover, with Bouguer anomalies and amplitudes of neotectonic movements. It varies from 8 km in margin areas of the Pacific Ocean basin to 60 km in the Tien Shan. Average crustal thicknesses within the platforms are in the region of 40 km, increasing to 50 km in the southern outlying areas of the East-European platform and to 46 km in areas of recent tectonic activity within the Turanian and Siberian Platforms.(4) In deep depressions, internal and outlying seas, the crustal thickness is reduced, and 'granite' layer (6.0–6.5 km/s) is absent. Mesozoic formations of the Far East and adjacent zones of transition from the Pacific Ocean to the continent are distinguished by reduced thicknesses. In Alpine formations the 'basalt' layer dominates over the 'granite' layer.(5) Subhorizontal crustal stratification complicated by deep faults has been found everywhere. Plicate geosynclinal folding is not reflected in deep crustal structures.(6) Temperature of the platform folded basement is mainly related to thickness and composition of the sediments.The transcontinental sections provide a clearer idea of the relationship between anomalous geophysical fields and deep and shallow crustal structures than is given in corresponding maps.

Profiles of High Conductivity Shallow Layers in Silicon Produced by Boron Ion Implantation†

Abstract The conductivity profiles produced when boron ions are implanted into silicon have been investigated as a function of ion energy, total ion flux and annealing schedule. The bombardments were carried out in a manner such that channelling was minimized. These profiles have been compared with those predicted theoretically by Lindhard et al. (1963) for the bombardment of amorphous solids. At doping concentrations up to ∼1018 acceptors/cm3 the electrically active profile is in close agreement with the theoretical profile both as regards depth of the peak doping and the cut-off. At concentrations ≳ 1020/cm3 there is slight modification of the back edge of the profile, presumably due to channelling. The conductivity produced by a given annealing schedule has been shown to be a function of the doping level and the results have been compared with those of Pavlov et al. (1965). The results show that this technique is capable of producing high conductivity layers with a very high degree of control of doping...

Self‐Activated Semiconductivity in CdS Single Crystals

AbstractThe results of this paper show that self‐activated conductivity is present in moderately doped CdS crystals above 220 °C. Measurements are made of the dark conductivity s̀ in vacuum between 220 and 400 °C, at different heating and cooling rates. For the stationary conductivity, two straight lines are obtained in a log s̀ vs T−1 plot, with activation energies of 0.67 and 0.98 eV below and above 300 °C, respectively, which are independent of doping. It is shown that Frenkel disorder is dominant and determines conductivity above 300 °C, whereas at lower temperatures its influence is exceeded by a surface layer of high conductivity. The formation energy ϵF of Frenkel defects is estimated to be between 1.2 and 3.2 eV.The influence of sulfur vapor on conductivity is investigated between 450 and 650 °C. The observed decrease in conductivity is attributed to the diffusion of singly ionized Cd interstitials from the interior to the surface of the crystal. The diffusion coefficient for this process is deduced to be 2.1 × 10−5 exp cm2 s−1. \documentclass{article}\pagestyle{empty}\begin{document}$ \left({\frac{{ - 0.6{\rm eV}}}{{kT}}} \right) $\end{document} Furthermore, a relation is obtained for the ionization energies of Cd interstitials and Cd vacancies, and an estimate given of the band gap for thermal excitation.

Temperatures under steadily heated floors

The temperature distribution below the heated floor of a kiln has been investigated both theoretically and by model experiments. The aim is to determine how much insulation is required to keep the temperature of the ground below that point (perhaps as low as 40° C) at which it tends to dry out and subside. The results show that a considerable thickness of insulation is necessary to achieve this, but that much less insulation is needed if a thin layer of high conductivity is placed below the insulation. Iron reinforcing mesh could act as a high conductivity layer. It also appears that there is an advantage both in cost and in effectiveness in making the insulation thinner toward the sides of the kiln. The conclusions also apply to the insulation of the floor of cold storage rooms and skating rinks where the aim is to prevent `frost heave'.