TRANSIENT ELECTROMAGNETIC FIELD OF A POLARIZABLE HALF-SPACE DUE TO VARIOUS CURRENT PULSES *

In this paper, the expressions for the induced voltage function V(T) and mutual impedance function Z_m(T) have been derived for specific range of time T and specific values of induced polarization parameters a for ramp and saw-tooth type of current pulses. The computed results for various cases are also presented. The low values of induced polarization parameter a represent the medium possessing membrane polarization, whereas high values exhibit electrode polarization medium. The method has practical applicability and is best suited for the interpretation of transient electromagnetic fields over a polarizable half-space.     

ON THE ELECTRICAL BEHAVIOUR OF A POLARIZABLE SAMPLE STUDIED BY THE NORMALIZED TIME-INTEGRAL AND BALLISTIC METHODS*

In this paper the experimental data obtained studying the decay of a sample of pisolitic bauxite both by a ballistic method and the normalized time-integral procedure are compared. This comparison allowed me to note two peculiarities. First, the apparent capacitance of the sample, as well as its normalized time-integral, seems to show a characteristic behaviour within the same interval of the charging time. Secondly, while the apparent capacitance plotted versus time (measured from the energization interruption instant) appears to assume values which seem to tend towards asymptotic value of the apparent capacitance, it is to be noted instead that in these circumstances the relative normalized time integral shows no tendency to reach any limiting value.     

EXPLORATION OF HIGH RESISTIVITY BASEMENT USING ELECTRICAL AND MAGNETIC FIELDS OF QUASI-STATIC POINT SOURCES*

Determination of thickness of sediments (usually of high conductivity) overlying a high-resistivity basement is one of the basic problems of electrical exploration methods. This paper proposes to determine horizontal electrical conductance on the basis of impedance calculated from electrical and magnetic fields of distant quasi-static (low-frequency) point sources. Using the proposed method, horizontal conductance of the sediments can be determined also from artificial quasi-static noise-impulses coming from sources of unknown position and intensity. The results of analogue modeling and field examples prove the potential of the proposed technique.     

THE TRANSIENT ELECTROMAGNETIC RESPONSE OF A CONDUCTING SPHERE IN AN IMPERFECTLY CONDUCTING HALF-SPACE*

The presence of a conducting environment about a spherical ore body must be considered when calculating the transient electromagnetic response of the ore body due to a step current flowing in a large circular loop at the earth's surface. Failure to do this can easily lead to errors in excess of 10% in numerical calculations. Moreover, there is only a limited time interval in which the response of the spherical conductor is easily seen. In a poorly conducting ground the resonance response of the sphere is the first to be excited. Later, however, the non-resonance or wave-type response is excited. These waves destructively interfere and finally the response of the sphere decays with time as t^?7/2. For a range of times and depths the best loop for detecting the sphere has about the same radius as the sphere.     

ELECTRICAL SOUNDING OF A HALF-SPACE WITH A MONOTONIC CONTINUOUS VARIATION OF THE RESISTIVITY WITH DEPTH*

The interpretation of electrical sounding data for a subsurface with monotonic continuous variation of the resistivity with depth is becoming increasingly necessary. The contribution of this article is the derivation of the solution for the Wenner and the Schlumberger apparent resistivity functions for a resistivity varying as a real power of a linear positive function of the depth. The interpretation of sounding data in these cases can be used to estimate the variation of the porosity or the salt content of the pore water with depth.     

INVERSION OF SURFACE AND BOREHOLE ELECTROMAGNETIC DATA FOR TWO-DIMENSIONAL ELECTRICAL CONDUCTIVITY MODELS*

A method for inverting electromagnetic fields induced by a line source in an earth of two-dimensional conductivity structure is developed. Certain unique features of the finite element method are used to construct an efficient algorithm for the accurate calculation of the Jacobian matrix of partial derivatives, and the resulting linearized equations are solved using the damped least squares method. Case studies of theoretical data generated from a simple model of interest in geophysical prospecting show that, in general, it is impossible to obtain, from surface data alone, accurate estimates of the conductivity of structures buried deeper than 0.2 skin depths under a conducting overburden. The addition of borehole data to the surface data is found to increase the resolving power of the electromagnetic method dramatically. In particular, the borehole data appear to stabilize the inverse when only a poor initial estimate of the likely structure is given.     

ELECTROMAGNETIC FIELD OF SOURCES AT THE SURFACE OF A HOMOGENEOUS CONDUCTING HALFSPACE WITH HORIZONTAL ANISOTROPY: APPLICATION TO FISSURED MEDIA*

The electric and magnetic fields generated by horizontal electric and vertical magnetic dipoles lying on the surface of a conducting medium with horizontal anisotropy are investigated. Full expressions of their Fourier transforms are given, and the fields for a vertical magnetic dipole are calculated numerically. The radial and vertical magnetic components are found to be independent of the receiver-transmitter direction, whereas the other magnetic and electric components strongly vary with this direction. These results give useful criteria for defining the direction and amplitude of anisotropy from ground data; a ground experiment on fissured limestone was found to confirm the expected variations of the various field components. It is believed that this electromagnetic method can be used in order to provide information about the direction and amplitude of electric anisotropy.     

THE INFLUENCE OF LATERAL INHOMOGENEITIES ON THE DIPOLE METHODS*

The theory of electrical dipole soundings proved that this method can produce resistivity measurements, which are comparable with those obtained by electrical soundings of the Wenner or Schlumberger type. Their main advantage is the use of short cable lengths, which is important if the depth of penetration should be large. A considerable disadvantage of the dipole method is the great sensitivity to lateral discontinuities. Though these have an influence on the Schlumberger arrangement as well, they can disturb a dipole sounding to such an extent than an interpretation based on a horizontal layer case is no more possible. There are six different dipole arrays, which differ from each other with respect to the angle enclosed by the two dipole orientations-the current dipole AB and the measuring dipole MN. The theoretical comparison of the dipole arrays with the Schlumberger array concerning their sensitivity to lateral discontinuities is a useful basis for the choice of the most suitable configuration. Considering geological subsurface conditions the right choice of a dipole array can give an optimal result, i.e. a dipole sounding for which the sensitivity to lateral discontinuities is as small as possible under the given circumstances.     

FREQUENCY ELECTROMAGNETIC SOUNDING USING A VERTICAL MAGNETIC DIPOLE*

A horizontal transmitter loop (vertical magnetic dipole) is used for frequency electromagnetic (FEM) soundings. The frequency ranges from approximately 6 Hz to about 4000 Hz. The vertical and radial magnetic field components are measured for 20 frequencies per decade several hundred meters from the transmitter loop. A very small bandwidth is selected for amplification using a reference signal. An Apple computer is used for data acquisition. A computer program for Marquardt inversion optimizes the parameters for the n-layer case: the resistivities and thicknesses of individual beds and a correction factor for the primary magnetic field. Interpretation of each component individually yields practically the same parameters. Examples from the field are given with interpretation; comparison with dc resistivity measurements is provided. The ratio of vertical/radial magnetic field components vs. frequency can be transformed simply into apparent resistivity vs. apparent depth. This can be done in the field to obtain an estimation of the depth of the layer boundaries. FEM results are compared with Schlumberger d.c. sounding obtained at the same site.     

ELECTROMAGNETIC RESPONSE OF TWO-DIMENSIONAL INHOMOGENEITIES IN A DISSIPATIVE HALF-SPACE FOR TURAM INTERPRETATION *

An integral equation technique based on the integral formulation of the electric field of a harmonically excited line-source in the presence of a two-dimensional scattering in-homogeneity is outlined. In this treatment the inhomogeneity is assumed to have an arbitrary cross-sectional shape and to be located in a layered, dissipative half-space. The scattering responses in terms of the horizontal and the vertical magnetic field components observed on the ground surface are studied for a variety of geologic models involving overburden layers, vertical or inclined veins and multiple inhomogeneities. The horizontal component of the total magnetic field appears to be the most diagnostic response parameter. A thin vein shaped target with a cross sectional area of 0.05 δ₂× 0.50 δ₂ (δ₂ being the skin-depth in the lower half-space) located 0.50 δ₂ away from the line-source and under a highly conductive overburden layer (σ₁/σ₂= 300) of thickness 0.02 δ₂ is easily detected with a moderate resolution. The scattering responses are also sensitive to changes in depths of burial, inclination and conductivity contrast with the surrounding half-space of the target inhomogeneity. An interpretation scheme as well as a number of characteristic detectability parameters are developed for the conventional Turam method used in mineral exploration.     

THE TRANSIENT ELECTROMAGNETIC RESPONSE OF A POLARIZABLE SPHERE IN A CONDUCTING HALF SPACE1

A polarizable sphere embedded in a conducting half-space can give rise to negative voltage transients in a coincident-loop time-domain electromagnetic system. Such transients have been observed in field situations. Our results are obtained from a model in which the contributions of the host rock, the currents in the sphere, and the interaction between the sphere and the host rock are separated and superposed. This model uses approximations to the integral equation solutions rather than finite-element or finite-difference approximations, and so allows very rapid computation. The theoretical demonstration suggests that interpretation of the negative voltage transients as a polarization response is valid, but more detailed interpretation of polarization properties may not be possible, because the superposition of the polarization response on the normal response depends strongly on the position of the target.     

THE STUDY OF THE GENESIS OF SURFACE WATERS BY OBSERVING THEIR ELECTRICAL RESISTIVITIES*

Specific electrical resistivity of natural waters contains information on their genesis. The authors propose to conduct mass and regime observations of this parameter in river and stream beds. The electrical resistivities in streams flowing from under a glacier reveal details formed at the same time as the glacier. Observations in the beds of big rivers show a gradual increase in water salinity overlain by reductions by inflowing glacial waters. The diurnal and annual trend of changes in the electrical conductivity of water associated with the change in the balance of glacial and ground waters has been established near to glaciers. Resistivity observations help to locate discharge sites of sub-permafrost waters, for water.     

ON THE TRANSFORMATION OF DIPOLE TO SCHLUMBERGER SOUNDING CURVES *

A method is described to transform a dipole sounding curve, obtained with any one of the common dipole arrays over a horizontally layered earth, to the form of a Schlumberger sounding curve. Starting from the general expression which relates the dipole apparent resistivity to the Schlumberger apparent resistivity and its derivative with respect to the spacing, it is possible with some approximations to derive an easy numerical computation procedure in order to perform the transformation. The applicability of the method is discussed briefly.     

THE EFFECT OF A SUPERPARAMAGNETIC LAYER ON THE TRANSIENT ELECTROMAGNETIC RESPONSE OF A GROUND*

A thin superparamagnetic layer on the earth's surface greatly affects the transient electromagnetic response of a conducting ground. The effect of the layer is most evident for singleloop transient electromagnetic data where transient voltages decay as 1/t. Even when a separate transmitter and receiver are used, the effect of the superparamagnetic layer is still pronounced. In this case the effect of the 1/t term in the equation is much less. More dominant now is a 1/t² term. The effect of the superparamagnetism can readily be seen in the analytical expressions for the apparent resistivities. If the presence of the superparamagnetic layer is not recognized, then the apparent resistivities decrease with time rather than approach the true value of the host rock.     

临震电磁幅射信号的准一静电磁场场强理论计算

本文应用班尼斯特和迪尤布水平电偶极子准—静电磁场的工程表达式,利用1979年7月9日溧阳六级地震和1984年5月21日南黄海6.2级地震临震电磁波超低频观测结果,估算了这两次地震电磁发射的峰值电流矩IL,其量级分别为10~7—10~8安米(溧阳)和10~9安米(南黄海)。与按地壳波导原理所估算的结果吻合。     

QUANTITATIVE INTERPRETATION OF TRANSIENT ELECTROMAGNETIC FIELDS OVER A LAYERED HALF SPACE*

The transient fields from a finite horizontal loop excited by a half sine wave current pulse have been computed numerically for a particular source receiver configuration at a height of 100 meters above a layered ground. The amplitude of the vertical component of the magnetic field has been chosen for the interpretation. Curves of apparent conductivity vs. time, plotted during the off-time of the signal, show that layering is easily resolved, that resonance effects are present and that polarization effects are detectable for certain types of polarization.     

一类非均匀介质中电磁场的延拓

本文讨论具垂向光滑变化的非均匀介质中电磁场延拓的数学物理问题及稳定化的算法原理。首先从电磁场的基本方程出发,导出了垂向非均匀介质下磁场的垂直分量所满足的二阶偏微分方程,并且依照实际背景将场的延拓问题化归为解相应二阶椭圆型方程的Cauchy问题。接着利用Fourier变换方法得到了场延拓的形式表达式,然后给出了几种特殊垂向非均匀介质下场延拓的精确解析公式。最后,应用正则化方法建立了场延拓的稳定化的公式,给出了所得公式的离散化形式,同时指出了相应正则化参数的选择原则。     

一类非均匀介质中电磁场的延拓

本文讨论具垂向光滑变化的非均匀介质中电磁场延拓的数学物理问题及稳定化的算法原理。首先从电磁场的基本方程出发,导出了垂向非均匀介质下磁场的垂直分量所满足的二阶偏微分方程,并且依照实际背景将场的延拓问题化归为解相应二阶椭圆型方程的Cauchy问题。接着利用Fourier变换方法得到了场延拓的形式表达式,然后给出了几种特殊垂向非均匀介质下场延拓的精确解析公式。最后,应用正则化方法建立了场延拓的稳定化的公式,给出了所得公式的离散化形式,同时指出了相应正则化参数的选择原则。     

中国大陆地壳水平运动统一速度场的建立与分析

收集了国内外中国大陆及周边地区GPS网的有关数据，并根据所收集的GPS网的不同数据，提出了GPS网速度场的不同融合方法．经过融合建立了中国大陆及周边地区统一的地壳运动速度场．该速度场使用的有效GPS站共423个，其覆盖面积为1200万平方千米．分析该速度场，初步总结出中国大陆及周边地区地壳水平运动空间分布的基本特征，并讨论了印度板块向欧亚板块碰撞力对中国大陆速度场的影响范围和印度板块作用力的主方向等问题．      

ELECTROMAGNETIC FIELDS DUE TO A THIN RESISTIVE LAYER*

Using approximate boundary conditions, expressions for electromagnetic fields have been derived for a thin, highly resistive layer lying between two homogeneous layers excited by an electric dipole grounded on the surface of the earth. The variations of the fields with the parameter T/T₁ (ratio of the transverse resistance of the thin layer to the transverse resistance of the first layer) were studied in relation to frequency, time, the normalized separation source—receiver, and the angle between the source and the radius to the observation point. For a value of h₂/h₁ (ratio of thickness of second layer to the thickness of the first layer) approximately equal to 0.2, the general three-layer medium case gives the same results as this approach. It was found that the electric fields have a very strong dependence on the parameter T (transverse resistance) which characterizes the thin, highly resistive layer. However, the magnetic fields depend only very weakly on this parameter.