共查询到20条相似文献，搜索用时 687 毫秒
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Most rocks display conductivity dispersion in the low-frequency range, when the usual displacement currents are neglected. The strong influence this low-frequency dispersion (LFD), including the response sign reversals, was revealed by field experiments with the coincident-loop configuration widely used in transient electromagnetics (EM). Mathematical modelling of LFD has been the subject of numerous studies. However, confirmation of the role of LFD or induced polarization (IP) by comparing mathematical modelling and field data is rather poor, because knowledge of the properties of rocks in the area of the field measurements is usually insufficient. For this reason physical modelling of LFD has been carried out at Moscow State Geological-prospecting Academy (Russia) in 1994-95. In order to observe criteria of similarity for both induction and polarization transients, a ring-shaped model was chosen and was represented by an electric circuit, consisting of lumped elements (real rock samples included). Qualitatively different transients for dispersive models and their non-dispersive ohmic equivalents were observed. 相似文献
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The article discusses the excitation of transient induced polarization responses using current and voltage sources. The first method has found a wide application in induced polarization surveys and—directly or indirectly—in the theory of the induced polarization method. Typically, rectangular current pulses are injected into the earth via grounding electrodes, and decaying induced polarization voltage is measured during the pauses between pulses. In this case, only the secondary field is recorded in the absence of the primary field, which is an important advantage of this method. On the other hand, since the current injected into the ground is fully controlled by the source, this method does not allow studying induced polarization by measuring the current in the transmitter line or associated magnetic field. When energising the earth with voltage pulses, the measured quantity is the transient induced polarization current. In principle, this method allows induced polarization studies to be done by recording the transmitter line current, the associated magnetic field, or its rate of change. The decay of current in a grounded transmitter line depends not only on the induced polarization of the earth but also on the polarization of the grounding electrodes. This problem does not occur when induced polarization transients in the earth are excited inductively. A grounded transmitter line is a mixed‐type source; hence, for a purely inductive excitation of induced polarization transients, one should use an ungrounded loop, which is coupled to the earth solely by electromagnetic induction. 相似文献
4.
Laboratory scale model experiments have been performed to obtain the electromagnetic response of a finitely conducting half plane embedded in resistive/conductive surrounding and excited by an oscillating magnetic dipole. Inphase and quadrature profiles are presented for two horizontal coplanar transmitter-receiver systems (inline and broadside) for normal and skew traverses and for different dips of the conductor. It is observed that the broadside system is more diagnostic in delineating the strike and dip of the conductor and is more sensitive to the conducting host rock. The broadside profile over a vertical or dipping half plane is characterized, when traversing perpendicular to strike, by two positive peaks flanking a zero response when the coils are over the top edge of the conductor. For skew traverses a negative peak replaces the zero response. An increasing asymmetry in the anomalies is caused by changing the dip of the conductor from the vertical in both the systems, but it is more pronounced for the broadside system. The quadrature response in the broadside system changes in a characteristic way when the target is surrounded by a conducting host rock. The comparative results of the two systems may, therefore, be useful in the induction prospecting for ore deposits approximated by a half plane, especially in delineating the strike, dip, and effect of conductive host rock. 相似文献
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S. E. HJELT 《Geophysical Prospecting》1974,22(3):546-567
The paper discusses the smallest obtainable parameter errors (variances) in the interpretation with the least-squares method. Useful approximations of the sum of squares contained in the minimum error expressions are obtained using results of numerical integration. The approximations lead to especially simple results for long interpretation profiles, when the parameter errors are proportional to the square root of the point separation. Formulae are developed and examples shown for minimum error calculation in gravimetric interpretation with the cylindrical model and in magnetic interpretation with the two-dimensional plate model. Smallest errors are obtained when the interpretation profile is chosen around the anomaly maximum except for dip and depth extent interpretation of magnetic plates. 相似文献
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Hesham M. El-Kaliouby Safwat A. Hussain Abd El-Rahim Bayoumi Essam A.El-diwany Essam A. Hashish 《Geophysical Prospecting》1995,43(5):595-603
The aim of this work is to consider the phenomenon of negative response and its detectability in a polarizable half-space formed of Clay-water mixture which commonly exists in water aquifers and which can be used as a simple indicator of the presence of Underground water. We make use of an analytical treatment for the transient voltage induced in a coincident loop lying on a half-space as a basis for our computations. A Cole-Cole model is used to represent the Clay with appropriate parameters. As saturated Clay is characterized by high conductivity and moderate chargeability, it is hardly detectable relative to the practical noise level. However, there are optimum Clay parameters that offer a maximum negative response. It is also observed that for a very short time constant τ in the Cole-Cole model, the amplitude of the negative response decreases with the decrease of the time constant due to the fast decay of the polarization current. Finally, it is found that there is an optimum value for the loop radius which gives the largest value of the negative response and this loop radius depends on the model parameters. 相似文献
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T. J. Lee 《Pure and Applied Geophysics》1980,119(2):309-338
The integral equation for the electromagnetic response of a sphere in a layered medium may be solved as follows. First, the unknown time harmonic electric field in the sphere is expanded in spherical vector waves. Secondly, the coefficients for these wave functions are found by a set of equations. The equations are found by multiplying the integral equation throughout by each wave function and integrating over the spherical conductor.Once the unknown coefficients have been determined, then the transient response may be found by taking the inverse Fourier transform. In carrying out the Fourier transform one learns that for most of the time range used in prospecting, only the lowest order vector wave function is significant. A study of the singularities of the spectrum of the transient shows that, for the time range considered, only a single branch cut is significant. There are no pole type responses. That is, the field does not decay exponentially. Previous studies of a sphere in free space reported only pole type responses. That is, at the later stages, the field decays exponentially. This study shows that, in order to model satisfactorily the effect of the host rock on transient electromagnetic fields, the sphere must be placed in layered ground. 相似文献
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SAMI SOLIMAN MOHAMED 《Geophysical Prospecting》1970,18(Z1):654-665
During the last decade many hypotheses were suggested to explain the phenomenon of induced electrical polarization in ionic conductive media. The most reliable of these is Fredricksberg's. Fredricksberg (1962) supposed that the pore spaces of a rock is composed of successively narrow (active zones) and wide (inactive zones). He simulated these pore spaces by a synthetic material that has an extremely high resistance. The pore spaces were generally in tube forms which exhibited some constrictions. He saturated these tubes with an electrolyte of a given concentration. An electric current was passed through this model. He observed an induced polarization voltage after current interruption. He attributed the formation of this voltage to a concentration gradient which took place due to the presence of excess charges in the active zones. Fredricksberg introduced a parameter (9) which described the relation among the lengths and cross sectional areas of the wide zones, the number of ions within each zone after current interruption with the recorded polarizability. The aim of this work is to correlate Fredricksberg's parameter with a parameter determined for natural rocks and to show experimentally the validity of this hypothesis when applying for some varieties of sandstones and volcanic rocks. The new parameter will help to evaluate a relationship between the polarizability and the water-collecting properties of rocks. Herein, we used the tortuosity T of sandstone samples instead of the parameter φ which was used by Fredricksberg to represent the pore geometry within his model (tortuosity of the passes within the model). It was shown that both φ and T have the same relationship with the polarizability ν of the rock samples and if φ or T have very low or very high values the polarizability ν tends to its minimum value, i.e. the curve representing the relation between ν and T has a maximum point corresponding to an intermediate value of T. This result supports Fredricksberg's hypothesis and confirm his results on synthetic models. For volcanic rocks the formation factor F was used since it was difficult to determine the porosity of the samples and consequently to calculate the tortuosity T as for sandstone samples. Experimental results confirm those obtained from sandstone. The grain constituents of sandstone samples were represented on equilateral triangle and the magnitude of induced polarization ν of each sample was deduced and represented on this triangle. Equipolarizability values ν drawn on this triangle showed that TJ will increase as the silty fractions of the rock increase, where the center of this triangle (represents minimum porosity) has polarizability less than 0.25%. An attempt was made to determine the coefficient of anisotropy of volcanic rock samples using the induced polarization method. For this reason the polarizability was deduced by measuring the induced polarization voltage for two perpendicular directions in a fractured cubes of andesitic basalt samples the coefficient of anisotropy was found to be equal 1.18. 相似文献
10.
It is now believed that the negative transients observed in coincident-loop transient electromagnetic (TEM) measurements are caused by polarizable bodies (bodies whose conductivity increases as a function of frequency). Ordinarily the TEM response of polarizable bodies is obtained by calculating the frequency-domain response at many frequencies and transforming it to the time domain via Fourier, Laplace or Hankel transforms. This is normally a computationally laborious task. However, for some simple non-polarizable bodies the time-domain response is analytical and can be computed easily. When these simple bodies are weakly polarizable an approximate response can be obtained by convolving the easily-calculated, non-polarizable response with the impulse response of the polarization. The approximate response is found to be very similar to the exact response for the polarizabilities normally seen in geological materials. 相似文献
11.
岩石物理模板采用测井解释的各类岩性矿物骨架点值,选取适合该地区的岩石物理模型,模拟在不同储层组合、不同孔隙及不同饱和度情况下,储层岩石物理参数变化引起的储层测井参数及地球物理响应特征的变化,定量地建立起储层参数同地球物理弹性参数间的解释关系模版.本文根据新场JS42气藏储层参数分析结果,尝试性地将岩石物理解释模板应用于储层定量解释中,对储层高产气区、含水区域进行定量解释,并预测了该气藏的气水界面,该预测结果与实钻井测试情况吻合,证实了该方法的科学性. 相似文献
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针对微地震裂缝解释的复杂性,从震源矢量场的重建开始研究,在研究VTI介质速度模型各向异性条件下的走时和透射系数的变化特征基础上,形成了各向异性条件下的群、相速度及透射系数的计算方法.针对多级检波器水平分量朝向的多向性特点,提出了多级检波器水平分量的偏振分析方法,得到了完整的水平特征矢量,克服了单级检波器水平分量偏振分析构建特征矢量信息不全的问题,形成了高精度微地震事件定位方法,实现各向异性VTI介质速度模型的高斯束微地震格林函数正演模拟.利用格林函数模拟场、观测记录场,从构建完整场研究入手,重建震源矢量场.根据重建的震源矢量场,提出了裂缝解释的全新的系列方法,包括单条裂缝、裂缝网络的解释方法.通过实际资料的测试分析,验证了研究技术的实用性. 相似文献
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A new approach is proposed in order to interpret field self-potential (SP) anomalies related to simple geometric-shaped models such as sphere, horizontal cylinder, and vertical cylinder. This approach
is mainly based on solving a set of algebraic linear equations, and directed towards the best estimate of the three model
parameters, e.g., electric dipole moment, depth, and polarization angle. Its utility and validity are demonstrated through
studying and analyzing synthetic self-potential anomalies obtained by using simulated data generated from a known model and
a statistical distribution with different random errors components. Being theoretically tested and proven, this approach has
been consequently applied on two real field self-potential anomalies taken from Colorado and Turkey. A comparable and acceptable
agreement is obtained between the results derived by the new proposed method and those deduced by other interpretation methods.
Moreover, the depth obtained by such an approach is found to be very close to that obtained by drilling information. 相似文献
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The induced polarization response of a horizontally multilayered earth with no resistivity contrast can rapidly be calculated on a desk calculator or minicomputer for any electrode array. The formulation is a simple series summation of the products of weighting coefficients and the true induced polarization responses for each of the layers. The coefficients are directly derivable from the corresponding resistivity model. This series approach to IP formulation was originally described by Seigel but has not been treated extensively in the present-day geophysical literature. This method can be applied to either time or frequency domain induced polarization measurements. Once the coefficients are known, apparent induced polarization response can readily be obtained by judicious substitution of known, suspected, or assumed values of the true induced polarization of each layer. Basic formulation is presented for the IP potential coefficients (pole-pole or two array) with no resistivity contrast between the layers. From these coefficients, response of any number of layers for any electrode array can be obtained by suitable differentiation. Some examples of Wenner array for a three-layered earth and dipole-dipole array for a four-layered earth are used to illustrate the application. The results of this technique are valid for many natural situations of modest resistivity contrast. However, they definitely cannot be used if there are highly contrasting resistivity layers present. Such an approach is conceptually simple and is useful for survey planning, checking or setting the “depth-of-penetration”of a given array. For field induced polarization data that fits reasonably well to the no-resistivity-contrast model, this simple approach facilitates quantitative interpretation. 相似文献
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A new method for the 2D inversion of induced polarization (IP) data in the time domain has been developed. The entire IP transients were observed and inverted into 2D Cole-Cole earth models, including resistivity, chargeability, relaxation time and the frequency constant. Firstly, a modified 1D time-domain electromagnetic algorithm was used to calculate the response of a layered polarizable ground. The transient signals were then inverted using the Marquardt method to derive the Cole-Cole parameters of each layer. However, model calculations showed that the EM effects could be neglected for the time range (>1 ms) and for the transmitter–receiver distances (<50 m) used in this study. Therefore, the induction effects were not considered for the solution of the 2D inverse problem and a DC solution was applied. An approximative forward algorithm was introduced in order to calculate the IP transients directly in the time domain and in order to speed up the inverse procedure. The approximation is highly accurate, and this is demonstrated by comparing the approximations with their exact solutions up to 3D. The inverse algorithm presented consists of two steps. The transient voltages of an array data set were inverted separately into a two-dimensional resistivity model for each time channel. The time-dependent resistivity of each cell was then interpreted as the response of a homogeneous half-space. In the 2D inversion algorithm, a 3D DC algorithm was used as a forward operator. The method only requires a standard 2D DC inversion and a homogenous half-space Cole-Cole inversion. The developed algorithm has been successfully applied to synthetic data sets and to a field data set obtained from a waste site situated close to Düren in Germany. 相似文献
16.
B. S. RATHOR 《Geophysical Prospecting》1978,26(1):175-183
The expressions for quasi-static electromagnetic fields of a horizontal electrical dipole placed on the surface of a polarizable half-space have been derived for low and high values of induced polarization parameter and presented along with numerical results. It has been observed that the polarity of the mutual impedance function is negative for low values and positive for high values of the induced polarization parameter in the entire time domain, whereas the induced voltage function is positive for low values of induced polarization parameter and becomes negative for high values. In the case of low values of induced polarization parameter the transient electromagnetic field in the beginning increases with time, later on it starts to decay with time. The present study shows that the decay rate of electromagnetic field is highly dependent upon the induced polarization parameter. In the light of the present study the conductivity of the polarizable medium may be determined more accurately. 相似文献
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《Advances in water resources》1998,21(7):581-590
Advances in computer power and in reservoir characterization allow simulation of pressure transients in complex reservoirs generated stochastically. Generally, interpretation of these transient gives useful information about the reservoir hydraulic properties: a major goal is to interpret these transients in the stochastic context. First we ensemble average the pressure over all the random permeability field realizations to derive an equation which drives the ensemble averaged pressure. We use the Fourier transform in space and the Laplace transform in time, in conjuction with a perturbation series expansion in successive powers of the permeability fluctuations to obtain an explicit solution. The Nth order term of this series involves the hydrodynamic interaction between N permeability heterogeneities and after averaging we obtain an expansion containing correlation functions of permeability fluctuations of increasing order.Next, Feynman graphs are introduced allowing a more attractive graphical interpretation of the perturbation series. Then series summation techniques are employed to reduce the graph number to be summed at each order of the fluctuation expansion. This in turn gives useful physical insights on the homogenization processes involved. In particular, it is shown that the sum of the so-called ‘one-particle irreducible graphs’ gives the kernel of a linear integro-differential equation obeyed by the ensemble average pressure. All the information about the heterogeneity structure is contained in this renormalized kernel, which is a limited range function.This equation on its own is the starting point of useful asymptotic results and approximations. In particular it is shown that interpretation of pumping tests yields the steady-state equivalent permeability after a sufficiently long time for an infinite reservoir, as expected. 相似文献
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Ajit K. Sinha 《Pure and Applied Geophysics》1990,132(4):733-756
The response of two-dimensional, inclined, sheet-like conductors with, low conductance values to plane wave electromagnetic fields in the very low frequency (VLF) range has been evaluated by using a numerical technique. The conductance values of the conductors considered are appropriate for those produced by water and/or clay-filled fracture and shear zones in the Precambrian crystalline rocks of the Canadian Shield. The surrounding host rock was assumed to be, resistive with resistivities in the 1–10 k.m range to reflect the high resistivities over the shield areas. No overburden was assumed in this analysis.The results of the computations are presented in the form of characteristic interpretation diagrams to interpret ground VLF data in the field, where facilities for direct numerical modelling may not be available. A method for interpreting ground VLF data using such characteristic diagrams has been proposed in this paper which requires a prior knowledge of the host rock resistivity and the inclination of the conductor. These two parameters may be derived from a VLF resistivity survey and from appropriate filtering of the VLF tilt angle response. The interpretation method was applied to a ground VLF anomaly obtained at a research site near Atikokan in NW Ontario, which yielded an interpretation compatible with information from geological mapping.Geological Survey of Canada Contribution No. 51888. 相似文献
19.
The Huntec Mark 3 time domain equipment provides information from which the nature of the transient voltage decay curves may be determined. A zone of disseminated mineralization extending above a normal fault zone was selected for a comparative study of decay curves. In a preliminary study in which steel stakes were used as current electrodes it was observed that at some stations the decay curves obtained with reciprocal dipole-dipole configurations were significantly different and that many decay curves initially increased from low values indicating the presence of a negative component. It was assumed that the decay curves were composite and were composed of the superposition of transient voltage components due to electromagnetic and polarization effects which could be described in terms of simple exponential functions. The initial negative component was thought to be an electromagnetic effect, but when the traverses were repeated using porous pots as current electrodes it was found that the initial negative component was usually completely eliminated, indicating that it was due to negative polarization arising from the steel stakes. When the effect of the current electrode polarization was completely eliminated identical decay curves were obtained with the reciprocal array configurations, but the remaining decay curve-which is due to induced ground polarization—cannot be described by a single simple exponential function. It is also demonstrated that the initial negative component due to stake polarization may also be present in decay curves obtained with gradient arrays indicating that separation of the current and potential electrodes does not avoid this effect which may significantly modify the decay curve in certain circumstances up to three seconds after switch-off. 相似文献
20.
《Journal of Applied Geophysics》2000,43(1):91-100
In well log interpretation, it is frequently necessary to correct logs for invasion. Invasion occurs in permeable formations when there is a radial differential pressure (RDP) between the borehole and formation. Other factors on which invasion depend include saturation, mobility, pressure (RDP) and capillary pressure, permeability and viscosity of fluids, and temperature transient effects associated with the mud filtrate injected into the formation. Thus, simulation of realistic invasion is not an easy task. This work reviews the famous Buckley–Leverett mathematical model in cylindrical coordinates appropriate for borehole geometries. The model predicts multiphase invasion in porous media when gravity, capillary pressure, and mud cake can be neglected. One application is to correct logging while drilling (LWD) and wireline resistivity logs for time-dependent invasion and formation temperature effects. This is important, for example, when there are possible large differences in formation and mud temperature. Modeling studies show these effects can be large enough to noticeably influence resistivity logs. However, after correction, difference in LWD and wireline logs arising from the time-dependent heat process are explained. Thus, the method, when coupled to a time-dependent heat flow model, and a response function formulation of resistivity, yields new insight into the influence of thermal and electrical transients in log interpretation. 相似文献