Electric Dipole Source Research Articles

Inversion of arbitrary segmented loop source TEM data over a layered earth

The loop source TEM method has been widely used in the detection of a mined out area in China. In the cases that the laying of traditional rectangle or square transmitting loop is limited due to the presence of obstacle on the path of the loop, the changing of the shape of the transmitting loop to bypass the obstacle is a labor saving solution. A numeric integration scheme is proposed to calculate the response and Jacobian of the segmented loop source from that of an electric dipole source. The comparison of forward response between the segmented loop and square loop shows the effect of loop geometry on the decay curves. In order to interpret the data from an irregular source loop, this paper presents an inversion scheme that incorporate the effect of loop geometry. The proposed inversion scheme is validated on the synthetic data, and then applied to the field data. The result reveals that the developed inversion scheme is capable of interpreting the segmented loop source TEM field data.

Three effective inverse Laplace transform algorithms for computing time-domain electromagnetic responses

The inverse Laplace transform is one of the methods used to obtain time-domain electromagnetic (EM) responses in geophysics. The Gaver-Stehfest algorithm has so far been the most popular technique to compute the Laplace transform in the context of transient electromagnetics. However, the accuracy of the Gaver-Stehfest algorithm, even when using double-precision arithmetic, is relatively low at late times due to round-off errors. To overcome this issue, we have applied variable-precision arithmetic in the MATLAB computing environment to an implementation of the Gaver-Stehfest algorithm. This approach has proved to be effective in terms of improving accuracy, but it is computationally expensive. In addition, the Gaver-Stehfest algorithm is significantly problem dependent. Therefore, we have turned our attention to two other algorithms for computing inverse Laplace transforms, namely, the Euler and Talbot algorithms. Using as examples the responses for central-loop, fixed-loop, and horizontal electric dipole sources for homogeneous and layered mediums, these two algorithms, implemented using normal double-precision arithmetic, have been shown to provide more accurate results and to be less problem dependent than the standard Gaver-Stehfest algorithm. Furthermore, they have the capacity for yielding more accurate time-domain responses than the cosine and sine transforms for which the frequency-domain responses are obtained by interpolation between a limited number of explicitly computed frequency-domain responses. In addition, the Euler and Talbot algorithms have the potential of requiring fewer Laplace- or frequency-domain function evaluations than do the other transform methods commonly used to compute time-domain EM responses, and thus of providing a more efficient option.

3D finite element modeling of marine controlled-source electromagnetic fields using locally refined unstructured meshes

We present a 3D forward modeling algorithm of marine controlled-source electromagnetic (CSEM) fields using finite element method based on unstructured meshes. As the unstructured mesh is incorporated, the algorithm can easily accommodate complex structures, such as dipping interfaces and rough topography. Secondary coupled-potential formulation of Maxwell's equations allows to avoid the singularities introduced by the sources. The primary electromagnetic fields excited by electric dipole source in layered isotropic media are derived from the semi-analytical formulas based on Schelkunoff potentials. To improve the accuracy and efficiency of the algorithm, we propose an effective local mesh refinement strategy, where the elements near the receiver locations and in the abnormal areas are refined using the volume constraints. We validate the finite element code against a 1D and 2D reservoir model and study the performance of the local mesh refinement strategy. The Canonical disc model illustrates the utility of the finite element method for 3D CSEM modeling. The bathymetry model study shows that the seafloor topography gives an important response which needs to be reproduced by numerical modeling to avoid the misinterpretation of measurements.

HIGH RESOLUTION NEAR-FIELD ELECTROMAGNETIC HOLOGRAPHY FOR DYNAMIC SOURCE IDENTIFICATION IN UNDERWATER MEDIUMS

This paper investigates the source reconstruction problem in underwater mediums using a compressive Near-Field Electromagnetic Holography (NEH) approach. More specifically we investigate the use of � 1 regularization for the purpose of decomposing near-field magnetic and/or electric surface measurements into electric and magnetic dipole sources. Our study indicates that not only do � 1 decompositions enable much higher resolution of sources than traditional � 2 approaches, but important features of the dipoles are preserved in the reconstruction. Our hypothesis are supported by numerical experiments as well as underwater physical measurements obtained in an earth field simulator facility.

Dyadic Green’s Functions for Dipole Excitation of Homogenized Metasurfaces

Dyadic Green’s functions for an electric dipole source over an infinite periodic metasurface are obtained using homogenized, nonlocal, anisotropic, generalized sheet-transition conditions. The homogenized Green’s functions can efficiently model near-field point source excitation of typical metasurface structures. The Green’s functions can be decomposed into discrete and continuous spectral components, providing physical insight into the wave dynamics. Several different metasurfaces are considered, and the results are validated by comparison with a full-wave array-scanning method, demonstrating computational efficiency of the proposed homogenized Green’s function approach.

STUDY OF THE SHIELDING EFFECTIVENESS OF DOUBLERECTANGULAR ENCLOSURESWITH APERTURES EXCITED BY AN INTERNAL SOURCE

An analytical formulation has been developed to evaluate the shielding effectiveness (SE) of two coplanar rectangular metallic enclosures with a circular aperture excited by an internal electric dipole source. The formulation consists of three parts: First, the near-field electromagnetic interference (EMI) of the electromagnetic leakage from the aperture is represented by the electric dipole in one enclosure. Then, the aperture equivalent magnetic and electric dipole moments are calculated according to the Bethe's small aperture coupling theory. Finally, the electric field of the other enclosure is derived by using the equivalent magnetic dipole field, equivalent electric dipole field and the corresponding enclosure's Green's functions in the two fields. In this formulation, the electric field of the enclosure can be expressed as a function of the observation point, the aperture's center point, source point, shape of the aperture and enclosure's conductivity. The formulation then is employed to analyze the effect of the above factors on the SE. The analytical results have been successfully compared with the full-wave simulation software Computer Simulation Technology (CST) from 0.3 ∼ 2. 4G Hz.

An Efficient Analytical Method for Electromagnetic Field to Transmission Line Coupling Into a Rectangular Enclosure Excited by an Internal Source

A simple and efficient analytical method has been developed for predicting the electromagnetic field coupling with a lossless transmission line (TL) located in a rectangular enclosure. The metallic enclosure is excited by an internal electrical short dipole or monopole source. The electric fields generated inside the rectangular enclosure and excited by some simple sources are calculated by using the electric-type dyadic Green's function. Our approach combines, on one hand, a lumped-pi circuit model of TL and, on the other hand, the Agrawal coupling model. A simple equivalent circuit has been derived which can give prediction for the coupling between the cavity field and the TL. This method is computationally less intensive compared to other numerical methods to solve such a problem. The analytical results have been successfully compared over a wide frequency band with the finite-difference time domain and experimental results carried out in different combinations of source and line position.

Propagation characteristics of power line harmonic radiation into the stratified anisotropic ionosphere

Power line harmonic radiation (PLHR), which specifically refers to the electromagnetic wave radiation observed in ionosphere or magnetosphere, is radiated by the transmission lines of power systems on the ground. PLHR is shown as a parallel spectrogram between 400 Hz and 5 kHz in frequency-time power spectrogram of electromagnetic field. And the frequency spacing of the parallel spectrogram is 50/100 Hz or 60/120 Hz. As an artificial pollution source in the near earth space, PLHR has attracted more and more attention. However, so far, there have been little proposed quantitative researches on the formation mechanism. This paper studies the propagation model for the electromagnetic waves generated by the electric dipole source above the non-ideal conductive ground in the stratified anisotropic ionosphere. Based on the method by Lehtinen(2008), a new full-wave finite element method is give to solve the problem. By recursively calculating reflection coefficients and mode amplitudes, the method contains no index increasing items. So it can effectively overcome the numerical overflow in programming calculations. In order to verify the correctness of the method, comparison are made between the existing analytical solutions and the solutions obtained from the proposed method, and they are in excellent agreement. Further more, using the present model, the new method and the associated parameters about practical power lines, ground and ionosphere, we have studied the effects of the frequency of dipole source, the bottom boundary height of ionosphere, the earth conductivity, and the geomagnetic field direction on PLHR propagation in the ionosphere. Results show that when the frequency of radiation source equals the cut off frequency of earth-ionosphere waveguide-guided wave modes, the strength of PLHR for penetrating into the ionosphere becomes larger. Keeping the harmonic current constant, a smaller ground conductivity would be accompanied by a larger power of PLHR. PLHR propagates along the direction of the geomagnetic field in the ionosphere. Therefore, it is much easier for a high-order harmonic radiation of transmission lines to penetrate into the ionosphere along the direction of the geomagnetic field in the areas with low ground conductivity under a certain condition. Results obtained in this paper may have important implications to explain the formation mechanism of PLHR.

On the electromagnetic response of a resistive cylinder buried in a conductive host

The frequency-domain electromagnetic response of a confined conductor buried in a resistive host has received much attention, particularly in the context of mineral exploration. In contrast, the problem of the electromagnetic response of a confined resistor buried in a conductive host has been less thoroughly studied. However, resistive targets are important in geotechnical and hydrologic studies, archaeological prospecting, and, more recently, offshore hydrocarbon exploration. I analytically address the problem of the electromagnetic response of a completely resistive cylindrical cavity buried in a conductive host in the presence of a simplified 2D electric dipole source. In contrast to the confined conductor, which channels and induces current systems, the confined resistor deflects current and produces additional eddy current systems in the conductive host. I apply this theory to model the response of a grounded electric dipole-dipole system operating over a range of frequencies from 0 Hz to 10 kHz, in the presence of a horizontal 5-m radius insulating cylinder located 1-m beneath the surface of a uniform earth. This represents a common hazard encountered during mining and civil engineering operations. Results show that such an insulating cavity increases the recorded electric field amplitude and phase delay at all transmitted frequencies. These observations suggest that a broadband electromagnetic prospecting system may provide additional information about the location and extent of a void, over and above a standard dipole-dipole resistivity survey. When the host skin depth is much larger than all other length scales, the response can be approximated by an equivalent single dipole unless the cylinder’s radius is much larger than its distance from the transmitter. This result provids a useful rule of thumb to determine the acceptable range over which a resistive target can be modeled by a distribution of dipoles.

Radiation reaction at the level of the action

The aim of this paper is to highlight a recently proposed method for the treatment of classical radiative effects, in particular radiation reaction, via effective field theory methods. We emphasize important features of the method and in particular the doubling of fields. We apply the method to two simple systems: a mass–rope system and an electromagnetic charge-field system. For the mass–rope system in 1 + 1 dimensions we derive a double-field effective action for the mass which describes a damped harmonic oscillator. For the EM charge-field system, i.e. the system of an accelerating electric charge in 3 + 1 dimensions, we show a reduction to a 1 + 1 dimensions radial system of an electric dipole source coupled to an electric dipole field (analogous to the mass coupled to the rope). For this system we derive a double-field effective action and reproduce in an analogous way the leading part of the Abraham–Lorentz–Dirac force.

Analytical solutions of EM fields due to a dipolar source inside an infinite casing

This paper studies the problem of electromagnetic fields observed outside of an infinite metallic casing due to dipolar excitations inside the pipe. Closed-form expressions are derived for the hertz vector potential driving the solution of the boundary value problem. The results indicates that the fields outside the casing are due to a distribution of vertical dipoles that decay in strength away from the true source. Analytical expressions are also obtained for the induced source distribution, as a function of the geometry and electric properties of the system. For the transverse magnetic mode (electric dipole source), the sources represent an electric current channeled vertically along the casing, whereas for the transverse electric mode (magnetic dipole source), they represent a distribution of induced magnetic moments. The strength of the current channeling decays exponentially away from the source, whereas the strength of the induced magnetic moment drops within the first few meters. The expressions obtained for the fields due to a magnetic dipole reveals that the effect of the casing yields a multiplicative complex constant, which attenuates the dipolar-type field. This effect has been observed experimentally in crosswell surveys, but it has never been formally proven. The fields due to an electric dipole excitation describe an inhomogeneous cylindrical surface wave propagating and attenuating in the radial and vertical directions.

Focusing of dipole radiation by a negative index chiral layer. 1. A thick layer as compared with the wavelength

We have derived and investigated the analytical expressions for the fields of scattered radiation of an electric dipole source by a chiral (bi-isotropic) layer with arbitrary permittivity and permeability and arbitrary thickness. It is shown that in the negative-index chiral layer the focus spot of dipole radiation is split due to excitation of right- and left-hand circularly polarised waves. The conditions are found under which the waves with one of the polarisations can be suppressed, which leads to a substantial improvement of the focusing properties of the chiral layer.

CT and MRI Assessment and Characterization Using Segmentation and 3D Modeling Techniques: Applications to Muscle, Bone and Brain.

This paper reviews the novel use of CT and MRI data and image processing tools to segment and reconstruct tissue images in 3D to determine characteristics of muscle, bone and brain. This to study and simulate the structural changes occurring in healthy and pathological conditions as well as in response to clinical treatments. Here we report the application of this methodology to evaluate and quantify: 1. progression of atrophy in human muscle subsequent to permanent lower motor neuron (LMN) denervation, 2. muscle recovery as induced by functional electrical stimulation (FES), 3. bone quality in patients undergoing total hip replacement and 4. to model the electrical activity of the brain.Study 1: CT data and segmentation techniques were used to quantify changes in muscle density and composition by associating the Hounsfield unit values of muscle, adipose and fibrous connective tissue with different colors. This method was employed to monitor patients who have permanent muscle LMN denervation in the lower extremities under two different conditions: permanent LMN denervated not electrically stimulated and stimulated. Study 2: CT data and segmentation techniques were employed, however, in this work we assessed bone and muscle conditions in the pre-operative CT scans of patients scheduled to undergo total hip replacement. In this work, the overall anatomical structure, the bone mineral density (BMD) and compactness of quadriceps muscles and proximal femoral was computed to provide a more complete view for surgeons when deciding which implant technology to use. Further, a Finite element analysis provided a map of the strains around the proximal femur socket when solicited by typical stresses caused by an implant press fitting. Study 3 describes a method to model the electrical behavior of human brain using segmented MR images. The aim of the work is to use these models to predict the electrical activity of the human brain under normal and pathological conditions by developing detailed 3D representations of major tissue surfaces within the head, with over 12 different tissues segmented. In addition, computational tools in Matlab were developed for calculating normal vectors on the brain surface and for associating this information with the equivalent electrical dipole sources as an input into the model.

Displacement currents in geoelectromagnetic problems

Abstract The influence of displacement currents in conventional geoelectromagnetic (GEM) methods using unimodal transversal electric (TE) or multimodal TE and TM (transversal magnetic) fields is only significant at very high frequencies in the frequency domain or at extremely early times in the time domain. The transient process in the latter includes three stages: the propagation through air, the propagation through earth and the diffusion within the earth. The influence of displacement currents is significant mainly during the former two stages, normally up to several tens to a few hundreds of nanoseconds. The behavior is essentially different in novel GEM methods using a vertical electric dipole (VED) or circular electric dipole (CED) sources of unimodal TM-fields. Under certain geoelectric conditions, the influence of displacement currents in these methods might be crucial at late times as well. This happens, if the model consists of insulating layers. In the absence of displacement currents, such layers would totally mask underlying structures. However, TM-fields including displacement currents depend on geoelectric parameters below insulating layers at late times.

Three-dimensional resistivity modelling of grounded electrical-source airborne transient electromagnetic (GREATEM) survey data from the Nojima Fault, Awaji Island, south-east Japan

An airborne electromagnetic (AEM) survey using the grounded electrical-source airborne transient electromagnetic (GREATEM) system was conducted over the Nojima Fault on Awaji Island, south-east Japan, to assess GREATEM survey applicability for studying coastal areas with complex topographic features. To obtain high-quality data with an optimised signal-to-noise ratio, a series of data processing techniques was used to acquire the final transient response curves from the field survey data.The 1D inversion results were feasible in that the horizontal resistivity contrast was not much higher than the true contrast, but they were not reasonable in that the horizontal resistivity values were greatly changed. To circumvent this problem, we performed numerical forward modelling using a finite-difference staggered-grid method (Fomenko and Mogi, 2002) adding a finite-length electrical dipole source routine to generate a three-dimensional (3D) resistivity structure model from GREATEM survey data of the Nojima Fault area. The 3D model was based on an initial model consisting of two adjacent onshore and offshore layers of different conductivity such that, a highly conductive sea of depth (10–40 m) is placed on top of a uniform half-space, assuming the presence of topographic features on the inland side. We examined the fit of the magnetic transient responses between field data and 3D forward-model computed data, the latter were convolved with the measured system response of the corresponding dataset. The inverted 3D resistivity structures showed that the GREATEM system has the capability to map underground resistivity structures as deep as 500 m onshore and offshore. The GREATEM survey delineated how seawater intrudes on the landside of the fault and indicated that the fault is a barrier to seawater invasion.

Delineation of Hydrocarbon with Variation of Overburden Thickness for Sea Bed Logging Applications

There are various methods being used to model and study behavior of electromagnetic (EM) waves in controlled source electromagnetic (CSEM) environment. Sea Bed logging (SBL) is using CSEM technique in detecting and characterizing hydrocarbon bearing reservoirs in deep water areas. It uses a mobile horizontal electric dipole (HED) source called transmitter that transmits low frequency of 0.1Hz to 10Hz, 30m - 40m above sea bed and an array of seafloor electric field receivers. These signals depend on the resistivity structure beneath the sea bed as hydrocarbon is known to have high resistivity value of 30 500 Ωm in contrast to sea water layer of 0.5 2 Ωm and sediments of 1-2 Ωm. Array of seafloor receivers detect EM energy that propagates through the sea and subsurface. Data collected is used for processing and modeling purposes to predict depth of resistive bodies. In this paper, synthetics data generated from developed simulator that is able to replicate SBL environment is compared to synthetics data generated from Computer Simulation Software (CST) and COMSOL software with same parameter setting to study trends between them. Percentage differences between data with hydrocarbon and without hydrocarbon are calculated and comparisons are made. Overburden thickness is varied from 1000m to 3000m (incremented by 500m) at frequency of 0.125Hz. It was found that all the data generated either from simulator, CST software and COMSOL showing the same trends. From these findings it shall conclude that the simulator is a reliable tool to model any sea bed logging environment and predicting present of hydrocarbon reservoir in SBL environment.

Electromagnetic Response for Horizontal Electric Dipole Source Considering Conduct & Displacement Current

Electromagnetic (EM) field is often referred to diffusion (quasi-static assumption) that displacement currents are neglected during data processing in geophysical application, while the ratio of conduct currents to displacement currents is higher than 10, that is ,we think EM field is diffusion dominated and wave dominate for the ratio less than 0.1. Our simulating with Horizontal Electric Dipole Field indicated that frequency range of wave dominated is that the ratio is less than 0.007, the magnitude curves of EM component and impedance referring to diffusion and wave are different from those of diffusion. And in transform zone () the curves are different from those of wave and diffusion.

Transient marine electromagnetics in shallow water: A sensitivity and resolution study of the vertical electric field at short ranges

We analyzed the sensitivity of transient step-off responses of shallow-water marine controlled-source electromagnetic configurations, specifically, horizontal electric dipole sources and vertical electric dipole receivers in the near field of the time domain ([Formula: see text]). The capabilities of this configuration were compared with commonly used configurations, specifically, horizontal electric dipole sources and horizontal electric dipole receivers in the frequency domain ([Formula: see text]). By examining some simplified models of local hydrocarbon reservoirs representing separate resistive bodies of limited size buried at different depths in the conductive sediments, and an example of a complex 3D geologic structure, the applications of [Formula: see text] and [Formula: see text] configurations and their limitations in terms of depth, size, and transverse resistance were evaluated.

Three Dimensional Electromagnetic Inversion Based on Electric Dipole Source Multiple Locations Excitation

In this paper, we use integral equation and damped least-squares method to invert three dimensional abnormal body's electromagnetic field through horizontal electric dipole source multiple locations excitation. Multiple groups electromagnetic field data in different excitation and receiving points to be uniform consideration in once inversion, the Jacobian matrix is obtained and divided into linear terms and nonlinear terms. At last, we use the forward simulation data fit the measured data, and gradually modify geoelectricity model parameter values, ultimately achieve optimal fitting, gain three dimensional abnormal body's resistivity. Model test shows that the inversion algorithm has a fast convergence speed, less dependents on the initial value; the inversion result is accurate and reliable. It is an effective solution to the inversion failure caused by insufficient amount of data. DOI: http://dx.doi.org/10.11591/telkomnika.v11i12.3985 Full Text: PDF

Efficient Closed‐Form Green's Functions for Layered Stripline Structures

ABSTRACTIn this article, simple and efficient closed‐form Green's function representations for various inhomogeneously‐filled (layered) stripline structures are derived in terms of cylindrical and spherical waves for a horizontal electric dipole source. The method is based on a two‐level approximation of the spectral‐domain Green's function. After using a fast pole‐location method to extract the surface‐wave modes that are guided by the dielectric slabs and the parallel‐plate modes that are trapped by the two ground planes, we first use a method that is similar to the first step in the traditional two‐level discrete complex image method (DCIM). This step provides a good approximation to the spectral‐domain Green's function for large values of the spectral variable , thus accounting for the fields at near and intermediate distances from the source. The remainder (smaller values of ) of the spectral‐domain Green's function is approximated using a few poles and residues by using the total least squares algorithm, which requires much less computational resources as compared to the vector fitting method or DCIM. This portion of the Green's function accounts well for the fields that are further away from the source. Our proposed method provides an efficient and accurate means for representing Green's functions in layered stripline structures both near and far from the source. The results are verified by comparing with those obtained using the traditional two‐level DCIM as well as direct numerical integration of the spectral‐domain Green's function. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2261–2265, 2013