scholarly journals Useful characteristics of shallow and deep marine CSEM responses inferred from 3D finite-difference modeling

Geophysics ◽  
2009 ◽  
Vol 74 (5) ◽  
pp. F67-F76 ◽  
Author(s):  
Yutaka Sasaki ◽  
Max A. Meju
Keyword(s):  
Magnetic Field ◽  
Finite Difference ◽  
Electric Field ◽  
Shallow Water ◽  
Phase Response ◽  
Staggered Grid ◽  
Small Scale ◽  
Shallow Waters ◽  
Near Surface ◽  
Electric And Magnetic Field

Hydrocarbon reservoirs can be mapped if sufficient resistivity contrasts exist between them and their confining layers, but practical problems remain in target discrimination in deep and shallow waters, especially in the presence of heterogeneous overburden. We have developed an efficient 3D staggered-grid finite-difference controlled-source electromagnetic (CSEM) modeling code that enables study of the physics underlying some practical problems. We undertook a comparative analysis of reservoir detection in [Formula: see text]- and [Formula: see text]-deep waters using the simulated electric and magnetic field responses of a simple 3D reservoir. We examined the effect of two types of near-surface heterogeneity (mimicking disconnected gas clouds and/or patchy geochemical alteration halos) on the 3D reservoir response. We found that small-scale, shallow heterogeneities cause distortions that are almost independent of the source frequency. These persist at all source-receiver offsets in the electric amplitude response in deep and shallow waters and phase response in shallow water. They decrease in magnitude with increasing offset in deepwater phase response. Large-scale near-surface heterogeneities distort the horizontal electric field response more significantly than the small-scale ones, but the near-surface response gets smaller in amplitude as the offset increases. The distortions in shallow water are much smaller in magnitude than those for the deepwater case, so that the reservoir signatures still are visible on the response profiles. This might be considered as a positive feature for shallow-water inline electric field exploration. The magnetic field responses for the orthogonal direction provide diagnostic target signatures that are similar to the inline electric field responses in deep water but that are different in shallow water. The magnetic responses are affected by the airwave in a different manner from the electric field, suggesting that combined 3D electric and magnetic field analysis might be vital for handling the airwave problem.

scholarly journals A parallel finite‐difference approach for 3D transient electromagnetic modeling with galvanic sources

Geophysics ◽  
2004 ◽  
Vol 69 (5) ◽  
pp. 1192-1202 ◽  
Author(s):  
Michael Commer ◽  
Gregory Newman
Keyword(s):  
Magnetic Field ◽  
Finite Difference ◽  
Electric Field ◽  
Electric Fields ◽  
Time Derivative ◽  
Stable Solution ◽  
Parallel Computer ◽  
Staggered Grid ◽  
Electromagnetic Modeling ◽  
Transient Electromagnetic

A parallel finite‐difference algorithm for the solution of diffusive, three‐dimensional (3D) transient electromagnetic field simulations is presented. The purpose of the scheme is the simulation of both electric fields and the time derivative of magnetic fields generated by galvanic sources (grounded wires) over arbitrarily complicated distributions of conductivity and magnetic permeability. Using a staggered grid and a modified DuFort‐Frankel method, the scheme steps Maxwell's equations in time. Electric field initialization is done by a conjugate‐gradient solution of a 3D Poisson problem, as is common in 3D resistivity modeling. Instead of calculating the initial magnetic field directly, its time derivative and curl are employed in order to advance the electric field in time. A divergence‐free condition is enforced for both the magnetic‐field time derivative and the total conduction‐current density, providing accurate results at late times. In order to simulate large realistic earth models, the algorithm has been designed to run on parallel computer platforms. The upward continuation boundary condition for a stable solution in the infinitely resistive air layer involves a two‐dimensional parallel fast Fourier transform. Example simulations are compared with analytical, integral‐equation and spectral Lanczos decomposition solutions and demonstrate the accuracy of the scheme.

Variations of electrical characteristics of near-surface atmosphere of the Earth during magnetic storm

2020 ◽  
Author(s):  
Svetlana Riabova ◽  
Alexander Spivak
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Magnetic Storms ◽  
Electrical Characteristics ◽  
Near Surface ◽  
Russian Academy Of Sciences ◽  
The Earth ◽  
Geosphere Dynamics ◽  
Academy Of Sciences ◽  
The Magnetic Field

<p>Temporal variations of the electric field in near-surface layer of the Earth are determined by many factors, among which strong disturbances of the magnetic field should be especially noted. Magnetic storms cause an increase in the ionospheric electric field, which leads to variations in the gradient of the electric field potential near the Earth's surface. We consider the effect of magnetic storms in variations in the electrical characteristics of the atmosphere at Geophysical observatory «Mikhnevo» of Sadovsky Institute of Geosphere Dynamics of Russian Academy of Sciences and at Center for geophysical monitoring of Moscow of Sadovsky Institute of Geosphere Dynamics of Russian Academy of Sciences. We used data from the continuous monitoring of three components of the magnetic field, vertical components of the atmospheric electric field and atmospheric current carried out in fair weather. Experimental data processing and analysis show that accompanying magnetic storms with geomagnetic K index more or equal 5 increased variations in the electric field and vertical atmospheric current are characterized by different morphological structures. It is currently difficult to interpret the data. Nevertheless, the research results can be of great help in the development and verification of theoretical and computational models for generating variations in the electric field as a result of strong geomagnetic disturbances.</p>

Three‐dimensional induction logging problems, Part 2: A finite‐difference solution

Geophysics ◽  
2002 ◽  
Vol 67 (2) ◽  
pp. 484-491 ◽  
Author(s):  
Gregory A. Newman ◽  
David L. Alumbaugh
Keyword(s):  
Finite Difference ◽  
Electric Field ◽  
Krylov Subspace ◽  
Inverse Operator ◽  
Three Dimensional ◽  
Staggered Grid ◽  
Linear System Of Equations ◽  
Order Of Magnitude ◽  
Finite Difference Solution ◽  
Difference Solution

A 3‐D finite‐difference solution is implemented for simulating induction log responses in the quasi‐static limit that include the wellbore and bedding that exhibits transverse anisotropy. The finite‐difference code uses a staggered grid to approximate a vector equation for the electric field. The resulting linear system of equations is solved to a predetermined error level using iterative Krylov subspace methods. To accelerate the solution at low induction numbers (LINs), a new preconditioner is developed. This new preconditioner splits the electric field into curl‐free and divergence‐free projections, which allows for the construction of an approximate inverse operator. Test examples show up to an order of magnitude increase in speed compared to a simple Jacobi preconditioner. Comparisons with analytical and mode matching solutions demonstrate the accuracy of the algorithm.

scholarly journals S-wave propagating in an anisotropic inhomogeneous elastic medium under the influence of gravity, initial stress, electric and magnetic field

2014 ◽  
Vol 41 (2) ◽  
pp. 141-157 ◽  
Author(s):  
Rajneesh Kakar ◽  
Shikha Kakar
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Initial Stress ◽  
Isotropic Medium ◽  
Shear Waves ◽  
S Wave ◽  
Incompressible Medium ◽  
Material Parameters ◽  
Electric And Magnetic Field ◽  
Inhomogeneous Elastic Medium

The purpose of this paper is to study the effect of gravity, initial stress, non-homogeneity, electric and magnetic field on the propagation of shear waves in an anisotropic incompressible medium. Various graphs are plotted to show the effect of direction of propagation, the anisotropy, magnetic field, electric field, non-homogeneity of the medium and the initial stress on shear waves. The dispersion equations for shear waves are obtained and discussed for different cases. In fact, in the absence of various material parameters, these equations are in agreement with the classical results for isotropic medium.

scholarly journals Health Hazards Associated with Electric and Magnetic Field Intensities around Mobile Base Stations in Katsina State, Nigeria

2020 ◽  
Vol 24 (2) ◽  
pp. 253-256
Author(s):  
M. Abdulsalam ◽  
S. Bello ◽  
Y.A. Sumaila ◽  
H. Abubakar ◽  
I.B. Muhammad ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Field Intensity ◽  
Health Hazard ◽  
Health Hazards ◽  
Base Stations ◽  
Reconnaissance Survey ◽  
Radiated Power ◽  
Electric And Magnetic Field ◽  
Mobile Base

This work was carried out to assess the health hazards associated with exposure to radiofrequency electromagnetic fields from mobile base stations (MBS) within Katsina, Nigeria. Seventy seven MBS were identified through reconnaissance survey. Received radiated power was measured at a distance of 0, 20, 40, 60 and 80m from the MBS using a handheld B and K precision spectrum analyser. Electric and magnetic field intensities (E and H) were calculated. E (mV/m) and H (μA/m) had average values of; 21.03 and 55.78 for MTN; 9.41 and 24.96 for GLO; 2.33 and 6.18, for Etisalat; 18.32 and 48.62 for Airtel. Our results indicated that the general public exposure from radiofrequency electromagnetic radiation from all the considered mobile base stations is within the acceptable threshold of 61 V/m for Electric field intensity and 0.16A/m for Magnetic field intensity. Keywords: Mobile base stations, Radiation, Katsina, Health hazard, Electric field, Magnetic field

Implications of the Born approximation for the magnetotelluric problem in three‐dimensional environments

Geophysics ◽  
1992 ◽  
Vol 57 (4) ◽  
pp. 587-602 ◽  
Author(s):  
Carlos Torres‐Verdín ◽  
Francis X. Bostick
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Born Approximation ◽  
Transfer Functions ◽  
Pass Filter ◽  
Near Surface ◽  
Low Pass ◽  
Surface Electric Field ◽  
Subsurface Resistivity ◽  
Lateral Variations

A first‐order Born approximation is obtained for the integral equations governing the surface magnetotelluric response over a three‐dimensional earth. Although accurate only in cases of low resistivity contrasts, the resulting expressions: (1) exhibit a linear relationship between a spatial perturbation in subsurface resistivity and the ensuing perturbation on the surface field response, and, more importantly, (2) allow arbitrary degrees of complexity in the geometrical characteristics of the subsurface. The linear system solutions derived from the Born approximation are studied by examining the properties of their associated kernels. These kernels may be thought of as a suite of horizontal magnetotelluric “wavelets” weighting the subsurface resistivity distribution at different depth levels. Analytical expressions for the wavelets are obtained in the wavenumber domain, thus generating a suite of magnetotelluric “transfer functions.” Expressions for the latter are particularized to the cases of one‐ and two‐dimensional geolectric media yielding results consistent with the characteristics of the magnetotelluric fields known to hold in these low‐order environments. Inspection of the electric transfer functions reveals severe sensitivity to near‐surface lateral variations of resistivity, which persists even at deep sensing frequencies. This near‐surface sensitivity is the result of an additive term in the electric field transfer functions, the static component, acting as a spatial highpass filter of the lateral variations of surface resistivity. A second additive component in the electric transfer functions, the induction component, functions as a spatial lowpass filter of the lateral variations in subsurface resistivity, and is primarily responsible for the inductive part of the surface electric field response. A common problem in magnetotelluric interpretation, the electric static effect can be reduced by inverting the role of the static component, i.e., by spatially low‐pass filtering the surface electric field. The suggested low‐pass filter for such an operation is one for which the cutoff wavenumber increases with frequency and is therefore insensitive to the response from the induction component. Low‐pass filtering of the surface electric field is best implemented in the field if the electric dipoles are deployed end‐to‐end continuously along a survey path. The magnetic field transfer functions, on the other hand, exhibit a single induction term with band‐pass filter properties which may actually lead to some amount of local distortion on the measured surface magnetic field. We propose to reduce this distortion by referring all electric field measurements to the primary magnetic field within the survey area. The primary magnetic field components, in turn, can be estimated by the spatial average of the magnetic measurements acquired at an array of magnetic stations. The suggested procedures for both the acquisition and processing of natural electric and magnetic field data encompass altogether a novel adaptation of the magnetotelluric method.

An Algorithm of 3-D ADI-R-FDTD Based on Non-Zero Divergence Relationship

2011 ◽  
Vol 317-319 ◽  
pp. 1172-1176
Author(s):  
Xiu Hai Jin ◽  
Yi Wang Chen ◽  
Pin Zhang
Keyword(s):  
Magnetic Field ◽  
Finite Difference ◽  
Electric Field ◽  
Time Domain ◽  
Finite Difference Time Domain ◽  
Fdtd Method ◽  
Memory Requirement ◽  
Alternating Direction ◽  
Relationship Of ◽  
Difference Time

In this letter, an alternating-direction reduced finite-difference time-domain (ADI-R-FDTD) method is presents. It is proven that the divergence relationship of electric-field and magnetic-field is non-zero even in charge-free regions, when the electric-field and magnetic-field are calculated with alternating-direction finite-difference time-domain (ADI-FDTD) method in 3 dimensions case, and the expression of the divergence relationship is derived. Based on the non-zero divergence relationship, the ADI-FDTD method is combined with the reduced finite-difference time-domain (R-FDTD) method. In the proposed method, the memory requirement of ADI-R-FDTD is reduced by1/12 of the memory requirement of ADI-FDTD averagely in 3D case. The formulation is presented and the accuracy and efficiency of the proposed method is verified by comparing the results with the conventional results.

On the Spectral Convergence of the Supercompact Finite-Difference Schemes for the f-Plane Shallow-Water Equations

2009 ◽  
Vol 137 (7) ◽  
pp. 2393-2406 ◽  
Author(s):  
S. Ghader ◽  
A. R. Mohebalhojeh ◽  
V. Esfahanian
Keyword(s):  
Finite Difference ◽  
Shallow Water ◽  
Potential Vorticity ◽  
Shallow Water Equations ◽  
Small Scale ◽  
Finite Difference Schemes ◽  
Vorticity Field ◽  
Eighth Order ◽  
Spectral Convergence ◽  
Nonlinear Flows

For the f-plane shallow-water equations, the convergence properties of the supercompact finite-difference method (SCFDM) are examined during the evolution of complex, nonlinear flows spawned by an unstable jet. The second-, fourth-, sixth-, and eighth-order SCFDMs are compared with a standard pseudospectral (PS) method. To control the buildup of small-scale activity and thus the potential for numerical instability, the vorticity field is damped explicitly by the application of a triharmonic hyperdiffusion operator acting on the vorticity field. The global distribution of mass between isolevels of potential vorticity, called mass error, and the representation of the balance and imbalance are used to assess numerical accuracy. In each of the quantitative measures, a clear convergence of the SCFDM to the PS method is observed. There is no saturation in accuracy up to the eighth order examined. Taking the PS solution as the reference, for the fundamental quantity of potential vorticity the rate of convergence to PS turns out to be algebraic and near-quadratic.

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