A Time-Harmonic Space/Spectral-Domain Solution of the Electromagnetic Fields Radiated by an Arbitrary Distribution of Electric and Magnetic Currents in a Three-Dimensional Cartesian Half-Space

2005 ◽  
Author(s):  
M.S. Gilbert
Keyword(s):  
Half Space ◽  
Electromagnetic Fields ◽  
Three Dimensional ◽  
Spectral Domain ◽  
Arbitrary Distribution ◽  
Harmonic Space ◽  
Time Harmonic

An evaluation of electromagnetic methods in the presence of geologic noise

Geophysics ◽  
1987 ◽  
Vol 52 (8) ◽  
pp. 1106-1126 ◽  
Author(s):  
Perry A. Eaton ◽  
Gerald W. Hohmann
Keyword(s):  
Half Space ◽  
Survey Design ◽  
Three Dimensional ◽  
Intermediate Frequency ◽  
Resolving Power ◽  
Step Response ◽  
Maximum Value ◽  
Time Harmonic ◽  
Electromagnetic Methods ◽  
Noise Measurements

An important element of electromagnetic (EM) prospecting is survey design; numerical modeling algorithms may be used to calculate signal‐to‐geologic‐noise (S/N) ratios to compare different survey configurations and measured responses quantitatively. Our models consist of a prismatic three‐dimensional (3-D) target in a conductive half‐space which may contain an overburden conductor; the models are energized by a time‐varying current transmitted in a loop of wire. The signal is the scattered or anomalous response of the target, while the geologic noise is either the response of the half‐space or the anomalous response of the overburden conductor. For typical loop sizes in exploration, the coincident‐loop configuration has a relatively high S/N ratio and thus a relatively high capability to resolve the target in the case of half‐space noise. Measurements made with the horizontal‐loop, moving‐coil configuration can be just as effective if the coil separation is one and one‐half to two times the depth of burial of the target and the transmitting and receiving coils are on opposite sides of the target. For coil positions on one side of the target, the S/N ratio decreases with increasing separation. The advantage in resolving power provided by the coincident loop’s superior S/N ratio diminishes as the size of the loop increases. For the case of noise due to the overburden conductor, the horizontal‐loop configuration with a large coil separation is optimal. If the depth of the target is unknown, the fixed‐loop, roving‐receiver configuration is useful for detecting the target but poor in resolving its depth because its S/N ratio is the least sensitive to the depth. With the fixed‐loop configuration, galvanic effects enhance the detectability of the target in a conductive half‐space, but inhibit detection if an overburden conductor is present. Regarding the S/N ratio, there does not appear to be any advantage in measuring the step response of a 3-D target in a conductive environment versus measuring the impulse response. The shapes of their respective S/N anomalies are essentially the same and the maximum impulse S/N ratio is 10 to 30 percent larger than the maximum step S/N ratio, though it occurs later in time by a factor of about 1.7. Although transient S/N ratios for a 3-D target in a conductive host reach a maximum value and then decrease with increasing time, harmonic S/N ratios do not necessarily reach a maximum value at an intermediate frequency. For all three survey configurations and both types of noise, target depths, and half‐space conductivities studied here, maximum transient S/N ratios are larger than harmonic S/N ratios. Peak step S/N ratios are 30 to 50 percent larger than corresponding in‐phase ratios in the case of half‐space noise, and several times larger in the case of the overburden conductor. A phase rotation of the target’s response due to the conductive host appears to amplify the quadrature S/N ratio relative to the in‐phase S/N ratio. However, in‐phase S/N ratios are always much larger than quadrature S/N ratios over the range of host resistivities used in this study.

Transient and Time-Harmonic Infinite Elements for Near-Surface Computations of Three-Dimensional Structures Submerged in a Half-Space

1995 ◽  
Author(s):  
Loukas F. Kallivokas ◽  
Jacobo Bielak
Keyword(s):  
Finite Element Method ◽  
Finite Element ◽  
Half Space ◽  
Three Dimensional ◽  
Scattering Problems ◽  
Near Surface ◽  
Infinite Element ◽  
Infinite Elements ◽  
Time Harmonic ◽  
Element Method

Abstract This paper is concerned with the numerical solution by the finite element method of transient and time-harmonic three-dimensional acoustic scattering problems in infinite and semi-infinite domains. Its main objective is to illustrate how a local second-order surface-only infinite element — either transient or time-harmonic — developed recently for the three-dimensional wave equation in a full-space can be applied readily to scattering problems with penetrable objects near a planar free surface. Taking a problem in structural acoustics as a prototype, the combined infinite element-finite element method is used here to determine the total and scattered pressure patterns generated when a traveling plane wave impinges upon a structure of general geometry submerged in an acoustic fluid in half-space. One key feature of this methodology is that the ordinary differential equations that result from the spatial discretization maintain the symmetry and sparsity associated with problems defined only over interior domains; the resulting equations can then be solved by standard step-by-step time integration techniques. Thus, the combination of low bandwidth matrices with the ease of use of the infinite elements places the method in an ideal position to meet the large computational demands typically associated with large-scale underwater acoustics problems.

scholarly journals Radio Base Stations and Electromagnetic Fields: GIS Applications and Models for Identifying Possible Risk Factors and Areas Exposed. Some Exemplifications in Rome

2020 ◽  
Vol 10 (1) ◽  
pp. 3
Author(s):  
Cristiano Pesaresi ◽  
Davide Pavia
Keyword(s):  
Risk Factors ◽  
Electromagnetic Fields ◽  
Digital Health ◽  
Three Dimensional ◽  
Health Information System ◽  
Medical Geography ◽  
Base Stations ◽  
Buffer Zones ◽  
North East ◽  
Gis Applications

This paper—which is contextualized in the discussion on the methodological pluralism and the main topics of medical geography, the complexity theory in geographies of health, the remaking of medical geography and ad hoc systems of data elaboration—focuses on radio base stations (RBSs) as sources of electromagnetic fields, to provide GIS applications and simplifying-prudential models that are able to identify areas that could potentially be exposed to hazard. After highlighting some specific aspects regarding RBSs and their characteristics and summarizing the results of a number of studies concerning the possible effects of electromagnetic fields on health, we have taken an area of north-east Rome with a high population and building density as a case study, and we have provided some methodological and applicative exemplifications for different situations and types of antennas. Through specific functionalities and criteria, drawing inspiration from a precautionary principle, these exemplifications show some particular cases in order to support: possible risk factor identification, surveillance and spatial analysis; correlation analysis between potential risk factors and outbreak of diseases and symptoms; measurement campaigns in heavily exposed areas and buildings; education policies and prevention actions. From an operative viewpoint, we have: conducted some field surveys and recorded data and images with specific geotechnological and geomatics instruments; retraced the routes by geobrowsers and basemaps and harmonized and joined up the materials in a GIS environment; used different functions to define, on aero-satellite images, concentric circular buffer zones starting from each RBS, and geographically and geometrically delimited the connected areas subject to high and different exposure levels; produced digital applications and tested prime three-dimensional models, in addition to a video from a bird’s eye view perspective, able to show the buildings in the different buffer zones and which are subject to a hazard hierarchy due to exposure to an RBS. A similar GIS-based model—reproposable with methodological adjustments to other polluting sources—can make it possible to conceive a dynamic and multiscale digital system functional in terms of strategic planning, decision-making and public health promotion in a performant digital health information system.

Guided Surface Waves on an Elastic Half Space

1971 ◽  
Vol 38 (4) ◽  
pp. 899-905 ◽  
Author(s):  
L. B. Freund
Keyword(s):  
Wave Propagation ◽  
Surface Wave ◽  
Surface Waves ◽  
Half Space ◽  
Body Wave ◽  
Three Dimensional ◽  
Elastic Half Space ◽  
Normal Displacement ◽  
Rigid Barrier ◽  
Wave Disturbances

Three-dimensional wave propagation in an elastic half space is considered. The half space is traction free on half its boundary, while the remaining part of the boundary is free of shear traction and is constrained against normal displacement by a smooth, rigid barrier. A time-harmonic surface wave, traveling on the traction free part of the surface, is obliquely incident on the edge of the barrier. The amplitude and the phase of the resulting reflected surface wave are determined by means of Laplace transform methods and the Wiener-Hopf technique. Wave propagation in an elastic half space in contact with two rigid, smooth barriers is then considered. The barriers are arranged so that a strip on the surface of uniform width is traction free, which forms a wave guide for surface waves. Results of the surface wave reflection problem are then used to geometrically construct dispersion relations for the propagation of unattenuated guided surface waves in the guiding structure. The rate of decay of body wave disturbances, localized near the edges of the guide, is discussed.

STRONG AND WEAK FORMS OF THE METHOD OF MOMENTS AND THE COUPLED DIPOLE METHOD FOR SCATTERING OF TIME-HARMONIC ELECTROMAGNETIC FIELDS

1992 ◽  
Vol 03 (03) ◽  
pp. 583-603 ◽  
Author(s):  
AKHLESH LAKHTAKIA
Keyword(s):  
Electromagnetic Fields ◽  
Method Of Moments ◽  
Electromagnetic Scattering ◽  
Final Section ◽  
The Other ◽  
Numerical Techniques ◽  
Scattering Problems ◽  
Time Harmonic

Algorithms based on the method of moments (MOM) and the coupled dipole method (CDM) are commonly used to solve electromagnetic scattering problems. In this paper, the strong and the weak forms of both numerical techniques are derived for bianisotropic scatterers. The two techniques are shown to be fully equivalent to each other, thereby defusing claims of superiority often made for the charms of one technique over the other. In the final section, reductions of the algorithms for isotropic dielectric scatterers are explicitly given.

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