Scattering by a two-layer spherical dielectric object in a lossy medium illuminated by a loop carrying an arbitrary azimuthal mode

1989 ◽  
Vol 67 (6) ◽  
pp. 617-623
Author(s):  
A. Sebak ◽  
L. Shafai
Keyword(s):  
Current Distribution ◽  
Scattered Field ◽  
Near Field ◽  
Electromagnetic Response ◽  
Field Zone ◽  
Spherical Object ◽  
Azimuthal Mode ◽  
Low Frequencies ◽  
Circular Loop ◽  
Lossy Medium

The electromagnetic response of a circular loop antenna in the vicinity of a two-layer dielectric spherical object located in a lossy medium is investigated analytically. For a loop carrying an azimuthally dependent current distribution, a technique based on the dyadic Green's functions is employed to determine the fields in all regions. The formulation is general and can be applied to a wide variety of electromagnetic sources. Numerical results are presented in the near field zone and at low frequencies to determine the effect of the coating on the scattered field and its influence on the degree of detectability of a coated object.

The effects of source polarization in CSAMT data over two massive sulfide deposits in Australia

Geophysics ◽  
1993 ◽  
Vol 58 (12) ◽  
pp. 1764-1772 ◽  
Author(s):  
Richard Kellett ◽  
John Bishop ◽  
Emmett Van Reed
Keyword(s):  
Structural Information ◽  
Near Field ◽  
Real Data ◽  
Black Shales ◽  
Electromagnetic Response ◽  
Resistivity Structure ◽  
Field Zone ◽  
Controlled Source ◽  
Source Field ◽  
Eastern Australia

Since the advent of the controlled‐source audio‐magnetotelluric method it has been recognized that the location and orientation of the bipole source is important in determining the response of the earth at the receiver. In this study, two‐dimensional (2-D) far‐field modeling has been used to illustrate the frequency-domain electromagnetic response of a simple conductive dike for two orthogonal polarizations of the source field. The current gathered from the surrounding media by the dike, when the electric field is parallel to the strike direction (E‐polarization), produces a strong anomaly not seen in the perpendicular H‐polarization. This model response has been identified in real data sets over the Rosebery and Flying Doctor orebodies of eastern Australia. In the case of Rosebery the E‐polarization data yielded little structural information because the penetration of the signal was reduced by strong current channeling in the orebody and adjacent black shales. At the Flying Doctor prospect the model predictions held but changes in the extent of the near‐field zone, for the two bipole locations, dominate the data. The changes in the source field observed over the Flying Doctor prospect are interpreted as evidence for anisotropy in the regional resistivity structure. The controlled‐source is a fundamental component of the CSAMT system, and the choice of the bipole location and orientation must be made considering the geology of the target region and the surrounding regional resistivity structure.

INDUCED CURRENT DISTRIBUTION IN A CONDUCTIVE TWO‐LAYER SPHERICAL BODY

Geophysics ◽  
1973 ◽  
Vol 38 (3) ◽  
pp. 530-544
Author(s):  
Chandra P. Gupta ◽  
Upendra Raval ◽  
Janardan G. Negi
Keyword(s):  
Current Distribution ◽  
Current Density ◽  
Spherical Body ◽  
Ore Deposits ◽  
Electromagnetic Response ◽  
Induced Current ◽  
Low Frequencies ◽  
Response Characteristics ◽  
The Core ◽  
Physical Insight

Under complex geologic situations the analysis of induced current distribution, rather than of the scattered field, may provide a useful physical insight into the electromagnetic response characteristics of a conducting ore body. Influence of a conducting cover and inhomogeneity in the conductivity on the currents induced in aspherical conductor excited by a uniform alternating magnetic field has, therefore, been investigated. The analysis shows (a) reduction of the current‐density amplitudes in the core by the cover, (b) occurrence of current‐density maxima inside the conducting system for some characteristic frequencies, (c) enhancement of the in‐phase component in some cases, and (d) reduction of current‐density in the shell with increase in the core conductivity and significant influence of the inhomogeneity in conductivity at low frequencies. Since there is a direct correspondence between the electromagnetic response of a target and the currents induced in it, these results are expected to aid in the interpretation of data over ore deposits having a halo‐zone of disseminated mineralization.

A Comparative Study on the Difference between the Multi-dipole Sources and Vector Synthesis Source

2020 ◽  
Vol 25 (4) ◽  
pp. 529-543
Author(s):  
Xian-Xiang Wang ◽  
Ju-Zhi Deng
Keyword(s):  
Electromagnetic Field ◽  
Near Field ◽  
Electromagnetic Response ◽  
Dipole Source ◽  
Mountainous Area ◽  
Field Zone ◽  
Specific Direction ◽  
Single Dipole ◽  
The Difference ◽  
Dipole Sources

CSAMT exploration generally adopts a single dipole as the transmitter. The single dipole source has the apparent disadvantages–there are weak areas for all components, Ey and Hx are weak in the area where Ex and Hy are reliable. Moreover, it is hard to deploy the source with a specific direction in a rugged mountainous area. Given the shortcomings of the single dipole source, multi-dipole sources are introduced into CSAMT exploration. Although the dipole sources follow the principle of vector synthesis, the length of the source in actual exploration can last for several kilometers and the offset is generally a few kilometers. In this case, the source can no longer be regarded as a single dipole in the near-field zone. The electromagnetic field in this region becomes relatively complicated. We first compare the similarities and differences of electromagnetic field generated by vector synthesis source and multi-dipole source through the Ex radiation patterns. Then, we study the factors that affect electromagnetic response due to the substitution of the double-dipole source with the vector synthesis source. The measured EM fields is affected by the source length, frequency, the source angle, the offset, and the resistivity.Finally, we apply the double-dipole source to the 1D and 3D geological model and compare the difference between the electromagnetic field generated by the double-dipole source and that generated by the vector synthesis source. Usually, the difference is very obvious in the near-field zone, and is almost negligible in the far-field zone.

Near-field zone analysis of the Faraday rotation of magneto-optical thin films

2000 ◽  
Vol 88 (5) ◽  
pp. 2541-2547 ◽  
Author(s):  
N. Richard ◽  
A. Dereux ◽  
E. Bourillot ◽  
T. David ◽  
J. P. Goudonnet ◽  
...  
Keyword(s):  
Thin Films ◽  
Faraday Rotation ◽  
Near Field ◽  
Field Zone

The harmonic and transient electromagnetic response of a thin dipping dike

Geophysics ◽  
1983 ◽  
Vol 48 (7) ◽  
pp. 934-952 ◽  
Author(s):  
P. Weidelt
Keyword(s):  
Formal Solution ◽  
Harmonic Excitation ◽  
Electromagnetic Response ◽  
Approximate Determination ◽  
Finite Conductivity ◽  
Circular Loop ◽  
Decay Modes ◽  
Transient Electromagnetic ◽  
Free Decay

An exact solution is given for the electromagnetic induction in a dipping dike of finite conductivity, represented as a thin half‐sheet in a nonconducting surrounding. The problem is formulated for arbitrary dipole or circular loop [Formula: see text] configurations. The formal solution obtained by the Wiener‐Hopf technique is cast into a rapidly convergent triple integral suitable for an effective numerical treatment. A good agreement is found between numerical results and analog measurements available for harmonic excitation. The transient response is obtained as a superposition of the half‐sheet free‐decay modes and is illustrated by some numerical examples for coincident loops, including a diagram for the approximate determination of conductance and depth of a vertical dike.

scholarly journals Hybrid multilevel plane wave based near-field far-field transformation utilising combined near- and far-field translations

2009 ◽  
Vol 7 ◽  
pp. 17-22 ◽  
Author(s):  
C. H. Schmidt ◽  
T. F. Eibert
Keyword(s):  
Near Field ◽  
Measurement Techniques ◽  
Plane Waves ◽  
Low Complexity ◽  
Far Field ◽  
Computation Complexity ◽  
Low Frequencies ◽  
Field Transformation ◽  
Translation Operators ◽  
Equivalent Sources

Abstract. The radiation of large antennas and those operating at low frequencies can be determined efficiently by near-field measurement techniques and a subsequent near-field far-field transformation. Various approaches and algorithms have been researched but for electrically large antennas and irregular measurement contours advanced algorithms with low computation complexity are required. In this paper an algorithm employing plane waves as equivalent sources and utilising efficient diagonal translation operators is presented. The efficiency is further enhanced using simple far-field translations in combination with the expensive near-field translations. In this way a low complexity near-field transformation is achieved, which works for arbitrary sample point distributions and incorporates a full probe correction without increasing the complexity.

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