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.

Controlled‐source audiofrequency magnetotelluric responses of three‐dimensional bodies

Geophysics ◽  
1991 ◽  
Vol 56 (2) ◽  
pp. 255-264 ◽  
Author(s):  
N. B. Boschetto ◽  
G. W. Hohmann
Keyword(s):  
Plane Wave ◽  
Electric Field ◽  
Near Field ◽  
Three Dimensional ◽  
Sole Source ◽  
Far Field ◽  
Field Zone ◽  
Wave Source ◽  
Controlled Source ◽  
Field Response

Modeling the controlled‐source audiofrequency magnetotelluric (CSAMT) responses of simple three‐dimensional (3-D) structures due to a grounded electric bipole confirms that the CSAMT technique accurately simulates plane‐wave results in the far‐field zone of the transmitter. However, at receiver sites located above large conductive or resistive bodies, the presence of the inhomogeneity extends or reduces, respectively, the frequency range of the far‐field zone. Measurements made on the surface beyond a large 3-D body display a small but spatially extensive effect due to decay of the artificial primary field. Situating a 3-D inhomogeneity beneath the source permits an evaluation of “source overprint” effects. When such a body is resistive, a slight shift in the near‐field response to higher frequencies occurs. When a body below the transmitter is conductive, it is possible to make far‐field measurements closer to the transmitter or lower in frequency. However, as the size of the conductor and its secondary‐field response increases, large transition‐zone responses distort the data. For both a plane‐wave source and a finite source, current channeling into a 3-D conductor from conductive overburden enhances the response of a target. The modeled response of a dike‐like conductor shows no better results for either the broadside or collinear configuration. The location and extent of such a body are better defined when measuring the electric field perpendicular to the strike of the prism, but resistivity estimates are better when using the electric field parallel to the strike of the prism, irrespective of transmitter orientation. Models designed from data collected at Marionoak, Tasmania, yield results which indicate that the thin, vertical graphitic unit intersected by drilling is detectable by the CSAMT method, but probably is not the sole source of the large anomaly seen in the CSAMT data.

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.

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.

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

Weakly Inhomogeneous Media Tomography

2003 ◽  
Vol 25 (2) ◽  
pp. 122-133 ◽  
Author(s):  
Robert Ferrière ◽  
Serge Mensah ◽  
Jean-Pierre Lefebvre
Keyword(s):  
High Resolution ◽  
Breast Imaging ◽  
Near Field ◽  
Wide Band ◽  
Field Zone ◽  
Reconstruction Procedure ◽  
Total Recovery ◽  
Spherical Waves ◽  
Good Penetration ◽  
Ultrasonic Scanner

Our objective is to develop an ultrasonic scanner for breast imaging. High resolution is obtained by using wide-band spherical waves transmitted and measured in the near field zone (i.e., close to the skin) all around the organ. The tomographic approach that we adopt allows us to use low central frequency waves (3–7 MHz) that are suitable for good penetration while maintaining high resolution and contrast. The procedure is thus suitable for early detection of tumors and increases the chances of total recovery. The novelty of the present reconstruction procedure is that it associates the signals acquired in transmission to the data measured in reflection over a large aperture. This enables us to correct the phase aberration induced by weak inhomogeneities whose sizes might be several wavelengths. Numerical tests based on Finite Difference Time Domain (FDTD) simulations demonstrate the greater fidelity of the reconstruction.

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