Reply to L. Szarka by the authors

Geophysics ◽  
1988 ◽  
Vol 53 (5) ◽  
pp. 727-729
Author(s):  
L. C. Bartel ◽  
R. D. Jacobson
Keyword(s):  
Half Space ◽  
Near Field ◽  
Three Dimensional ◽  
Far Field ◽  
Audio Frequency ◽  
Opportunity To Respond ◽  
Controlled Source ◽  
Electrical Structure ◽  
Homogeneous Half Space ◽  
Correction Scheme

We welcome the opportunity to respond to comments by Szarka on our recent paper. The main points he raised on our near‐field correction scheme for controlled‐source audio‐frequency magnetotelluric (CSAMT) data are the application of the correction scheme and the near‐field/far‐field demarcation in the presence of layers and the application in the presence of electrical structure beneath the transmitter location. In our paper, we addressed the application for three‐dimensional electrical structure beneath the receiver location with the transmitter over a homogeneous half‐space. In this reply we wish to clarify these points and point out possible limitations of our correction scheme.

Inversion of controlled source audio‐frequency magnetotellurics data for a horizontally layered earth

Geophysics ◽  
1999 ◽  
Vol 64 (6) ◽  
pp. 1689-1697 ◽  
Author(s):  
Partha S. Routh ◽  
Douglas W. Oldenburg
Keyword(s):  
Field Data ◽  
Near Field ◽  
Dipole Source ◽  
Earth Model ◽  
Far Field ◽  
Audio Frequency ◽  
Controlled Source ◽  
The Earth ◽  
Data Points ◽  
Corrected Data

We present a technique for inverting controlled source audio‐frequency magnetotelluric (CSAMT) data to recover a 1-D conductivity structure. The earth is modeled as a set of horizontal layers with constant conductivity, and the data are apparent resistivities and phases computed from orthogonal electric and magnetic fields due to a finite dipole source. The earth model has many layers compared to the number of data points, and therefore the solution is nonunique. Among the possible solutions, we seek a model with desired character by minimizing a particular model objective function. Traditionally, CSAMT data are inverted either by using the far‐field data where magnetotelluric (MT) equations are valid or by correcting the near‐field data to an equivalent plane‐wave approximation. Here, we invert both apparent resistivity and phase data from the near‐field transition zone and the far‐field regions in the full CSAMT inversion without any correction. Our inversion is compared with that obtained by inverting near‐field corrected data using an MT algorithm. Both synthetic and field data examples indicate that a full CSAMT inversion provides improved information about subsurface conductivity.

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.

Three-dimensional inversion of controlled-source audio-frequency magnetotelluric data based on unstructured finite-element method

2020 ◽  
Vol 17 (3) ◽  
pp. 349-360
Author(s):  
Xiang-Zhong Chen ◽  
Yun-He Liu ◽  
Chang-Chun Yin ◽  
Chang-Kai Qiu ◽  
Jie Zhang ◽  
...  
Keyword(s):  
Finite Element Method ◽  
Finite Element ◽  
Three Dimensional ◽  
Audio Frequency ◽  
Magnetotelluric Data ◽  
Controlled Source ◽  
Element Method

Computation of Green’s tensor integrals for three‐dimensional electromagnetic problems using fast Hankel transforms

Geophysics ◽  
1984 ◽  
Vol 49 (10) ◽  
pp. 1754-1759 ◽  
Author(s):  
Walter L. Anderson
Keyword(s):  
Half Space ◽  
Three Dimensional ◽  
Hankel Transform ◽  
Electromagnetic Modeling ◽  
Electromagnetic Problems ◽  
Hankel Transforms ◽  
Homogeneous Half Space ◽  
Green's Tensor ◽  
Green’S Tensor ◽  
The Many

A new method is presented that rapidly evaluates the many Green’s tensor integrals encountered in three‐dimensional electromagnetic modeling using an integral equation. Application of a fast Hankel transform (FHT) algorithm (Anderson, 1982) is the basis for the new solution, where efficient and accurate computation of Hankel transforms are obtained by related and lagged convolutions (linear digital filtering). The FHT algorithm is briefly reviewed and compared to earlier convolution algorithms written by the author. The homogeneous and layered half‐space cases for the Green’s tensor integrals are presented in a form so that the FHT can be easily applied in practice. Computer timing runs comparing the FHT to conventional direct convolution methods are discussed, where the FHT’s performance was about 6 times faster for a homogeneous half‐space, and about 108 times faster for a five‐layer half‐space. Subsequent interpolation after the FHT is called is required to compute specific values of the tensor integrals at selected transform arguments; however, due to the relatively small lagged convolution interval used (same as the digital filter’s), a simple and fast interpolation is sufficient (e.g., by cubic splines).

On: “Results of a controlled‐source audiofrequency magnetotelluric survey at the Puhimau thermal area, Kilauea Volcano, Hawaii, by L. C. Bartel and R. D. Jacobson (GEOPHYSICS, 52, 665–677, May 1987)

Geophysics ◽  
1988 ◽  
Vol 53 (5) ◽  
pp. 726-727 ◽  
Author(s):  
Lásaló Szarka
Keyword(s):  
Near Field ◽  
Kilauea Volcano ◽  
Far Field ◽  
Gradual Change ◽  
Demarcation Line ◽  
Controlled Source ◽  
Layered Earth ◽  
Kīlauea Volcano ◽  
Subsurface Layers

A growing number of papers being published on the CSAMT-MT curve transformation, which — as the authors state — allows a simpler magnetotelluric interpretation of the corrected CSAMT curves. The concept of near‐field corrections is based on electromagnetic relations over a homogeneous earth, and the effects of subsurface layers or lateral inhomogeneities are usually neglected. Bartel and Jacobson (1987) especially suppress the bounds of the near‐field correction: After presenting several near‐field correction curves over a homogeneous earth in their Figure 2 (which includes an idealistic demarcation line instead of a gradual change between near‐field and far‐field regions), they simply add that “…for a layered earth a similar demarcation occurs between the far‐ and near‐field regimes.” Further, the problem of lateral inhomogeneities is not mentioned in the paper. Such a description might lead to an oversimplification. I should like here to underline both limitations.

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