Orthogonality in CSAMT and MT measurements

Geophysics ◽  
1993 ◽  
Vol 58 (7) ◽  
pp. 924-934 ◽  
Author(s):  
David E. Boerner ◽  
Ron D. Kurtz ◽  
Alan G. Jones
Keyword(s):  
Magnetic Fields ◽  
Plane Wave ◽  
Field Data ◽  
Necessary Condition ◽  
Far Field ◽  
Single Plane ◽  
Wave Source ◽  
Near Surface ◽  
Layered Earth ◽  
Electric And Magnetic Fields

The electric and magnetic fields from a single plane‐wave source on a one dimensional (1-D) earth, or a plane‐wave source polarized parallel or perpendicular to strike on a two-dimensional (2-D) earth, are orthogonal. On a layered earth and in the far‐field of a controlled source, the electric and magnetic fields are also orthogonal. Therefore, orthogonality of E and H data is a necessary condition to justify the application of 1-D or 2-D modeling algorithms having a plane wave source. A strict criterion to prove orthogonality, and thus provide a rationale for the choice of interpretation methods, can be defined directly in terms of field data. However, field data acquired in the intermediate and near‐field of any electromagnetic (EM) source are generally not orthogonal, even on a plane‐layered earth. Representing these nonorthogonal data in an orthogonal coordinate system can be misleading, particularly for the minor axis components of the polarization ellipses. Nonorthogonality also arises because of 3-D scattering, with one common example being the electric field response of near surface structure. An example of field data illustrates the nonorthogonality in CSAMT measurements caused by the response of surficial geology. In these EM data, the angle between E and H is a sensitive indicator of geological contacts and faults. Quantitative analysis of these data can be performed with the assumptions of a “bulk” 1-D earth (i.e., orthogonal E and H in the far‐field) and purely galvanic scattering of the EM fields.

3D plane-wave migration in tilted coordinates: A field data example

Geophysics ◽  
2009 ◽  
Vol 74 (6) ◽  
pp. WCA199-WCA209 ◽  
Author(s):  
Guojian Shan ◽  
Robert Clapp ◽  
Biondo Biondi
Keyword(s):  
Plane Wave ◽  
Field Data ◽  
Synthetic Data ◽  
Data Set ◽  
Wave Source ◽  
Surface Data ◽  
The One ◽  
Wave Migration ◽  
Wavefield Extrapolation ◽  
Slant Stacking

We have extended isotropic plane-wave migration in tilted coordinates to 3D anisotropic media and applied it on a Gulf of Mexico data set. Recorded surface data are transformed to plane-wave data by slant-stack processing in inline and crossline directions. The source plane wave and its corresponding slant-stacked data are extrapolated into the subsurface within a tilted coordinate system whose direction depends on the propagation direction of the plane wave. Images are generated by crosscorrelating these two wavefields. The shot sampling is sparse in the crossline direction, and the source generated by slant stacking is not really a plane-wave source but a phase-encoded source. We have discovered that phase-encoded source migration in tilted coordinates can image steep reflectors, using 2D synthetic data set examples. The field data example shows that 3D plane-wave migration in tilted coordinates can image steeply dipping salt flanks and faults, even though the one-way wave-equation operator is used for wavefield extrapolation.

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.

The borehole controlled‐source audiomagnetotelluric response of a three‐dimensional fracture zone

Geophysics ◽  
1988 ◽  
Vol 53 (2) ◽  
pp. 215-230 ◽  
Author(s):  
Richard C. West ◽  
Stanley H. Ward
Keyword(s):  
Magnetic Fields ◽  
Half Space ◽  
Fracture Zone ◽  
Body Position ◽  
Imaginary Component ◽  
Model Parameters ◽  
Simple Relationship ◽  
Real Component ◽  
Wave Source ◽  
Electric And Magnetic Fields

Approximate theoretical borehole CSAMT profiles of the normalized vertical electric and magnetic fields [Formula: see text] and [Formula: see text] were computed for several models of practical interest using a 3-D MT computer program based on the method of integral equations. [Formula: see text] and [Formula: see text] are the horizontal fields measured at the surface. A conductive tabular prism in a layered and homogeneous half‐space was chosen to simulate a 3-D fracture zone composed of individual, interconnected fractures. Model parameters varied during this study were depth and dip of the tabular body, the conductivity and layering of the half‐space, the frequency of the plane‐wave source, and the separation between the borehole and target. In addition, a model composed of two horizontal prisms was investigated. Decreasing the host conductivity, the depth of the prism, or the separation between the borehole and prism increases the magnitude of the subsurface normalized vertical electric and magnetic fields. Depth to the top of a dipping prism in a half‐space can be determined from the crossover of the profile of the [Formula: see text] real component. Peak amplitudes of the [Formula: see text] profiles provide information about the location of the maximum current density within the prism, which is quite variable for the imaginary component. There is no simple relationship between a small borehole‐to‐prism separation and the separation between the antisymmetric peaks of the [Formula: see text] profiles. Without knowledge of the borehole‐to‐prism separation, the dip of a nonhorizontal prism cannot be determined accurately. However, the down‐dip side of a dipping prism is indicated by the larger peak anomaly in the real component of the profile. The normalized [Formula: see text] anomalies of the conductive prism seem more sensitive to body position and also to variation in the host resistivity than are the normalized [Formula: see text] anomalies, thus making the former parameter more susceptible to geologic noise. The resistivity of the half‐space and overburden and the frequency of the source significantly influence the amplitude and phase of the secondary electric fields. Current channeling is a significant contributor to the response of the prism within a half‐space of even moderate resistivity at frequencies of 10 to 100 Hz. The [Formula: see text] field is little influenced by horizontal layering, so the borehole profiles reflect mainly subsurface inhomogeneities. The two main advantages of this technique are that the signal is much larger than the level of natural‐field noise and data acquisition is rapid because of the high frequency of the source (10–1000 Hz). However, the borehole profiles can be significantly affected by nearby inhomogeneities and by the incident field when the borehole and sensor are not vertically aligned.

Estimation of the depth of investigation in the magnetotelluric method from the phase

Geophysics ◽  
2019 ◽  
Vol 84 (6) ◽  
pp. E377-E385 ◽  
Author(s):  
Ujjal K. Borah ◽  
Prasanta K. Patro
Keyword(s):  
Magnetic Fields ◽  
Inverse Modeling ◽  
Field Data ◽  
Previous Method ◽  
Phase Lag ◽  
New Approach ◽  
Electric And Magnetic Fields ◽  
Magnetotelluric Method ◽  
Depth Of Investigation ◽  
Instrument Sensitivity

Proper computation for depth of investigation (DOI) of the magnetotelluric (MT) method is vital because the standard approximate depth transforms provide the depth corresponds to the interpreted conductivities instead of the actual DOI of MT. Previous works on DOI estimation of the MT method are based on 1D inversion. Although the previous works are significant, the methods of DOI calculation are lengthy and affected by non-uniqueness of inverse modeling. Therefore, to overcome these problems, we developed a simple and direct approach of DOI computation for the MT method using phase, which applies to a layered earth. For this purpose, we calculate the instantaneous phases of electric and magnetic fields, which correspond to the minimum evaluation frequency using the solution of the 1D electromagnetic (EM) diffusion equation and instrument sensitivity. From the calculated instantaneous phases, we derive an expression that connects the phase-lag between the electric and magnetic fields at the minimum evaluation frequency to its corresponding DOI. To examine the effectiveness of our method, we applied the new approach to 1D synthetic and field data and compared the calculated DOIs with the DOIs obtained from a previous method. We determine that our new approach is faster in computation and overcomes the effect of non-uniqueness of inverse modeling, unlike the previous methods.

scholarly journals Timing of the martian dynamo: New constraints for a core field 4.5 and 3.7 Ga ago

2020 ◽  
Vol 6 (18) ◽  
pp. eaba0513 ◽  
Author(s):  
A. Mittelholz ◽  
C. L. Johnson ◽  
J. M. Feinberg ◽  
B. Langlais ◽  
R. J. Phillips
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Field Data ◽  
Magnetic Minerals ◽  
Near Surface ◽  
Magnetic Field Data ◽  
Core Field ◽  
Before And After ◽  
Basin Edge ◽  
Weak Fields

The absence of crustal magnetic fields above the martian basins Hellas, Argyre, and Isidis is often interpreted as proof of an early, before 4.1 billion years (Ga) ago, or late, after 3.9 Ga ago, dynamo. We revisit these interpretations using new MAVEN magnetic field data. Weak fields are present over the 4.5-Ga old Borealis basin, with the transition to strong fields correlated with the basin edge. Magnetic fields, confined to a near-surface layer, are also detected above the 3.7-Ga old Lucus Planum. We conclude that a dynamo was present both before and after the formation of the basins Hellas, Utopia, Argyre, and Isidis. A long-lived, Earth-like dynamo is consistent with the absence of magnetization within large basins if the impacts excavated large portions of strongly magnetic crust and exposed deeper material with lower concentrations of magnetic minerals.

Transformation of VLF anomaly maps into apparent resistivity and phase

Geophysics ◽  
2003 ◽  
Vol 68 (2) ◽  
pp. 497-505 ◽  
Author(s):  
Michael Becken ◽  
Laust B. Pedersen
Keyword(s):  
Magnetic Field ◽  
Plane Wave ◽  
Electric Field ◽  
Transfer Functions ◽  
Magnetic Data ◽  
Impedance Tensor ◽  
Wave Source ◽  
Layered Earth ◽  
Magnetic Transfer ◽  
The Magnetic Field

We investigate a transformation of magnetic transfer functions into the tangential‐electric mode part of the impedance tensor in the scope of the plane‐wave electromagnetic tensor–VLF method. The transformation, which is applicable to any 2D data representing the response of arbitrary 3D geoelectric structures, overcomes the difficulties of quantitative interpretation of magnetic transfer functions, which predominantly provide a measure of the lateral changes of the electrical conductivity in the earth. We require densely sampled magnetic transfer functions of one frequency as input data. These may be decomposed into their normal and anomalous parts (deviation from the response of a layered earth) for a unit external plane‐wave source field using the Hilbert transform relationship between the magnetic field components. Faraday's law then directly provides the anomalous toroidal electric field. Unfortunately, there is no chance to estimate the normal electric field from magnetic data, since the magnetic field is not sensitive to a layered earth. This constant must be provided as a boundary condition, e.g., from one ground measurement, to derive an impedance tensor and related apparent resistivities and phases.

Electrical and magnetometric fields in a layered earth containing buried electrodes

Geophysics ◽  
1990 ◽  
Vol 55 (12) ◽  
pp. 1605-1612 ◽  
Author(s):  
D. Veitch ◽  
M. W. Asten ◽  
E. H. van Leeuwen
Keyword(s):  
Half Space ◽  
Field Data ◽  
Massive Sulfide ◽  
Magnetic Field Measurement ◽  
Layered Earth ◽  
Homogeneous Half Space ◽  
Electric And Magnetic Fields ◽  
Borehole Logs ◽  
Two Layer Model ◽  
The Magnetic Field

The number of analytical magnetometric resistivity (MMR) results available for basic earth geometries is limited compared to that of electrical resistivity methods, which is unfortunate since MMR has advantages for certain classes of problems. This paper extends the list of MMR results by deriving the response for the homogeneous half‐space and the multilayered earth. Both are calculated for arbitrary source and receiver positions. We show how the result for the layered earth reduces to that of the half‐space when there is only one layer. Necessary procedures for successful numerical evaluation of results for the layered case are given. Sample borehole logs of electric and magnetic fields for an example of a two‐layer model illustrate an advantage of the magnetic‐field measurement; namely, that it is sensitive to the position of layer boundaries rather than to the position of transmitter electrodes. The algorithm is also applied to interpretation of MMR field data from two boreholes drilled near massive sulfide conductors. The magnetometric influence (or background) due to borehole geometry in an electrically layered earth may be computed and subtracted from the field data, after which it is possible to perform quantitative modeling of the residual MMR anomaly to define the location of ore‐related conductors.

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