Invariant TE and TM magnetotelluric impedances: application to the BC87 dataset

Gómez‐Treviño, Enrique; Muniz, Y.; Cuellar, Mayra; Calderón-Moctezuma, Armando

doi:10.1186/s40623-018-0900-y

Cited by 4 publications

(2 citation statements)

References 25 publications

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“…However, this is how it should be since the undistorted impedances are the ones that are ultimately interpreted. If data are available for several nearby sites the factors can be estimated using existing inverse routines in anti-Occam mode (Gómez-Treviño et al, 2014b;2018).…”

Section: Metodology the Groom-bailey Decompositionmentioning

confidence: 99%

“…However, if this is not the case the situation can become helpless. Interpreting the well-known BC87 data set, much to our dismay we had to recur to the algorithm STRIKE simply to identify the modes (Gómez-Treviño et al, 2018), although the rest of the analysis was based on the ± formulae. To simply identify the modes there are other alternatively.…”

Section: Introductionmentioning

confidence: 99%

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Linking strike directions to invariant TE and TM impedances of the magnetotelluric impedance tensor

Arellano-Castro

Gómez‐Treviño

2021

Preprint

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The traditional transverse electric (TE) and transverse magnetic (TM) impedances of the magnetotelluric tensor can be decoupled from the strike direction with significant implications when dealing with galvanic distortions. Distortion-free impedances are obtainable combining a quadratic equation with the phase tensor. In the terminology of Groom-Bailey, the quadratic equation provides amplitudes and phases that are immune to twist and the phase tensor provides phases immune to both, twist and shear. On the other hand, distortion-free strike directions can be obtained using Bahr’s approach or the formula provided by the phase tensor. In principle, this is all that is needed to proceed to a two-dimensional (2D) interpretation. However, the resulting impedances are strike-ignorant because they are invariant under rotation and, if they are to be related to a geological strike they must be linked to a particular direction. This is an extra ambiguity beside the classical of 90 degrees which must be resolved independently. In this work we use the distortion model of Groom-Bailey to resolve the ambiguity by bringing back the coupling between impedances and strike in the presence of galvanic distortions. Considering that most quantities are already known, fitting the responses of the model to the data requires minimizations only over the single variable of twist, instead of the original approach of having to minimize not only twist, shear and strike, but also the impedances themselves. Our approach is a hybrid between existing numerical and analytical approaches that reduces the problem to a binary decision. The fusion of the two approaches is illustrated using synthetic and field data.

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Section: Metodology the Groom-bailey Decompositionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Linking strike directions to invariant TE and TM impedances of the magnetotelluric impedance tensor

Arellano-Castro

Gómez‐Treviño

2021

Preprint

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How close can we get to the classical magnetotelluric sounding?

Calderón-Moctezuma

Gómez‐Treviño

Yutsis

et al. 2022

Journal of Applied Geophysics

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Invariant TE and TM impedances in the marine magnetotelluric method

Montiel-Álvarez

Romo

Constable

et al. 2020

Geophysical Journal International

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SUMMARY The magnetotelluric (MT) impedance tensor has a nil diagonal when one of the axes of the coordinate system coincides with the strike of a 2-D structure. In general, real data are full tensors either because of 3-D effects or measurements not aligned to the geological strike. The usual practice to adapt the field tensor to the 2-D assumption is to rotate to a new system of coordinates. In most cases, there is no single angle of rotation that warranties that the diagonal elements become zeros as in the ideal 2-D case. Even maximizing the off-diagonal elements does not necessarily produce a nil diagonal. Consequently, the 2-D inversions proceed by neglecting whatever there is left in the diagonals. In this work, we explore an alternative that places no constraints on direction but assures a nil diagonal. We use two rotational invariants that compact the four elements of the tensor into only two and reduce in 2-D to the TE and TM impedances. These are obtained readily by solving a quadratic equation. We explore four different scenarios: (1) using the invariants, (2) rotating the tensor perpendicular to the profile, (3) rotating to the average maximum orientation for each station and (4) maximizing the off-diagonal elements of the tensor for each site, frequency to frequency. These approaches were applied to 3-D synthetic and field data. The field data correspond to two marine magnetotelluric surveys in the Gulf of California. In one of them, there is no information on the instrument orientation because the compasses failed. In this case, the rotational invariants come handy to overcome the problem. In the other survey, there was orientation information and the 2-D inversions illustrate the better performance of the invariants relative to the traditional approaches.

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Invariant TE and TM magnetotelluric impedances: application to the BC87 dataset

Cited by 4 publications

References 25 publications

Linking strike directions to invariant TE and TM impedances of the magnetotelluric impedance tensor

Linking strike directions to invariant TE and TM impedances of the magnetotelluric impedance tensor

How close can we get to the classical magnetotelluric sounding?

Invariant TE and TM impedances in the marine magnetotelluric method

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