Fast and Stable Two-dimensional Inversion of Magnetotelluric Data

1995 ◽  
Author(s):  
M.S. Zhdanov ◽  
P.P de Lugao ◽  
O. Portniaguine
Keyword(s):  
Two Dimensional ◽  
Magnetotelluric Data

DIAGRAMS FOR MAGNETOTELLURIC DATA

Geophysics ◽  
1976 ◽  
Vol 41 (4) ◽  
pp. 766-770 ◽  
Author(s):  
F. E. M. Lilley
Keyword(s):  
Impedance Tensor ◽  
Two Dimensional ◽  
Magnetotelluric Data ◽  
One Dimensional ◽  
Magnetic Components ◽  
Strike Direction ◽  
Electric And Magnetic Fields ◽  
The Relationship ◽  
Special Case ◽  
Phase Differences

Observed magnetotelluric data are often transformed to the frequency domain and expressed as the relationship [Formula: see text]where [Formula: see text] [Formula: see text] and [Formula: see text] [Formula: see text] represent electric and magnetic components measured along two orthogonal axes (in this paper, for simplicity, to be north and east, respectively). The elements [Formula: see text] comprise the magnetotelluric impedance tensor, and they are generally complex due to phase differences between the electric and magnetic fields. All quantities in equation (1) are frequency dependent. For the special case of “two‐dimensional” geology (where structure can be described as having a certain strike direction along which it does not vary), [Formula: see text] with [Formula: see text]. For the special case of “one‐dimensional” geology (where structure varies with depth only, as if horizontally layered), [Formula: see text] and [Formula: see text].

COHERENCE FUNCTIONS FOR MAGNETOTELLURIC ANALYSIS

Geophysics ◽  
1974 ◽  
Vol 39 (3) ◽  
pp. 312-320 ◽  
Author(s):  
I. K. Reddy ◽  
D. Rankin
Keyword(s):  
Data Analysis ◽  
Linear System ◽  
System Approach ◽  
Partial Coherence ◽  
Impedance Method ◽  
Two Dimensional ◽  
Magnetotelluric Data ◽  
The Earth ◽  
Principal Axes ◽  
Tensor Impedance

A multiinput linear system approach is used to study the magnetotelluric phenomena in the presence of lateral conductivity inhomogeneities in the earth. The three types of coherence functions (ordinary, multiple, and partial) are defined, and their use in magnetotelluric data analysis is illustrated with a field example. Partial coherence functions are used to determine the principal axes in the case of two‐dimensional type inhomogeneities, and as measures of three‐dimensionality in the case of non‐two‐dimensional type structures. The results obtained using coherence functions are compared with those obtained with the conventional tensor impedance method.

scholarly journals METHODOLOGY TO ESTIMATE THE MAXIMUM DEPTH OF RELIABILITY OF TWO-DIMENSIONAL GEOELECTRICAL MODELS DERIVED FROM MAGNETOTELLURIC DATA

2013 ◽  
Vol 31 (1) ◽  
pp. 97
Author(s):  
Andréa Cristina Lima dos Santos ◽  
Antônio Lopes Padilha ◽  
Ícaro Vitorello ◽  
Marcelo Banik de Pádua ◽  
Augusto César Bittencourt Pires
Keyword(s):  
Heuristic Method ◽  
Transverse Magnetic ◽  
Empirical Method ◽  
Maximum Depth ◽  
Northeastern Brazil ◽  
Perfect Conductor ◽  
Two Dimensional ◽  
Magnetotelluric Data ◽  
Starting Point ◽  
Forward Calculation

An empirical technique is proposed to estimate the maximum depth of reliability of two-dimensional (2D) geoelectrical models derived from magnetotelluric surveys conducted in regions with different conductivity. The results are then compared to those derived from a heuristic methodology well established in the literature. Experimental data from a linear profile cutting across the major structures in the SE portion of the Borborema Province, northeastern Brazil, are used. The data were collected with modern instrumentation, processed by robust techniques and modeled using inversion algorithms available nowadays to the research community of electromagnetic induccion inside the Earth. Sensitivity tests have shown that the 2D geoelectrical section is robust and accurately represents the conductivity distribution below the profile. The 2D section is used as a starting point for the empirical method employed, which consists of introducing a perfect conductor (or resistor) at different depths of the 2D model. The effect of adding this structure to the data fitting is checked through forward calculation and by comparing the RMS misfit. The results show that the maximum depth of reliability of the 2D geoelectrical model is usually given by the phase of the transverse electric (TE) mode, whereas the maximum depth of propagation of the EM signal is usually given by the phase of transverse magnetic (TM) mode. The empirical approach shows similar variations in depth when compared to the results from the heuristic method, but provides lateral variations more compatible with the diffusive process of EM wave propagation. RESUMO: Uma técnica empírica para estimar a profundidade máxima de confiabilidade de modelos geoelétricos bidimensionais (2D), obtidos a partir de sondagens magnetotelúricas realizadas em regiões de diferentes condutividades, é aqui proposta e seus resultados são comparados àqueles derivados de uma metodologia heurística já consagrada na literatura. Para tanto, são utilizados dados experimentais obtidos em um perfil linear que corta transversalmente as principais estruturas e terrenos na porção SE da Província Borborema, região Nordeste do Brasil. Os dados utilizados foram coletados com instrumentação moderna, processados por técnicas robustas e modelados por algoritmos de inversão atualmente disponíveis para a comunidade de estudos de indução eletromagnética no interior da Terra. A seção geoelétrica 2D derivada desse procedimento é robusta em relação a diferentes testes de sensibilidade e representa adequadamente a distribuição de condutividade elétrica sob o perfil, sendo aqui utilizada como ponto de partida para o método empírico empregado. A técnica empírica aqui proposta é bastante simples, baseada na introdução de um condutor (ou um resistor) perfeito em diferentes profundidades do modelo de distribuição de condutividades e verificando seu efeito no ajuste dos dados (RMS) por cálculo direto usando o programa empregado na inversão dos dados. Os resultados obtidos mostram que a profundidade limite de validade da interpretação 2D do modelo geoelétrico é geralmente dada pela fase do modo transverso elétrico (TE) de propagação do sinal eletromagnético (EM), enquanto o limite máximo de propagação desse sinal é dado pela fase do modo transverso magnético (TM). Em comparação com as profundidades de investigação obtidas pelo método heurístico, a metodologia empírica mostra comportamento semelhante nas variações de profundidade, mas fornece variações laterais mais compatíveis com o processo difusivo de propagação das ondas EM.Palavras-chave: sondagem magnetotelúrica; modelo geoelétrico bidimensional; profundidade máxima de confiabilidade do modelo 

scholarly journals Two-dimensional determinant inversion of marine magnetotelluric data and a field example from the Gulf of California, Mexico

Geophysics ◽  
2021 ◽  
Vol 86 (1) ◽  
pp. E37-E57
Author(s):  
Shunguo Wang ◽  
Steven Constable ◽  
Valeria Reyes-Ortega ◽  
Hormoz Jahandari ◽  
Colin Farquharson ◽  
...  
Keyword(s):  
Gulf Of California ◽  
Subduction Zones ◽  
Field Data ◽  
Real Data ◽  
Transverse Magnetic ◽  
Two Dimensional ◽  
Magnetotelluric Data ◽  
Earth Structures ◽  
3D Effects ◽  
2D Data

Two-dimensional marine magnetotelluric (MT) observations are useful for offshore geologic studies, such as natural resource exploration, fault mapping, fluid estimation at subduction zones, and the delineation of the lithosphere-asthenosphere boundary beneath the seafloor. Earth structures are often assumed to be two dimensional, which allows MT data to be decomposed into a transverse electric (TE) mode and a transverse magnetic (TM) mode. The 2D assumption can effectively reduce acquisition and computational costs. However, offline 3D effects and other problems such as lack/failure of the compass on instruments are often encountered, making it difficult to decompose data into the TE and TM modes. In these cases, 2D inversion may be misleading or may not provide an acceptable misfit to the marine MT observations. Thus, we have developed a 2D determinant inversion to the marine MT method to mitigate these difficulties, implemented in the MARE2DEM code, and we tested its utility using synthetic examples and a field example. In the synthetic examples, the determinant inversion demonstrates an ability to overcome 3D effects caused by 3D anomalies and bathymetry. With confidence from the synthetic tests, we interpreted real data acquired in the Gulf of California, Mexico, where not only is the bathymetry 3D in nature, but the external compasses failed to record the orientation. The field data can not only be fit to a reasonable misfit with a determinant inversion, but the resolved conductive zones also have a good correlation with known faults. A comparison between the resistivity model from the field data and a seismic reflection section shows that a previously interpreted fault, the Wagner Fault, should be shifted 5 km toward the southwest and made slightly steeper. Thus, the implementation of the determinant inversion may provide a new approach for using problematic 2D data.

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