DARWIN Digitale Dissertationen German Version Strich

FU Berlin
Digitale Dissertation

Wolfgang Soyer :
Analysis of geomagnetic variations in the Central and Southern Andes
Analyse geomagnetischer Variationen in den Zentralen und Südlichen Anden

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Abstract

This study is concerned with the (re)analysis of long period electromagnetic variation data recorded between 1993 and 2000 during several field campaigns in the Central (~21°S) and Southern (~39°S) Andes to derive the spatial distribution of subsurface electrical conductivity at this active continental margin. Taking particular advantage of the simultaneity of the data, additional to local transfer functions also inter-station transfer functions have been calculated and analyzed in terms of a full array data analysis.

From uniqueness theorems for 2-D inversion solutions of ideal local data sets, it is shown that also for magnetic inter-station data, the 2-D inversion problem has in principle an unique solution. The analysis of such geomagnetic perturbation data is rarely performed and modern inversion programs are all designed for the analysis of local transfer functions. Therefore a recently published 2-D inversion code for local data has been extended to the inversion of geomagnetic inter-station transfer functions. Synthetic modelling studies demonstrate that over laterally extended conductivity anomalies an investigation of horizontal perturbation data can resolve the vertical conductivity distribution and thus support an analysis of impedance data, which are more likely subject to distortion effects due to superficial heterogeneities. An inversion of data with various references is also possible, though most probably not equally effective.

Since in the Central Andes not all inter-station data can be related to the same reference, the potential of such data analysis could not fully be exploited here. Inclusion of geomagnetic data in the modelling generally confirms the anomalies derived in previous analysis - a good conductor below the Chilean Precordillera and a vast zone of high conductivity below the Bolivian Altiplano -, but indicates laterally enhanced conductivities in the eastern part of the Altiplano, which is not reflected in inversions of impedance data alone.

The conductive anomaly below the volcanic arc in the Southern Andes is comparably weak, and therefore horizontal perturbations are small, so that they can not give significantly more insight into the subsurface conductivity distribution than local data. However, magnetic data in South Chile have clear signature of a continental mid to lower crustal horizontal electrical anisotropy. The anisotropy strike is not well constrained, but clockwise oblique to the structural resp. morphological strike. One possible explanation for the proposed anisotropy are conductive magmatic dykes, which are oriented parallel to the direction of maximum horizontal stress, and not just confined to a narrow band below the volcanic arc.

Table of Contents

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Title, Contents 1
1. Introduction 5
2. Methodical fundamentals 9
2.1. The physical model 10
2.2. Calculation of transfer functions 11
2.3. Analysis of transfer function tensors 13
3. On the study areas 19
3.1. The Central Andes 19
3.2. The Southern Andes 29
4. Field data analysis 33
4.1. Processing techniques 33
4.2. Application to the processed data 35
4.3. Combination of processed arrays 39
5. Presentation of the processed data 41
5.1. Central Andes 41
5.2. Southern Andes 51
6. 2-D inversion of inter-station transfer functions 55
6.1. General considerations ... 55
6.1.1. ... on the amount of information 55
6.1.2. ... on the uniqueness of inversion solutions 56
6.2. The inversion scheme of the employed program 57
6.3. Calculations of sensitivities 59
6.4. Synthetic examples 61
6.4.1. Model from Siripunvaraporn & Egbert 62
6.4.2. A very crude ANCORP resistivity model 64
6.4.3. Inversion of data with various references 66
7. Isotropic 2-D modelling 71
7.1. The ocean effect 71
7.2. On the sensitivity and accuracy of the two polarizations 72
7.3. Central Andes 74
7.4. Southern Andes 82
8. Anisotropic 2-D modelling of magnetic data from the Southern Andes 89
9. Discussion and conclusions 97
10. Summary 105
Acknowledgements 107
Bibliography 109
Appendix 119
A Calculation of forward responses in REBOCC 119
A.1. Solution of the induction problem 119
A.2. Calculation of field components 121
B Data and joint inversion responses 123
C Station runtimes 129
D Analysis of an instrumental test recording 132

More Information:

Online available: http://www.diss.fu-berlin.de/2002/134/indexe.html
Language of PhDThesis: english
Keywords: geoelectromagnetic induction, magnetotellurics, inter-station transfer function, 2-D inversion
DNB-Sachgruppe: 31 Geowissenschaften
Date of disputation: 22-May-2002
PhDThesis from: Fachbereich Geowissenschaften, Freie Universität Berlin
First Referee: Prof. Dr. Volker Haak
Second Referee: Prof. Dr. Hans-Jürgen Götze
Contact (Author): w.soyer@geophysik.fu-berlin.de
Contact (Advisor): vhaak@gfz-potsdam.de
Date created:24-Jul-2002
Date available:26-Jul-2002

 

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