geoelectromagnetic induction, magnetotellurics, inter-station transfer function, 2-D inversion
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.
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