South Africa - Inkaba.org

Study of the geo-electrical anisotropy in the Cape Fold Belt
using magnetotelluric
X. Chen 1, 2 , U. Weckmann 1
1. Helmholtz Centre Potsdam – German Research Centre for Geosciences, Potsdam, Germany
2. Institute of Earth and Environmental Sciences, University of Potsdam, Potsdam, Germany
xiaoming@gfz-potsdam.de , uweck@gfz-potsdam.de
ABSTRACT
Within the framework of the German - South African geo-scientific research initiative Inkaba yeAfrica a series of
magnetotelluric (MT) field experiments were conducted along the Agulhas-Karoo Transect in South Africa. This
transect crosses several continental collision zones between the Cape Fold Belt, the Namaqua Natal Mobile Belt
and the Kaapvaal Craton. Along the Cape Fold Belt profile we can identify areas (> 10km) where MT sites exhibit
phases over 90 degrees. This phenomenon usually occurs in presence of electrical anisotropy. Due to the dense site
spacing we are able to observe this behaviour consistently at several sites.
With isotropic 2D inversion we are able to explain most features in the MT data but not the abnormal phase
behaviour. In order to explain the abnormal phase behaviour we have to include zones of electrical anisotropy.
Currently, we can use 2D anisotropic forward modelling to reproduce the general trend of phases leaving the
quadrant. To explain the MT data in more detail, 2D anisotropic inversion would be desirable. In this presentation
we focus on a spatially constraint 2D anisotropic inversion. In a first step, we use simple 2D models with
embedded zones of electrical anisotropy to study the general influence of anisotropy parameters. In a second step,
isotropic 2D inversion results are used as background models in which we include anisotropic zones in order to fit
the abnormal phase curves in the data. For these forward modelling studies we implemented the 2D anisotropic
code by Pek & Verner (1997) into the modular EM modelling and inversion software (Egbert, 2006; Egbert &
Siripunvaraporn, 2008). The modular system provides an ideal environment to implement this inversion scheme.
KEYWORDS: Cape Fold Belt, magnetotelluric, anisotropy, modelling, constraint inversion.
14