PhD thesis,
Entwicklung eines Verfahrens zur Abbildung krustaler Leitfähigkeitsstrukturen anhand von Magnetotellurikdaten aus Namibia
Freie Universität Berlin, Berlin, Germany, (2002)
Abstract
Within the framework of an integrated geophysical and geological project
in Namibia we recorded MT and GDS data in a broad frequency range
between 0.001s and 1000s at 107 sites. In my thesis I concentrate
on a subset of sites across the Waterberg Fault / Omaruru Lineament
(WF/OL), a major tectono-stratigraphic zone boundary in the Central
Zone of the Damara Belt in Namibia. Most of the sites are aligned
along two parallel 18 km long profiles with a site spacing of 500
m and 2 km, respectively. These dense profiles together with 20 additional
sites to the East and West provide a good spatial coverage to study
the WF/OL in detail. The MT results at some sites in the vicinity
of the Fault are strongly affected by 3D effects - we observe high
skews, phases over 90 deg and a strong correlation of parallel electric
and magnetic field components for long periods. The interpretation
of such a complicated magnetotelluric data set with 3D forward modelling
is difficult and very time consuming. To image the measured data
directly - without a priori information - requires the transformation
of the complex impedance tensor in a physically meaningful quantity.
Normal apparent resistivities calculated from the impedance tensor
do not retain the tensor properties. Therefore I developed a method
to derive an apparent resistivity tensor. Combining the MT admittance
equation with Maxwell's equations we yield a tensorial equation for
the propagation number. This quantity is subsequently separated into
two tensors - an apparent permittivity and an apparent conductivity
tensor. From the latter an apparent resistivity tensor can be computed.
This method is called Propagation Number Analysis (PNA). Applying
PNA to MT data, we obtain an image of the conductivity distribution
in form of resistivity ellipses. The PNA is also compared with two
already known methods for imaging conductivity anomalies: Eggers'
Eigenstate Analysis and LaTorraca's Singular Value Decomposition.
All imaging methods presented are applied to synthetic data and measured
MT data from Namibia. The resistivity ellipses obtained by the PNA
indicate that the WF/OL is not a narrow fault, but a wider fault
zone extended to the North. Most of the observed 3D effects are explained
by the combination of (i) a shallow conductive ring structure and
(ii) an anisotropic zone in the upper crust and a lower crust which
itself is also anisotropic. The anisotropic zone is approximately
10km wide and modelling studies suggest that the fault zone might
continue down to at least 14 km. urn:nbn:de:kobv:188-2002000247
