Imaging 3D effects in magnetotelluric data across the Waterberg Fault, Namibia

Within the framework of an integrated geophysical and geological project we recorded MT and GDS data in a broad frequency range between 0.001s and 1000s at 107 sites. In this paper we concentrate on a subset across the Waterberg Fault / Omaruru Lineament (WFOL), a major tectonic zone boundary in the Central Zone of the Damara Belt. 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 WFOL in detail. Also seismic investigations have been carried out along both profiles. Explosions in 500m spacing were recorded with 180 seismic sensors (100 m spacing). 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^o and a strong correlation of parallel electric and magnetic field components for long periods. We explain most of the observed effects with generic 3D models which suggest current gathering. Applying Propagation Number Analysis (PNA), we obtain an image of the conductivity distribution by plotting current ellipses. They indicate that the WFOL is not a narrow fault, but a wider fault zone extended to the North with a strongly preferred direction of current flow running parallel. A wider fault zone is in agreement with the seismic results. The tomographic inversion of the direct P- and S-wave travel time data reveals the complex velocity structure of the shallow (<1km) subsurface. The WFOL is imaged as a region of decreased velocities.