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Magnetotelluric Investigations in the Rwenzori Region, Uganda

Prominent geological structures within the Earth's crust and mantle often are associated with electrical conductivity anomalies, mainly due to the influence of their mineral composition and of their fluid and/or partial melt content on the electrical conductivity. Thus the 3D image of the electrical conductivity distribution of the subsurface gives valuable information for the reconstruction of tectonic formations and processes.




Fig. 1: Site distribution: Orange diamonds refer to telluric sites, red diamonds to magnetotelluric sites. The inlet shows the position of the survey area (black rectangle) within East Africa



Most of our knowledge about the Earth's conductivity distribution at greater depth comes from observing natural time variations of the Earth's magnetic and electric (=telluric) fields at the Earth's surface. The magnetic field variations originate from electric current systems in the Ionosphere and Magnetosphere, and they induce secondary electric currents within the subsurface, whereas the strength and direction of these currents depend on the electrical conductivity distribution of the underground. At the Earth's surface the superposition of the primary and secondary field variations is observed at selected sites and explained by adequate models of the electrical conductivity distribution underneath the area of observation.

Within the Magnetotelluric (=MT) method the horizontal and vertical components of the magnetic field variations and the horizontal components of the telluric field are observed simultaneously during a certain time interval at selected field sites. The locations are chosen according to the geological problem and logistic constraints. The time length of the observations depends on the signal to noise ratio and the favoured depth of investigation, as the signals are attenuated while penetrating into the subsurface according to the period of the time variation and the conductivity of the underground - the penetration depth increases with longer periods and lower conductivity (skin effect).

Within the Riftlink project 23 MT sites were installed in the Rwenzori region during two field campaigns in 2007 and 2008 (Fig. 1). Each field campaign lasted about 2 months. At 8 sites the magnetic field variations and at all sites the telluric field variations were observed. Most observations were of excellent quality and showed high spatial correlation (Fig. 2). The horizontal components of the magnetic field were spatially rather homogeneous whereas the vertical component and the telluric horizontal components turned out to be rather location dependent and thus reflect the conductivity situation underneath.



Fig. 2: Examples for time variations of horizontal magnetic and telluric field components at different sites for a 4 hours time interval



In our data analysis we concentrate on the period range between 10 - 10.000 s which corresponds to a depth range of 1-200 km. The information about the conductivity structure is hidden in the relations between the horizontal components of the magnetic field and the vertical magnetic field as well as the horizontal components of the telluric field. The relations are best displayed as transfer functions in the frequency domain, reflecting amplitude and phase relations between the observed field components for certain frequencies resp. periods and thus, due to the skin effect, for certain depth ranges.



Fig. 3a and 3b display the phase tensor invariants φmin and φmax at most of the locations for the periods 156.2 and 1562 seconds. The colours of the bars are coded by the values of φmin and φmax. Thus the colours reflect the change of φmin and φmax with period resp. depth.



Four transfer functions are estimated between the horizontal magnetic and telluric field components at each site. The phase relations are displayed at two selected periods, 156 and 1562 seconds, in Fig. 3: The lengths of the bars point into the direction of minimum and maximum phases at each location, the colours of the bars reflect the phase values. It is obvious that the pattern of the phases is spatially very consistent for the long period, whereas the short period data display significant differences between the Northern and Southern part. Overall, there is a conspicuous large difference between each minimum and maximum phase value which is referred to as phase anisotropy.



Fig. 4: Top view and cross section of the 3D anisotropic model (dashed line marks the position of the cross section).



The data can be explained by a complex anisotropic 3D conductivity model (Fig. 4): The Rift sediments have rather low resistivity compared to the Rwenzori Mountain area and the basement. It is striking that the data can only be explained by introducing an electrical anisotropy increasing by a factor of 100 within a depth range from 20-50 km, the high conducting direction pointing perpendicular to the Rift axis. Furthermore the observations demand a high conducting area to the South of the Rwenzoris at about 20 km depth (dark shaded block in Fig. 4). The conductivity increases within the Upper Mantle at about 200 km depth.



Fig. 5: Comparison of modelled (grey) and observed (black) phase tensor invariants φmin(.) and φmax(o) at sites KASE (left) and induction vectors at site UTCK (right).



The excellent consistency between observed and modelled data is shown exemplarily for the telluric transfer functions and the tipper vectors (transfer functions between vertical and horizontal magnetic field components) at sites KASE and UTCK (Fig. 5). Again the phase anisotropy is striking; the change of direction with period of the imaginary tipper vector is a clear indication for the three-dimensionality of the conductivity structure.The high conducting body at the Southern part of the measurement area matches rather well a zone of low seismic velocity (project A1). This result gives evidence for weak crustal layer and might indicate the existence of partial melt. An unsolved mystery is the electrical anisotropy: It is reported in literature that a rather high electrical anisotropy can be caused by Olivin crystals under the influence of water. However, within the measurement area Olivin has not been found in Xenoliths (project A3).