Magnetotelluric fieldwork adventures in Africa

The magnetotelluric (MT)method is away of probing the electrical resistivity (or its inverse, electrical conductivity) distribution of the subsurface. It is based on the principle of electromagnetic induction, in which natural magnetic field sources external to the Earth induce eddy currents within it whose geometry and decay depend on the subsurface resistivity structure. The method is therefore entirely passive, and it can be employed in environmentally sensitive areas. The resistivity of the subsurface depends on its temperature, mineralogy and fluid content, both interstitial and partial melt, and is sensitive to even small amounts of melt and residual fluids and its connectivity. Thus it can be a good discriminant between different rock types, and complements information available from other geophysical methods. Here, I outline the basics of the MT method, indicate how data acquisition, processing and modelling are undertaken, and illustrate the uses of MT to investigate tectonic and structural problems through three case studies from Africa.