Magnetic distortion of GDS transfer functions : An example from the Penninic Alps of Eastern Switzerland revealing a crustal conductor

We carried out sixty-four MT and GDS soundings in the eastern Swiss Alps to obtain information on the electrical conductivity distribution. One of the main findings is an anomalous directional behaviour of the real induction arrows over the entire period range (T = 1–300; s) on the Bündnerschiefer. The Bündnerschiefer occurs between the crystalline Aar and Gotthard massives to the North and the crystalline Penninic nappes (Adula, Tambo, Suretta) to the South within the investigation area. The sediments form an elongated eastwards plunging ramp with a possible conductive link to the Northern Swiss Permo-carboniferous trough (Molasse basin). We consider electric currents induced in various local and regional conductive structures and leaking into the Bündnerschiefer as possible causes for the observed effect upon the electromagnetic field. Applying Hypothetical Event Analysis (HEA) we find a spatial decoupling of the induction processes with depth and a conductivity anomaly presumably due to the northward indentation by the Adriatic plate.