Magnetometer array study in a tectonically active region of Quebec, Canada

Summary. An array of 26 magnetometers deployed in a tectonically active area of Quebec has mapped a boundary in terrestrial electrical conductivity for 200 km along strike. The contrast in conductivity across the boundary, from previous magnetotelluric soundings, is about one order of magnitude. Anomalous variation fields associated with electric currents flowing along the boundary are readily detected at pulsation periods only when the horizontal field is polarized transverse to the structure (the E-polarization case). The anomaly is hardly visible in transfer functions from substorms, for a number of reasons: a predominant H-polarization orientation of the substorm fields, the single order-of-magnitude contrast in conductivity, and the probably small depth extent of the structure. Attempts were made to estimate the response of a one-dimensional earth via the inductive scale length q0) =z/(ax/ax t a y/aY) with gradients evaluated from polynomial surfaces fitted to the smoothly varying substorm fields. The results were inconsistent, owing to vertical fields with strong external components and to horizontal fields with scale lengths too small relative to their penetration distances.

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