Two years of magnetotelluric measurements in Abitibi, western Québec, using a telephone line

Continuous magnetotelluric measurements were made over a period of 600 days, with 100-m-, 30-km- and 100-km-long dipoles and a period range of 40–4000 s. Data analysis for different dipole lengths indicates the presence of static shift at various scales. It is shown that the longer the telluric dipole, the less statically shifted the resistivity curves; nevertheless, static shifts can still be present due to geological structures causing anomalies exhibiting wavelengths comparable to the dipole length. Also, a relationship is observed between the coherence and the main impedance components. This relation is explained in terms of signal-to-noise ratio. A way to reduce the bias on the impedance estimates is suggested. The apparent resistivities and phases computed from three different impedance estimates using 100-km-long dipoles are then compared to those observed in similar studies made near our observation region.