Acquisition, Processing and Interpretation of Chambon-La-Forêt magnetotelluric data

Séminaire associé, en collaboration avec les observatoires magnétiques et l'équipe de géomagnétisme. We conducted MT and Audio-MT data acquisition in the vicinity of the French magnetic observatory of Chambon-La-Forêt (CLF, Loiret), to characterize the electrical resistivity of the subsurface around the magnetic observatory. High-frequency data show anthropogenic signal with large amplitude, preventing the use of standard processing schemes. To recover proper MT soundings, we developed new numerical schemes, based on the application of the continuous wavelet transform. Geomagnetic and atmospheric waves with large signal-to-noise ratio are first sought in MT times series, based on their time-frequency signature, prior to the robust computation of MT response functions, allowing a large improvement in the quality of MT soundings. From the analysis of MT response functions, we show that the conductivity distribution can be assumed to be only vertically varying for depths smaller than 2 km, with values corresponding to standard sedimentary basins. Below 2 km, MT soundings show 2D geometry, with geological strike and depth corresponding to the signature of the magnetic anomaly of the Paris basin.