## 2 Materials and Methods
### Magnetotelluric Sounding (MT)  
The MT method studied the penetration and propagation of electromagnetic waves inside the Earth, associated with the action of electrical storms and/or the incidence of the solar wind on the Earth [6]. The method was based on measurements taken on the Earth's surface of the natural electric field (employing two perpendicular electric dipoles, Ex and Ey) and the magnetic field (in our case, using a triaxial flux magnetometer with components Bx, By, and Bz). The Earth's surface partially reflected the fluctuating electromagnetic fields that originated in the ionosphere, and the ionosphere again reflected the returning fields due to their conductive characteristics. It repeatedly happened so that the fields eventually had a strong vertical component and could be considered vertically propagating plane waves, characterized by covering a broad spectrum of frequencies. These fields penetrated the ground and induced telluric (electric) currents, generating secondary magnetic fields. The telluric currents, detected by two pairs of electrodes, each pair of which composed a dipole, were perpendicularly oriented. The three components of the magnetic fields were measured: the vertical component and two horizontal components, parallel to each one of the electrical components [9; 10]. This method provided information about the resistivity (conductivity) values for much greater depths than artificial source induction methods. Using long-period signals ranging from tens to thousands of seconds, the MT method reached investigation depths that may sample the entire lithosphere [9]. In this work, the permanent station guaranteed continuous datasets for years and, consequently, large depths of investigation. Details for estimating the apparent resistivity structure of the subsoisils for an instant and using the MT method can be found in [10] and [6]. Because an electromagnetic wave in the subsoil decays its amplitude due to the resistivity of the medium (assuming a homogeneous half-space), the depth at which the amplitude reaches the factor \(1/e\approx 0.37\) (skin depth, \(\delta\)) can be estimated using the expression \(\delta(\omega)~{}=~{}\sqrt{2/\omega\mu\sigma}\)[6; 10], which can be simplified to:  
\[\delta\approx 503\sqrt{\rho T}, \tag{1}\]  
where \(\omega\) is the angular frequency under the assumption of \(e^{i\omega t}\) time dependence, \(\mu\) is the magnetic permeability, \(\sigma\) is the electrical conductivity, \(\rho\) is the resistivity, and \(T\) is the period evaluated. Thus, the procedure implemented with the data acquired for this work is briefly described as follows:  
1. Time windows of 5000 s were chosen. An overlap of 500 s was used to estimate the resistivity's temporal evolution. Each time window was tapered with a Hanning window, and its frequency spectrum was calculated using the FFT. 2. The orthogonal components of the natural electric (\(\mathbf{E}\)) and magnetic (\(\mathbf{B}\)) fields were related to the impedance tensor \(\mathbf{Z}\) of the subsoil in the following way [10]: \[ \begin{pmatrix}E_{x}\\ E_{y}\end{pmatrix} ~{}=~{} \begin{pmatrix}Z_{xx}&Z_{xy}\\ Z_{yx}&Z_{yy}\end{pmatrix}\begin{pmatrix}\frac{\mathbf{B}_{x}}{\mu_{0}}\\ \frac{\mathbf{B}_{y}}{\mu_{0}}\end{pmatrix} ,\] (2) where \(\mathbf{E}~{}=~{}\mathbf{Z}_{\mu_{0}}^{\mathbf{\frac{\mathbf{B}}{2}}}\) and \(\mu_{0}\) is the magnetic permeability of free space \(\left[\mathrm{V}~{}\mathrm{s}~{}\mathrm{A}^{-1}~{}\mathrm{m}^{-1}~{}=~{} \mathrm{H}~{}\mathrm{m}^{-1}\right]\). From the complex coefficients of the frequency spectrum, the \(Z_{xy}\) component under the station was estimated. 3. The Schmucker-Weidelt (\(C\)) transfer function was estimated as follows: \[C(\omega)~{}=~{}\frac{E_{x}}{\mathrm{i}\omega B_{y}}~{}=~{}\frac{Z_{xy}}{ \mathrm{i}\omega}.\] (3)
4. The \(\rho_{a}\) was calculated using the following expression: \[\rho_{a}(\omega)~{}=~{}|C(\omega)|^{2}\mu_{0}\omega.\] (4)5.