## 4 Results
### Electrical Anisotropy
#### 4.2.2 The Ellipticity of the Phase Tensor  
Evidence of the phase split can be observed in the presence of anisotropy for the long periods where near surface galvanic distortion has less effect on the regional phase response. In general cases, this phenomenon can be hidden or difficult to observed for the short periods due to the near surface galvanic distortion effects which are significant [24]. Thus, phase split derived from impedance phase is distorted in the same way as the impedance tensor is distorted for the short periods, whereas the phase tensor is not affected by the galvanic distortion for both the long and the short periods [22; 24]. The maximum and minimum phases of the principal values of phase tensor [22; 27] are invariant quantities, and their geometric mean \(\varphi_{\mathbf{mean}}\) can be calculated to determine the magnitude of the phase tensor response. Moreover, the ellipticity of phase tensor can help to assess the presence of electrical anisotropy [24]. The major axes of ellipses are generally oriented N43\({}^{\circ}\)E at short periods, but for the long periods their direction tends to be north-south (Figure 7). Furthermore, ellipticity and the shape of ellipses for many MT sites are related to the phase split between the \(xy\) and \(yx\) polarization in a regional 2D resistivity where the minor axis of the ellipse decreased. The consistency of this feature at many periods (Figure 7) may be produced either by 2D conductivity structure or electrical anisotropic medium or an interface between isotropic and anisotropic medium. Logically, single large 2D electrical heterogeneity is not expected at depth in the study area over the entire 2D profile that can generate the consistent ellipticity because there is not one unique direction of induction vectors in Figure 5 that support the assumption of single large 2D electrical heterogeneity. Consequently, the potential cause of this consistent ellipticity with approximate preferred direction can be explained by the presence of anisotropy. The greatest values of the geometric mean \(\varphi_{\mathbf{mean}}\) = (\(\Phi_{\mathbf{max}}\)\({}^{*}\)\(\Phi_{\mathbf{min}}\))\({}^{1/2}\) that are coincident with obvious ellipticity are observed for intermediate and long periods in the center and in the north-east indicating conductivity increase (Figure 7). The pattern displayed by the values of \(\varphi_{\mathbf{mean}}\) is somehow different for MT sites between the center and the south-west where this pattern is more localized for the intermediate periods. A relative increase in \(\varphi_{\mathbf{mean}}\) values appears in some MT sites in the center, south-west and north-east for short and intermediate periods, which can be interpreted as high contrast resistivity inside a macro-anisotropic medium and the surrounding rocks for the corresponding periods. For long periods, ellipticity reduces in the north-east and at the center of the rift, with an increase in \(\varphi_{\mathbf{mean}}\) values. This increase in \(\varphi_{\mathbf{mean}}\) values can suggest a possible interface between two anisotropic media with different geoelectric strikes and a high conductivity contrast between the two media where the deepest medium may be more conductive than the shallow medium (Figure 7).